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High Power of a Self-Q-Switched Tm:YAP Laser

Zhaoyu Ma, Qianqian Hao, Huanli Wang, Xudong Sun, and Linjun Li

DOI: 10.1364/OL.518655 Received 22 Jan 2024; Accepted 28 Mar 2024; Posted 28 Mar 2024  View: PDF

Abstract: For the first time, the output performance of a self-Q-switched Tm:YAP laser has been controlled by adjusting the cavity length. By using a concise concave-flat cavity, a pulsed laser emitting at 1993.08 nm is produced without any additional modulation device. Under a stable self-Q-switched mode, the maximum average output power of 9.76 W is achieved from the laser when the incident pump power is 28.78 W, corresponding to a slope efficiency of 36.86% and an optical-to-optical conversion efficacy of 33.91%. Also, the narrowest pulse width of 485 ns at 48.97 kHz is obtained from the laser with a single pulse energy of 199.31 µJ. As far as we know, this laser has the highest average power and narrowest pulse width compared to other self-Q-switched Tm:YAP lasers.

Strategically constructed AlGaN doping barriers for efficient deep ultraviolet light-emitting diodes

Ziqi Zhang, Qianxi Zhou, Xu Liu, Zhenxing Lv, Bin Tang, Hansong Geng, Shengli Qi, and Shengjun Zhou

DOI: 10.1364/OL.522212 Received 26 Feb 2024; Accepted 28 Mar 2024; Posted 28 Mar 2024  View: PDF

Abstract: Here, we propose a sandwich-like Si-doping scheme (undoped/Si-doped/undoped) in Al0.6Ga0.4N quantum barriers (QBs) to simultaneously promote the optoelectronic performances and reliability of deep ultraviolet light-emitting diodes (DUV-LEDs). Through experimental and numerical analyses, in the case of DUV-LEDs with conventional uniform Si-doping QB structure, severer operation-induced reliability degradation including the increase of reverse leakage current (IR) and reduction of light output power (LOP) will offset the enhancement of optoelectronic performances as the Si-doping levels increase to an extent, which hinders further development of DUV-LEDs. According to transmission electron microscope characterization and numerical simulation, an improved interfacial quality in multiple quantum wells (MQWs) and more uniform carrier distribution within MQWs are demonstrated for our proposed Si-doping structure in comparison to the uniform Si-doping structure. Consequently, the proposed DUV-LED shows superior wall-plug efficiency (4%), IR at -6 V reduced by almost one order of magnitude, and slower LOP degradation after 168-h 100 mA-current-stress operation. This feasible doping scheme provides a promising strategy for the high-efficiency and cost-competitive DUV-LEDs.

Hyperspectral optical vortex modulation from optically-thick liquid crystal structured waveplates

Etienne Brasselet and Vagharshak Hakobyan

DOI: 10.1364/OL.521749 Received 19 Feb 2024; Accepted 27 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: Shaping the orbital angular momentum of optical pulses in the spectral domain is a means of managing light in space and time that offers many possible applications. However, these are limited by the small number of techniques available, whose flexibility does not yet rival that of the continuous regime. We propose here to implement a tunable hyperspectral management of the orbital angular momentum of a polychromatic light field. The main idea is to exploit the dispersive nature of geometric phase optical elements by intentionally choosing to work in a regime of high anisotropic optical retardance. Experimental proof-of-principle is demonstrated in the visible range using a supercontinuum laser and an optically thick, electrically controllable, liquid crystal structured waveplate.

High-Q Two-dimensional Photonic Crystal Nanocavity on Glass with an Upper Glass Thin Film

Ryusei Kawata, Akinari Fujita, Natthajuks Pholsen, Satoshi Iwamoto, and Yasutomo Ota

DOI: 10.1364/OL.522068 Received 27 Feb 2024; Accepted 26 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: We numerically analyze two-dimensional photonic crystal (PhC) nanocavities on glass with a thin glass film on top of the structure. We investigated a multi-step heterostructure GaAs PhC nanocavity located on glass. We found that covering the structure even with a very-thin glass film efficiently suppresses unwanted polarization mode conversion occurring due to the asymmetric refractive index environment around the PhC. We also uncovered that the glass-covered structure can exhibit a higher Q factor than that observed in the structure symmetrically cladded with thick glass. We point out that the mode mismatch between the PhC nanocavity and modes in the upper glass film largely contributed to the observed Q-factor enhancement. These observations were further analyzed through the comparison among different types of on-glass PhC nanocavites covered with thin glass films. We also discuss that the in-plane structure of the upper glass film is important for additionally enhancing Q factor of the nanocavity.

Spatial resolution limit of single pixel imaging of complex light fields

Dennis Scheidt and Pedro Quinto-Su

DOI: 10.1364/OL.519587 Received 25 Jan 2024; Accepted 26 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: Complex light fields with arbitrary amplitudes and phases can be measured by sampling them with an orthogonal basis (i.e. canonical, Hadamard) and performing single pixel interferometric measurements of the focused modes. In this work we show that when the spatial resolution of the sampling basis is coarser than the spatial resolution of the phase in the complex field, the measured reconstructed amplitude exhibits crosstalk with the phase, that is, phase information appears in the amplitude. To demonstrate this phenomenon, we encode an arbitrary amplitude and phase with a spatial light modulator and compare measurements with simulations.

High-power 2.3 µm Tm:YLF laser with intracavity upconversion pumping by a Nd:ASL laser at 1051 nm

Hippolyte Dupont, TIMOTHÉE LENFANT, Lauren Guillemot, Pavel Loiko, Xavier Delen, Pascal Loiseau, Bruno viana, Thierry Georges, Patrick Georges, Patrice Camy, and Frederic Druon

DOI: 10.1364/OL.523059 Received 05 Mar 2024; Accepted 26 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: A Tm:LiYF4 laser operating on the 3H4 → 3H5 transition is embedded in a high-power diode-pumped Nd:ASL laser for intracavity upconversion pumping at 1.05 µm. This leads to a record-high output power at 2.3 µm for any bulk Thulium laser pumped by an upconversion process. The continuous-wave Tm:LiYF4 laser delivers 1.81 W at 2.3 μm for 32 W of laser-diode pump power, making this kind of pumping competitive with direct diode pumping. The intracavity pumping process allows for counteracting the low absorption inherent to upconversion pumping and to dispatch the thermal loads on two separate laser crystals. The proposed laser architecture also features a relatively weak heating of the Tm:LiYF4 crystal, and an increased tolerance to Tm3+ absorption. This laser design opens a new paradigm that holds great promise for high-power 2.3-µm solid-state lasers based on thulium ions. © 2024 Optical Society of America

Noninvasive holographic imaging through dynamically scattering media

Naoki Matsuda, Jun Tanida, Makoto Naruse, and Ryoichi Horisaki

DOI: 10.1364/OL.516083 Received 21 Dec 2023; Accepted 26 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: We present a noninvasive method for quantitative phase imaging through dynamically scattering media. A complex amplitude object, illuminated with coherent light, is captured through a dynamically scattering medium and a variable coded aperture, without the need for interferometric measurements or imaging optics. The complex amplitude of the object is computationally retrieved from intensity images that use multiple coded aperture patterns, employing a stochastic gradient descent algorithm. We demonstrate the proposed method both numerically and experimentally.

Imaging Focused Laser Differential Interferometry

Andrew Marsh, Alice Kramer, Kevin Maranto, and Yi Mazumdar

DOI: 10.1364/OL.520660 Received 02 Feb 2024; Accepted 26 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: Focused laser differential interferometry (FLDI) is an important diagnostic for measuring density fluctuations in high speed flows. Currently, however, high dynamic range FLDI is limited to single-point measurements. In order to spatially resolve multiple locations within complex flows, we present a novel refractive-optic imaging FLDI concept that not only produces two-dimensional images without scanning, but also reduces the measurement noise floor. To demonstrate this concept, a 33x33 grid of FLDI points is first generated using a microlens array. Then, the beams are split and recombined using two polarized Mach-Zehnder interferometers to maximize flexibility in beam separation and optimize signal sensitivity. Next, the FLDI points are collected slightly out-of-focus on a high speed camera in order to increase the number of pixels n per FLDI point, thereby reducing noise floor by √n . Finally, an under-expanded jet with a characteristic screech at 14.1 kHz is tested with the imaging FLDI setup, showing clear barrel and reflected shock features as well as spatially varying turbulence densities. Overall, this unique concept enables the creation of both two-dimensional and reduced-noise-floor FLDI datasets for the study of supersonic and hypersonic flows.

Improved power and temperature performance of half-disk diode microlasers

Fedor Zubov, Artem Beckman, Yuri Shernyakov, Nikolay Kalyuzhnyy, Sergey Mintairov, Yulia Guseva, Marina Kulagina, Vladimir Dubrovsky, and Mikhail Maximov

DOI: 10.1364/OL.521450 Received 12 Feb 2024; Accepted 25 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: The power and temperature characteristics of Ø200 μm half-disk microlasers with a half-ring metal contact and high-density InGaAs/GaAs quantum dots are studied. In continuous wave (CW) mode the maximal optical power at 20°C was 134 mW, and the maximal CW lasing temperature reached 113°C. In pulsed regime the maximal optical power of 1.6 W, limited by catastrophic degradation, was achieved. By comparing CW and pulsed current-voltage characteristics, the dependence of a microlaser temperature on CW pumping current was determined. At CW currents corresponding to the maximal wall-plug efficiency, the maximal optical power and complete lasing quenching, the laser temperatures were 60, 99 and 149°C, respectively.

Partially coherent Pearcey-Gauss source of the spatial spectrum with hyperbolic sine correlation

JingHuan Huang, Yanghong Li, Yejin Liu, jingyi shi, YingJun Luo, Yongzheng Yang, Peiwen Peng, Shigen Ouyang, and Dongmei Deng

DOI: 10.1364/OL.519074 Received 18 Jan 2024; Accepted 25 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: By designing the intricate coherence structure, we are able to create a desired beam profile and trajectory with exceptional beam quality and flexible propagation characteristics even in different conditions. Our research focus lies on the Fourier plane, specifically emphasizing the coherence of spatial frequencies, and we find it can be seen as a constant system response. A theoretical framework is developed and experimental studies are conducted to generate a light field of the spatial spectrum with a complex correlation model using the pseudo-modes superposition method. We successfully produce partially coherent Pearcey-Gauss beams whose spatial spectrum is hyperbolic sine correlational. Interestingly, these beams maintain the distinctive propagation properties of the Pearcey pattern while simultaneously exhibiting the remarkable ability to split the mainlobe into two separate lobes.

Tailoring nondiffracting fields with non-Markovian phase imprint

Zhuhe Jing, Jingjing Zhang, Huaijian Chen, Dandan Huang, Pei Zhang, Hong Gao, fuli li, and Ruifeng Liu

DOI: 10.1364/OL.519524 Received 24 Jan 2024; Accepted 25 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: We experimentally generate nondiffracting speckles that carry non-Markovian properties by encoding the wavefront of a monochromatic laser beam with ring-shaped non-Markovian phases. The resulting non-Markovian nondiffracting fields present a ring-shaped pattern and central dark notches, which are analyzed with an expression of the orbital angular momentum (OAM) spectra of the wavefront possessing ring-shaped non-Markovian phases. Furthermore, we demonstrate that the intensity profiles of these non-Markovian nondiffracting fields exhibit stability over multiple Rayleigh ranges, and their statistical properties could be controlled with the non-Markovianity of the input phase masks. This work presents an approach for simultaneously tailoring the diffracting property and non-Markovanity of optical fields, and provides a deeper understanding of non-Markovian processes.

TT-type resonator-based differential photoacoustic spectroscopy for trace gas detection

wang fupeng, Liyan Fu, Jianguo Zhang, Ze Han, shuo Pang, Qingsheng Xue, Diansheng Cao, Qian Li, and Qiang Wang

DOI: 10.1364/OL.520154 Received 30 Jan 2024; Accepted 25 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: A novel TT-type resonator was proposed for the first time to realize differential photoacoustic (PA) detection for trace gas measurement. Special design of the TT-type resonator allows us to conveniently install the microphones at the resonant center of the acoustic field, which avoids the probing loss when inserting the microphones as conventional Helmholtz resonators. To meet the requirement of low gas consumption and easy integration, the TT-type resonator-based PA cell was fabricated as a fiber-coupled module with an inner volume of only 1.1 mL. Based on the TT-type PA cell, a 30 mW distributed feedback (DFB) laser at 1531 nm was used to construct a photoacoustic spectroscopy (PAS) sensor for acetylene detection. As a result, an excellent linearity of 0.99999 was achieved in the concentration range of 0-5000 ppm. A 1σ equivalent sensitivity of 102 ppb was measured with a response time of 10 s and the limit of detection (LOD) was evaluated to be 0.2 ppm.

Photonic-assisted high-order vector millimeter-wave signal generation enabled by one-bit DSM

Acai Tan, Yanyiw Wang, Zhengxuan Li, Yingxiong Song, and Jianjun Yu

DOI: 10.1364/OL.520250 Received 30 Jan 2024; Accepted 25 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: We propose a virtual-carrier-assisted optical dual-single-sideband (SSB) modulated 16384 quadrature amplitude modulation (QAM) photonic vector millimeter-wave (mm-wave) signal generation scheme based on one-bit delta-sigma modulation (DSM). With the aid of DSM, the severe nonlinear distortion of envelope detection for high-order QAM modulation signals in wireless communication can be effectively resolved. For the validation of our proposed scheme, we experimentally demonstrate the generation of a 40 GHz 16384-QAM orthogonal frequency division multiplexing (OFDM) photonic vector mm-wave signal and transmission over 25-km standard single-mode fiber (SSMF) and 1-m wireless link with the bit error ratio (BER) reach the hard-decision forward-error-correction (HD-FEC) threshold of 3.8Χ10-3.

Hyperspectral microscopy of boron nitride nanolayers using hybrid femto/picosecond coherent anti-Stokes Raman scattering

elodie lin, Michael Scherman, aurelie pierret, Brigitte Attal-Tretout, amandine andrieux, Laure Tailpied, Takashi Taniguchi, Kenji Watanabe, and A. Loiseau

DOI: 10.1364/OL.519571 Received 23 Jan 2024; Accepted 25 Mar 2024; Posted 28 Mar 2024  View: PDF

Abstract: The rise in interest in two-dimensional (2D) nanomaterials, particularly hexagonal boron nitride (h-BN), has been notable in recent years. In particular, hexagonal boron nitride (h-BN), recognized as an optimal substrate for enhancing graphene properties, holds promise for electronic applications. However, the widely employed spontaneous Raman microscopy, a gold standard for graphene study, faces strong limitations in h-BN due to its large bandgap and low cross-section. In this paper, high-resolution femto/picosecond coherent anti-Stokes Raman scattering (fs/ps-CARS) spectroscopy is used for fast hyperspectral imaging of nanometric h-BN layers. Our study establishes that CARS signal effectively enhances Raman signature related to in-plane ring vibrations, thus providing valuable quantitative insights into sample thickness and crystalline quality, also corroborated by additional AFM measurements.

High-average-power single-frequency pulse OPO based on pulse-integrated seed-injection automatic locking

Jiaqi Song, Jiliang Qin, Cao Xuechen, Cheng Donglin, Chen Yanan, and Huadong Lu

DOI: 10.1364/OL.521977 Received 20 Feb 2024; Accepted 25 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: Near-infrared nanosecond (ns) single-longitudinal-mode (SLM) pulse light generated from an optical parametric oscillator (OPO) is an important source in nonlinear optics and high precision spectral analysis. In this Letter, a stable SLM near-infrared ns pulse light source generated from the OPO is presented, which is achieved by developing a seed-injection automatic locking technique based on a pulse-integrated photodetector (PIPD). Depending on the PIPD, the peak power of the pulse light detected by the photodiode is converted to the average power by integrating several pulses. As a result, the detector saturation is thoroughly eliminated and the interference signal including the resonance point between seed and pulse lights can easily be attained by scanning the resonator length. On the basis, a microcontroller unit (MCU) is employed to realize automatic locking by looking for the minimum value of the interference signal. Finally, a SLM 824 nm pulse light source with an output power of 20.5 W and a linewidth of 51.42 MHz is obtained. The presented method can pave the way to implement a low-cost, and compact high-average-power SLM pulse OPO.

Blind Frequency Offset Estimation Method Based on Minimum Phase Correction Error for Full Spectrum Modulated NFDM System

Jianqing He, Li Jianping, Yuwen Qin, Xinkuo Yu, Tao Huang, Gai Zhou, and Songnian Fu

DOI: 10.1364/OL.519766 Received 24 Jan 2024; Accepted 25 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: To improve spectral efficiency of full spectrum modulated nonlinear frequency division multiplexing (FS-NFDM) system, a blind frequency offset estimation (FOE) method has been proposed. The approach based on the minimum phase correction error can achieve high estimation accuracy of sub-MHz without need of any training symbols. Furthermore, in order to reduce the computational complexity, eigenvalue-shift method is used to get coarse search interval of FO, and then the one-dimensional optimization algorithm based on golden section search and parabolic interpolation is used to get the optimal FOE for the coarse search interval. The feasibility and reliability of the proposed blind FOE approach have been demonstrated in both BTB and fiber transmission scenarios. Compared with the grid search method, the proposed solving scheme can save several hundred times of searches. The experimental results reveal that the proposed method is robust to the amplified spontaneous emission noise and phase noise, and has the capabilities of a wide FOE range and a high FOE accuracy.

Geometry optimization of cantilever-based optical microphones

Shen Tian, Pengbo Chen, mingqi jiao, Kaijun mu, Yang Gao, Yingying Qiao, lei li, and Chongxin Shan

DOI: 10.1364/OL.519135 Received 18 Jan 2024; Accepted 25 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: The introduction of cantilever-based fiber-optic microphones (FOMs) has proven to be effective in acoustic sensing. Further improvements in cantilevers face two key constraints: the challenge of achieving minimal sizes with sufficient reflective area and the trade-off between sensitivity and acoustic response bandwidth. Herein, we present a geometry optimization framework for a cantilever-based FOM that addresses this issue. Employing drumstick-shaped cantilevers housed within a Fabry-Perot (F-P) interferometric structure, we showcase a heightened sensitivity of 302.8 mV/Pa at 1 kHz and a minimum detectable acoustic pressure (MDP) of 2.35 µPa/ Hz^1/2. Notably, these metrics outperform those of the original rectangular cantilever with identical dimensions. Furthermore, our proposed micromachined cantilever effectively mitigates the reduction in resonance frequencies, thereby improving the response bandwidth. This geometry optimization framework offers considerable design flexibility and scalability, making it especially suitable for high-performance acoustic sensing applications.

26.5625 Gbaud PAM-4 Short-reach Transmission at 75°C Using 850 nm VCSELs With Strategic Guiding Layer Positioning

Yun-Cheng Yang, Wei-Hsin Chen, Chih-Chuan Chiu, and Chao-Hsin Wu

DOI: 10.1364/OL.521023 Received 06 Feb 2024; Accepted 24 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: This article presents an all-epitaxy approach to reduce the root mean square (RMS) spectral width of 850 nm oxide-confined VCSELs with a large aperture of 7 µm through strategic optimization of the guiding layer within the epitaxy structure. At 75°C, the VCSEL demonstrates an impressive ~0.3 nm RMS spectral width at a bias current of 7.5 mA. Furthermore, the VCSEL achieves successful transmission of 26.5625 Gbaud PAM-4 modulation over a short-reach OM4 fiber link without the need for equalizations, while maintaining a TDECQ budget below the 4.5 dB specified by 50G IEEE Ethernet standards.

Dispersive wave generation in single higher-order modes of a large-core silica step-index fiber with pulse energies up to 12nJ

Andrea Arduin, Lars Rishoj, and Jesper Laegsgaard

DOI: 10.1364/OL.521173 Received 14 Feb 2024; Accepted 24 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: The generation of light in a laser system is constrained by the gain medium, limiting the available wavelengths. We demonstrate in-fiber generation of ultrafast pulses between $\sim$550nm-800nm via dispersive wave generation, in higher order modes. Using higher order modes enables power scaling, due to their large effective area compared to the fundamental modes of single mode fibers, and dispersion engineering, even in simple step-index fibers. The process occurs in a single higher order mode, which we excite using passive glass components (an axicon and two telescopes). The output pulses have energies up to 12nJ at the biologically relevant wavelength of 705nm.

Crackless high-aspect-ratio processing of silica glass with a temporally shaped ultrafast laser

Guoqi Ren, Huijie Sun, Keiichi Nakagawa, Naohiko Sugita, and Yusuke Ito

DOI: 10.1364/OL.522052 Received 22 Feb 2024; Accepted 24 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: In this letter, we propose a crackless high-aspect-ratio processing method based on a temporally shaped ultrafast laser. The laser pulse is temporally split into two sub pulses: one with smaller energy is used to excite electrons but without ablation so that the applied pressure to the sample is weak, and the other one is used to heat the electrons and achieve material removal after it is temporally stretched by a chirped volume Bragg grating (CVBG). Compared with the conventional ultrafast laser processing, the crack generation is almost suppressed by using this proposed method. The hole depth increases more than 3.3 times, and the aspect-ratio is improved at least 2.2 times. Moreover, processing dynamics and parameter dependence are further experimentally studied. It shows that the processing highly depends on the density of electrons excited by the first pulse (P1) and the energy of the second pulse (P2). This novel method provides a new route for the precise processing of wide-bandgap materials.

Programmable Spin and Transport of Living Shrimp Egg through Photoacoustic Pressure

Xichuan Zhao, Ruoqin Zhang, Jinzhi Li, Di Zhou, Feng Li, and Honglian Guo

DOI: 10.1364/OL.518231 Received 12 Jan 2024; Accepted 24 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: In the fields of biomedicine and microfluidics, the non-contact capture, manipulation, and rotation of micro-particles hold great importance. In this study, we propose a programmable non-contact manipulation technique that utilizes photoacoustic effect to spin and transport living shrimp eggs. By directing a modulated pulsed laser towards a liquid-covered stainless-steel membrane, we can excite patterned Lamb waves within the membrane. These Lamb waves occur at the interface between the membrane and the liquid, enabling the manipulation of nearby particles. Experimental results demonstrate successful capture, spin and transport of shrimp eggs in diameter of 220 μm over a distance of about 5 mm. Calculations indicate that the acoustic radiation force and torque generated by our photoacoustic manipulation system can reach 299.5 nN and 187.4 nN·mm, respectively. The system surpasses traditional optical tweezers in terms of force and traditional acoustic tweezers in terms of flexibility. Consequently, this non-contact manipulation system significantly expands the possibilities for applications in various fields, including embryo screening, cell manipulation, and microfluidics.

High performance thin-film lithium niobate polarizer based on width-tapered Euler bending

weixi liu, Chengfeng Wen, Chenkun Lei, lijia song, Daoxin Dai, and Yaocheng Shi

DOI: 10.1364/OL.511708 Received 13 Nov 2023; Accepted 24 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: In this paper, we propose and demonstrate an integrated polarizer on thin film lithium niobite (TFLN). The polarizer consists of width-tapered 180° Euler bending waveguide featuring thin thickness and bilevel mode convertors. Notably, the TE0 mode is efficiently confined in the waveguide while the TM0 mode confronts significant bending losses. The measurements reveal that the excess loss remains below 1.5 dB, and the extinction ratio surpasses 19 dB within a working bandwidth spanning from 1480 nm to 1578 nm. The proposed polarizer holds considerable promise for enhancing polarization handling within TFLN photonic circuits.

Polarization-insensitive wide-angle resonant acousto-optic phase modulator

Okan Atalar and Amin Arbabian

DOI: 10.1364/OL.514333 Received 15 Dec 2023; Accepted 24 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: Phase modulators are commonly used devices in optics. Free-space phase modulators are typically constructed from optically anisotropic crystals exhibiting the Pockels effect. To preserve the light's polarization state as it propagates through the crystal, it is essential to align the polarization and angle of incidence of the light with respect to the crystal. In this study, we demonstrate the feasibility of constructing free-space resonant phase modulators with a broad acceptance angle and minimal dependence on the polarization state of light using an acousto-optic approach. These modulators operate in the megahertz frequency range, require modest power levels, have aperture sizes exceeding one square centimeter, and feature sub-millimeter thickness.

Wavelength extension beyond 3 μm in Ho3+/Pr3+ co-doped AlF3-based fiber laser

NianNian Xu, Pengfei Wang, Shunbin Wang, Mo Liu, Juan Wang, zhenrui Li, and Zhiyong Yang

DOI: 10.1364/OL.520761 Received 02 Feb 2024; Accepted 24 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: In this letter, we report fiber lasers with wavelengths beyond 3 μm in homemade Ho3+/Pr3+ co-doped AlF3-based glass fibers. The laser cavity was established through the integration of a dichroic mirror (DM, HR@3-3.1 μm) positioned at the pump end and a partial reflectivity (PR) fiber Bragg grating (FBG) situated at the laser emission end. The FBGs in AlF3-based glass fibers were fabricated by a fs laser direct-writing method and the resonant wavelengths were 3.009 μm, 3.036 μm and 3.064 μm, respectively. Under the pump of 1.15 μm laser, a maximum unsaturated output power of 1.014 W was obtained at 3.009 μm with an overall laser efficiency of 11.8% and FWHM bandwidth of 0.88 nm. Furthermore, in order to enhance the optical-thermal stability, the FBG was heat-treated at 200 °C for 30 min and a higher output power of 1081 mW (348 mW without heat-treatment) at 3.036 μm was achieved. To the best of our knowledge, this is the first demonstration of 3-3.1 μm lasers by using FBGs in Ho3+/Pr3+ co-doped AlF3-based fibers.

Long range topography by dispersion unmatched spectral-domain interferometry based on virtually imaged phased array modes

Tao Han, Lu Yang, Yun Tang, Changyong Chen, Cui Ma, Zhiyi liu, and Zhihua Ding

DOI: 10.1364/OL.517249 Received 29 Dec 2023; Accepted 24 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: Long range topography with high depth resolution of samples with irregular surfaces by spectral-domain interferometry is challengeable due to increased samplings based on Nyquist principle and reduced sensitivity versus increased depth. In this letter, we propose to realize long range topography by dispersion unmatched spectral-domain interferometry based on virtually imaged phased array (VIPA) modes. By filtering the continuous spectrum of a broadband supercontinuum source through a side-entrance Fabry-Perot etalon configured at two input angles, two groups of VIPA modes are generated. A method based on unmatched dispersion is proposed for non-aliasing reconstruction of the true depth from the interference spectrum under-sampled at two groups of VIPA modes without resorting to the complex signal detection. With the high spectral resolution provided by the VIPA modes instead of the grating-based spectrometer, only a 10 dB fall-off in sensitivity over a range of 10 mm was demonstrated. The feasibility of the proposed method was confirmed by topography of a sample of gauge blocks and a model of three-dimensional (3D) printed tooth. Occlusal surface of the tooth model was further quantitatively evaluated, demonstrating its potential application in long range 3D topography.

Cryogenic Lithium Niobate on Insulator Optical Filter

Yujie Cheng, Lantian Feng, Jianghao He, Xinyu Song, Xu Han, Yuyang Ding, Cheng Wang, Guang-can Guo, Ming Zhang, Daoxin Dai, and Xifeng Ren

DOI: 10.1364/OL.518418 Received 12 Jan 2024; Accepted 24 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: Photonic integrated circuits have garnered significant attention and experienced rapid development in recent years. To provide fundamental building blocks for scalable optical classical and quantum information processing, one important direction is to develop cryogenic compatible photonic integrated devices. Here, we prepare one optical filter on lithium-niobate-on-insulator (LNOI) platform based on a multimode waveguide grating and verify its availability at temperature from 295 K down to 7 K. We find that the integrated optical filter still shows good quality under cryogenic conditions, and the shift of the working wavelength at different temperatures is well explained by the index variation of the material. These results advance LNOI integrated optical devices in applications at cryogenic conditions.

Direct generation of 635nm red random lasers based on praseodymium(Pr)-doped ZBLAN fiber

zhipeng dong, yongjia yao, mingyue he, xinyun yang, Wencheng Jia, Hang Wang, and Zhengqian Luo

DOI: 10.1364/OL.519425 Received 18 Jan 2024; Accepted 23 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: Visible random fiber lasers have garnered significant attention due to their unique emission properties and potential applications in various fields. We first demonstrated, to the best of our knowledge, a compact all-fiber structure, red wavelength, random fiber laser (RFL) based on double-clad Pr-doped ZBLAN fiber. The simple half-open cavity consists of a high-reflectivity fiber pigtail mirror and the Pr-doped ZBLAN fiber. The Pr-doped ZBLAN fiber not only served as a gain medium but also offered random backward scattering. We investigated the effects of different lengths on output power and slope efficiency of the RFL. For 21 meters Pr-doped fiber, the RFL emitted a maximum output power of 208.50 mW with a slope efficiency of 11.09%. For 15 meters Pr-doped fiber, the maximum power decreased to 120.18 mW with the slope efficiency of 7.27%. We are also numerical simulating the output power versus pump power at different fiber lengths based on power steady-state light propagation equations. This novel RFL has the potential for broad applications in fields such as display technology, spectroscopy, biomedical imaging, and optical sensing due to its unique properties and simple all-fiber structure.

Structural and optical performance of novel NiV/Ti multilayer mirrors for Z-pinch plasma diagnostic at the wavelength region of 350-500 eV

Zile Wang, Zhe Zhang, Jialian He, Angelo Giglia, Qiya Zhang, Runze QI, Qiushi Huang, Shengzhen Yi, Zhong Zhang, and Zhanshan Wang

DOI: 10.1364/OL.519527 Received 22 Jan 2024; Accepted 22 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: This Letter reports on investigations of novel NiV(Ni93V7)/Ti multilayer mirrors for operation in the wavelength region of 350-500 eV. Such mirrors are promising optical components for the Z-pinch plasma diagnostic. The NiV/Ti multilayers show superior structural and optical performance compared to conventional Ni/Ti multilayers. Replacing Ni with NiV in multilayers decreases interface widths and enhances the contrast of refractive index between the absorber and spacer layers. The improvement of interface quality contributes to the enhancement in reflectance. Under the grazing incidence of 13°, a peak reflectivity of 25.1% at 429 eV is achieved for NiV/Ti multilayers, while 17.7% at 427 eV for Ni/Ti.

All-solid highly sensitive fiber-tip magnetic field sensor based on Fabry-Perot interferometer with breakpoint structure

Yingfang Zhang, Xingchao Ma, Ben Xu, Jianqing Li, Hui Fang Chen, Juan Kang, Chunliu Zhao, and Shangzhong Jin

DOI: 10.1364/OL.521138 Received 06 Feb 2024; Accepted 22 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: An all-solid fiber-tip Fabry-Perot interferometer (FPI) coated with nickel film is proposed and experimentally verified for magnetic field sensing with high sensitivity. It is fabricated by splicing a segment of thin-wall capillary tube to a standard single-mode fiber (SMF), then inserting a tiny segment of fiber with a smaller diameter into the capillary tube, creating an ultra-narrow air-gap at the SMF end to form an FPI. When the device is exposed to magnetic field, the capillary tube is strained due to the magnetostrictive effect of nickel film coated on its outer surface. In further, owing to the unique breakpoint sensitivity-enhancement structure of the air-gap FPI, the elongation of the capillary tube whose length is over one hundred times longer than the air-gap width, is entirely transferred to the cavity length change of the FPI, the sensor is extreme sensitive to magnetic field as proved by our experiments, achieving a high sensitivity of up to 2. 6 nm/mT for a linear magnetic field range from 40 to 60 mT, as well as a low temperature cross-sensitivity of 56 µT/℃. The all-solid stable structure, compact size (total length of ~3.0 mm), and reflective working mode with high magnetic field sensitivity indicate this sensor has good application prospects.

Microwave re-excitation of femtosecond laser tagging for highly flexible velocimetry

Fynn Reinbacher, Sarang Bidwai, and James Michael

DOI: 10.1364/OL.522115 Received 22 Feb 2024; Accepted 22 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: Molecular tagging velocimetry is typically species specific and limited by excited state/species lifetimes. We utilize laser-generated ionization, long-lived anions, and a time-delayed microwave pulse to monitor the tagged region up to several milliseconds. This non-resonant excitation and microwave interaction is demonstrated in a range of gas mixtures. Signal levels show up to 1000-fold improvement, and the flexibility in interrogation time allows for velocity measurements over a large dynamic range (1-100 m/s) with single-shot precision of <5%. This approach has the potential for wide application over a range of relevant gas compositions, temperatures, and pressures.

Deep-ultraviolet n-ZnGa2O4/p-GaN heterojunction photodetector fabricated by pulsed laser deposition

Ning Cao, Lichun Zhang, Xin Li, Xianling Meng, Doudou Liang, Yadan Zhu, and fengzhou Zhao

DOI: 10.1364/OL.519668 Received 25 Jan 2024; Accepted 22 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: Zinc gallium oxide (ZnGa2O4) has attracted considerable interest in deep-ultraviolet photodetectors, due to the ultra-wideband gap, high transmittance in the ultraviolet (UV) region, and excellent environmental stability. In this study, ZnGa2O4 thin films were deposited on p-GaN epi-layers using pulsed laser deposition, resulting in improved crystalline quality. The ZnGa2O4 film exhibited a bandgap of 4.93eV, calculated through absorption spectra. A heterojunction photodetector (PD) was constructed, demonstrating a rectification effect, an on/off ratio of 12,697 at -5.87V, a peak responsivity of 14.5 mA/W, and a peak detectivity of 1.14×1012 Jones (262 nm, -6V). The PD exhibited fast response time (39 ms) and recovery time (30 ms) under 262 nm illumination. The band diagram based on the Anderson model elucidates the photoresponse and carrier transport mechanism. This work paves the way for advancing next-generation optoelectronic

All-Optical High-Contrast Femtosecond Switching using Nonlinearity from Epsilon-Near-Zero effect in Plasmonic Metamaterials

TANMAY BHOWMIK, BODHAN CHAKRABORTY, and Debabrata Sikdar

DOI: 10.1364/OL.521075 Received 07 Feb 2024; Accepted 22 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: Metamaterials opened a new realm to control light–matter interactions at sub-wavelength scale by engineering meta-atoms. Recently, the integration of several emerging nonlinear materials with metamaterial structures enables ultrafast all-optical switching at the nanoscale and thus, brings enormous possibilities to realize next-generation optical communication systems. This letter presents a novel design of plasmonic metamaterial for high-contrast femtosecond all-optical switching. We leverage magnetic plasmon (MP) resonance combined with the nonlinear effects of epsilon-near-zero (ENZ) material. The proposed design comprises a periodic array of two closely-spaced Au-nanograting deposited on an optically thick Au-substrate to excite MP-resonance. To enable dynamically tunable resonance, the nanogrooves in meta-atoms are filled with an ENZ-material, cadmium-oxide (CdO). The intraband transition-induced optical nonlinearities in the ENZ-medium are studied using two-temperature model. We observe that the pump-induced refractive index change in CdO-layer causes red-shift of the MP-resonance dip wavelength in the reflectance spectrum leading to a high modulation depth of 0.83 at 1.55 µm. With ultrafast response time of 776 fs while maintaining a low pump-fluence requirement of 75 µJ/cm², the proposed metamaterial could help in realizing switches for next-generation optical computation systems.

All-optical neuromorphic XOR and XNOR operation utilizing a photonic spiking neuron based on a passive add-drop microring resonator

Qiang Zhang, Ning Jiang, Anran Li, Yiqun Zhang, Gang Hu, Yongsheng Cao, and Kun Qiu

DOI: 10.1364/OL.518392 Received 10 Jan 2024; Accepted 21 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: We propose a concise hardware architecture supporting efficient photonic exclusive OR (XOR) and exclusive NOR (XNOR) operations, by employing a single photonic spiking neuron based on a passive add-drop microring microresonator (ADMRR). The threshold mechanism and inhibitory dynamics of the ADMRR-based spiking neuron is numerically discussed on the basis of the coupled mode theory. It is shown that precise XOR operation in the ADMRR-based spiking neuron can be achieved by adjusting temporal differences within the inhibitory window. Additionally, within the same framework, the XNOR function can also be carried out by accumulating the input power over time to trigger excitatory behavior. This work presents a novel and pragmatic technique for optical neuromorphic computing and information processing utilizing passive devices.

Mn2+/Mg2+ co-doped AlON ceramic with ultra-narrow-band green-emitting combining high transparency toward wide gamut backlight application

Xiaolan Zhou, Shengyuan Chen, Cong Zhang, Xu Huang, Kailei Lu, Jian Qi, and tiecheng lu

DOI: 10.1364/OL.520495 Received 31 Jan 2024; Accepted 21 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: Narrow-band green-emitting, combined with superior physicochemical stability and thermal performance, is regarded as a common pursuit in backlight display applications. However, mainstream phosphor-converted materials composed of resin or silicone resin easily encounter the dilemma of thermal decomposition and chemical corrosion for practical use. To overcome this problem, in this work, Mn2+/Mg2+ co-doped AlON ceramic is successfully realized ultra-narrow-band green-emitting and high transparency. The luminescent property of AlON: Mn2+-Mg2+ ceramic exhibits narrow band green emitting centered at 509 nm with a full width at half maximum of 36 nm, which is smaller than the corresponding powder counterpart (44 nm). Moreover, AlON: Mn2+-Mg2+ ceramic presents a wide color gamut (103.6%) and high color purity (74%). Concurrently, high transmittance of this ceramic, at 82%, unveils a novel transparent device. This work may facilitate the development of narrow-band green light-emitting converters based on AlON: Mn2+-Mg2+ transparent ceramics in large color gamut backlight display applications.

Coherent radiation at visible wavelengths from sub-keV electron beams

Dolev Roitman, Aviv Karnieli, Shai Tsesses, zahava barkay, and Ady Arie

DOI: 10.1364/OL.521354 Received 08 Feb 2024; Accepted 21 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: The Smith-Purcell effect allows for coherent free-electron–driven compact light sources over the entire electromagnetic spectrum. Intriguing interaction regimes, with prospects for quantum optical applications, are expected when the driving free electron enters the sub-keV range, though this has until now remained an experimental challenge. Here, we demonstrate Smith-Purcell light emission from UV to visible using engineerable, fabricated gratings with periodicities as low as 19 nm, and with electron energies as low as 300 eV. Our findings constitute a major step towards broadband, highly tunable, on-chip light sources, observation of quantum recoil effects, and tunable EUV and x-ray sources from swift electrons.

Exploring global symmetry-breaking superradiant phase via phase competition

Haichao Li, Wen Huang, and Wei Xiong

DOI: 10.1364/OL.522886 Received 01 Mar 2024; Accepted 21 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: Superradiant phase transitions play a fundamental role in understanding the mechanism of collective light-matter interaction at the quantum level. Here we investigate multiple superradiant phases and phase transitions with different symmetry-breaking patterns in a two-mode V-type Dicke model. Interestingly, we show that there exists a quadruple point where one normal phase, one global symmetry-breaking superradiant phase and two local symmetry-breaking superradiant phases meet. Such a global phase results from the phase competition between two local superradiant phases and can not occur in the standard Λ- and Ξ-type three-level configurations in quantum optics. Moreover, we exhibit a sequential first-order quantum phase transition from one local to the global again to the other local superradiant phase. Our study opens up a perspective of exploring multi-level quantum critical phenomena with global symmetry breaking.

Influence of static and dynamic ocular aberrations on full-field OCT for in-vivo high resolution retinal imaging

Yao Cai, Olivier Thouvenin, Kate Grieve, and Pedro Mece

DOI: 10.1364/OL.515749 Received 14 Dec 2023; Accepted 21 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: Under spatially incoherent illumination, Time-domain Full-Field Optical Coherence Tomography (FFOCT) offers the possibility to achieve in-vivo retinal imaging at cellular resolution over a wide field-of-view. Such performance is possible albeit the presence of ocular aberrations even without the use of classical Adaptive Optics. While the effect of aberrations in FFOCT has been debated these past years, mostly on low order and static aberrations, we present for the first time a method enabling a quantitative study of the effect of statistically representative static and dynamic ocular aberrations on FFOCT image metrics, such as SNR, resolution, and image similarity. While we show that ocular aberrations can decrease FFOCT SNR and resolution by up to 14 dB and 5-fold, we take advantage of such quantification to discuss different possible compromises between performance gain and Adaptive Optics complexity and speed, to optimize both sensor-based and sensorless FFOCT high-resolution retinal imaging.

Excitation of optically trapped single particles using femtosecond pulses

Kyle Latty, Justin Borrero, Thiago Arnaud, and Kyle Hartig

DOI: 10.1364/OL.519790 Received 25 Jan 2024; Accepted 21 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: Excitation from optically trapped particles are examined through laser-induced breakdown spectroscopy following interactions with mJ-level fs-pulses. The optical emissions from sub-ng ablation of precisely positioned cupric oxide microparticles are used as a method to spatially resolve the laser-particle interactions resulting in excitation. External focusing lenses are often used to change the dynamics of nonlinear self-focusing of fs-pulses to form laser filaments, or alternatively, to form very intense air plasmas. Given the significant implications external focusing has on the laser propagation and plasma conditions, single-particle emissions are studied with focusing lenses ranging from 50-300 mm. It is shown that, while single particles are less excited at longer focal lengths due to limited energy transfer through laser-particle interactions, the cooler plasma results in a lower thermal background to reveal resolved single-shot emission peaks. By developing an understanding in the fundamental interaction that occurs between single-particles and fs-pulses and filaments, practical improvements can be made for atmospheric remote sensing of low-concentration aerosols.

High-sensitivity optical tomography of instabilities in supersonic gas flow

Marek Raclavský, Kavya H Rao, Uddhab Chaulagain, Marcel Lamac, and Jaroslav Nejdl

DOI: 10.1364/OL.510289 Received 25 Oct 2023; Accepted 20 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: Characterization of gas targets relies largely on conventional optical techniques, providing millisecond time resolution, which not only overlooks the fluctuations occurring at shorter timescales but also often challenges the sensitivity limits of optical probing as their refractive index is close to unity. Hence, the ability to resolve these fluctuations needs to be addressed as it is paramount for accurate gas jet characterization for their applications, including laser-matter interaction in laser wakefield electron acceleration or plasma X-ray sources. In this letter, we introduce an advanced gas jet characterization system capable of visualizing fast density fluctuations by Schlieren imaging, combined with density characterization by interferometric tomography, both with increased sensitivity due to the four-pass probing configuration. We demonstrate that combining the two modalities provides a substantial advancement in achieving a comprehensive, both quantitative and qualitative, characterization of gas jets.

Efficient single-cycle mid-infrared femtosecond laser pulse generation by spectrally-temporally cascaded optical parametric amplification with pump energy recycling

Yuan Hao, Pei huang, Tongyu Feng, Yahui Ma, Xianglin Wang, Huabao Cao, Yishan Wang, Wei Zhao, and Yuxi Fu

DOI: 10.1364/OL.519729 Received 24 Jan 2024; Accepted 20 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: We proposed spectrally-temporally cascaded optical parametric amplification (STOPA) using pump energy recycling to simultaneously increase spectral bandwidth and conversion efficiency in optical parametric amplification (OPA). Using BiB3O6 and KTiOAsO4 nonlinear crystals, near-single-cycle mid-infrared (MIR) pulses with maximum energy conversion and quantum efficiencies exceeding 25% and 55%, respectively, in simulations. We successfully demonstrated sub-two-cycle, CEP-stable pulses generation at 1.8 µm using a four-step STOPA system in experiment. This method overcomes the limitations of gain bandwidth of nonlinear crystals and the low conversion efficiency in broadband OPA systems, which is significant for intense attosecond pulse generation and strong laser field physics studies.

Super-resolution radial fluctuations microscopy for optimal resolution and fidelity

yanru li, Selene Roberts, Lixin Liu, and Lin Wang

DOI: 10.1364/OL.514964 Received 11 Dec 2023; Accepted 20 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: Fluorescence fluctuations super-resolution microscopy (FF-SRM) has emerged as a promising method for fast, low-cost, and uncomplicated imaging of biological specimens beyond the diffraction limit. Among the FF-SRM techniques, super-resolution radial fluctuations (SRRF) microscopy is a popular technique but is prone to artifacts, resulting in low fidelity, especially under conditions of high-density fluorescence. In this paper, we propose a novel approach, namely VeSRRF, that combines intensity and gradient variance reweighted radial fluctuations (VRRF) and enhanced-SRRF (eSRRF) algorithms, leveraging the enhanced resolution achieved through intensity and gradient variance analysis in VRRF and the improved fidelity obtained from the radial gradient convergence transform in eSRRF. Our method is validated using microtubules in mammalian cells as a standard biological model system. Our results demonstrate that VeSRRF consistently achieves the highest resolution and fidelity compared to those obtained from other algorithms in both single molecule localization microscopy and fluorescence fluctuations microscopy. Moreover, we have developed the VeSRRF software package that is freely available on the open-source ImageJ/Fiji software platform, to facilitate the use of VeSRRF in the broader community of biomedical researchers.

Symbol-Level Fiber-longitudinal Power Profile Estimation

Yingjie Jiang, Du Tang, Zhengkang Wang, Ji Luo, Bofang ZHENG, and Yaojun Qiao

DOI: 10.1364/OL.518389 Received 11 Jan 2024; Accepted 20 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: Symbol-level fiber-longitudinal power profile estimation (PPE) greatly reduces the implementation complexity compared with the waveform-level PPE using oversampled data. However, symbol-rate data cannot account for the inter-sample interaction, which leads to inaccuracy of the absolute power estimation. To realize an accurate symbol-level PPE, we provide an in-depth analysis of the differences between symbol-level and waveform-level perturbation matrices and propose a power calibration method based on the trace of the inverse matrix. Evaluated in the probabilistic constellation shaping (PCS) 64QAM 130 Gbaud 5 × 50 km optical links, the root mean square error (RMSE) of symbol-level PPE decreases by 0.98 dB and 0.62 dB at erbium-doped fiber amplifier (EDFA) positions and all estimated positions with the aid of matrix calibration.

A modulated-symbol domain matched filtering scheme for photonics-assisted integrated sensing and commu-nication system based on a single OFDM waveform

Lede Yin and Jing He

DOI: 10.1364/OL.518695 Received 15 Jan 2024; Accepted 20 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: In this Letter, a modulated-symbol domain matched filtering scheme based on orthogonal frequency division multiplexing (OFDM) is pro-posed for the photonics-assisted W-band integrated sensing and com-munication (ISAC) system. And the photonics-assisted ISAC system based on a single OFDM waveform is experimentally demonstrated for the first time. By using the single OFDM waveform, it can improve the spectrum efficiency and achieve dual functionalities of communi-cation and radar detection simultaneously. The optical heterodyne scheme is used to generate the W-band OFDM signal for the ISAC system. The experimental results show that the data rate of OFDM is up to 4.56 Gbit/s, meanwhile, it can achieve the radar ranging with a ranging resolution of 1.88 cm and a ranging error of millimeter level in the OFDM-ISAC system.

Yb-doped mode-locked fiber laser with a hybrid structure of NPR and Lyot filter as the saturable absorber

Yongjie Pu, Pan Guo, Yuan Gao, Qihao Sheng, Zhicheng Zhang, MINYU FAN, and Sha Wang

DOI: 10.1364/OL.516495 Received 21 Dec 2023; Accepted 20 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: Passively mode-locked fiber laser based on nonlinear polarization rotation (NPR) have attracted much attention due to their ability to generate short pulses with wide spectra and high peak power. However, environmental perturbations can easily cause laser to lose mode-locked state, and making it a challenge for the practical application. The aim of this research is to improve the laser stability by inserting a Lyot filter into the mode-locked laser cavity. The experimental results indicate that the mode-locked state can be maintained when radius of the fiber loop is changed from 7.5 cm to 1.5 cm, while the signal-to-noise ratio of the fundamental frequency remains almost the same. The tunability of the output power can be achieved by adding a half-wave plate (HWP) in the laser cavity without changing the pump power, while the mode-locked state remains stable. By adjusting the angle of the HWP2, the output power can be adjusted from 1.02 mW to 63.9 mW at repetition rate of 29.7MHz.

Auxiliary-cavity enhanced quantum estimation of optorotational coupling strength

Jia-Xin Peng, Zeng-Xing Liu, Muhammad Asjad, and Naeem Akhtar

DOI: 10.1364/OL.509425 Received 17 Oct 2023; Accepted 20 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: A scheme is proposed to achieve significantly enhanced quantum estimation of opto-rotational-coupling (ORC) strength by coupling a driven auxiliary-cavity to a Laguerre-Gaussian (L-G) rotational-cavity, where theORC originates from the exchange of orbital angular momentum between L-G light and rotational mirror. The results indicate that, by appropriately designing the auxiliary-cavity mechanism, the estimation error ofORC parameter is significantly reduced and revealing the estimation precision has a much stronger thermal noise and dissipation robustness in comparison with the unassisted case. Our study paves a way towardsachieving high-precision quantum sensors.

High-Order QAM NANF Transmission Utilizing MIMO Equalizer Integrated with Low-Complexity Decision-Directed Carrier Phase Estimation

Chen Wang, KAIHUI WANG, Jianyu Long, Wen Zhou, Feng zhao, Lei Shen, Peng Li, and Jianjun Yu

DOI: 10.1364/OL.519042 Received 15 Jan 2024; Accepted 20 Mar 2024; Posted 25 Mar 2024  View: PDF

Abstract: We experimentally realized a high-speed nested anti-resonant nodeless fiber (NANF) transmission with the assistance of polarization division multiplexing (PDM) and probabilistic shaping (PS) technology. In this system, a low-complexity multiple-input multiple-output (MIMO) real-valued equalizer (RVE) integrated with decision-directed carrier phase estimation (DDCPE), which is robust against the IQ crosstalk and tiny phase disturbance between PS symbols. By using the proposed MIMO-RVEDDCPE, the 60-GBaud PDM-PS-256QAM signal has been delivered through 2-km NANF satisfying the soft-decision forward error correction (SD-FEC).

Fractional topological charge measurement through optical correlation

ALLARAKHA SHIKDER, JYOTI MOHAPATRA, and Naveen Nishchal

DOI: 10.1364/OL.523154 Received 05 Mar 2024; Accepted 20 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: The emerging field of optical vortex beams having fractional topological charges (TC) is of high interest due to its usefulness in various applications. The efficiency of result depends on the precise measurement of orbital angular momentum information tied to the fractional TC. This letter demonstrates a novel and simple technique to measure the fractional TC of optical vortex beams through a hybrid digital-optical correlator with the help of auto-correlation between fork-shaped interference patterns corresponding to integer and fractional TCs. Unlike machine learning-based approaches, the proposed method does not require a complex architecture, which lowers computational cost and enables real-time implementation.

Plasmonic nanosensor and pressure-induced transparency based on coupled resonator in a nanoscale system

Zhao Chen, Baixu Zhu, Xinxin Ma, Shijie Zhang, Linhao Li, Tong Li, Yilin Wang, and Zhi-Ling Hou

DOI: 10.1364/OL.522612 Received 29 Feb 2024; Accepted 20 Mar 2024; Posted 27 Mar 2024  View: PDF

Abstract: Plasmonic nanosensors and the dynamic control of light fields are of the utmost significance in the field of micro- and nano-optics. Here, our study successfully demonstrates a plasmonic nanosensor in a compact coupled resonator system and obtains the pressure-induced transparency phenomenon for the first time. The proposed structure consists of a groove and slot cavity coupled in the metal-insulator-metal waveguide, whose mechanical and optical characteristics are detailed investigated using the finite element method. Simulation results show that we construct a quantitative relationship among the resonator deformation quantity, the applied pressure variation, and the resonant wavelength offset by combining the mechanical and optical properties of the proposed system. The physical features contribute to highly efficient plasmonic nanosensors for refractive index and optical pressure sensing with sensitivity of 1800 nm/RIU (RIU: per unit variation of the refractive index) and 7.4 nm/MPa, respectively. Furthermore, the light waves are coupled to each other in the resonators, which are detuned due to the presence of pressure, resulting in the pressure-induced transparency phenomenon. It is noteworthy to emphasize that, unlike previously published works, our numerical results take structural deformation-induced changes in optical properties into account, making them trustworthy and practical. The proposed structure introduces a novel approach for the dynamic control of light fields and has special properties that can be utilized for the realization of various integrated components.

Determining triple half-twist of optical Möbius strips in electromagnetic fields with non-planar structure

Kirill Grigoriev and Vladimir Makarov

DOI: 10.1364/OL.522834 Received 01 Mar 2024; Accepted 19 Mar 2024; Posted 19 Mar 2024  View: PDF

Abstract: We found an analytical expression that determines the number of twist of polarization ellipse strip built on a small circular contour, in the center of which lies the circular polarization singularity point and the plane of which coincides with the plane of polarization at this point. Necessary and sufficient conditions of formation of strips with one and three half-twists are found. A set of five parameters of electromagnetic field at the polarization singularity point is found that definitely determines the value of the twist coefficient of the strip. These parameters are written in coordinateless form without references to special sets of coordinate axes.

Can Linear Pre-emphasis Mitigate the Nonlinear Distortion of LEDs?

Juliusz Bojarczuk, Michal Marzecki, and Grzegorz Stepniak

DOI: 10.1364/OL.517490 Received 02 Jan 2024; Accepted 19 Mar 2024; Posted 20 Mar 2024  View: PDF

Abstract: Transmitter pre-emphasis is one of the techniques proposed to improve the relatively poor frequency responses of the Light-Emitting Diodes (LEDs) used in Visible Light Communications (VLC) as light sources. However, according to the communications theory in linear channels, pre-emphasis may only be as good as the not particularly efficient feed-forward equalizer (FFE). Given the increase in PAPR caused by pre-emphasis, its use in place of receiver equalisation can be considered counterproductive. In this study, we show that the performance of transmitter pre-emphasis is indeed equivalent to the FFE at low input powers, where the LED is a linear system. However, at higher modulation voltages, the transmitter pre-emphasis has a clear advantage over FFE as it mitigates the nonlinear distortion of the LED. This mechanism is for the first time explained in both theory and experiment.

Multi-Channel Delay Sampling to Extend Imaging Depth in High-Speed Swept-Source OCT Systems

Yaping Shi, Jian Liu, Zhaoyu Gong, Christopher Burgner, Vijaysekhar Jayaraman, and Ruikang Wang

DOI: 10.1364/OL.517493 Received 04 Jan 2024; Accepted 19 Mar 2024; Posted 20 Mar 2024  View: PDF

Abstract: We present a multi-channel delay sampling method to extend imaging depth in high-speed swept-source optical coherence tomography (OCT). A balanced detector captures interference signals, converting them into electrical signals, which are then split into N channels, each with fixed time delays determined by the length of the electrical cables. Subsequently, they are digitized by an N-channel acquisition card. A calibration procedure is utilized to compensate for non-uniform phase shifts resulting from fixed time delays. The N-channel signals are merged in k-space and resampled to obtain a linearized spectrum, which increases the sampling rate by a factor of N, thereby extending the ranging distance by N times, all without altering external triggering or sacrificing imaging speed. In addition, our technique enhances the signal-to-noise ratio and sensitivity within the original depth range. This advancement contributes to the overall improvement in the performance of OCT system.

Experimental reservoir computing with diffractively coupled VCSELs

Moritz Pflüger, Daniel Brunner, Tobias Heuser, James Lott, Stephan Reitzenstein, and Ingo Fischer

DOI: 10.1364/OL.518946 Received 02 Feb 2024; Accepted 19 Mar 2024; Posted 19 Mar 2024  View: PDF

Abstract: We present experiments on reservoir computing (RC) using a network of vertical-cavity surface-emitting lasers (VCSELs) that we diffractively couple via an external cavity.Our optical reservoir computer consists of 24 physical VCSEL nodes.We evaluate the system's memory and solve the 2-bit XOR task and the 3-bit header recognition (HR) task with bit error ratios (BER) below 1 % and the 2-bit digital-to-analog conversion (DAC) task with a root-mean-square error (RMSE) of 0.067.In its current form, and at an input rate of 0.45 GHz, our system consumes 1.8 nJ per inference.

DAC-less PAM4 Signal Generation using Silicon Dual-Drive Push-Pull MZI Modulator

Kaizhong Chen, Hengsong Yue, Jing Huang, and Tao Chu

DOI: 10.1364/OL.521455 Received 12 Feb 2024; Accepted 19 Mar 2024; Posted 20 Mar 2024  View: PDF

Abstract: In this study, we demonstrate DAC-less PAM-4 signal generation by driving a silicon MZI modulator based on a series push-pull configuration with two independent binary signals of varying amplitudes. Such configuration boosts transfer speed between PAM4 levels, leading to enhanced performance metrics compared to the differential driving scheme. Experimentally, our scheme demonstrated a lower bit-error rate compared to the differential scheme, proving its cost-efficiency and feasibility for PAM4 generation.

Optimized Adaptation Algorithms for Low-complexity Adaptive Equalizers

Shuo Zheng, Min Yang, Guofeng Yan, Yanjun Zhu, Hua Zhang, Chaonan Yao, Yuchen Shao, and Jian Wang

DOI: 10.1364/OL.521757 Received 20 Feb 2024; Accepted 19 Mar 2024; Posted 20 Mar 2024  View: PDF

Abstract: In order to address the high-power consummation issue of conventional multi-input and multi-output (MIMO) adaptive equalizer (AEQ) for short-reach coherent transmissions, several state-of-the-art low-complexity AEQs have been proposed. The property of low-complexity AEQs is that crosstalk between polarizations and impairments of each polarization channel are compensated separately at different sections. In our work, optimized adaptation algorithms for low-complexity real valued (RV) AEQs with different structures are analyzed. The approach to avoid introducing additional computational complexity due to optimized adaptation process is proposed here. The advantages of proposed optimized adaptation algorithms are experimentally demonstrated in a 25 Gbaud dual-polarization 16-quadrature-amplitude-modulation (DP-16QAM) back-to-back (BtB) intradyne system with an overall bandwidth limitation of 14 GHz. Experiment results show that a similar performance as conventional AEQ could be achieved by using proposed adaptation algorithms and reduce the number of multiplications with up to ~51%.

High-energy, low-jitter, narrowband ps probe laser for kHz-rate fs/ps coherent anti-Stokes Raman scattering

Erik Braun, Karna Patel, Venkat Athmanathan, Terrence Meyer, Sukesh Roy, and Mikhail Slipchenko

DOI: 10.1364/OL.519396 Received 18 Jan 2024; Accepted 18 Mar 2024; Posted 19 Mar 2024  View: PDF

Abstract: Hybrid fs/ps coherent anti-Stokes Raman scattering (CARS) thermometry often utilizes ps probe pulses derived from pulse shaping or spectrally filtering the primary laser source or through synchronization with a low repetition rate external laser. This results in limited energy, spectral resolution, and/or repetition rate of the ps probe. In this work, a master-oscillator power-amplifier (MOPA) laser was synchronized to the oscillator of a Ti:Sapphire regenerative amplifier to achieve high-energy (600 µJ), narrowband (58 ps) probe pulses at kHz repetition rates. Temporal filtering allows the pulse characteristics to be adjusted for each application. At 25 torr, relevant to high-speed flows, the kHz-rate MOPA system generated signal-to-noise ratios 3x higher in nitrogen and improved the precision relative to a 10 ps probe derived from spectral filtering and a narrowband spectral amplifier. The MOPA system also enabled single-shot ro-vibrational hybrid fs/ps CARS thermometry in 650 K heated air.

Multi-toric optical element to compensate ocular astigmatism with increased tolerance under rotation

Diana Gargallo, Anabel Martínez Espert, SARA PERCHES, María Collados, Laura Martin, and Jorge Ares

DOI: 10.1364/OL.518973 Received 16 Jan 2024; Accepted 18 Mar 2024; Posted 20 Mar 2024  View: PDF

Abstract: A new optical element designed to compensate regular astigmatism while exhibiting increased tolerance to rotational misalignment is introduced. The element incorporates an optical design based on concentric annular regions with slightly different cylindrical axis angular positions. To assess visual quality performance as a function of rotation, retinal image simulation and clinical assessments with an adaptive optics visual simulator were carried out. The results demonstrate the superior performance of the newly proposed element in the presence of rotational errors when compared to traditional solutions.

Tunable and mode-locked Tm,Ho:GdScO3 laser

Jian Liu, Ning Zhang, Qingsong Song, Heng Ding, Yinyin Wang, Peng Chen, Zebin Wang, Yanyan Xue, Jie Xu, Yongguang Zhao, Xiaodong Xu, kheirreddine Lebbou, and Jun Xu

DOI: 10.1364/OL.514957 Received 05 Dec 2023; Accepted 18 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: A novel Tm,Ho:GdScO3 crystal grown using the Czochralski method was investigated for its polarized spectroscopic properties and laser performance in both the tunable continuous wave (CW) and mode-locked regimes. The crystal's multisite structure (Gd3+/Sc3+ site) and Tm3+/Ho3+ dopants contributed to spectral broadening, enabling tunable laser operation from 1914 nm to 2125 nm (with a broad range of 215 nm). Additionally, a pulse duration of 72 fs was achieved for E ǁ b polarization. These results demonstrate the potential of the Tm,Ho:GdScO3 perovskite crystal as a promising gain material for ultrafast lasers operating around 2 μm.

Broandband long-wavelength upconversion in ultra-short nonlinear crystals

Peter Tidemand-Lichtenberg and Christian Pedersen

DOI: 10.1364/OL.518776 Received 12 Jan 2024; Accepted 18 Mar 2024; Posted 19 Mar 2024  View: PDF

Abstract: Since the inception of second-order nonlinear frequency conversion in 1961, enhancing the inherent low conversion efficiency has been a primary objective. This goal has been successfully accomplished through the utilization of cm-long nonlinear crystals characterized by high quality and nonlinearity, coupled with versatile phase matching strategies and high-power mixing lasers. However, the reliance on lengthy nonlinear crystals and the necessity for precise phase matching introduce stringent tolerances on acceptance angles and spectral bandwidths for the interacting fields, thereby constraining its widespread applicability in scientific and industrial domains. This challenge is addressed by combining a broadly tunable, ~5 mW Quantum Cascade Laser operating in the 9.5-12.5 µm range with upconversion detection in ~100 µm long AGS crystals. Using a tightly focused continuous wave Nd:YVO4 laser with 20 mW output power, and spatial filtering of the upconverted beam leads to a SNR of 55 for 50 µs averaging time sufficient for many applications.

Slow light enhanced Brillouin scatterting with integrated Bragg grating

Mingyu Xu, Peng Lei, Yunhui Bai, Zhangyuan Chen, and Xiaopeng Xie

DOI: 10.1364/OL.520165 Received 30 Jan 2024; Accepted 18 Mar 2024; Posted 19 Mar 2024  View: PDF

Abstract: Advancements in photonic integration technology have enabled the effective excitation of simulated Brillouin scattering (SBS) on a single chip, boosting Brillouin-based applications such as microwave photonic signal processing, narrow-linewidth lasers, and optical sensing. However, on-chip circuits still require large pump power and centimeter-scale waveguide length to achieve considerable Brillouin gain, making them both power-inefficient and challenging for integration. Here, we exploit the slow-light effect to significantly enhance SBS, presenting the first demonstration of a slow-light Brillouin-active waveguide on the Silicon-On-Insulator (SOI) platform. By integrating a Bragg grating with a suspended ridge waveguide, a 2.1-fold enhancement of the forward Brillouin gain coefficient is observed in a 1.25 mm device. Furthermore, this device shows a Brillouin gain coefficient of 1,693 m^(-1)W^(-1) and a mechanical quality factor of 1,080. The short waveguide length reduces susceptibility to inhomogeneous broadening, enabling the simultaneous achievement of a high Brillouin gain coefficient and a high mechanical quality factor. This approach introduces a novel dimension to enhance acousto-optic interaction efficiency and holds significant potential for microwave photonic filters and high spatial resolution sensing.

Functional OCT Reveals Anisotropic Changes of Retinal Flicker-Evoked Vasodilation

Taeyoon Son, Guangying Ma, and Xincheng Yao

DOI: 10.1364/OL.520840 Received 02 Feb 2024; Accepted 18 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: The purpose of this study is to verify the effect of anisotropic property of retinal biomechanics on vasodilation measurement. A custom-built optical coherence tomography (OCT) was used for time-lapse imaging of flicker stimulation evoked vessel lumen changes in mouse retinas. Comparative analysis revealed significantly larger (18.21%) lumen dilation in axial direction compared to lateral (10.77%) direction. The axial lumen dilation predominantly resulted from the top vessel wall movement toward the vitreous direction, whereas the bottom vessel wall remained stable. This observation indicates that traditional vasodilation measurement in the lateral direction may result in underestimated value.

The isochemical crystallization in condensed borate LaMgB₅O₁₀ glass-ceramics doped with optical probe Eu³+

Bartosz Bondzior

DOI: 10.1364/OL.521244 Received 09 Feb 2024; Accepted 18 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: The isochemical glass-ceramics doped with Eu³+ were prepared by the heat-treatment of lanthanum magnesium borate glass. The crystalline phase was chemically identical to glass matrix and consisted of condensed borate LaMgB₅O₁₀. The isochemical crystallization process begins with the formation of rings by BO₄ groups. The emergence of ordered crystalline phase give rise to intense charge transfer absorption of Eu³+, allowing the efficient luminescence under UV. The analysis of Judd-Ofelt parameters and comparison to purely crystalline samples obtained by solid-state synthesis reveals a switch of parameter relations from Ω₂> Ω₄ for glass to Ω₂< Ω₄ for crystals, but also a maximum value of Ω₆ for glass-ceramic sample, which indicates enhanced structural rigidity and results in superior luminescence output. The quantum yield measurements confirmed higher luminescence efficiency for glass-ceramics compared to both pure glass and pure crystalline samples.

Remote erasing and writing of ferroelectric domains by femtosecond laser in lithium niobate

Fengchang Li, Qiang Cao, Xiaoliang Wang, and Ruonan Wang

DOI: 10.1364/OL.519935 Received 25 Jan 2024; Accepted 18 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: We experimentally demonstrate the highly-efficient remote erasing and writing of ferroelectric domains by femtosecond laser in lithium niobate. Based on the induction of a focused infrared femtosecond laser without any relative displacement or additional treatment, the original multiple ferroelectric domains can be either erased (erasing operation) or elongated (writing operation) simultaneously in the crystal, depending on the laser focusing depth and the laser pulse energy. In the erasing operation, the original multiple ferroelectric domains can be cleared completely by just one laser induction, while in the writing operation, the average length of the ferroelectric domains can be elongated up to 5 μm by three laser inductions. A theoretical model has been proposed in which a thermoelectric field and a space charge field are used cooperatively to successfully explain the mechanism of remote erasing and writing. This method greatly improves the efficiency and flexibility of tailoring ferroelectric domain structures, paving the way to large-scale all-optical industrial production for nonlinear photonic crystals and nonvolatile ferroelectric domain wall memories.

A reconfigurable origami hologram based on deep neural networks

Kang Wang, dashuang liao, and Hao Gang Wang

DOI: 10.1364/OL.520781 Received 05 Feb 2024; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Reconfigurable and multifunctional metasurfaces are becoming indispensable in a variety of applications due to their capability to execute diverse functions acrossvarious states. However, many of these metasurfacesincorporate complex active components, thereby escalating structural complexity and bulk volume. In this research, we propose a reconfigurable passive hologrambased solely on an origami structure, enabling the successful generation of holograms depicting the ‘Z’ and ‘L’ illuminated by a Right-Hand Circular Polarization (RHCP) wave in two distinct states: planar and zigzagconfiguration, respectively. The transformation between the 2D planar metasurface and the 3D zigzag structurewith slant angles of 35 is achieved solely throughmechanically stretching and compressing the origami metasurface. The phases on the origami metasurface are trained through a Deep Neural Network which operates on the NVIDIA Tesla k80 GPU, with the total training process costing 11.88 seconds after 100 epochs. The reconfigurable scheme proposed in this research provides flexibility and ease of implementation in the fields of imaging and data processing.

Vortex ring beams in nonlinear PT-symmetric systems

Cristian Mejía Cortés, Mario Molina, and Jesús Muñoz-Muñoz

DOI: 10.1364/OL.521865 Received 20 Feb 2024; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: In this paper, we investigate a two-dimensional photonicarray featuring a circular shape and an alternatinggain and loss pattern. Our analysis revolves arounddetermining the presence and resilience of ring modeswith varying vorticity values. This investigation is conductedwith respect to both the array’s length and thestrength of the non-Hermitian parameter. For largervalues of array’s length, we observe a reduction in thestability domain as the non-Hermitian parameter increases.However, a rise up of the vorticity originatesfull stability windows for shorter values of the array’slength.

Low-loss and compact photonic lantern based on step-index double cladding fiber

Cong Zhang, Yue Wang, senyu zhang, Meng Xiang, Songnian Fu, and Yuwen Qin

DOI: 10.1364/OL.516211 Received 15 Jan 2024; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: The fulfillment of the adiabatic criterion is indispensable for the realization of a low-loss photonic lantern (PL), concurrently imposing a stringent striction on the taper transition length of the PL. Here, by relaxing the adiabatic criterion, a low-loss and compact PL based on step-index double cladding fiber (SI-DCF) is theoretically proposed and experimentally demonstrated. The use of SI-DCF can reduce the mode field diameter (MFD) expansion ratio during the tapering processing, and greatly decrease the taper transition length required for adiabatic tapering. We initially evaluate the variation of both MFD and effective refractive index (RI) along the fiber tapering region based on three types of fiber structures, including the modified standard single mode fiber (SSMF), the graded index fiber (GIF), and the proposed SI-DCF. In comparison with the commonly used fiber geometry, the SI-DCF can reduce the MFD expansion ratio from 77.73% to 38.81%, leading to more than half reduction of tapering length for both 3-mode and 6-mode PLs. Then, two kinds of SI-DCF with different core diameters are fabricated to realize a 3-mode PL. The fabricated PL possesses a 1.5 cm tapering length and less than 0.2 dB insertion loss (IL). After splicing with the commercial few-mode fiber, the PL has an average IL of 0.6 dB and more than 13 dB LP11 mode purity over the C-band. Finally, a back-to-back power matrix measurement indicates that the fabricated PLs have a mode coupling of less than -10 dB at 1550 nm.

Ultra low phase noise self injection locked diode laser with a high Q fiber resonator – model and experiment

Safia Mohand Ousaid, Germain Bourcier, arnaud fernandez, Olivier LLOPIS, Julien Lumeau, Antonin Moreau, Thomas Bunel, Matteo Conforti, Arnaud Mussot, Vincent Crozatier, and Stéphane Balac

DOI: 10.1364/OL.514778 Received 30 Nov 2023; Accepted 17 Mar 2024; Posted 21 Mar 2024  View: PDF

Abstract: Low phase noise and narrow linewidth lasers are achieved by implementing self-injection locking of a DFB laser on two distinct fiber Fabry-Perot resonators. More than 45 dB improvement of the laser phase or frequency noise is observed when the laser is locked. In both cases, a frequency noise floor below 1 Hz2/Hz is measured. The integrated linewidth of the best of the two lasers is computed to be in the range of 400 Hz and appears to be dominated by vibration noise close to the carrier. The results are then compared with a model based on the retro-injected power and the Q factors ratio between the DFB laser and the resonator. This straightforward model facilitates the extraction of the theoretical performance of these sources close to the carrier, a characteristic still hidden by vibration noise.

Quickly tunable ultra-narrow filter via metal film waveguide

Hongrui Shan, Qiheng Wei, Hailang Dai, and Xianfeng Chen

DOI: 10.1364/OL.517815 Received 05 Jan 2024; Accepted 17 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: The development of fast, efficient and cost-effective tunable optical filters is a tireless pursuit of the goal in the field of optical signal processing and communications. However, the traditional filters have been limited by their complex structures, slow tuning speed and high cost. To address this challenge, we present a tunable ultra-narrow bandpass filter which is fabricated by metal layer cladded in a high-parallelism and high-precision piezoelectric ceramic for interlayer. Experimental results show a remarkable full width at half maximum of 51 pm and a fast response time of 800 ns. In addition, by cascading double filters, the wavelength of output light has been fine-tuned from a vernier effect. Then, we realize a tunable filter to select and output several ultra-narrow single peaks with 56% efficiency in the 2nm range. Furthermore, it offers a wide tunable range, exceptional narrowband filtering performance and fast piezoelectric response times. Hence, it is particularly well suited to applications requiring precise wavelength selection and control, opening new possibilities in the field of tunable optical filters.

Widely tunable S-band ring-cavity Tm3+-doped fluorotellurite fiber laser

Linghao Cui, Zhixu Jia, Junjie Wang, Chuanze Zhang, Feng Tian, Fanchao Meng, Yasutake Ohishi, Weiping Qin, and Guanshi Qin

DOI: 10.1364/OL.521853 Received 19 Feb 2024; Accepted 17 Mar 2024; Posted 26 Mar 2024  View: PDF

Abstract: Tm3+-doped fluorotellurite fibers (TDFTFs) are fabricated by using a rod-in-tube method. A 2.1 m long TDFTF is used as the gain medium, in which both ends of the TDFTF are connected to a short piece of silica fiber by direct fusion splicing. By inserting the above TDFTF and a tunable optical bandpass filter into a ring-cavity, and employing a 1400/1570 nm dual-wavelength pumping technique, tunable lasing from 1460 to 1526 nm is obtained, which almost cover the whole S-band. To the best of our knowledge, this is the first report of tunable Tm3+-doped fiber laser with a tunable range almost covering the whole S-band. Furthermore, by removing the tunable optical bandpass filter from the ring-cavity, free running multi-wavelength lasers at 1500 and 1901 nm are achieved. Our results show that TDFTFs are promising gain media for constructing S-band fiber lasers.

Terahertz metalens for generating multi-polarized focal points and images with uniform intensity distributions

Binbin Lu, Yefei Fu, Teng Zhang, Zuanming Jin, XiaoFei Zang, and Yiming Zhu

DOI: 10.1364/OL.519605 Received 23 Jan 2024; Accepted 16 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Metasurfaces have provided a flexible platform for designing ultracompact metalenses with unusual functionalities. However, traditional multi-foci metalenses are limited to generating circularly-polarized (CP) or linearly-polarized (LP) focal points, and the intensity distributions are always inhomogeneous/chaotical between the multiple focal points. Here, an inverse design approach is proposed to optimize the in-plane orientation of each meta-atom in a terahertz (THz)multi-foci metalens that can generate multi-polarized focal points with nearly uniform intensity distributions. As a proof-of-principle example, we numerically and experimentally demonstrate an inversely designed metalens for simultaneously generating multiple CP- and LP-based focal points with homogeneous intensity distributions, leading to a multi-polarized image (rather than the holography). Furthermore, the multi-channel and multi-polarized images consisting of multiple focal points with homogeneous intensity distributions are also numerically demonstrated. The unique approach for inversely designing multi-foci metalens that can generate multi-polarized focal points and images with uniform intensity distributions will enable potential applications in imaging and sensing.

Contrast-enhanced phase-resolved second harmonic generation microscopy

Zhanshan Wang, Canyu Hong, Zeyuan Sun, Shuang Wu, Bokai Liang, Xidong Duan, Weitao Liu, and Shiwei Wu

DOI: 10.1364/OL.520814 Received 06 Feb 2024; Accepted 16 Mar 2024; Posted 20 Mar 2024  View: PDF

Abstract: The characterization of inverted structures (crystallographic, ferroelectric or magnetic domains) is crucial in the development and application of novel multi-state devices. However, determining these inverted structures needs a sensitive probe capable of revealing their phase correlation. Here a contrast-enhanced phase-resolved second harmonic generation (SHG) microscopy is presented, which utilizes a phase-tunable Soleil-Babinet compensator and the interference between the SHG fields from the inverted structures and a homogeneous reference. By this means, such inverted structures are correlated through the π-phase difference of SHG and the phase difference is ultimately converted into the intensity contrast. As a demonstration, we have applied this microscopy in two scenarios to determine the inverted crystallographic domains in two-dimensional van der Waals material MoS2. Our method is particularly suitable for applying in vacuum and cryogenic environments while providing optical diffraction-limited resolution and arbitrarily adjustable contrast. Without loss of generality, this contrast-enhanced phase-resolved SHG microscopy can also be used to resolve other non-centrosymmetric inverted structures, e.g. ferroelectric, magnetic, or multiferroic phases.

Thermally-recovered Mechanoluminescence in Ca6BaP4O17:Eu2+

SU ZHOU, Yao Cheng, and Yuansheng Wang

DOI: 10.1364/OL.519863 Received 05 Feb 2024; Accepted 15 Mar 2024; Posted 19 Mar 2024  View: PDF

Abstract: Stable reproducibility of Mechanoluminescence (ML) is of vital importance for trap-controlled ML materials. Photo/electric excitation is usually needed for ML recovery of trap-controlled materials. In this work, it is demonstrated that thermal treatment can be applied to achieve recovering of ML, which is ascribed to the unique trap level configuration. The Ca6BaP4O17:Eu2+ performing robust trap-controlled ML has been proposed and the corresponding repetitive ML can be realized by thermal treatment. TL spectra reveals that the thermally-induced reproducible ML benefits from the dual defect level electronic structure of Ca6BaP4O17:Eu2+. The ML intensity is dependent on the electrons in shallow traps and the electron transfer from deep traps to shallow traps induced by thermal treatment leads to repetitive ML.

Large and Tunable Optoelectronic Chromatic Dispersion in PIN-type photodiodes

Ayuushi Dutta, Egor Liokumovitch, Ziv Glasser, and Shmuel Sternklar

DOI: 10.1364/OL.519164 Received 16 Jan 2024; Accepted 15 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: It is known that PN-type photodiodes possess high optoelectronic chromatic dispersion (OED). Here we present a theoretical and experimental study of OED in PIN-type photodiodes. Applying the modulation phase-shift technique, a Ge PIN photodiode exhibits ~0.5 deg/nm phase-shift sensitivity at 10MHz modulation, corresponding to a dispersion of 1.4x〖10〗^9 ps/(nm∙km), many orders of magnitude larger than high-dispersion optical materials. A striking feature of the PIN device is the ability to tune the amount and sign of the OED through the bias voltage. Electronic tuning between -0.8 deg/nm to +0.5 deg/nm is shown. The PIN photodiode is an on-chip device possessing significant tunable dispersion, for applications in optical sensing and spectroscopy.

Frequency-domain Searching Algorithm in Deflectometry for measuring the surface of transparent planar element

Wanxing Zheng, Dahai Li, Ruiyang Wang, Zekun Zhang, Renhao Ge, and Manwei Chen

DOI: 10.1364/OL.522243 Received 23 Feb 2024; Accepted 15 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: This letter presents the Frequency-domain Searching Algorithm in Deflectometry (FSAD). By encoding specialized multi-frequency fringe patterns and employing a correlation searching algorithm, the limitations of existing frequency-domain methods can be overcome to some extent, thereby separating front and back surface reflections to obtain a complete measurement data. The principles of FSAD are described in detail. In the experiment, a piece of window glass with thickness of 10mm and a square area of 96×96mm is measured to verify the proposed method.

Integrated Segmented IQ-Modulator for Orthogonal Sampling and Multi-Level High-Bandwidth Signal Generation

Younus Mandalawi, Mohamed Hosni, Janosch Meier, Karanveer Singh, Souvaraj De, Ranjan Das, and Thomas Schneider

DOI: 10.1364/OL.519345 Received 18 Jan 2024; Accepted 15 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: Photonics-assisted signal processing of high-bandwidth signals emerges as a solution for challenges encountered in electronic-based processing. Here we present a concept for a compact, photonics-assisted digital-to-analog converter (DAC) and optical IQ-modulator in one single integrated device based on two innovative concepts: a segmented Mach-Zehnder modulator and orthogonal sampling. For electrically driving the modulator, only a single radio frequency oscillator and no pulse source or electrical DAC is required. The presented and simulated proof-of-concept device with six segments can generate a multi-level and high-bandwidth signal from low-bandwidth electronic drivers. As an example, we show the generation of a 120 Gbps data rate, 16-quadrature amplitude modulation (16-QAM, 30 Gbaud) signal solely based on low-bandwidth (5 GHz) non-return-to-zero (NRZ) signals. Integrated on a silicon photonics platform, the device provides fixable speed and bandwidth operations, positioning it as a viable solution for diverse communication systems.

Pump-power-controlled L-band wavelength-tunable mode-locked fiber laser utilizing all polarization maintaining nonlinear polarization rotation

Guanyu YE, Bowen Liu, Maolin Dai, Yifan Ma, Takuma Shirahata, Shinji Yamashita, and Sze Set

DOI: 10.1364/OL.518882 Received 15 Jan 2024; Accepted 15 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: For the first time, we present the pump power-controlled wavelength-tunable mode-locked fiber laser in the L-band (1565 nm to 1625 nm), achieved by all-polarization maintaining (all-PM) nonlinear polarization rotation (NPR). The wavelength of the laser can be tuned over 20 nm, from 1568.2 nm to 1588.9 nm simply by controlling the pump power from 45 mW to 115 mW. In contrast to conventional wavelength tuning mechanisms such as optical bandpass filters, our tuning method is non-mechanical and electrically controllable, featuring simplicity and cost-effectiveness in a superior all-fiber design.

Large optoelectronic chromatic dispersion in PN-type silicon photodiodes and photovoltaic cells

Sapna Mudgal, Pawan Dubey, Ziv Glasser, and Shmuel Sternklar

DOI: 10.1364/OL.514906 Received 01 Dec 2023; Accepted 14 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Optoelectronic chromatic dispersion (OED) is a significant source of effective chromatic dispersion in photodiodes. We present results of an experimental and theoretical study of OED in PN-type Si photodiodes and photovoltaic cells, which points to a very large effective chromatic dispersion in these devices. As measured with the modulation phase-shift technique at a modulation frequency of 4 kHz for these slow devices, the OED spectral sensitivity for a commercial Si photodiode is approx. 0.02 deg/nm in the 720nm-850nm wavelength band and increases to 0.25 deg/nm at λ=1μm. For a Si photovoltaic cell, the OED is approx. 0.09 deg/nm in this spectral region. These values translate into an effective chromatic dispersion parameter of approx. 10^12 ps/(nm∙km) for these sub-millimeter device lengths, which is over 8 orders of magnitude larger than high-dispersion optical materials such as TiO2. The enormous dispersion in these sub-millimeter sized silicon-based devices can be utilized for on-chip optoelectronic sensors such as wavelength monitoring and spectroscopy. Picometer-range wavelength monitoring is demonstrated. The substantial OED of photovoltaic cells can be utilized for characterization and optimization and opens new applications for optical sensing with these self-powered devices.

Ultrabroadband Two-Dimensional Electronic Spectroscopy in the Pump–Probe Geometry Using Conventional Optics

Matthew Barclay, Nicholas Wright, Paul Cavanaugh, Ryan Pensack, Eric Martin, and Daniel Turner

DOI: 10.1364/OL.519387 Received 18 Jan 2024; Accepted 14 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: We report ultrabroadband two-dimensional electronic spectroscopy (2D ES) measurements obtained in the pump–probe geometry using conventional optics. A phase-stabilized Michelson interferometer provides the pump-pulse delay interval, τ1, necessary to obtain the excitation-frequency dimension. Spectral resolution of the probe beam provides the detection-frequency dimension, ω3 . The interferometer incorporates active phase stabilization via a piezo stage and feedback from interference of a continuous-wave reference laser detected in quadrature. To demonstrate the method, we measured a well-characterized laser dye sample and obtained the known peak structure. The vibronic peaks are modulated as a function of the waiting time, τ2, by vibrational wavepackets. The interferometer simplifies ultrabroadband 2D ES measurements and analysis.

Switchable hybrid-order optical vortex lattice

Xueyun Qin, Hao Zhang, Miaomiao Tang, Yujie Zhou, Yuping Tai, and Xinzhong Li

DOI: 10.1364/OL.515906 Received 14 Dec 2023; Accepted 14 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: Optical vortex (OV) modulation is a powerful technique for enhancing the intrinsic degrees-of-freedom in structured light. In particular, the optical vortex lattice (OVL) involving multiple OVs has captured significant interest owing to their exceptional applications in optical tweezers and condensed matter physics. However, all the OVs in the OVL possess the same order, which cannot be modulated individually, and thus, limits its versatile application. Herein, we propose a novel concept, called hot-swap method, to design a switchable hybrid-order OVL (SHO-OVL), where each OV is easily replaced by arbitrary orders. We experimentally generated the SHO-OVL and studied its characteristics, including interferograms, retrieved phase, energy flow and orbital angular momentum. Further, the significant advantages of the SHO-OVL are showcased through the independent manipulation of multiple yeast cells. This study provides a novel scheme for accuracy control and modulation of OVL, which greatly facilitates the diverse applications of optical manipulation and particle trapping and control.

High-power, gigahertz repetition frequency self-mode-locked Ho:GdVO₄ laser resonantly pumped by a Tm-doped fiber laser

Panqiang Kang, Xinlu Zhang, Xiaofan Jing, Changchang Shen, Jinjer Huang, Yulei Wang, and Zhiwei Lv

DOI: 10.1364/OL.519796 Received 24 Jan 2024; Accepted 13 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: A self-mode-locked Ho:GdVO₄ laser with the GHz pulse repetition frequency oscillation near 2061.5 nm was demonstrated for the first time, to our best knowledge. The output performances of the self-mode-locked Ho:GdVO₄ laser were investigated for a few of output coupler transmittances at the pulse repetition frequency of 1.89 GHz. At the incident pump power of 8.12 W, the maximum average output power was as high as 2.28 W, corresponding to the slope efficiency and optical-to-optical efficiency of 36.3% and 28.1%, respectively. To the best of our knowledge, this is the maximum average output for the 2 µm self-mode-locked laser with a GHz pulse repetition frequency. This work provides a new way for generating efficient and high-power ultrafast pulse laser with a GHz repetition frequency in the 2 µm waveband.

Nonreciprocal magnon blockade via optical nonlinearity

Han-Qiu Zhang, Shuang-Shuo Chu, Zhang Song, Wenxue Zhong, and Guangling Cheng

DOI: 10.1364/OL.520578 Received 02 Feb 2024; Accepted 13 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: We present an alternative scheme to achieve nonreciprocal unconventional magnon blockade (NUMB) in a hybrid system formed by two microwave cavities and one yttrium iron garnet (YIG) sphere, where the pump and signal cavities interact with each other nonlinearly and the signal cavity is coupled to the YIG sphere. Based on the dispersive interactions among three bosonic modes with the large detuning, the second-order nonlinear coupling occurs between the pump cavity and magnon and meanwhile optical Kerr nonlinearity is present in the pump cavity, which leads to the magnon blockade effect with the help of the weak parametric driving of the pump cavity. By analyzing the second-order correlation functions via numerical simulations and analytical calculations, the nonreciprocity of magnon blockade could be achieved by changing the effective Kerr nonlinearity. The responsible mechanism of unconventional magnon blockade (UMB) within the regime of weak coupling strength is the destructive interference of the two excitation pathways. The present work provides an alternative method to achieve nonreciprocal magnon blockade based on optical nonlinearity, which may be helpful for quantum information processing.

Polarization feature fusion and calculation of birefringence dynamics in complex anisotropic media

Rui Hao, Nan Zeng, Zheng Zhang, Honghui He, Chao He, and Hui Ma

DOI: 10.1364/OL.515983 Received 14 Dec 2023; Accepted 13 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: As a complex anisotropic medium, the variation in birefringence within biological tissues is closely associated with numerous physiological behaviors and phenomena. In this Letter, we propose a polarization feature fusion method and the corresponding polarimetric parameters, which exhibit excellent performance of capturing the birefringence dynamic variation process in complex anisotropic media. By employing the feature fusion method, we combine and transform polarization basis parameters (PBPs) to derive fused polarization feature parameters (FPPs) with explicit expressions. Subsequently, we conduct Monte Carlo simulation to demonstrate the effectiveness of the proposed FPPs from two variation dimensions of birefringence direction θ and modulus ∆n. Leveraging mathematical modeling and linear transformations, we investigate and abstract their response patterns concerning θ and ∆n. Finally, the experiments confirm that the FPPs show superior adaptability and interpretability in characterizing the birefringence dynamic process of turbid media. The findings presented in this study provide new methodological insights of information extraction for computational polarimetry in biomedical research.

Enhanced Performance of Quantum Dot Light-Emitting Diodes Enabled by Zirconium Doped SnO2 as Electron Transport Layers

Jinxing Zhao, Zhongwei Man, Shuaibing wang, Chaoqi Hao, Zhenzhen Yu, Xu Li, and Aiwei Tang

DOI: 10.1364/OL.521324 Received 12 Feb 2024; Accepted 13 Mar 2024; Posted 13 Mar 2024  View: PDF

Abstract: Next-generation display and lighting based on quantum dot light-emitting diodes (QLEDs) require a balanced electron injection of electron transport layers (ETLs). However, classical ZnO nanoparticles (NPs) as ETL face inherent defects such as excessive electron injection and positive aging effects, urgently requiring the development of new types of ETL materials. Here, we show that high stability SnO2 NPs as ETL can significantly improve the QLED performance to 100567 cd m-2 (luminance), 14.3% (maximum external quantum efficiency), and 13.1 cd A-1 (maximum current efficiency) using traditional device structures after optimizing film thickness and annealing temperature. Furthermore, experimental tests reveal that by doping Zr4+ ions, the size of SnO2 NPs will reduce, dispersion will improve and energy level will shift up. As expected, when using Zr-SnO2 NPs as the ETL, the maximum external quantum efficiency can reach 16.6%, which is close to the state-of-the-art QLEDs based on ZnO ETL. This work opens the door for developing novel type ETL for QLEDs.

GaN Integrated Optical Devices for Measuring Viscosity of Glycerol Droplets

YUMENG LUO, Binlu Yu, HONGYU YU, and Kwai Hei Li

DOI: 10.1364/OL.518090 Received 08 Jan 2024; Accepted 13 Mar 2024; Posted 19 Mar 2024  View: PDF

Abstract: This letter presents the fabrication and characterization of a chip-scale GaN optical device for measuring glycerol viscosity. The monolithically integrated GaN chip with a size of 1 × 1 mm2 comprises a light-emitting diode (LED) and a photodiode (PD) on a transparent sapphire substrate. The glycerol droplet applied to the device acts as a medium for coupling light from the LED to the PD. When a mechanical impulse is applied, the droplet undergoes a damped vibration that depends on its viscosity, causing a change in its shape and altering the path of light propagation. The viscosity of the glycerol sample can be determined by obtaining the rate of attenuation of the measured photocurrent signals. The proposed unit offers a fast time response in microseconds and requires only a small sample volume of 5 μL. The developed device is highly suitable for the practical measurement of glycerol viscosity due to its miniaturization, low cost, and ease of operation without the need for external optical components.

Efficient and high-speed coupling modulation of silicon racetrack ring resonators at 2 μm waveband

Xi Wang, Jianing Wang, Yong Yao, Shumin Xiao, Qinghai Song, and Ke Xu

DOI: 10.1364/OL.518729 Received 11 Jan 2024; Accepted 13 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: Significantly increased interests have been witnessed for the 2 μm waveband which is considered to be a promising alternative window for fiber and free space optical communications. However, the less mature device technology at this wavelength range is one of the primary obstacles towards practical applications. In this work, we demonstrate an efficient and high-speed silicon modulator based on carrier depletion in a coupling tunable resonator. A benchmark high modulation efficiency of 0.75 V‧cm is achieved. The 3-dB electro-optic bandwidth is measured to be 26 GHz allowing for upto 34 Gbit/s on-off-keying modulation with a low energy consumption of ~0.24 pJ/bit. It provides a solution for silicon modulator with high-speed and low power consumption in 2-μm waveband.

Variational Autoencoder-Assisted Unsupervised Hardware Fingerprint Authentication in Fiber Network

Yilin Qiu, Xinyong Peng, Xinran Huang, Zhi Chai, Mingye Li, Weisheng Hu, and Xuelin Yang

DOI: 10.1364/OL.518952 Received 15 Jan 2024; Accepted 12 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: Physical-layer authentication (PLA) based on hardware fingerprints can safeguard optical networks against large-scale masquerade or active injection attacks. However, traditional schemes rely on massive labeled close-set data. Here, we propose an unsupervised hardware fingerprint authentication based on a variational autoencoder (VAE). Specifically, the triplets are generated through variational inference on unlabeled optical spectra and then applied to train the feature extractor, which has excellent generalization ability and enables fingerprint feature extraction from previously unknown optical transmitters. The feasibility of the proposed scheme is experimentally verified by the successful classification of 8 optical transmitters after a 20 km standard single-mode fiber transmission, to distinguish efficiently the rogue from legal devices. A recognition accuracy of 99%, and a miss alarm rate of 0% are achieved even under the interference of multiple rogue devices. Moreover, the proposed scheme is verified to have a comparable performance with the results obtained from supervised learning.

Active feedback stabilization of super-efficient microcombs in photonic molecules

Israel Rebolledo, Óskar Helgason, Vicente Duran, Marcello Girardi, Martin Zelan, and Victor Torres Company

DOI: 10.1364/OL.514761 Received 04 Dec 2023; Accepted 12 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: Dissipative Kerr soliton frequency combs, when generated within coupled cavities, exhibit exceptional performance concerning controlled initiation and power conversion efficiency. Nevertheless, to fully exploit these enhanced capabilities, it is necessary to maintain the frequency comb in a low-noise state over an extended duration. In this study, we demonstrate the control and stabilization of super-efficient microcombs in a photonicmolecule. Our findings demonstrate that there is a direct relation between the effective detuning and soliton power, allowing the latter to be used as a set point in a feedback control loop. Employing this method, we achieve the stabilization of a highly efficient microcomb indefinitely, paving the way for its practical deployment in optical communications and dual-comb spectroscopy applications.

Intense emission at 605nm from Pr3+-doped fluorotellurite glass fibers

Jinming Yan, Zhixu Jia, Junjie Wang, Chuanze Zhang, Fangning Wang, Fanchao Meng, Yasutake Ohishi, Daming Zhang, Weiping Qin, Fei Wang, and Guanshi Qin

DOI: 10.1364/OL.518023 Received 08 Jan 2024; Accepted 12 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: Pr3+-doped fluorotellurite glass fibers (PDFTFs) were fabricated by using a rod-in-tube method. By using a 976/1400 nm dual-wavelength upconversion pump technique, intense emission at 605 nm was obtained from a 6 cm long PDFTF, which was attributed to the transition 1D2→3H4 of Pr3+ ions. With an increase of the power of 976 nm and 1400 nm lasers, the spectral bandwidth of the 605 nm emission decreased and the intensity of the 605 nm emission increased monotonically, indicating the generation of 605 nm amplified spontaneous emission (ASE). To the best of our knowledge, this is the first report of 605 nm ASE in PDFTFs. Our results showed that PDFTFs had the potential for constructing red fiber lasers and amplifiers.

Bidirectional Raman soliton-like combs with unidirectional pump in a spherical microresonator

Alexey Andrianov and Elena Anashkina

DOI: 10.1364/OL.516842 Received 21 Dec 2023; Accepted 12 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: We experimentally demonstrate bidirectional Raman soliton-like combs in a whispering gallery mode microresonator with unidirectional pump, for the first time, to the best of our knowledge. We develop a relatively simple theoretical model and find an analytical solution for forward- and backward-propagating Raman sech2-shaped solitons in an anomalous dispersion region under unidirectional pumping in a normal dispersion region. Raman solitons exist thanks to the balance between losses and Raman gain from a CW wave (which is equal in both directions) as well as between dispersion and Kerr nonlinearity.

Direct Object Detection with Snapshot Multispectral Compressed Imaging in Short-Wave Infrared Band

Naike Wei, Yingying Sun, tingting jiang, and Qiong Gao

DOI: 10.1364/OL.517284 Received 28 Dec 2023; Accepted 12 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Snapshot multispectral imaging (SMSI) has attracted much attention in recent years for its compact structure and superior performance. High-level image analysis based on SMSI, such as object classification and recognition, usually takes the image reconstruction as the first step, which hinders its application in many important real-time scenarios. Here we demonstrate the first reconstruction-free strategy for object detection with SMSI in the short-wave infrared (SWIR) band. The implementation of our SMSI is based on a modified 4f system which modulates the light with a random phase mask, and the distinctive point spread function (PSF) in each narrow band endows the system with spectrum resolving ability. A deep learning network is trained to detect small object by constructing a dataset with the PSF of our SMSI system and the sky images as background. Our results indicate that a small object with spectral feature can be detected directly with the compressed image output by our SMSI system, and the inferred object category and location information is reliable with moderate noise. This work paves the way towards the use of SMSI to detect multispectral object in practical applications.

Simple few-shot method for spectrally resolving the wavefront of an ultrashort laser pulse

Slava Smartsev, Aaron Liberman, Igor Andriyash, Antoine Cavagna, ALESSANDRO FLACCO, Camilla Giaccaglia, Jaismeen Kaur, Josephine Monzac, Sheroy Tata, Aline Vernier, Victor Malka, Rodrigo Lopez-Martens, and Jerome Faure

DOI: 10.1364/OL.502000 Received 31 Jul 2023; Accepted 12 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: We present a novel and straightforward approach for the spatio-spectral characterization of ultrashort pulses. This minimally intrusive method relies on placing a mask with specially arranged pinholes in the beam path before the focusing optic and retrieving the spectrally-resolved laser wavefront from the speckle pattern produced at focus. We test the efficacy of this new method by accurately retrieving chromatic aberrations, such as pulse front tilt, pulse front curvature, and higher-order aberrations introduced by a spherical lens. The simplicity and scalability of this method, combined with its compatibility with single-shot operation, make it a promising candidate to become a new standard diagnostic tool in high-intensity laser facilities.

Collison dynamics between soliton molecules and a single soliton: exploding soliton pair and periodic soliton explosions

Runmin Liu, Defeng Zou, Youjian Song, and Ming-lie Hu

DOI: 10.1364/OL.516363 Received 21 Dec 2023; Accepted 12 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: Inherent periodic collisions in dual-wavelength mode-locked fiber lasers (MLFLs) stimulate various intra-cavity collision dynamic phenomena. Analogous to the collision of mater particles, collisions between optical soliton pair (SPs) and single soliton (SSs) have been observed by the real-time spectral measurements. It is demonstrated that the energy accumulation after collision caused by internal motion within bound pulses leads to SP explosions, while the periodic soliton explosions with another cavity parameter setting are almost unaffected by the collision. Additionally, the collision between a SP and a SS is reproduced through numerical simulations, and the collision-induced double Hopf-type bifurcation of SP is predicted. These findings provide novel insights for further understanding the complex collision dynamics in dual-wavelength MLFLs and will help in the design of high-performance dual-comb sources.

Nonlinear errors elimination using the fusion of PGC-DCM, geometric fitting and Atan algorithms

Gang Zhang, Qiang Ge, Linguang Xu, Xuqiang Wu, and Benli Yu

DOI: 10.1364/OL.516756 Received 21 Dec 2023; Accepted 12 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: Phase generated carrier (PGC) demodulation scheme are always accompanied by nonlinear errors. We propose a fusion of PGC differential and cross multiplying (PGC-DCM), geometric fitting and arctangent (Atan) algorithms for fiber optic interferometric sensors to eliminate nonlinear errors. The output amplitude of PGC-DCM algorithm is used to judge whether the Lissajous figure of quadrature signals is larger than 1/2 ellipse arc. When the Lissajous figure exceeds 1/2 ellipse arc, the contaminated quadrature signals are corrected by the ellipse correction parameters calculated from the geometric fitting, otherwise the previous fitting parameters are employed for correction. Geometric fitting is realized by minimizing the Sampson error and its failure problem under small signals is solved by using the temporary stability of fitting results. Finally, desired signals are extracted from the corrected quadrature signals by the Atan algorithm. Experimental results show that the fusion combines the merits of the three algorithms and expands the application of the geometric fitting in PGC demodulation schemes.

Magnetically Tunable Brewster Angle in Uniaxial Magneto-Optical Metamaterials for Advanced Integration of High-Resolution Sensing Devices

SULMA Sarmiento, Edwin Moncada, and Jorge Mejía-Salazar

DOI: 10.1364/OL.520552 Received 01 Feb 2024; Accepted 11 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: In this letter, we introduce a concept to produce high-resolution, highly integrable biosensing devices. Our idea exploits the highly absorbing modes in multilayered metamaterials to maximize the transverse magneto-optical Kerr effect (TMOKE). Results are discussed in the context of dielectric uniaxial (ε₁ε₂>0) and hyperbolic metamaterial (ε₁ε₂<0) regimes. For applications in gas sensing, we obtained sensitivities of S=46.02 deg/RIU and S=73.91 deg/RIU when considering the system working in the uniaxial and hyperbolic regimes, respectively, with figures-of-merit (resolution) in the order of 310 or higher. On the contrary, when considering the system for biosensing applications (incidence from an aqueous medium), we observed that the proposed mechanism can only be successfully used in the uniaxial regime, where a sensitivity of 56.87 deg/RIU was obtained.

On-chip optical wavefront shaping by transverse spin induced Pancharatanam-Berry phase

Wanyue Xiao and Shubo Wang

DOI: 10.1364/OL.521060 Received 06 Feb 2024; Accepted 11 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Pancharatnam-Berry (PB) metasurfaces can be applied to manipulate the phase and polarization of light within subwavelength thickness. The underlying mechanism is attributed to the geometric phase originating from the longitudinal spin of light. Here, we demonstrate a new type of PB geometric phase derived from the intrinsic transverse spin of guided light. Using full-wave numerical simulations, we show that the rotation of a metallic nano bar sitting on a metal substrate can induce a geometric phase covering 2π full range for the surface plasmons carrying intrinsic transverse spin. Specially, the geometric phase is different for the surface plasmons propagating in opposite directions due to spin-momentum locking. We apply the geometric phase to design metasurfaces to manipulate the wavefront of surface plasmons to achieve steering and focusing. Our work provides a new mechanism for on-chip light manipulations with potential applications in designing ultra-compact optical devices for imaging and sensing.

Echelon grating refractive index sensor

Haotian Zhang, Xiaoping Li, Yue Pan, Hongzhong Cao, Yunjie Xia, and Rende Ma

DOI: 10.1364/OL.520742 Received 02 Feb 2024; Accepted 11 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: There are few reports on optical refractive index sensors that have both high resonant-wavelength resolution (RWR) and high refractive index sensitivity (RIS). Herein, based on an echelon grating, we design a refractive index sensor that combines the two advantages together. The principal fringe of echelon grating has a small full width at half maximum and a good signal-to-noise ratio, leading to a high RWR. The wavefront splitting interference makes the sensor have high RIS. The large free spectral range (FSR) of the principal fringes expands the dynamic range of the sensor. The experimentally realized RWR, RIS, and FSR are 3.76 × 10¯³ nm, 1.14 × 10⁴ nm/RIU (RIU: Refractive Index Unit), and 130 nm, respectively. The detection limit of refractive index is 3.3 × 10¯⁷ RIU. The dynamic range of the sensor is 1.14 × 10¯² RIU. Moreover, the theoretically predicted detection limit of refractive index is around the order of 10¯⁸ RIU. The echelon grating refractive index sensor features low detection limit, low cost, high stability, and good robustness.

Nonlinear-Tolerant Two-Dimensional Distribution Matcher Scheme for Probabilistic Shaping

yanan luo, Bin Chen, and Qin Huang

DOI: 10.1364/OL.519549 Received 22 Jan 2024; Accepted 11 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: This letter proposes a nonlinear-tolerant two-dimensional distribution matcher (2D-DM) scheme. It removes the corner points of probabilistically shaped quadrature amplitude modulation (QAM) to obtain better nonlinear tolerance. Because the remaining number of points is not a power of two, we propose to divide constellation points into different layers and symbols. Then, the proposed 2D-DM performs matching using one-dimensional shapers, which generates the in-phase and quadrature components of QAM together. In fact, it realizes two-dimensional shapers from one-dimensional shapers. Simulation results show that two-dimensional shapers generated by the proposed 2D-DM have higher tolerance to power amplifier nonlinearity and fiber nonlinearity compared to one-dimensional shapers.

Optical Snake States in Photonic Graphene

Olha Bahrova, Sergei Koniakhin, Anton Nalitov, and Evgenia Cherotchenko

DOI: 10.1364/OL.519717 Received 25 Jan 2024; Accepted 11 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: We propose an optical analogue of electron snake states based on artificial gauge magnetic field in photonic graphene implemented by varying distance between cavity pillars.We develop an intuitive and exhaustive continuous model based on tight-binding approximation and compare it with numerical simulations of a realistic photonic structure.The allowed lateral propagation direction is shown to be strongly coupled to the valley degree of freedom and the proposed photonic structure may be used a valley filter.

2.05-μm Tm,Ho:YLF waveguide lasers

Zichen Bai, Zhixiang Chen, yujie xiong, Hongliang Liu, Siying Gao, Yingying Ren, Xingjuan Zhao, Fengqin Liu, Yuechen Jia, and Feng Chen

DOI: 10.1364/OL.520576 Received 05 Feb 2024; Accepted 10 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: In this work, we report on the first 2.05-μm laser based on femtosecond-laser direct written (FsLDW) Tm,Ho:YLF cladding waveguides. A channel waveguide with a 90-μm diameter “fiber-like” low-index cladding is fabricated in a 6 at.% Tm3+, 0.4 at.% Ho3+:LiYF4 crystal by FsLDW. Pumped by Ti:sapphire laser at 795.1 nm, the fabricated waveguide supports efficient lasing oscillation at 2050 nm with a maximum output power of 47.5 mW, a minimum lasing threshold of 181 mW, and a slope efficiency of 20.1%. The impacts of cavity conditions and polarizations of the pump light on the obtained lasing performance are well studied. The experimental results obtained in this study demonstrate the great potential of utilizing Tm,Ho:YLF and FsLDW for the development of durable mid-infrared lasers featuring compact designs.

Nanoprinted microstructure-assisted light incoupling into high numerical aperture multimode fibers

Matthias Zeisberger, Henrik Schneidewind, Torsten Wieduwilt, Oleh Yermakov, and Markus Schmidt

DOI: 10.1364/OL.521471 Received 13 Feb 2024; Accepted 10 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: The coupling of light into optical fibers is limited by the numerical aperture. Here, we show that large-area polymer axial-symmetric microstructures printed on silica multimode fibers improve their in-coupling performance by 2-3 orders of magnitude beyond the numerical aperture limit. A ray-optical mathematical model describing the impact of the grating-assisted light coupling complements the experimental investigation. This study clearly demonstrates the improvement in-coupling performance by nanoprinting microstructures on fibers, opening new horizons for multimode fibers applications in life sciences, quantum technologies and ‘lab-on-fiber’ devices.

Hybrid Graphene Anti-resonant Fiber with Tunable Light Absorption

Kang She, Guo Sheng, Zhengping Shan, Piaorong Xu, and Exian Liu

DOI: 10.1364/OL.520824 Received 07 Feb 2024; Accepted 10 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Controlling the output light-intensity and realizing the light-switch function in hollow-core anti-resonant fibers (HC-ARFs) is crucial for their applications in polarizers, lasers, sensors systems. Here we theoretically propose a hybrid light-intensity-tunable HC-ARF deposited with the sandwiched graphene/hBN/graphene based on the typical six-circular-tube and the nested structures. Changing the external drive voltage from 12.3V to 31.8V, the hybrid HC-ARF experiences a high-low alterative attenuation coefficient with a modulation depth 3.87 dB/cm and 1.91 dB/cm for the six-circular-tube and nested structures respectively, serving as a well-performance light-switch at the optical communication wavelength of 1.55 μm. This response is attributed to the variation of the Fermi level of graphene and is obviously influenced by the core size, fiber length, and the number of graphene and hBN layers. Moreover, one weak attenuation peak originating from the surface plasmon resonance is also found simultaneously and shifts with the incident wavelength. Our design provides a feasible paradigm for integrating graphene with anti-resonant fibers and high-performance electro-optic modulators.

Efficient photorefractive effect triggered bypyroelectricity in magnesium doped LiNbO3 films

Anton Perin, Ludovic gauthier-Manuel, Florent Bassignot, and Mathieu Chauvet

DOI: 10.1364/OL.516930 Received 22 Dec 2023; Accepted 10 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: An attractive photorefractive effect triggered by pyroelectricity is displayed in slab waveguides constituted of magnesium oxide (MgO) doped LiNbO3 film on insulator. Microwatts CW 532 nm guided mode can remarkably self-trapped to form 10 µm FWHM beam triggered by a few degrees temperature increase of the sample. Fast self-focusing response time on the order of milliseconds is measured for milliwatts of injected beam, which is more than two orders of magnitude faster than in undoped LiNbO3 film. Long living 2-D induced waveguides are found to be memorized in the films.

Generation of high-energy, sub-20 fs deep-UV pulses in a twin-crystal third harmonic generation scheme

Peter Susnjar, Gabor Kurdi, Paolo Cinquegrana, Alexander Demidovich, Ivaylo Nikolov, Paolo Sigalotti, and M Danailov

DOI: 10.1364/OL.519486 Received 23 Jan 2024; Accepted 10 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Ultrashort deep ultraviolet (DUV) pulses serve as indispensable tools for investigating molecular dynamics on the femtosecond scale. Nonlinear frequency up-conversion of near-infrared (NIR) light sources in a sequence of nonlinear crystals is a common method for their generation. However, preserving the temporal duration of the starting source encounters challenges owing to phase-matching bandwidth limitations within the harmonic generation process. Here we propose an approach for circumventing this limitation and demonstrate it for the case of generation of the third harmonic of 800 nm pulses in a two-stage scheme (second harmonic generation succeeded by sum-frequency mixing of the fundamental and second harmonic pulses). Expanding the bandwidth of the DUV pulse involves the utilization for the last mixing process of two nonlinear crystals, detuned to convert opposite sides of the spectrum. The implementation of this approach yields 20 µJ, 263 nm DUV pulses as short as 19 fs after compression. The setup is very compact and extremely stable due to the common-path scheme which makes it very interesting for a variety of advanced ultrafast spectroscopy applications.

Dual high-order QAM-modulated mm-wave signal transmission in the Q-band enabled by simple IM/DD architecture and bandpass delta-sigma modulation

Hengxin Yan, Xinying Li, Xiaolong Pan, tangyao Xie, Liye Fang, Jiahao Bi, Han Jiang, and Xiangjun Xin

DOI: 10.1364/OL.521343 Received 08 Feb 2024; Accepted 09 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: The intensity-modulation (IM)/direct-detection (DD) systems have been proven effective and low-cost due to their simple system architecture. However, the Mach‒Zehnder modulator (MZM) of the IM/DD systems only reserves its driving signal intensity. Therefore, the IM/DD systems are generally unable to transmit vector signals and have a restricted spectrum efficiency and channel capacity. Similarly, the radio-over-fiber (RoF) transmission systems based on IM/DD are limited by their simple architecture and generally cannot transmit high-order quadrature amplitude modulation (QAM) signals, which hinders the improvement of their spectrum efficiency. To address the challenges, we propose a novel scheme to simultaneously transmit the dual independent high-order QAM-modulated millimeter-wave (mm-wave) signals in the RoF system with a simple IM/DD architecture, enabled by precoding-based optical carrier suppression (OCS) modulation and bandpass delta-sigma modulation (BP-DSM). The dual independent signals can carry different information, which increases channel capacity and improves spectrum efficiency and system flexibility. Based on our proposed scheme, we experimentally demonstrate the dual 512-QAM mm-wave signal transmission in the Q-band (33-50GHz) under three different scenarios: 1) dual single-carrier (SC) signal transmission, 2) dual orthogonal-frequency-division-multiplexing (OFDM) signal transmission, and 3) hybrid SC and OFDM signal transmission. We achieve high-fidelity transmission of dual 512-QAM vector signals over a 5 km single-mode fiber-28 (SMF-28) and a 1-m single-input single-output (SISO) wireless link operating in the Q-band, with the bit error rates (BERs) of all three scenarios below the hard decision forward error correction (HD-FEC) threshold of 3.8 10-3. To the best of our knowledge, this is the first time to achieve dual high-order QAM-modulated mm-wave signal transmission in a RoF system with a simple IM/DD architecture.

A liquid crystal waveplate operating close to 18 THz

Fabio Novelli, Patrick Friebel, Marta Murillo-Sanchez, J. Michael Klopf, and Laura Cattaneo

DOI: 10.1364/OL.519177 Received 23 Jan 2024; Accepted 09 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: Controlling the properties of mid and far-infrared radiation can provide a means to transiently alter the properties of materials for novel applications. However, a limited amount of optical elements are available to control its polarization state. Here we show that a 15-micron thick liquid crystal cell containing 8CB (4-Octyl-4'-cyanobiphenyl) in the ordered, smectic A phase can be used as a phase retarder or waveplate. This was tested using the bright, short-pulsed (~1 ps) radiation centered at 16.5 μm (18.15 THz) that is emitted by a free electron laser at high repetition rate (13 MHz). These results demonstrate a possible tool for the exploration of the mid and far-infrared range and could be used to develop novel metamaterials or extend multidimensional spectroscopy to this portion of the electromagnetic spectrum.

Tunable liquid crystal lens with symmetric bipolar operation

Zhanna Zemska and Tigran Galstian

DOI: 10.1364/OL.519493 Received 19 Jan 2024; Accepted 09 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: We describe an electrically tunable liquid crystal lens that can dynamically generate symmetric wavefront profiles. The curvature of these profiles may be inversed enabling thus a bipolar response (focusing and defocusing). Different wavefronts, including non-monotonic, are predicted theoretically and demonstrated experimentally. The optical performance of the devices is characterized experimentally in an imaging scheme.

Learning-enabled data transmission with up to 32 multiplexed orbital angular momentum channels through a commercial multi-mode fiber

Jihong Tang, Yaling Yin, Jingwen Zhou, Yong Xia, and Jianping Yin

DOI: 10.1364/OL.518681 Received 15 Jan 2024; Accepted 09 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Multiplexing orbital angular momentum (OAM) modes enables high-capacity optical communication. However, the highly similar speckle patterns of adjacent OAM modes produced by strong mode coupling in common fibers prevents the utility of OAM channel demultiplexing. In this paper, we propose a machine learning-supported fractional OAM-multiplxed data transmission system to sort highly scattered data from up to 32 multiplexed OAM channels propagating through a commercial multi-mode fiber parallelly with an accuracy of >99.92%, which is the largest bit number of OAM superstates reported to date. Here, by learning limited samples, unseen OAM superstates during the training process can be predicted precisely, which reduces the explosive quantity of dataset. To verify its application, both gray and colored images, encoded by the given system, have been successfully transmitted with error rates of <0.26%. Our work might provide a promising avenue for high-capacity OAM optical communication in scattering environments.

Fabrication of elliptical-silica microfiber for ultrasound detection

Haokun Yang, GERARD TATEL, Yuan Wang, Liang Chen, and Xiaoyi Bao

DOI: 10.1364/OL.513749 Received 20 Nov 2023; Accepted 08 Mar 2024; Posted 18 Mar 2024  View: PDF

Abstract: Photons can excite many high order modes in asymmetric fiber which form in-fiber Mach-Zehnder (MZI) interferometers for time dependent displacement sensing without the need of average, due to high contrast. In this paper, we present the design and fabrication of an ultra-compact elliptical-silica microfiber utilizing off-axis flame-drawing for ultrasound detection. This microfiber is engineered with the purpose to excite multiple high-order modes and multi-mode interference. Consequently, this design results in a transmission spectrum with a remarkably sharp slope and high contrast, it gives the high level of detection sensitivity. With a major-axis diameter of 6.25 μm, the elliptical-silica microfiber sensor exhibits a broadband ultrasound frequency response spanning from 20 kHz to 38.5 MHz. Furthermore, it achieves a signal-to-noise ratio (SNR) of up to 80 dB at 1 MHz which is resonance frequency of the microfiber and the linear response for different driving voltages of the PZT ultrasound generator. This low-cost microfiber sensor offers exceptional sensitivity across a broad ultrasonic bandwidth response, making it an ideal choice for non-destructive testing (NDT) and medical imaging applications. Its compact size and immunity to electric and magnetic fields further enhance its utility in various environments.

Untrained neural network enables fast structured illumination microscopy

zitong ye, Xiaoyan Li, Yile Sun, YURAN HUANG, Xu Liu, Yubing Han, and Cuifang Kuang

DOI: 10.1364/OL.511983 Received 21 Nov 2023; Accepted 08 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: Structured illumination microscopy (SIM) offers a two-fold resolution enhancement beyond the optical diffraction limit. At present, SIM requires several raw structured illuminated image frames to reconstruct a super-resolution (SR) image, which limits its imaging speed. Considering this, we herein propose an untrained structured-illumination reconstruction neural network (USRNN), that applies the untrained convolutional neural network (CNN) to reduce the amount of raw data required for SIM reconstruction and thus improve the temporal resolution of SIM. Benefiting from the unsupervised optimizing strategy and CNNs structure priors, the high-frequency information is obtained from the network without a requirement of a large dataset and thus a high-fidelity SR image can be reconstructed using fewer image frames, which allows for SIM imaging with higher speed and greatly reduced photo-bleaching. Experiments on reconstructing non-biological and biological samples demonstrate the high-speed and high-universality capabilities of our method.

Nearest Neighbor Bit Assisted Decision Scheme for ISI Mitigation in Optical Camera Communications

Jin Shi and Jing He

DOI: 10.1364/OL.520900 Received 05 Feb 2024; Accepted 08 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: To enable higher transmission rate in optical camera communication (OCC) systems, severe ISI occurs owing to the reduction of the number of pixel-row-per-bit (PPB). Therefore, those pixels representing logic bit 0 or 1 may have same grayscale values, significantly deteriorating the bit decision when using conventional thresholding scheme. In this letter, a simple yet efficient scheme, referred to as nearest neighbor bit assisted decision (NNBAD) scheme, is proposed and experimentally demonstrated for signal decision in optical camera communication (OCC) systems. NNBAD leverages the nearest neighbor bit to jointly assist bit decision for pixels with severe ISI. Experimental results show that, for OCC systems with OOK modulation, those pixels with severe ISI cannot be distinguished by conventional thresholding scheme. Yet, NNBAD scheme exhibits strong robustness against ISI, remarkably improving the BER performance. The proposed scheme can achieve a throughput of 8.2 kbps with OOK modulation under an illuminace of 600 lx.

Deep Learning Empowers Photothermal Microscopy with Super-Resolution capabilities

Yonghui Wang, zhuoyan yue, Fei Wang, Peng Song, and Junyan Liu

DOI: 10.1364/OL.517164 Received 28 Dec 2023; Accepted 08 Mar 2024; Posted 12 Mar 2024  View: PDF

Abstract: In the past two decades, Photothermal microscopy (PTM) has reached the sensitivity of single particle or molecule, and it has been used in the fields of material science and biology. PTM is a far-field imaging method, its resolution is restricted by the diffraction limits. In our previous work, the modulated difference PTM (MDPTM) was proposed to improve the lateral resolution, but, its resolution improvement was seriously constrained by the information loss and the artifacts. In this letter, a deep learning approach of the cycle generative adversarial network (Cycle GAN) is employed for further improving the resolution of PTM, called DMDPTM. The point spread functions (PSF) of both PTM and MDPTM are optimized and act as the second generator of Cycle GAN. Besides, the relationship between the sample’s volume and the photothermal signal is taken into account during dataset construction. The images of both PTM and MDPTM are used as the input of the Cycle GAN, as well as bring more information to the network. In the simulation, the distance of 80nm between two nanoparticles (diameter of 60nm) can be quantitively determined by DMDPTM, and this replies that the DMDPTM has a 3.3fold resolution enhancement over the conventional PTM. Consequently, the super-resolution of DMDPTM is experimentally verified by restored images of Au nanoparticles, achieving the resolution of 114nm. Finally, the DMDPTM is successfully applied for the imaging of carbon nanotubes with diameters of 10-30nm. Therefore, the DMDPTM will serve as a powerful tool to improve the lateral resolution of PTM.

A Visible Light Positioning System Using Smartphone’s Built-in Ambient Light Sensor and Inertial Measurement Unit

Yuan Zhuang, Yaxin Wang, xiansheng yang, and Tianbing Ma

DOI: 10.1364/OL.519674 Received 25 Jan 2024; Accepted 08 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: In recent years, the visible light positioning field has experienced remarkable advancements. However, smartphones find it difficult to identify LED and extract each LEDs light signal intensity due to the low-frequency and uneven sampling of built-in Ambient Light Sensors (ALS, which is a photodiode that measures ambient light in lux units). Thus, traditional visible light positioning systems cannot be directly applied to smartphones. In this paper, we propose a single-light visible light positioning system using a non-modulated LED as an emitter, the built-in ALS as the receiver, and the inertial measurement unit of the smartphone to assist in measuring the smartphones attitude. It only requires the user to turn the smartphone by a few angles in a stationary position to estimate its current 3D spatial position. This method does not require modification of the existing lighting system and consumes less power than the camera-based VLP systems. We have built an experimental site measuring 5m × 5m × 2.2m to evaluate the performance of the positioning system, and the preliminary results show that the proposed system achieves sub-meter level positioning accuracy.

Free spectral range measurement based on multi-longitudinal mode laser self-mixing ellipse fitting degree detection technique

Zhen Huang, HAOZHONG XU, and Dongyu Li

DOI: 10.1364/OL.519913 Received 25 Jan 2024; Accepted 08 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: A method by detecting the ellipse fitting degree of the trajectory equation formed by two self-mixing (SM) signals in the multi-longitudinal mode laser self-mixing system with a Wollaston prism is presented to test the free spectral range (FSR) of the laser. By utilizing the orthogonal vector and phase-shift characteristics between adjacent longitudinal modes, the variations in multi-mode self-mixing effects caused by changes in the external cavity length are converted to changes in the trajectory consisting of two orthogonal SM signals. The FSR is calculated by detecting the difference in external cavity lengths between the two positions, where the trajectory of the two SM signals has the best fit to an ellipse. To achieve automatic FSR measurement, the ellipse fitting degree is proposed as the criterion for positioning external cavity mode. Experimental results indicate that the FSR of the laser diode is measured to be 85. GHz with a resolution of 0.48 GHz while the corresponding external cavity resolution is 10 μm and the resolution of the ellipse fitting degree is less than 1. The compact and straightforward design, coupled with high sensitivity, automated measurements, and immunity to optical feedback, holds significant promise as a robust tool for measuring FSR and assessing laser performance.

Coherent beam combining of femtosecond third-harmonic generators: Towards high-power, high-beam-quality UV light generation

Genyu Bi, Chenming Yu, Bowen Liu, Jintao Fan, Yuxi Chu, Xiaohui Zhang, and Ming-lie Hu

DOI: 10.1364/OL.520637 Received 02 Feb 2024; Accepted 07 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: Coherent beam combining (CBC) of two femtosecond third-harmonic (TH) generators is proposed and demonstrated. By applying phase modulation to one of the fundamental laser pulses, the feedback loop effectively eliminates both phase and pointing errors between the two TH femtosecond laser beams. The system delivers 345-nm femtosecond laser pulses with 22-W average power at 1-MHz repetition rate. The average combining efficiency is 91.5% over approximately one hour of testing. The beam quality of the combined ultraviolet (UV) laser beam is near-diffraction-limited with M² factors of M²x=1.36, M²y=1.24, which are similar to those of the individual channels. This scheme exhibits promising potential for increasing high-beam-quality UV laser power.

Integrated photon pair source based on a silicon nitride micro-ring resonator for quantum memories

Juan Durán-Gómez, Roberto Ramirez Alarcón, Mauricio Gómez Robles, Patricia Tavares Ramírez, Gerardo Rodríguez Becerra, Erasto Ortíz-Ricardo, and Rafael Salas-Montiel

DOI: 10.1364/OL.519784 Received 24 Jan 2024; Accepted 07 Mar 2024; Posted 08 Mar 2024  View: PDF

Abstract: We report the design of an integrated photon pair source based on spontaneous four wave mixing (SFWM), implemented in an integrated micro-ring resonator in the silicon nitride platform (Si₃N₄). The signal photon is generated with emission at 606 nm and bandwidth of 3.98 MHz, matching the spectral properties of Praseodymium ions (Pr), while the idler photon is generated at 1430.5 nm matching the wavelength of a CWDM channel in the E-band. This novel device is designed to interact with a quantum memory based on a Y₂SiO₅ crystal doped with Pr³+ ions, in which, we used cavity-enhanced SFWM along with dispersion engineering to reach the required wavelength and the few megahertz signal photon spectral bandwidth.

Pulse pattern manipulation of dichromatic soliton complexes by a twistable tapered fiber filter

Bowen Liu, Shinji Yamashita, and Sze Set

DOI: 10.1364/OL.517054 Received 15 Jan 2024; Accepted 06 Mar 2024; Posted 14 Mar 2024  View: PDF

Abstract: Soliton complexes highlight the particle-like dynamics of dissipative pulse motions. Whereas simple and reliable manipulation of bound solitons remains challenging. Here, we report controllable pulse patterns of robustly coexisting dichromatic soliton complexes in an all-polarization-maintaining fiber laser based on a twistable tapered fiber filter. According to the twist angle, dichromatic pulses are switched between different patterns. Components at each wavelength can also be independently manipulated depending on the twist direction, enabling extended encoding formats from the time to frequency domain. To the best of our knowledge, it is the first experimental demonstration of dual-wavelength soliton complexes that different bound pulse patterns coexist at separated wavebands.

Decoding of compressive data pages for optical data storage utilizing FFDNet

Zehao He, Yan Zhang, Daping Chu, and Liangcai Cao

DOI: 10.1364/OL.516785 Received 08 Jan 2024; Accepted 06 Mar 2024; Posted 15 Mar 2024  View: PDF

Abstract: Coded aperture-based compression has proven to be an effective approach for high-density cold data storage. Nevertheless, its limited decoding speed represents a significant challenge for its broader application. We introduce a novel decoding method leveraging the fast and flexible denoising network (FFDNet), capable of decoding a coded aperture-based compressive data page within 30.64 s. The practicality of the method has been confirmed in the decoding of monochromatic photo arrays, full-color photos, and dynamic videos. In experimental trials, the variance between decoded results obtained via the FFDNet-based method and the FFDNet-absent method in terms of average PSNR is less than 1 dB, while realizing a decoding speed enhancement of over 100-fold when employing the FFDNet-based method.

Terahertz-triggered ultrafast nonlinear optical activities in two-dimensional centrosymmetric PtSe₂

Xin Chen, Jianhua Sang, kang wang, zhuorui zheng, Yifei Fang, Jun Wang, Xiaojun Wu, Liwei Song, Ye Tian, Yuxin Leng, and Ruxin Li

DOI: 10.1364/OL.520416 Received 01 Feb 2024; Accepted 06 Mar 2024; Posted 11 Mar 2024  View: PDF

Abstract: The nonlinear mechanisms of polarization and optical fields can induce extensive responses in materials. In this study, we report on two kinds of nonlinear mechanisms in the topological semimetal PtSe₂ crystal under the excitation of intense terahertz pulses, which are manipulated by the real and imaginary parts of the nonlinear susceptibility of PtSe₂. Regarding the real part, the broken inversion symmetry of PtSe₂ is achieved through terahertz electric field polarization approach, which is characterized by second harmonic generation (SHG) measurements. The transient terahertz-laser-induced SHG signal occurs within 100 fs and recombines to the equilibrium state within 1 ps, along with a high signal-to-noise ratio (~51 dB) and a high on/off ratio (~10²). Regarding the imaginary part, a nonlinear absorption change can be generated in the media. We reveal a terahertz-induced absorption enhancement in PtSe₂ via nonlinear transmittance measurements, and the sheet conductivity can be modulated up to 42% by terahertz electric fields in our experiment. Therefore, the terahertz-induced ultrafast nonlinear photoresponse reveals the application potential of PtSe₂ in photonic and optoelectronic devices in terahertz technology.

Tracking bandwidth limitations in strong optical-turbulence conditions

Matthew Kalensky, Darren Getts, and Mark Spencer

DOI: 10.1364/OL.521092 Received 06 Feb 2024; Accepted 06 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: We derive a modified fundamental tracking frequency, which is applicable for beam-control systems that do not employ adaptive-optics compensation. Specifically, we show that when higher-order phase aberrations are appreciably present (when D/r0>4, where D is the aperture diameter and r0 is the Fried parameter), there are diminishing returns on tracking faster than the modified fundamental tracking frequency. This conclusion results from beam spreading being the dominate driver for decreased system performance, as opposed to beam jitter.

17.5 Gb/s physical layer key distribution over 100 km fiber link based on channel physical intrinsic property and polarization reciprocity

Taihang Qiu, Lei Deng, Qi Yang, Xiaoxiao Dai, Deming Liu, and Mengfan Cheng

DOI: 10.1364/OL.517847 Received 05 Jan 2024; Accepted 06 Mar 2024; Posted 08 Mar 2024  View: PDF

Abstract: Secure key distribution (SKD) schemes based on fiber channel reciprocity provide information-theoretic security as well as a simple symmetric structure. However, the nonlinear effects and backscattering effects introduced during the bidirectional transmission process degrade the channel reciprocity. Recent unidirectional SKD schemes avoid non-reciprocal factors but require additional negotiation mechanisms to aggregate the transmitter and receiver data. Here, we propose a unidirectional SKD scheme based on channel physical intrinsic property and polarization reciprocity. The designed loopback structure constructs asymmetry between legitimate and illegitimate parties while aggregating data. The deployment of a broadband chaotic entropy source significantly improves the key generation rate (KGR). In the experiment, the KGR reaches 17.5 Gb/s and the distribution distance reaches 100km.

Co-LSTM based fiber link modeling with ASE noise tracking for long-haul coherent optical transmission

Jiayu Zheng, tianhong zhang, and Fan Zhang

DOI: 10.1364/OL.517041 Received 03 Jan 2024; Accepted 05 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: In this letter, a novel channel modeling scheme based on cascading chromatic dispersion-nonlinearity feature decoupling modules is proposed with the center-oriented long-short term memory (Co-LSTM) network structure adopted for modeling nonlinearity of each optical fiber span. By tracking amplified spontaneous emission (ASE) noise at the output of each fiber span, the Co-LSTM based channel modeling scheme achieves a high waveform accuracy for long-haul coherent optical transmission compared with the conventional split-step Fourier transform method (SSFM), while saving calculation time by one order of magnitude.

Architecture for Low-cost and Highly Flexible Metro-access Networks using SOA-based OADM Nodes and Digital Subcarrier Multiplexing with Power Loading

Zhouyi Hu, Shiyi Xia, Henrique Freire Santana, Marijn Rombouts, Bin Shi, and Nicola Calabretta

DOI: 10.1364/OL.514171 Received 23 Nov 2023; Accepted 05 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: Metro-access networks exploiting wavelength division multiplexing (WDM) to cope with the ever-growing bandwidth demands are sensitive to cost and need to be fast-configurable to meet the requirements of many new network services. Optical add-drop multiplexers (OADMs) are a key component in enabling fast dynamic wavelength allocation and optimization. In this Letter, we propose and demonstrate a novel architecture for high-performance metro-access networks that utilizes semiconductor optical amplifier (SOA) -based OADM nodes, digital subcarrier multiplexing (DSCM), low-cost direct detection receivers, and power loading techniques, which makes the designed metro-access network cost-effective, fast reconfigurable, and flexible for bandwidth allocation on demand. Through a proof-of-concept experiment, we have successfully demonstrated a prototype horseshoe optical network consisting of up to 4 SOA-based OADM nodes at 40 Gb/s per wavelength channel by leveraging the proposed scheme. Flexible bandwidth allocation and dynamic add and drop operations have also been achieved in an emulated WDM optical network. All results indicate the great scalability and flexibility of the proposed architecture.

New Fast Nonlinear Fourier Transform Algorithms forOptical Data Processing

Sergey Medvedev, Irina Vaseva, Dmitry Kachulin, Igor Chekhovskoy, and Mikhail Fedoruk

DOI: 10.1364/OL.515200 Received 06 Dec 2023; Accepted 05 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: The nonlinear Fourier transform (NFT) is an approach that is similar to a conventional Fourier transform. In particular, NFT allows to analyze the structure of a signal governed by the nonlinear Schrödinger equation(NLSE). Recently, NFT applied to NLSE has attracted special attention in applications of fiber-optic communication. Improving the speed and accuracy of the NFT algorithms remains an urgent problem in optics. We present an approach that allows to find all variants of symmetric exponential splitting schemes suitable for the fast NFT (FNFT) algorithms with low complexity. One of the obtained schemes showed good numerical results comparing with other fast fourth order NFT schemes.

Compact and broadband silicon polarization splitter-rotator using adiabaticity engineering

Yung-Jr Hung, Chih-Hsien Chen, Hung-Ching Chung, Jun-Zhu Lai, and Shuo-Yen Tseng

DOI: 10.1364/OL.518607 Received 12 Jan 2024; Accepted 05 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: We propose and demonstrate a short and broadband silicon mode-conversion polarization splitter-rotator (PSR) consisting of a mode-conversion taper and an adiabatic coupler-based mode sorter both optimized by adiabaticity engineering (AE). AE is used to optimize the distribution of adiabaticity parameter over the length of the PSR, providing shortcut to adiabaticity at a shorter device length. The total length of the PSR is 85 μm. The design is compatible with standard silicon photonicsplatforms and requires only one patterning step. Fabricated PSR has a polarization crosstalk of less than -20 dB over the entire O-band for the TE polarization, and a polarization crosstalk of less than -15 dB from 1267 to 1348 nm for the TM polarization. Overall, the PSR shows low polarization crosstalk (-15 dB) over a bandwidth of 81 nm in the O-band. Cross-wafer measurements show that the PSR has good fabrication tolerance.

Instantaneous frequency measurement based on photonic compressive sensing with sub-Nyquist pseudo-random binary sequences

Runcheng Li, shuna yang, Bo Yang, Yiran Gao, Hongxia He, and Hao Chi

DOI: 10.1364/OL.520471 Received 01 Feb 2024; Accepted 05 Mar 2024; Posted 06 Mar 2024  View: PDF

Abstract: We propose a novel approach to realizing instantaneous frequency measurement with ultra-high measurement bandwidth, which utilizes three-channel photonic compressive sensing (CS) with sub-Nyquist pseudo-random binary sequences (PRBSs). In each CS channel, an alias frequency is recovered due to the sub-Nyquist property of the applied PRBS. A frequency identification algorithm is employed to determine the frequency of the signal under measurement according to the three alias frequencies. The proposed approach significantly reduces the bit rate of the applied PRBSs and the sampling rate required by the digitizers in CS. A proof-of-concept experiment for measuring frequency in the Ku band is demonstrated using PRBSs at 1 Gb/s and digitizers with a sampling rate of 250 MS/s.

Unsupervised Spectral Reconstruction from RGB images under Two Lighting Conditions

Xuheng Cao, Yusheng Lian, Zilong Liu, Jin Li, and Kaixuan Wang

DOI: 10.1364/OL.517007 Received 27 Dec 2023; Accepted 04 Mar 2024; Posted 07 Mar 2024  View: PDF

Abstract: Unsupervised spectral reconstruction (SR) aims to recover the hyperspectral image (HSI) from corresponding RGB images without annotations. Existing SR methods achieve SR from single RGB image, hindered by the significant spectral distortion. Although several deep learning-based methods increase the SR accuracy by adding RGB image, their network are always designed for other image recovery task, leaving a huge room for improvement. To overcome this problem, we propose a novel approach that reconstructs the HSI from a pair of RGB images captured under two illuminations, significantly improving reconstruction accuracy. Specifically, an SR iterative model based on two illuminations is constructed at first. By unfolding the proximal gradient algorithm solving this SR model, an interpretable unsupervised deep network is proposed. All the modules in the proposed network have precise physical meanings, which enables our network have both superior performance and good generalization capability. Experimental results on two public datasets and our real-world images show the proposed method significantly improves both visually and quantitatively as compared with state-of-the-art methods.

Polarized vortex Smith–Purcell radiation with cascaded metasurfaces

Wen-xia Xu, Wenjia Li, Yehan Wang, Chunhua Qin, Botian Sun, Chunying Guan, Jianlong Liu, and Jin-hui Shi

DOI: 10.1364/OL.503537 Received 17 Aug 2023; Accepted 04 Mar 2024; Posted 06 Mar 2024  View: PDF

Abstract: We introduce the concept of polarized vortex Smith–Purcell radiation by the interaction of electron beam and cascaded metasurfaces. The spin and orbital angular momenta of Smith–Purcell radiation are determined by the cascaded metasurface that consists of a grating and a phase gradient metasurface. The grating converts the electron beam radiation into the desired polarized light, while the phase gradient metasurface generates vortex light. Furthermore, the vortex Smith–Purcell radiation with linear and circular polarizations can be achieved by the various cascaded metasurfaces. In particular, the conversion of chirality in Smith–Purcell radiation carrying circular polarization is accompanied by the alteration of positive and negative topological charges. This work paves the way for generating polarized vortex electron radiation and is beneficial to promote the development of free electron-driven devices.

Extending the Reach of Multi-core Fiber via Voronoi Constellations with Concatenated Multi-level Coding

Can zhao, Bin Chen, Yi Lei, Shen Li, Jiaqi Cai, Daohui Hu, Wenkai fang, and Lin Sun

DOI: 10.1364/OL.517409 Received 12 Jan 2024; Accepted 04 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: This paper proposes a novel coded modulation scheme for randomly-coupled multi-core fiber (RC-MCF) via multi-dimensional constellation with concatenated two-level multi-level coding (MLC). In the proposed system, the 16-dimensional (16D) Voronoi constellation (VC), naturally fitting with the sixteen degrees of freedom of a four-core fiber (two quadratures, two polarizations, and four cores), is generated by a latticed-based shaping method to provide higher shaping gains. Moreover, combining it with the concatenated two-level MLC can further achieve better performance-complexity tradeoff. It is demonstrated by simulation results of long-haul multi-channel RC-MCF transmission that, the proposed coded modulation scheme for four-core fiber transmission offers 77% reduction in the number of decoding operations, and up to 21% (585km) reach increase over the conventional bit-interleaved coded modulation scheme for quadrature amplitude modulation.

Optical phase encoding in pulsed approach to reservoircomputing

Johan Henaff, Matthieu Ansquer, Miguel Cornelles Soriano, Roberta Zambrini, Nicolas Treps, and Valentina Parigi

DOI: 10.1364/OL.518505 Received 23 Jan 2024; Accepted 04 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: The exploitation of the full structure of multimode light fields enables compelling capabilities in many fields including classical and quantum information science. We exploit data-encoding on the optical phase of the pulses of a femtosecond laser source for a photonic implementation of a reservoir computing protocol. Rather than intensity detection, data-reading is done via homodyne detection that accesses combinations of amplitude and phase of the field. Numerical and experimental results on NARMA tasks and laser dynamic predictions are shown. We discuss perspectives for quantum enhanced protocols.

Fast-beam-switching optical phased array for moving objects in wireless optical communication networks

Shichong Yang, Guihan Wu, Kaifei Tang, Fuhao Yu, Xiang Ji, Yu Xin, and Wei Jiang

DOI: 10.1364/OL.517454 Received 23 Jan 2024; Accepted 04 Mar 2024; Posted 04 Mar 2024  View: PDF

Abstract: For optical wireless communication systems, mechanical beam steering struggles to timely switch between multiple users or search for moving users. Here we demonstrate a fast-beam-switching optical phased array (OPA) for agile wireless communications networks. For point-to-multi-point (P2MP) scenarios, a setup of OPA-based fast beam switching between two aligned receivers was developed. A loss-free image transmission experiment was used to demonstrate the stability of switching. Furthermore, we have developed an approach to using the fast-switching OPA to follow the trajectory of moving objects so as to help enable agile random-access switching between moving objects. These results could help offer fast switching and reconfiguration for indoor wireless optical communications.

Sub-Doppler spectroscopy of the near-UV Cs atom 6S$_{1/2}$ - 7P$_{1/2}$ transition in a microfabricated vapor cell

Emmanuel Klinger, Andrei Mursa, Carlos Rivera-Aguilar, Remy Vicarini, Nicolas Passilly, and Rodolphe Boudot

DOI: 10.1364/OL.514866 Received 01 Dec 2023; Accepted 04 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: We report on the characterization of sub-Doppler resonances detected by probing the 6S$_{1/2}$ - 7P$_{1/2}$ transition of Cs atom at 459 nm in a microfabricated vapor cell. The dependence of the sub-Doppler resonance (linewidth, amplitude) on some key experimental parameters, including the laser intensity and the cell temperature, is investigated. These narrow atomic resonances are of interest for high-resolution spectroscopy, instrumentation, and may constitute the basis of a near-UV microcell optical standard.

Design and demonstration of high efficiency perfectly vertical grating couplers with random structure

Xin Jin, Jinbin Xu, Cuiwei Xue, chenxing guo, Liucheng Fu, Min Liu, Yunliang Shen, Xueling Quan, and Xiu-Lan Cheng

DOI: 10.1364/OL.519489 Received 22 Jan 2024; Accepted 04 Mar 2024; Posted 08 Mar 2024  View: PDF

Abstract: Utilizing an automated optimization method, we propose a perfectly vertical grating coupler (PVGC) characterized by random structure, superior performance, simplified fabrication process, and increased minimum feature size (MFS). Within the range of MFS from 60 nm to 180 nm, the optimized PVGC exhibited a simulated coupling efficiency of approximately -2.0 dB at 1550 nm with a 34 nm 1-dB bandwidth. Experimental results for the corresponding structure demonstrated coupling efficiencies ranging from -2.5 dB to -2.8 dB with a 32 nm 1-dB bandwidth, while maintaining high manufacturing tolerances. This represents the most outstanding experimental outcome to date regarding the coupling performance of a PVGC fabricated on 220nm silicon on insulator (SOI), without requiring any complex processes as reported in existing literature.

Tailored fabrication of self-rolled-up AlGaN/GaN tubular structure with photoelectrochemical etching

Hyunsu Hwang, Seonghun Ahn, Hyun Gyu Song, Kie Young Woo, and Yong-Hoon Cho

DOI: 10.1364/OL.518076 Received 09 Jan 2024; Accepted 03 Mar 2024; Posted 06 Mar 2024  View: PDF

Abstract: Group III-nitride semiconductors with tubular structure have been considered to be applicable to various fields, such as optics, electronics, and chemical sensors. Thereafter, the method of tailored fabrication of the tubular structure is needed. In this research article, micro-sized tubular structures were fabricated through the rolling of a planar heterostructure composed of group III-nitride alloys, employing the photoelectrochemical (PEC) etching method. The rolling direction was precisely controlled using a triangular-shaped pattern with a stem structure created through photolithography. To customize the geometry of the tubular structure, an analytical calculation of strain and deformation was conducted for the AlGaN/GaN heterostructure. Subsequent to the calculation, an AlGaN/GaN/InGaN/n-GaN/sapphire structure was designed and fabricated using metal-organic chemical vapor deposition (MOCVD). Photolithography and PEC etching were employed to selectively etch the sacrificial layer, which in this study was a 5-nm thick InGaN layer. Micro-photoluminescence (μ-PL) and micro Raman spectroscopy were employed to examine the position-dependent strain in the tubular structure. The polarization-resolved PL measurement revealed strain anisotropy in the rolled-up microtube structure, emphasizing the importance of appropriate pattern design for achieving strain uniformity across the entire tubular structure.

A data-driven method of super-resolution image recovery for speckle-illumination photoacoustic computed tomography

Tianhua Zhou, Boyi Li, Xin liu, and Dean Ta

DOI: 10.1364/OL.509788 Received 16 Nov 2023; Accepted 01 Mar 2024; Posted 05 Mar 2024  View: PDF

Abstract: Methods have been proposed in recent years aimed at pushing photoacoustic imaging resolution beyond the acoustic diffraction limit, among which those based on random speckle illumination show particular promise. In this Letter, we propose a data-driven deep learning approach to processing the added spatiotemporal information resulting from speckle illumination, where the neural network learns the distribution of absorbers from a series of different samplings of the imaged area. In ex-vivo experiments based on the tomography configuration with prominent artifacts, our method successfully breaks the acoustic diffraction limit, and delivers better results in identifying individual targets when compared against a selection of other leading methods.

Cladding-pumped laser and amplifier for E- and S-bands based on multimode bismuth-doped GRIN fibers: toward “watt-level” output power

Alexander Vakhrushev, Andrey Umnikov, Alexandr Dostovalov, Konstantin Riumkin, Sergey Alyshev, Elena Firstova, Aleksandr Khegai, Mikhail Melkumov, Sergey Babin, and Sergey Firstov

DOI: 10.1364/OL.514236 Received 23 Nov 2023; Accepted 01 Mar 2024; Posted 01 Mar 2024  View: PDF

Abstract: In this paper, we investigated the potential scalability of output power of a cladding-pumped laser and a power amplifier (booster) based on multimode Bi-doped fiber (BDF) using mode-selection approach. We fabricated the multimode double-clad graded-index (GRIN) fiber with a confined Bi-doped germanosilicate glass core having a diameter of ≈30 and ≈60 μm. Using femtosecond (fs) inscription technology with high spatial resolution, Bragg gratings of a special transverse structure allowing the selection of low-order modes were written into the core of BDFs. The operation features of the cladding-pumped multimode bismuth-doped GRIN fiber lasers with the inscribed Bragg gratings having various reflection coefficients were investigated. In addition, the behaviorof the output power and beam quality (M² parameter) of the optical radiation of the developed devices was studied. The CW laser and booster operating at near 1.45 µm with a maximum output power of ≈0.8 W and ≈1 W, correspondingly, based on the 60-µm-core BDF under pumping by multimode laser diodes at 808 nm were developed, which are to the best of our knowledge the most powerful cladding-pumped BDF devices to date. Near single-mode lasing (M²<1.3) is demonstrated for 30-µm-core fiber. The experimental data open new possibilities to achieve higher powers in cladding-pumped BDF sources, which are more cost-effective compared to core-pumped counterparts.

Longitudinal imaging of vitreal hyperreflective foci in mice with acute optic nerve damage using visible-light optical coherence tomography

Weijia Fan, David Miller, Shichu Chang, JUNGHUN KWON, Wei-Hong Yeo, Marta Grannonico, Xiaorong Liu, and Hao Zhang

DOI: 10.1364/OL.512029 Received 14 Dec 2023; Accepted 25 Feb 2024; Posted 04 Mar 2024  View: PDF

Abstract: Hyperreflective foci (HRF) appear in optical coherence tomography (OCT) images of the retina and vitreous of patients with various ocular diseases, including uveitis, diabetic retinopathy, age-related macular degeneration, and glaucoma. HRF are hypothesized to be immune cells that appear in response to ischemia or tissue damage. To accurately identify HRF and establish their clinical significance, it is necessary to replicate the detection of similar patterns in vivo in a small animal model. We combined visible-light OCT with temporal speckle averaging (TSA) to visualize and track vitreal HRF (VHRF) densities for three days after optic nerve crush (ONC) injury. Resulting vis-OCT images revealed that VHRF density significantly increased approximately 10-fold at 12 hours after ONC and returned to baseline three days after ONC.

Optical wireless communication using flexible and waterproof perovskite color converter

Feifei Qin, Feng Chen, Yue Cao, Linning Wang, Chenwei Wang, Yanan Liao, Yuhang Dai, Junfeng Lu, Xinru Lan, Xu Wang, xianwu tang, Xiaoyan Liu, Gangyi Zhu, and yongjin wang

DOI: 10.1364/OL.518687 Received 12 Jan 2024; Accepted 25 Feb 2024; Posted 15 Mar 2024  View: PDF

Abstract: In this paper, the CH3NH3PbBr3 nanocrystals are embedded into the interstices of the fluorine (polyvinyl fluoride/polyvinylidene fluoride, PVF/PVDF) matrix on polyethylene terephthalate (PET) substrate to introduce a new advantage, such as flexible and waterproof, while maintaining the high optical performance of perovskites. The sample's photoluminescence (PL) spectra under 325 nm laser is green emission peaked at 537 nm with full width at half maximum (FWHM) of about 21.5 nm and a fast PL decay time. As a color converter, it shows high optical absorption and can transform light from solar-blind ultraviolet to blue region into green region in air, water, and bending conditions. While excited by 270 nm ultraviolet light emitting diode (LED), the system's observed -3dB bandwidth with the color converter is near 4.4 MHz in air and water condition with well-eye diagrams at a data rate of 30 Mbps. Finally, we demonstrate an audio transmission application with an ultraviolet light source, color conversion layer, and low-cost silicon-based photodetector.

Dual-comb spectroscopy at a fast rate in the water-transparent 8-12 μm region

Luca Moretti, Mathieu Walsh, Nawaf Abualsaud, Davide Gatti, Marco Lamperti, Jérôme Genest, Aamir Farooq, and Marco Marangoni

DOI: 10.1364/OL.515199 Received 07 Dec 2023; Accepted 23 Feb 2024; Posted 28 Feb 2024  View: PDF

Abstract: We introduce a dual-comb spectrometer based on Erbium-fiber oscillators at 250 MHz that operates in the 8-12 μm spectral range over optical bandwidths up to 9 THz with a multi-kHz acquisition rate. Over an observation bandwidth of 0.8 THz, the signal-to-noise ratio per spectral point reaches 168 Hz⁰·⁵ at an acquisition rate of 26 kHz, which allows investigation of transient processes in the gas phase at high temporal resolution. The system also represents an attractive solution for multispecies atmospheric gas detection in open paths due to the water transparency of the spectral window, the use of thermo-electrically cooled detectors and the out-of-loop phase correction of the interferograms.

Computer-generated holography with ordinary display

Otoya Shigematsu, Makoto Naruse, and Ryoichi Horisaki

DOI: 10.1364/OL.516005 Received 19 Dec 2023; Accepted 18 Feb 2024; Posted 20 Feb 2024  View: PDF

Abstract: We propose a method of computer-generated holography (CGH) using incoherent light emitted from a mobile phone screen. In this method, we suppose a cascade of holograms in which the first hologram is a color image displayed on the mobile phone screen. The hologram cascade is synthesized by solving an inverse problem with respect to the propagation of incoherent light. We demonstrate three-dimensional color image reproduction using a two-layered hologram cascade composed of an iPhone and a spatial light modulator.

High-Efficiency Radiation-Balanced Yb-Doped Silica Fiber Laser With 200-mW Output

Enkeleda Balliu, Bailey Meehan, Mary Ann Cahoon, Thomas Hawkins, John Ballato, Peter Dragic, Tommy Boilard, Lauris Talbot, Martin Bernier, and Michel Digonnet

DOI: 10.1364/OL.517568 Received 02 Jan 2024; Accepted 18 Feb 2024; Posted 21 Feb 2024  View: PDF

Abstract: The focus of this study was the development of a second generation of fiber lasers internally cooled by anti-Stokes fluorescence. The laser consisted of a length of single-mode fiber spliced to fiber Bragg gratings to form the optical resonator. The fiber was single-moded at the pump (1040 nm) and signal (1064 nm) wavelengths. Its core was heavily doped with Yb, in the initial form of CaF2 nanoparticles, and co-doped with Al to reduce quenching and improve the cooling efficiency. After optimizing the fiber length (4.1 m) and output-coupler reflectivity (3.3%), the fiber laser exhibited a threshold of 160 mW, an optical efficiency of 56.8%, and a radiation-balanced output power (no net heat generation) of 192 mW. On all three metrics, this performance is significantly better than the only previously reported radiation-balanced fiber laser, which is even more meaningful given that the small size of the single-mode fiber core (7.8-µm diameter). At the maximum output power (~2 W), the average fiber temperature was still barely above room temperature (470 mK). This work demonstrates that with anti-Stokes pumping, it is possible to induce significant gain and energy storage in a small-core Yb-doped fiber while keeping the fiber cool.

Dynamic beam steering for wireless optical power transfer in IoT applications

NGOC-LUU NGUYEN, Khanh-Hung Nguyen, Nadeem Javed, and Jinyong Ha

DOI: 10.1364/OL.518243 Received 12 Jan 2024; Accepted 13 Feb 2024; Posted 21 Feb 2024  View: PDF

Abstract: The alignment of a receiver with pencil beam in a wireless optical power transfer (WOPT) system employing resonance beam charging (RBC) technology limits the establishment of a resonance cavity. Accurate tracking necessitates precise and dependable monitoring, which requires the exact placement of transmitting and receiving devices. Herein, we present a concept of a two-dimensional (2D) beam steering mechanism for RBC-based WOPT systems utilizing dispersed laser beams. The proposed approach allows significant improvement, including reduction of scanning times and minimization of errors, in relation to conventional pencil-beam-based systems. Experimental results reveal an improvement of 121.01% in the efficiency of acquisition time, a reduction of 81.09% in pointing errors, and 75.4% increase in tracking performance. These results establish the prerequisites for the implementation of dispersed beam steering in the RBC-based WOPT system. This capability empowers the system to charge movable devices and Internet of Things devices consistently in smart factories.

Off-plane quartz-enhanced photoacoustic spectroscopy

Huijian Luo, Junming Li, Haohua Lv, Jiabao Xie, Chenglong Wang, Haoyang Lin, Ruobin Zhuang, Wenguo Zhu, Yongchun Zhong, Ruifeng Kan, JianHui Yu, and Huadan Zheng

DOI: 10.1364/OL.506650 Received 25 Sep 2023; Accepted 01 Jan 2024; Posted 31 Jan 2024  View: PDF

Abstract: In this work, we developed off-plane quartz-enhanced photoacoustic spectroscopy (OP-QEPAS). In the OP-QEPAS the light beam neither went through the prong spacing of the quartz tuning fork (QTF), nor in the QTF plane. The light beam is in parallel with the QTF with an optimal distance, resulting in low background noise. A radial-cavity (RC) resonator was coupled with the QTF to enhance the photoacoustic signal by radial resonance mode. By offsetting both the QTF and the laser position from the central axis, we enhance the effect of acoustic radial resonance and prevent the noise generated by direct laser irradiation of the QTF. Compared to IP-QEPAS based on a bare QTF, the developed OP-QEPAS with a RC resonator showed a >10× signal-to-noise ratio (SNR) enhancement. The OP-QEPAS system has great advantages in the use of light emitting devices (LEDs), long-wavelength laser sources such as mid-infrared quantum cascade lasers, and terahertz sources. When employing a LED as excitation source, the noise level was suppressed by ~2 orders of magnitude. Furthermore, the radial and longitudinal resonance modes can be combined to further improve the sensor performance.

Extreme-value statistics in nonlinear optics

Aleksei Zheltikov

DOI: 10.1364/OL.510419 Received 26 Oct 2023; Accepted 27 Dec 2023; Posted 13 Feb 2024  View: PDF

Abstract: We show that, although nonlinear optics may give rise to a vast multitude of statistics, all these statistics converge, in their extreme-value limit, to one of a few universal extreme-value statistics. Specifically, in the class of polynomial nonlinearities, such as those found in the Kerr effect, weak-field harmonic generation, and multiphoton ionization, the statistics of the nonlinear-optical output converges, in the extreme-value limit, to the exponentially tailed, Gumbel distribution. Exponentially growing nonlinear signals, on the other hand, such as those induced by parametric instabilities and stimulated scattering, are shown to reach their extreme-value limits in the class of the Fréchet statistics, giving rise to extreme-value distributions with heavy, manifestly nonexponential tails, thus favoring extreme-event outcomes and rogue-wave buildup.