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Virtual Journal for Biomedical Optics

Virtual Journal for Biomedical Optics

| EXPLORING THE INTERFACE OF LIGHT AND BIOMEDICINE

  • Editors: Andrew Dunn and Anthony Durkin
  • Vol. 7, Iss. 8 — Aug. 2, 2012
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High peak-power picosecond pulse generation at 1.26 µm using a quantum-dot-based external-cavity mode-locked laser and tapered optical amplifier

Y. Ding, R. Aviles-Espinosa, M. A. Cataluna, D. Nikitichev, M. Ruiz, M. Tran, Y. Robert, A. Kapsalis, H. Simos, C. Mesaritakis, T. Xu, P. Bardella, M. Rossetti, I. Krestnikov, D. Livshits, Ivo Montrosset, D. Syvridis, M. Krakowski, P. Loza-Alvarez, and E. Rafailov  »View Author Affiliations


Optics Express, Vol. 20, Issue 13, pp. 14308-14320 (2012)
http://dx.doi.org/10.1364/OE.20.014308


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Abstract

In this paper, we present the generation of high peak-power picosecond optical pulses in the 1.26 μm spectral band from a repetition-rate-tunable quantum-dot external-cavity passively mode-locked laser (QD-ECMLL), amplified by a tapered quantum-dot semiconductor optical amplifier (QD-SOA). The laser emission wavelength was controlled through a chirped volume Bragg grating which was used as an external cavity output coupler. An average power of 208.2 mW, pulse energy of 321 pJ, and peak power of 30.3 W were achieved. Preliminary nonlinear imaging investigations indicate that this system is promising as a high peak-power pulsed light source for nonlinear bio-imaging applications across the 1.0 μm - 1.3 μm spectral range.

© 2012 OSA

1. Introduction

High peak-power ultrafast laser systems are key elements in a number of biomedical imaging applications. This is the case for nonlinear microscopy (NLM), where excitation occurs only when two (or more) photons coincide in space and time. Therefore, a high photon flux density (in space and time) is required. The first condition can be achieved through the use of a high numerical aperture microscope objective and the second by employing ultrashort pulsed lasers. The combination of these two conditions produces the required peak intensities for generating a nonlinear effect (while maintaining average powers at a suitable level compatible with biological samples). These conditions also ensure that any generated light is only produced within the optical section to be imaged [1

1. W. Denk, J. H. Strickler, and W. W. Webb, “2-photon laser scanning fluorescence microscopy,” Science 248(4951), 73–76 (1990). [CrossRef] [PubMed]

]. Currently, most NLMs employ the classic ultrashort pulsed Ti:Sapphire laser source, extensively used in research laboratories as it combines a broad tunability range, together with the required output peak powers to generate nonlinear images [2

2. D. Yelin, D. Oron, E. Korkotian, M. Segal, and Y. Silberberg, “Third-harmonic microscopy with a titanium-sapphire laser,” Appl. Phys. B-Lasers Opt. 74(9), S97–S101 (2002). [CrossRef]

, 3

3. L. Moreaux, O. Sandre, M. Blanchard-Desce, and J. Mertz, “Membrane imaging by simultaneous second-harmonic generation and two-photon microscopy,” Opt. Lett. 25(5), 320–322 (2000). [CrossRef] [PubMed]

]. However, such lasers are bulky, expensive and normally require mechanical isolation tables. Furthermore, such laser systems include a high-power pump laser and are normally composed of several elements which make the whole system difficult to operate. They require, therefore, continuous maintenance (including realignment and cleaning) or need to rely on expensive vacuum-sealed computer-controlled enclosing boxes to achieve efficient operation. In principle, an ideal excitation source for nonlinear imaging should be able to excite several fluorophores, be cost-effective, easy to use, reliable and compact. If these conditions are met, such excitation sources could potentially replace the expensive and bulky solid-state ultrafast laser sources. In order to be able to make relative comparisons between the applicability of laser sources for NLM applications, a figure-of-merit (FOM)can be defined as a function of their average power, pulse duration, and repetition rate [4

4. R. Aviles-Espinosa, G. Filippidis, C. Hamilton, G. Malcolm, K. J. Weingarten, T. Südmeyer, Y. Barbarin, U. Keller, S. I. Santos, D. Artigas, and P. Loza-Alvarez, “Compact ultrafast semiconductor disk laser: targeting GFP based nonlinear applications in living organisms,” Biomed. Opt. Express 2(4), 739–747 (2011). [CrossRef] [PubMed]

]. The FOM is defined as the product of the average power and the peak power (Pavg × Ppeak), which can be recast into the equation:Pavg2 / (frep·Δτ), where frep and Δτ are the repetition rate and the pulse duration, respectively. Moreover, this value is useful for determining if the laser source is suitable for nonlinear excitation as the detected signal level from a two-photon process is proportional to this FOM [5

5. Y. Li, M. Breivik, C. Y. Feng, B. O. Fimland, and L. F. Lester, “A low repetition rate all-active monolithic passively mode-locked quantum-dot laser,” IEEE Photon. Technol. Lett. 23(14), 1019–1021 (2011). [CrossRef]

].

Ultrafast lasers within the wavelength range of ~700 nm to 1000 nm, particularly at wavelengths near 800 nm (where the Ti:Sapphire lasers operate most efficiently), have been used as excitation sources for two-photon excited fluorescence imaging (TPEF) [6

6. H. Kano and H. O. Hamaguchi, “In-vivo multi-nonlinear optical imaging of a living cell using a supercontinuum light source generated from a photonic crystal fiber,” Opt. Express 14(7), 2798–2804 (2006). [CrossRef] [PubMed]

] and other nonlinear imaging techniques such as those using second harmonic generation [7

7. S. Tang, T. B. Krasieva, Z. Chen, G. Tempea, and B. J. Tromberg, “Effect of pulse duration on two-photon excited fluorescence and second harmonic generation in nonlinear optical microscopy,” J. Biomed. Opt. 11(2), 020501 (2006). [CrossRef] [PubMed]

, 8

8. S. M. Zhuo, J. X. Chen, S. S. Xie, L. Q. Zheng, Z. B. Hong, and X. S. Jiang, “Nonlinear optical microscopy for visualizing dermal structural assembly in normal and pathological human dermis,” Laser Phys. Lett. 6(10), 764–767 (2009). [CrossRef]

].In the past years, a few semiconductor laser diode systems with amplification schemes have been successfully demonstrated as light sources for NLM applications at the abovementioned waveband [9

9. H. Yokoyama, A. Sato, H. C. Guo, K. Sato, M. Mure, and H. Tsubokawa, “Nonlinear-microscopy optical-pulse sources based on mode-locked semiconductor lasers,” Opt. Express 16(22), 17752–17758 (2008). [CrossRef] [PubMed]

11

11. K. Taira, T. Hashimoto, and H. Yokoyama, “Two-photon fluorescence imaging with a pulse source based on a 980-nm gain-switched laser diode,” Opt. Express 15(5), 2454–2458 (2007). [CrossRef] [PubMed]

]. These laser diode systems typically involved two or more stages of amplification (based on semiconductor and/or fiber amplifiers), as well as extra-cavity dispersion compensation schemes. Very recently, NLM imaging has been demonstrated with a compact, picosecond, vertical external-cavity surface-emitting laser emitting at 965 nm [4

4. R. Aviles-Espinosa, G. Filippidis, C. Hamilton, G. Malcolm, K. J. Weingarten, T. Südmeyer, Y. Barbarin, U. Keller, S. I. Santos, D. Artigas, and P. Loza-Alvarez, “Compact ultrafast semiconductor disk laser: targeting GFP based nonlinear applications in living organisms,” Biomed. Opt. Express 2(4), 739–747 (2011). [CrossRef] [PubMed]

]. This device was able to efficiently produce TPEF images of several fluorescent markers including green fluorescent protein, as the laser’s emission wavelength virtually matched the maximum two-photon action cross section of this protein.

In addition to the above, ultrafast lasers operating at longer wavelengths are of great interest for deep tissue imaging. Indeed, high contrast imaging of different tissues at approximately twice the depth has been demonstrated with 1280-nm excitation, when compared to 775-nm excitation (generated by a Ti:Sapphire-pumped optical parametric oscillator and a Ti:Sapphire laser, respectively) [12

12. D. Kobat, M. E. Durst, N. Nishimura, A. W. Wong, C. B. Schaffer, and C. Xu, “Deep tissue multiphoton microscopy using longer wavelength excitation,” Opt. Express 17(16), 13354–13364 (2009). [CrossRef] [PubMed]

]. The advantages of this spectral band for NLM applications are therefore linked to the larger penetration depths which result from decreased scattering [12

12. D. Kobat, M. E. Durst, N. Nishimura, A. W. Wong, C. B. Schaffer, and C. Xu, “Deep tissue multiphoton microscopy using longer wavelength excitation,” Opt. Express 17(16), 13354–13364 (2009). [CrossRef] [PubMed]

], and the reduced probability of photo-damage [13

13. I. H. Chen, S. W. Chu, C. K. Sun, P. C. Cheng, and B. L. Lin, “Wavelength dependent damage in biological multi-photon confocal microscopy: A micro-spectroscopic comparison between femtosecond Ti: sapphire and Cr: forsterite laser sources,” Opt. Quantum Electron. 34(12), 1251–1266 (2002). [CrossRef]

]. As a result, the use of ultrafast lasers emitting within the ~1.2-1.3μmspectral range is showing a great promise for commercial NLM applications. In this context, compact Cr:Forsterite lasers emitting at 1230 nm have been demonstrated as excitation sources for NLM with great success [14

14. M. C. Chan, T. M. Liu, S. P. Tai, and C. K. Sun, “Compact fiber-delivered Cr:forsterite laser for nonlinear light microscopy,” J. Biomed. Opt. 10(5), 054006 (2005). [CrossRef] [PubMed]

, 15

15. W. J. Lee, C. F. Lee, S. Y. Chen, Y. S. Chen, and C. K. Sun, “Virtual biopsy of rat tympanic membrane using higher harmonic generation microscopy,” J. Biomed. Opt. 15(4), 046012 (2010). [CrossRef] [PubMed]

].

In this paper, we present the first semiconductor pulsed laser diode system within the spectral range between 1.0 µm and 1.3 µm with characteristics which are compatible with NLM applications. The laser system here demonstrated is a master-oscillator power-amplifier (MOPA) based on a QD-ECMLL, amplified by a tapered QD-SOA, with emission in the 1.26 μm spectral band. Both devices were based on InAs/GaAs QD structures, as these materials can be routinely grown to cover the spectral range between 1.0 µm and 1.3 µm, in addition to offering important advantages in the context of ultrashort-pulsed lasers such as ultrafast carrier dynamics, broad gain/absorption bandwidth, excellent noise characteristics and low optical losses [16

16. E. U. Rafailov, M. A. Cataluna, and W. Sibbett, “Mode-locked quantum-dot lasers,” Nat. Photonics 1(7), 395–401 (2007). [CrossRef]

20

20. M. A. Cataluna, Y. Ding, D. I. Nikitichev, K. A. Fedorova, and E. U. Rafailov, “High-power versatile picosecond pulse generation from mode-locked quantum-dot laser diodes,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1302–1310 (2011). [CrossRef]

]. Moreover, QD-based SOAs are extremely suitable for boosting the power of ultrashort pulses due to their high gain saturation characteristics, broad gain bandwidth, fast gain recovery times and low noise figure exhibited by these devices [21

21. T. W. Berg and J. Mork, “Saturation and noise properties of quantum-dot optical amplifiers,” IEEE J. Quantum Electron. 40(11), 1527–1539 (2004). [CrossRef]

]. The choice of an external-cavity mode-locking configuration in this work is associated with the possibility to access lower repetition rates than what are typically achievable with monolithic lasers, as well as offering the possibility to easily vary the pulse repetition rate [20

20. M. A. Cataluna, Y. Ding, D. I. Nikitichev, K. A. Fedorova, and E. U. Rafailov, “High-power versatile picosecond pulse generation from mode-locked quantum-dot laser diodes,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1302–1310 (2011). [CrossRef]

, 22

22. Y. Ding, M. A. Cataluna, D. Nikitichev, I. Krestnikov, D. Livshits, and E. Rafailov, “Broad repetition-rate tunable quantum-dot external-cavity passively mode-locked laser with extremely narrow radio frequency linewidth,” Appl. Phys. Express 4(6), 062703 (2011). [CrossRef]

24

24. M. Xia, M. G. Thompson, R. V. Penty, and I. H. White, “External-cavity mode-locked quantum-dot laser diodes for low repetition rate, sub-picosecond pulse generation,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1264–1271 (2011). [CrossRef]

].

2. Experimental setup

The schematic of the experimental setup is shown in Fig. 1
Fig. 1 Configuration of a QD-MOPA system and the experimental setup. CBG OC: chirped Bragg grating output coupler; TS: motorized translation stage; OI: optical isolator; HWP: half wave plate; SOA: semiconductor optical amplifier; SMF: single-mode fiber; FS: fiber splitter; OSA: optical spectrum analyzer; PC: Personal computer; Autoco: autocorrelator; Osc: oscilloscope; PD: photo detector; RFSA: RF spectrum analyzer.
. The QD-MOPA system consists of a QD-ECMLL based on a two-section gain chip or superluminescent diode (SLD) and a chirped volume Bragg grating (CBG)used as an external cavity output coupler together with a tilted and tapered QD-SOA for amplifying the pulse power from QD-ECMLL.

Both the SLD and tapered SOA were fabricated from wafers with similar epitaxial structure grown on an n+-GaAs (100) substrate by molecular beam epitaxy which contained 10 layers of self-assembled InAs/GaAs quantum dots. The wavelength and optical mode matching between QD lasers and QD amplifiers were important factors in the decision to adopt identical QD structures as active regions of both devices in this work. Both wafers were grown by Innolume, but not in the same epitaxial batch. The two-section gain chip was fabricated by Innolume, whereas the tapered SOA was fabricated by III-V Lab according to the design and simulations carried out by the teams at the Politecnico di Torino and the National and Kapodistrian University of Athens(further details on the simulation and design of this and other tapered QD-SOAs will be published elsewhere).

The two-section SLD chip used for the QD-ECMLL consisted of a gain section and a saturable absorber section. The waveguide in the gain section was bent and terminated at an angle of 7° relative to the cleaved facet with an antireflection (AR) coating (R~10−5); the back facet was high-reflection coated (R~95%). The total chip length was 4 mm, with an800 μm-long saturable absorber section (20% absorber-to–total-length ratio) placed near the back facet.

As mentioned above, the output coupler employed was a CBG, sourced from Optigrate Corp. The CBG is a reflective Bragg grating inscribed in photo-thermal-refractive glass. The chirp is made possible through the grating period variation in the direction of beam propagation, which imparts different delays to different spectral components reflected along the CBG. In this work we used a CBG with an optical aperture of 5 mm x 6 mm, a thickness of 2 mm and a center wavelength of about 1262 nm. The CBG’s reflectivity(or diffraction efficiency) was around ~12-15%. The CBG was mounted on a motorized translation stage in order to obtain an accurate tunability of the pulse repetition rate.

A collimating aspherical lens with a numerical aperture of 0.55 was used to couple light to and from the SLD chip. The output beam from the seed laser source was coupled onto the input facet of SOA after optical isolation and polarization control.

The total length of the SOA chip was 6 mm. The waveguide width changed from 14 μm at the input facet to 80 μm at the output facet. Both the input and output facets of the SOAs were AR-coated, which in combination with a tilted waveguide resulted in a residual reflectivity of ~10−5.

The output beam from the SOA was collimated and focused onto a single-mode fiber-splitter and coupled to an autocorrelator, a radio-frequency (RF) spectrum analyzer and an optical spectrum analyzer (OSA) for characterization of the MOPA output. Using a mirror flipper inserted between the oscillator and amplifier, the pulsed output from the QD-ECMLL was also characterized, in order to compare the pulse characteristics of the input pulses to the SOA and resulting amplified pulses. Both the SLD and SOA chips were mounted on AlN submounts and copper heatsinks, and were kept at 20þC using a thermoelectric temperature controller during the experimental work.

3. Experimental results and discussion

3.1 Continuous-wave operation of the MOPA system

The continuous-wave (CW) operation of the MOPA system was characterized prior to the investigation of the MOPA’s performance under pulsed operation.

In order to achieve CW operation, a forward bias was applied to the gain section of the two-section SLD in the QD-ECMLL, while the saturable absorber section was left unbiased. The CW amplified power (red) from the SOA and its gain (black) as a function of the current applied to the SOA are represented in Fig. 2
Fig. 2 CW output power (red) and gain (black) versus SOA current at 20 °C.
. The output power represented is the power obtained from the SOA, after subtracting the power contribution from the broadband amplified spontaneous emission generated by the SOA (which was measured without the oscillator input). For example, under an injection current of 3120 mA applied to the SOA, the measured output power from the SOA and the ASE power were 676 mW and 224 mW, respectively – resulting in an output power of 452 mW, as represented in Fig. 2.The gain of the SOA presented here is the ratio of the output power (the power after collimation from the SOA output facet, measured at point D in Fig. 1, subtracted of all the ASE power) to the input power (the power before coupling towards the SOA input facet, measured at point B in Fig. 1). The real SOA chip gain is unknown because the input coupling efficiency cannot be accurately determined. It is important to note that in this graph and throughout the paper, only the “worst-case” scenario is represented, as we have subtracted all the ASE contributions from the output powers and corresponding gains. We have therefore assumed that no ASE power is converted into the oscillator input (whether under CW or pulsed operation), in order to simplify the analysis and avoid a possible overestimation of the output power, as there is a degree of uncertainty in the quantification of the amount of “useful” ASE that is coupled to the SOA input (whether pulsed or CW), and thus only a range of such coupled ASE power can be estimated, rather than an actual value – this is illustrated in the recent paper by Koda et al. [26

26. R. Koda, T. Oki, T. Miyajima, H. Watanabe, M. Kuramoto, M. Ikeda, and H. Yokoyama, “100 W peak-power 1 GHz repetition picoseconds optical pulse generation using blue-violet GaInN diode laser mode-locked oscillator and optical amplifier,” Appl. Phys. Lett. 97(2), 021101 (2010). [CrossRef]

].

The 3-dB saturation output power of the SOA chip for CW input is about 316 mW (~25 dBm) under 2000-mA injection current. Beyond the 3-dB saturation output power point, the gain dropped quickly with increasing input power. Knowing the 3-dB saturation output power of the SOA, one can estimate the possible maximum pulse peak power produced by the MOPA system under pulsed operation, considering the inverse of the duty cycle associated with both a particular pulse duration and pulse repetition frequency (and assuming that the amplification for the pulsed and CW regimes is not distinct). A gradually saturated output power can be observed with an increasing SOA current, especially close to 3000 mA, when the input CW power is 22.7 mW (13.5 dBm).From the trend of the gain increase with increasing SOA current in Fig. 2, we know that a higher power can be obtained under a higher SOA current, but accompanied with the risk of thermal saturation problems and pulse broadening for the pulsed input.

Finally, it should also be noted that the output power from the SOA measured with an integrating sphere photodiode power sensor at point C was 1.17-1.25 times higher than with a conventional (non-integrating-sphere type) detector at point D, which is caused by the collimating lens loss. We estimated the lens efficiency to be a maximum of 85% and used this value to estimate the actual power generated by SOA according to the measured value from the conventional detector [26

26. R. Koda, T. Oki, T. Miyajima, H. Watanabe, M. Kuramoto, M. Ikeda, and H. Yokoyama, “100 W peak-power 1 GHz repetition picoseconds optical pulse generation using blue-violet GaInN diode laser mode-locked oscillator and optical amplifier,” Appl. Phys. Lett. 97(2), 021101 (2010). [CrossRef]

]. As such, the actual maximum CW output power is estimated to be 532 mW (452mW/0.85), under the bias conditions investigated in this work.

3.2 Picosecond operation of the MOPA system

We investigated the performance of the MOPA system under pulsed operation for two different pulse repetition rates of the QD-ECMLL oscillator: 1.1 GHz and 648 MHz.

Under mode-locked operation at a pulse repetition rate of 1.1 GHz, the pulsed output of the QD-ECMLL was characterized (without amplification). In Fig. 4
Fig. 4 (a) Autocorrelation trace, (b) optical spectrum, (c) RF spectrum with 200-MHz span, and (d) RF spectrum with 20-GHz span, at reverse bias of 4 V and forward current of 200 mA with a pulse repetition rate of 1.1 GHz directly from the QD-ECMLL at 20 °C.
, the autocorrelation trace, optical spectrum, RF spectrum with both 200-MHz span and 20-GHz span are represented, for the case of a reverse bias of 4 V and forward current of 200 mA applied to the two-section SLD. The autocorrelation was fitted by a Lorentzian function, as it provided the best fit among conventional fitting functions. Moreover, the corresponding optical spectrum was also well fitted by a Lorentzian function (this was also the case for all the autocorrelation traces and optical spectra represented in this paper). An average power of ~10 mW was measured at the position after the optical isolator and half-wave plate (point B in Fig. 1). 1W peak power and about 9-ps pulses were generated directly from the QD-ECMLL.

While keeping the same bias conditions applied to the QD-ECMLL gain chip, the MOPA output was characterized as a function of the current applied to the QD-SOA, ranging from 1000 mA to 3000 mA (pulse repetition rate of 1.1 GHz).The autocorrelation trace, optical spectrum and RF spectra are shown in Fig. 5
Fig. 5 (a) Autocorrelation trace, (b) optical spectrum, (b) RF spectrum with 200-MHz span, and (d) RF spectrum with 20-GHz span, for a pulsed input at a pulse repetition rate of 1.1 GHz and a current of 3000 mA applied to the SOA at 20 °C.
, for a 3000-mA current applied to the SOA. Comparing the results represented in Fig. 4(a) with Fig. 5(a), we can find that the pulse broadening after 3000-mA SOA amplification is about 12.5%. The RF signal exhibits a high dynamic contrast and a large number of harmonics in Fig. 5(d), indicating that the high quality of mode-locking was kept after amplification. Some residual ASE contribution can still be seen in the optical spectrum in Fig. 5(b), due to a strong ASE at a high current, despite the fact that a bandpass filter was used here (filter with a central wavelength of 1262 nm and a bandwidth of 10 nm).

For the same repetition rate (1.1 GHz), we subsequently investigated the dependence of the peak power, gain, average power, and FOM on SOA current after collimation (point D in Fig. 1). The results are represented in Fig. 6
Fig. 6 (a) and (b), Peak power (red), gain (black), average power (black), and FOM (red) against SOA current for a 1.1-GHz repetition rate.
. Using the same approach as detailed in section 3.1, we assumed the worst-case scenario for the representation of the output power and subtracted all the ASE power (measured without input) to the output power (this approach is followed throughout the paper).

Note that, as in section 3.1, the measured (and represented) output power should be further multiplied by an inverse factor of 0.85 to correct for the 85% collimating lens efficiency in order to estimate the amplified pulse power generated directly by the SOA. As shown in Fig. 6(a), the highest peak power of 22.35 W divided by 0.85 resulted in a peak power of 26.3 W. Likewise, a corrected average power of 294 mW (250/0.85 mW) (267 pJ pulse energy) and FOM of 7.73 W2 (5.59/(0.85 × 0.85) W2) can be obtained at the highest SOA current of 3000 mA.

In order to investigate the amplification performance with a lower pulse repetition rate, we extended the cavity length of the QD-ECMLL, thus changing its pulse repetition rate from 1.1 GHz (10mW power, see Fig. 3 red line) to 648 MHz (6mW power, see Fig. 3 magenta line), while keeping the same bias conditions applied to the two-section SLD (reverse bias of 4 V and forward current of 200 mA). The autocorrelation trace, optical spectrum and RF spectra measured directly from the QD-ECMLL at a pulse repetition rate of 648 MHz are shown in Fig. 7
Fig. 7 (a) Autocorrelation trace, (b) optical spectrum, (c) RF spectrum with 200-MHz span, and (d) RF spectrum with 20-GHz span, at reverse bias of 4 V and forward current of 200 mA with a pulse repetition rate of 648 MHz for the QD-ECMLL without amplification at 20 °C.
. An average power of ~6 mW was measured at point B in Fig. 1.Similarly with the case at 1.1 GHz repetition rate, a peak power of around 1 W and about a 9ps pulse can be achieved from the QD-ECMLL, because the constant pulse energy required to saturate the absorber is independent of the repetition rate under certain operation conditions, which is consistent the conclusions reported in Ref. 20

20. M. A. Cataluna, Y. Ding, D. I. Nikitichev, K. A. Fedorova, and E. U. Rafailov, “High-power versatile picosecond pulse generation from mode-locked quantum-dot laser diodes,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1302–1310 (2011). [CrossRef]

. The wavelength for both repetition rates is around 1262.3-1262.5 nm which is determined by the CBG central wavelength.

The QD-ECMLL’s pulsed output (as illustrated in Fig. 7) was then used as an input to the tapered QD-SOA. Figure 8
Fig. 8 (a) Autocorrelation trace, (b) optical spectrum, (c) RF spectrum with 200-MHz span, and (d) RF spectrum with 20-GHz span, for a pulse repetition rate of 648 MHz and SOA current of 2500 mA at 20 °C.
exhibits the autocorrelation trace, optical spectrum and RF spectra of the amplified pulsed output at 648 MHz, for a 2500mA current applied to the SOA. The measured RF signal displays a high dynamic contrast and a large number of harmonics in Fig. 8(d), indicating that the quality of mode-locking is preserved after amplification, similar to the performance previously obtained for the MOPA configuration with 1.1-GHz pulse repetition rate.

The dependence of the peak power, gain, average power, and FOM for the 648MHz repetition rate was then investigated as a function of the current applied to the SOA (from 1000 mA to 2500 mA), as depicted in Fig. 9
Fig. 9 (a) Peak power (red), gain (black), (b) average power (black), and FOM (red) against SOA current for a 648-MHz repetition rate.
.

The average power as represented in Fig. 9 was measured at point D in Fig. 1—after collimation. Therefore, the same technique can be applied as previously to account for the collimation lens losses (15%) and we estimate that the highest peak power of 30.3 W can be achieved directly from the SOA under these conditions (25.8 W/0.85). Likewise, a corrected average power of 208 mW (177/0.85 mW) and FOM of 6.3 W2 (4.56/(0.85 × 0.85) W2) can be obtained at the highest SOA current of 2500 mA (Fig. 9(b)). The corresponding pulse energy is 321 pJ, which represents a ~13-fold increase when compared to the current state-of-the-art for semiconductor lasers at the 1.26μm waveband [25

25. Y. Ding, D. I. Nikitichev, I. Krestnikov, D. Livshits, M. A. Cataluna, and E. U. Rafailov, “Quantum-dot external-cavity passively modelocked laser with high peak power and pulse energy,” Electron. Lett. 46(22), 1516–1517 (2010). [CrossRef]

].

3.3 Discussion of the results

For demonstrating the results presented in this paper, we harnessed the advantage of using a QD-ECMLL as an oscillator, enabling the delivery of a low-repetition-rate pulsed output, which resulted in a low duty cycle and a relatively low average power, when compared to the monolithic mode-locked lasers. This approach allowed us to boost the achievable gain from the tapered QD-SOA, thus enabling the generation of high-peak power pulses from the MOPA system.

Regarding the overall performance of the MOPA system, a comparison of the amplification results obtained for the 1.1 GHz and648MHz pulsed output with the same peak power highlights that the SOA gain was enhanced, as expected, from 12.3 dB for 1.1GHz repetition rate to 13.7 dB for 648 MHz (under the same applied SOA current of 2000 mA, corresponding to a SOA current density of ~600 A/cm2). Indeed, the corresponding input average power was 10 mW (10 dBm) for the 1.1GHz case, as opposed to 6mW (7.78 dBm) for the 648MHzrepetition rate. Therefore, even though the peak power and pulse energy at the SOA input can be kept constant under different repetition rates, a higher peak power as well as a higher pulse energy can be achieved from the SOA output for a lower repetition rate. However, the average power and FOM after amplification became lower for a decreased repetition rate.

Additionally, it is also possible to lower the repetition rate of this QD-ECMLL further [22

22. Y. Ding, M. A. Cataluna, D. Nikitichev, I. Krestnikov, D. Livshits, and E. Rafailov, “Broad repetition-rate tunable quantum-dot external-cavity passively mode-locked laser with extremely narrow radio frequency linewidth,” Appl. Phys. Express 4(6), 062703 (2011). [CrossRef]

]. With the insights gained from this work, it is foreseeable that lower repetition rates will lead to even higher values of peak power.

3.4 Nonlinear imaging application of the picosecond MOPA system

The QD-MOPA prototype was installed and tested as an ultrashort pulsed light source for nonlinear imaging applications. The collimated SOA output was air-coupled to an inverted microscope (Nikon, Eclipse TE 2000U). The beam was expanded to fill the back aperture of a CFI Apochromatic, oil immersion, 60xmicroscope objective with a numerical aperture of 1.4 (λS, Nikon). The sample was scanned in the x-y directions using a translation stage (Tango, Märzhäuser Wetzlar). To acquire the TPEF image, a band pass filter (KG3, Schott) was placed in front of a photomultiplier tube (Hamamatsu, H9305-03), as shown in Fig. 10
Fig. 10 The left panel is the TPEF image of 15µm Crimson fluorescent beads obtained at ICFO with the QD-MOPA system. The resulting image was obtained by averaging 10 frames to improve the signal to noise ratio. The right panel is a simplified schematic of the nonlinear microscope setup. L#: lenses; M#: mirror; HM: dichroic mirror; F: bandpass filter; PMT: photomultiplier tube.
. The two-section SLD was operated under a reverse bias of 4 V and a forward current of 200 mA, with the QD-ECMLL generating optical pulses at a repetition rate of 648 MHz. The tapered SOA current was 2000 mA. Both semiconductor devices operated at 20°C.

For the nonlinear imaging demonstration, 15-µm crimson fluorescent microspheres (Invitrogen, F-8839) were used. These were placed in water solution, sandwiched between two cover glasses. This sample can be excited using ~625 nm if a linear excitation scheme is considered; however, in our case the two-photon action cross-section of Crimson dye at 1260 nm is ~16 GM [27

27. M. Drobizhev, N. S. Makarov, S. E. Tillo, T. E. Hughes, and A. Rebane, “Two-photon absorption properties of fluorescent proteins,” Nat. Methods 8(5), 393–399 (2011). [CrossRef] [PubMed]

] - under this condition a few Watts of peak power together with the laser repetition rate would enable the production of nonlinear images of such a sample [4

4. R. Aviles-Espinosa, G. Filippidis, C. Hamilton, G. Malcolm, K. J. Weingarten, T. Südmeyer, Y. Barbarin, U. Keller, S. I. Santos, D. Artigas, and P. Loza-Alvarez, “Compact ultrafast semiconductor disk laser: targeting GFP based nonlinear applications in living organisms,” Biomed. Opt. Express 2(4), 739–747 (2011). [CrossRef] [PubMed]

]. The available peak power at the sample plane was ~3-4 W at the operating wavelength (1.26μm) of the MOPA system (the microscope objective measured transmission was ~28%).Despite the low peak powers, the imaged sample could be observed as shown in Fig. 10. To the best of our knowledge this is the first demonstration of TPEF imaging obtained with a semiconductor-based ultrafast laser system within the spectral range of 1.0μm - 1.3μm.

Although this system operates at a fixed wavelength, it could also be used to efficiently excite a wide range of fluorescent dyes via TPEF, such as: the mkate variants; TurboFP635;TurboFP650;mRaspberry;mPlum;mGrape3;Neptune;NirFP; mNeptune; etc, as these have similar or higher two-photon action cross-sections compared to the Crimson dye [27

27. M. Drobizhev, N. S. Makarov, S. E. Tillo, T. E. Hughes, and A. Rebane, “Two-photon absorption properties of fluorescent proteins,” Nat. Methods 8(5), 393–399 (2011). [CrossRef] [PubMed]

].

In the present work, an ultrashort pulsed laser system based on chip-scale devices, having modest power requirements, and based on QDs, has been developed. Its output peak power characteristics have enabled TPEF imaging as a proof-of-concept demonstration.

4. Conclusion

For a pulse repetition rate of 1.1GHz, an average power of 294 mW, a pulse energy of 267 pJ, a peak power of 26.3 W and FOM of 7.73 W2wereachieved from the QD-MOPA system. Importantly, by lowering the repetition rate to 648 MHz, a higher peak power of 30.3 W was achieved together with a 208.2-mW average power, a 321-pJ pulse energy and a 6.31W2 FOM. Using a constant pulse energy and peak power as the input, we have demonstrated that a lower repetition rate - corresponding to a lower average input power—enabled a higher SOA gain and consequently the generation of higher peak power (and energy) pulses, due to the gain saturation characteristics of the SOA.

Preliminary TPEF imaging results indicate that this QD laser system is promising as a high peak-power pulsed light source, with a wavelength emission that could potentially cover 1.0 μm - 1.3 μm, which is very useful for nonlinear bio-imaging applications.

Acknowledgments

The work was funded within the Seventh Framework Program “FAST-DOT”, through Grant No. 224338. M. A. Cataluna acknowledges also financial support through a Royal Academy of Engineering/EPSRC Research Fellowship. Y. Ding acknowledges financial support from a Marie Curie Fellowship. This work is supported by the Generalitat de Catalunya grant 2009-SGR-159, the Spanish government grant TEC2009-09698, the NoE P4L, and Laserlaboptobio. We thank Nikon instruments Spain for providing the microscope objective used in this work. This research has been partially supported by FundacióCellex Barcelona and has been partially conducted at ICFO’s “Super-Resolution Light Microscopy and Nanoscopy Facility” (SLN@ICFO).

References and links

1.

W. Denk, J. H. Strickler, and W. W. Webb, “2-photon laser scanning fluorescence microscopy,” Science 248(4951), 73–76 (1990). [CrossRef] [PubMed]

2.

D. Yelin, D. Oron, E. Korkotian, M. Segal, and Y. Silberberg, “Third-harmonic microscopy with a titanium-sapphire laser,” Appl. Phys. B-Lasers Opt. 74(9), S97–S101 (2002). [CrossRef]

3.

L. Moreaux, O. Sandre, M. Blanchard-Desce, and J. Mertz, “Membrane imaging by simultaneous second-harmonic generation and two-photon microscopy,” Opt. Lett. 25(5), 320–322 (2000). [CrossRef] [PubMed]

4.

R. Aviles-Espinosa, G. Filippidis, C. Hamilton, G. Malcolm, K. J. Weingarten, T. Südmeyer, Y. Barbarin, U. Keller, S. I. Santos, D. Artigas, and P. Loza-Alvarez, “Compact ultrafast semiconductor disk laser: targeting GFP based nonlinear applications in living organisms,” Biomed. Opt. Express 2(4), 739–747 (2011). [CrossRef] [PubMed]

5.

Y. Li, M. Breivik, C. Y. Feng, B. O. Fimland, and L. F. Lester, “A low repetition rate all-active monolithic passively mode-locked quantum-dot laser,” IEEE Photon. Technol. Lett. 23(14), 1019–1021 (2011). [CrossRef]

6.

H. Kano and H. O. Hamaguchi, “In-vivo multi-nonlinear optical imaging of a living cell using a supercontinuum light source generated from a photonic crystal fiber,” Opt. Express 14(7), 2798–2804 (2006). [CrossRef] [PubMed]

7.

S. Tang, T. B. Krasieva, Z. Chen, G. Tempea, and B. J. Tromberg, “Effect of pulse duration on two-photon excited fluorescence and second harmonic generation in nonlinear optical microscopy,” J. Biomed. Opt. 11(2), 020501 (2006). [CrossRef] [PubMed]

8.

S. M. Zhuo, J. X. Chen, S. S. Xie, L. Q. Zheng, Z. B. Hong, and X. S. Jiang, “Nonlinear optical microscopy for visualizing dermal structural assembly in normal and pathological human dermis,” Laser Phys. Lett. 6(10), 764–767 (2009). [CrossRef]

9.

H. Yokoyama, A. Sato, H. C. Guo, K. Sato, M. Mure, and H. Tsubokawa, “Nonlinear-microscopy optical-pulse sources based on mode-locked semiconductor lasers,” Opt. Express 16(22), 17752–17758 (2008). [CrossRef] [PubMed]

10.

M. Kuramoto, N. Kitajima, H. C. Guo, Y. Furushima, M. Ikeda, and H. Yokoyama, “Two-photon fluorescence bioimaging with an all-semiconductor laser picosecond pulse source,” Opt. Lett. 32(18), 2726–2728 (2007). [CrossRef] [PubMed]

11.

K. Taira, T. Hashimoto, and H. Yokoyama, “Two-photon fluorescence imaging with a pulse source based on a 980-nm gain-switched laser diode,” Opt. Express 15(5), 2454–2458 (2007). [CrossRef] [PubMed]

12.

D. Kobat, M. E. Durst, N. Nishimura, A. W. Wong, C. B. Schaffer, and C. Xu, “Deep tissue multiphoton microscopy using longer wavelength excitation,” Opt. Express 17(16), 13354–13364 (2009). [CrossRef] [PubMed]

13.

I. H. Chen, S. W. Chu, C. K. Sun, P. C. Cheng, and B. L. Lin, “Wavelength dependent damage in biological multi-photon confocal microscopy: A micro-spectroscopic comparison between femtosecond Ti: sapphire and Cr: forsterite laser sources,” Opt. Quantum Electron. 34(12), 1251–1266 (2002). [CrossRef]

14.

M. C. Chan, T. M. Liu, S. P. Tai, and C. K. Sun, “Compact fiber-delivered Cr:forsterite laser for nonlinear light microscopy,” J. Biomed. Opt. 10(5), 054006 (2005). [CrossRef] [PubMed]

15.

W. J. Lee, C. F. Lee, S. Y. Chen, Y. S. Chen, and C. K. Sun, “Virtual biopsy of rat tympanic membrane using higher harmonic generation microscopy,” J. Biomed. Opt. 15(4), 046012 (2010). [CrossRef] [PubMed]

16.

E. U. Rafailov, M. A. Cataluna, and W. Sibbett, “Mode-locked quantum-dot lasers,” Nat. Photonics 1(7), 395–401 (2007). [CrossRef]

17.

X. D. Huang, A. Stintz, H. Li, L. F. Lester, J. Cheng, and K. J. Malloy, “Passive mode-locking in 1.3 µm two-section InAs quantum dot lasers,” Appl. Phys. Lett. 78(19), 2825–2827 (2001). [CrossRef]

18.

E. U. Rafailov, M. A. Cataluna, W. Sibbett, N. D. Il'inskaya, Y. M. Zadiranov, A. E. Zhukov, V. M. Ustinov, D. A. Livshits, A. R. Kovsh, and N. N. Ledentsov, “High-power picosecond and femtosecond pulse generation from a two-section mode-locked quantum-dot laser,” Appl. Phys. Lett. 87(8), 081107 (2005). [CrossRef]

19.

M. G. Thompson, A. R. Rae, R. V. Mo Xia, Penty, and I. H. White, “InGaAs quantum-dot mode-locked laser diodes,” IEEE J. Sel. Top. Quantum Electron. 15(3), 661–672 (2009). [CrossRef]

20.

M. A. Cataluna, Y. Ding, D. I. Nikitichev, K. A. Fedorova, and E. U. Rafailov, “High-power versatile picosecond pulse generation from mode-locked quantum-dot laser diodes,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1302–1310 (2011). [CrossRef]

21.

T. W. Berg and J. Mork, “Saturation and noise properties of quantum-dot optical amplifiers,” IEEE J. Quantum Electron. 40(11), 1527–1539 (2004). [CrossRef]

22.

Y. Ding, M. A. Cataluna, D. Nikitichev, I. Krestnikov, D. Livshits, and E. Rafailov, “Broad repetition-rate tunable quantum-dot external-cavity passively mode-locked laser with extremely narrow radio frequency linewidth,” Appl. Phys. Express 4(6), 062703 (2011). [CrossRef]

23.

Y. Ding, D. I. Nikitichev, I. Krestnikov, D. Livshits, M. A. Cataluna, and E. U. Rafailov, “Fundamental and harmonic mode-locking with pulse repetition rate between 200 MHz and 6.8 GHz in a quantum-dot external-cavity laser,” in Lasers and Electro-Optics Europe (CLEO EUROPE/EQEC),2011Conference on and 12th European Quantum Electronics Conference, (Munich, Germany, 2011), p. CF_P23.

24.

M. Xia, M. G. Thompson, R. V. Penty, and I. H. White, “External-cavity mode-locked quantum-dot laser diodes for low repetition rate, sub-picosecond pulse generation,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1264–1271 (2011). [CrossRef]

25.

Y. Ding, D. I. Nikitichev, I. Krestnikov, D. Livshits, M. A. Cataluna, and E. U. Rafailov, “Quantum-dot external-cavity passively modelocked laser with high peak power and pulse energy,” Electron. Lett. 46(22), 1516–1517 (2010). [CrossRef]

26.

R. Koda, T. Oki, T. Miyajima, H. Watanabe, M. Kuramoto, M. Ikeda, and H. Yokoyama, “100 W peak-power 1 GHz repetition picoseconds optical pulse generation using blue-violet GaInN diode laser mode-locked oscillator and optical amplifier,” Appl. Phys. Lett. 97(2), 021101 (2010). [CrossRef]

27.

M. Drobizhev, N. S. Makarov, S. E. Tillo, T. E. Hughes, and A. Rebane, “Two-photon absorption properties of fluorescent proteins,” Nat. Methods 8(5), 393–399 (2011). [CrossRef] [PubMed]

OCIS Codes
(140.4050) Lasers and laser optics : Mode-locked lasers
(230.5590) Optical devices : Quantum-well, -wire and -dot devices
(250.5980) Optoelectronics : Semiconductor optical amplifiers
(180.4315) Microscopy : Nonlinear microscopy

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 16, 2012
Revised Manuscript: May 14, 2012
Manuscript Accepted: June 5, 2012
Published: June 12, 2012

Virtual Issues
Vol. 7, Iss. 8 Virtual Journal for Biomedical Optics

Citation
Y. Ding, R. Aviles-Espinosa, M. A. Cataluna, D. Nikitichev, M. Ruiz, M. Tran, Y. Robert, A. Kapsalis, H. Simos, C. Mesaritakis, T. Xu, P. Bardella, M. Rossetti, I. Krestnikov, D. Livshits, Ivo Montrosset, D. Syvridis, M. Krakowski, P. Loza-Alvarez, and E. Rafailov, "High peak-power picosecond pulse generation at 1.26 µm using a quantum-dot-based external-cavity mode-locked laser and tapered optical amplifier," Opt. Express 20, 14308-14320 (2012)
http://www.opticsinfobase.org/vjbo/abstract.cfm?URI=oe-20-13-14308


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References

  1. W. Denk, J. H. Strickler, and W. W. Webb, “2-photon laser scanning fluorescence microscopy,” Science248(4951), 73–76 (1990). [CrossRef] [PubMed]
  2. D. Yelin, D. Oron, E. Korkotian, M. Segal, and Y. Silberberg, “Third-harmonic microscopy with a titanium-sapphire laser,” Appl. Phys. B-Lasers Opt.74(9), S97–S101 (2002). [CrossRef]
  3. L. Moreaux, O. Sandre, M. Blanchard-Desce, and J. Mertz, “Membrane imaging by simultaneous second-harmonic generation and two-photon microscopy,” Opt. Lett.25(5), 320–322 (2000). [CrossRef] [PubMed]
  4. R. Aviles-Espinosa, G. Filippidis, C. Hamilton, G. Malcolm, K. J. Weingarten, T. Südmeyer, Y. Barbarin, U. Keller, S. I. Santos, D. Artigas, and P. Loza-Alvarez, “Compact ultrafast semiconductor disk laser: targeting GFP based nonlinear applications in living organisms,” Biomed. Opt. Express2(4), 739–747 (2011). [CrossRef] [PubMed]
  5. Y. Li, M. Breivik, C. Y. Feng, B. O. Fimland, and L. F. Lester, “A low repetition rate all-active monolithic passively mode-locked quantum-dot laser,” IEEE Photon. Technol. Lett.23(14), 1019–1021 (2011). [CrossRef]
  6. H. Kano and H. O. Hamaguchi, “In-vivo multi-nonlinear optical imaging of a living cell using a supercontinuum light source generated from a photonic crystal fiber,” Opt. Express14(7), 2798–2804 (2006). [CrossRef] [PubMed]
  7. S. Tang, T. B. Krasieva, Z. Chen, G. Tempea, and B. J. Tromberg, “Effect of pulse duration on two-photon excited fluorescence and second harmonic generation in nonlinear optical microscopy,” J. Biomed. Opt.11(2), 020501 (2006). [CrossRef] [PubMed]
  8. S. M. Zhuo, J. X. Chen, S. S. Xie, L. Q. Zheng, Z. B. Hong, and X. S. Jiang, “Nonlinear optical microscopy for visualizing dermal structural assembly in normal and pathological human dermis,” Laser Phys. Lett.6(10), 764–767 (2009). [CrossRef]
  9. H. Yokoyama, A. Sato, H. C. Guo, K. Sato, M. Mure, and H. Tsubokawa, “Nonlinear-microscopy optical-pulse sources based on mode-locked semiconductor lasers,” Opt. Express16(22), 17752–17758 (2008). [CrossRef] [PubMed]
  10. M. Kuramoto, N. Kitajima, H. C. Guo, Y. Furushima, M. Ikeda, and H. Yokoyama, “Two-photon fluorescence bioimaging with an all-semiconductor laser picosecond pulse source,” Opt. Lett.32(18), 2726–2728 (2007). [CrossRef] [PubMed]
  11. K. Taira, T. Hashimoto, and H. Yokoyama, “Two-photon fluorescence imaging with a pulse source based on a 980-nm gain-switched laser diode,” Opt. Express15(5), 2454–2458 (2007). [CrossRef] [PubMed]
  12. D. Kobat, M. E. Durst, N. Nishimura, A. W. Wong, C. B. Schaffer, and C. Xu, “Deep tissue multiphoton microscopy using longer wavelength excitation,” Opt. Express17(16), 13354–13364 (2009). [CrossRef] [PubMed]
  13. I. H. Chen, S. W. Chu, C. K. Sun, P. C. Cheng, and B. L. Lin, “Wavelength dependent damage in biological multi-photon confocal microscopy: A micro-spectroscopic comparison between femtosecond Ti: sapphire and Cr: forsterite laser sources,” Opt. Quantum Electron.34(12), 1251–1266 (2002). [CrossRef]
  14. M. C. Chan, T. M. Liu, S. P. Tai, and C. K. Sun, “Compact fiber-delivered Cr:forsterite laser for nonlinear light microscopy,” J. Biomed. Opt.10(5), 054006 (2005). [CrossRef] [PubMed]
  15. W. J. Lee, C. F. Lee, S. Y. Chen, Y. S. Chen, and C. K. Sun, “Virtual biopsy of rat tympanic membrane using higher harmonic generation microscopy,” J. Biomed. Opt.15(4), 046012 (2010). [CrossRef] [PubMed]
  16. E. U. Rafailov, M. A. Cataluna, and W. Sibbett, “Mode-locked quantum-dot lasers,” Nat. Photonics1(7), 395–401 (2007). [CrossRef]
  17. X. D. Huang, A. Stintz, H. Li, L. F. Lester, J. Cheng, and K. J. Malloy, “Passive mode-locking in 1.3 µm two-section InAs quantum dot lasers,” Appl. Phys. Lett.78(19), 2825–2827 (2001). [CrossRef]
  18. E. U. Rafailov, M. A. Cataluna, W. Sibbett, N. D. Il'inskaya, Y. M. Zadiranov, A. E. Zhukov, V. M. Ustinov, D. A. Livshits, A. R. Kovsh, and N. N. Ledentsov, “High-power picosecond and femtosecond pulse generation from a two-section mode-locked quantum-dot laser,” Appl. Phys. Lett.87(8), 081107 (2005). [CrossRef]
  19. M. G. Thompson, A. R. Rae, R. V. Mo Xia, Penty, and I. H. White, “InGaAs quantum-dot mode-locked laser diodes,” IEEE J. Sel. Top. Quantum Electron.15(3), 661–672 (2009). [CrossRef]
  20. M. A. Cataluna, Y. Ding, D. I. Nikitichev, K. A. Fedorova, and E. U. Rafailov, “High-power versatile picosecond pulse generation from mode-locked quantum-dot laser diodes,” IEEE J. Sel. Top. Quantum Electron.17(5), 1302–1310 (2011). [CrossRef]
  21. T. W. Berg and J. Mork, “Saturation and noise properties of quantum-dot optical amplifiers,” IEEE J. Quantum Electron.40(11), 1527–1539 (2004). [CrossRef]
  22. Y. Ding, M. A. Cataluna, D. Nikitichev, I. Krestnikov, D. Livshits, and E. Rafailov, “Broad repetition-rate tunable quantum-dot external-cavity passively mode-locked laser with extremely narrow radio frequency linewidth,” Appl. Phys. Express4(6), 062703 (2011). [CrossRef]
  23. Y. Ding, D. I. Nikitichev, I. Krestnikov, D. Livshits, M. A. Cataluna, and E. U. Rafailov, “Fundamental and harmonic mode-locking with pulse repetition rate between 200 MHz and 6.8 GHz in a quantum-dot external-cavity laser,” in Lasers and Electro-Optics Europe (CLEO EUROPE/EQEC),2011Conference on and 12th European Quantum Electronics Conference, (Munich, Germany, 2011), p. CF_P23.
  24. M. Xia, M. G. Thompson, R. V. Penty, and I. H. White, “External-cavity mode-locked quantum-dot laser diodes for low repetition rate, sub-picosecond pulse generation,” IEEE J. Sel. Top. Quantum Electron.17(5), 1264–1271 (2011). [CrossRef]
  25. Y. Ding, D. I. Nikitichev, I. Krestnikov, D. Livshits, M. A. Cataluna, and E. U. Rafailov, “Quantum-dot external-cavity passively modelocked laser with high peak power and pulse energy,” Electron. Lett.46(22), 1516–1517 (2010). [CrossRef]
  26. R. Koda, T. Oki, T. Miyajima, H. Watanabe, M. Kuramoto, M. Ikeda, and H. Yokoyama, “100 W peak-power 1 GHz repetition picoseconds optical pulse generation using blue-violet GaInN diode laser mode-locked oscillator and optical amplifier,” Appl. Phys. Lett.97(2), 021101 (2010). [CrossRef]
  27. M. Drobizhev, N. S. Makarov, S. E. Tillo, T. E. Hughes, and A. Rebane, “Two-photon absorption properties of fluorescent proteins,” Nat. Methods8(5), 393–399 (2011). [CrossRef] [PubMed]

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