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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 7 — Apr. 8, 2013
  • pp: 8231–8239
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In-line polarization-dependent microfiber interferometers and their applications in UWB signal generation

Ping Zhao, Jihua Zhang, Yuan Yu, Jianji Dong, Lei Shi, Yang Liu, and Xinliang Zhang  »View Author Affiliations


Optics Express, Vol. 21, Issue 7, pp. 8231-8239 (2013)
http://dx.doi.org/10.1364/OE.21.008231


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Abstract

A novel in-line polarization-dependent microfiber interferometer (PD-MFI) is proposed and experimentally demonstrated, which is tapered from a commercial polarization-maintaining fiber. Different from conventional MFIs, the transmission spectra of such MFIs are highly polarization-dependent, due to the mode-sensitive birefringence. The experimental results agree well with the theoretical predictions. Moreover, exploiting the polarization-dependent property of PD-MFIs, we demonstrate a simple and flexible scheme of generating polarity-switchable ultra-wideband pulses in the optical domain. Doublet pulses with a central frequency of 6.28 GHz and a 10-dB bandwidth of 7.86 GHz are obtained. Hence, with the advantages of being fiberized, simple fabrication and robustness, these PD-MFIs can be attractive elements in optical signal processing, optical sensing, optical fiber communication, and microwave photonics.

© 2013 OSA

1. Introduction

In the past few years, optical microfibers have attracted intensive interest [1

1. L. M. Tong, R. R. Gattass, J. B. Ashcom, S. L. He, J. Y. Lou, M. Y. Shen, I. Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature 426(6968), 816–819 (2003). [CrossRef] [PubMed]

3

3. L. Tong, F. Zi, X. Guo, and J. Lou, “Optical microfibers and nanofibers: A tutorial,” Opt. Commun. 285(23), 4641–4647 (2012). [CrossRef]

] due to advantages of ultralow loss, flexibility, strong field confinement and large evanescent fields. And microfiber-based devices have been applied in the areas of optical sensing [4

4. J. Y. Lou, L. M. Tong, and Z. Z. Ye, “Modeling of silica nanowires for optical sensing,” Opt. Express 13(6), 2135–2140 (2005), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-6-2135. [CrossRef] [PubMed]

7

7. Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. Wang, P. Shum, and X. Zhang, “Refractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,” Opt. Express 18(25), 26345–26350 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-25-26345. [CrossRef] [PubMed]

], nonlinear optics [8

8. S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St J Russell, and M. W. Mason, “Supercontinuum generation in submicron fibre waveguides,” Opt. Express 12(13), 2864–2869 (2004), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-13-2864. [CrossRef] [PubMed]

], atom optics [9

9. F. Le Kien and K. Hakuta, “Slowing down of a guided light field along a nanofiber in a cold atomic gas,” Phys. Rev. A 79(1), 013818 (2009). [CrossRef]

], micro- and nano-photonics [10

10. L. M. Tong, J. Y. Lou, R. R. Gattass, S. L. He, X. W. Chen, L. Liu, and E. Mazur, “Assembly of silica nanowires on silica aerogels for microphotonic devices,” Nano Lett. 5(2), 259–262 (2005). [CrossRef] [PubMed]

12

12. P. Zhao, Y. Li, J. Zhang, L. Shi, and X. Zhang, “Nanohole induced microfiber Bragg gratings,” Opt. Express 20(27), 28625–28630 (2012), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-20-27-28625. [CrossRef] [PubMed]

] and optical signal processing [13

13. Y. Zhang, E. M. Xu, D. X. Huang, and X. L. Zhang, “All-optical format conversion from RZ to NRZ utilizing microfiber resonator,” IEEE Photon. Technol. Lett. 21(17), 1202–1204 (2009). [CrossRef]

15

15. Z. Yu, Z. Xin-Liang, C. Guo-Jie, X. En-Ming, and H. De-Xiu, “Photonic generation of millimeter-wave ultra-wideband signal using microfiber ring resonator,” Opt. Commun. 284(7), 1803–1806 (2011). [CrossRef]

]. Especially, in-line microfiber interferometers (MFIs) are widely studied since they offer numeric exciting properties, such as all fiber components, easy fabrication and robustness. The principle of MFI primarily stems from mode beating behaviors [16

16. F. Gonthier, J. Lapierre, C. Veilleux, S. Lacroix, and J. Bures, “Investigation of power oscillations along tapered monomode fibers,” Appl. Opt. 26(3), 444–449 (1987). [CrossRef] [PubMed]

]. Besides, MFIs have large evanescent waves and can act as platforms for the interaction between guided optical waves and surrounding medium. So far, they have been extensively employed for optical sensing [17

17. Z. B. Tian, S. S. H. Yam, and H. P. Loock, “Refractive index sensor based on an abrupt taper Michelson interferometer in a single-mode fiber,” Opt. Lett. 33(10), 1105–1107 (2008). [CrossRef] [PubMed]

20

20. J. Li, L.-P. Sun, S. Gao, Z. Quan, Y.-L. Chang, Y. Ran, L. Jin, and B.-O. Guan, “Ultrasensitive refractive-index sensors based on rectangular silica microfibers,” Opt. Lett. 36(18), 3593–3595 (2011). [CrossRef] [PubMed]

] and fiber lasers [21

21. K. Kieu and M. Mansuripur, “Tuning of fiber lasers by use of a single-mode biconic fiber taper,” Opt. Lett. 31(16), 2435–2437 (2006). [CrossRef] [PubMed]

, 22

22. Z. B. Tian, M. Nix, and S. S. H. Yam, “Laser beam shaping using a single-mode fiber abrupt taper,” Opt. Lett. 34(3), 229–231 (2009). [CrossRef] [PubMed]

]. Li et al. have demonstrated that MFIs due to polarimetric interference can enable ultrahigh sensing of ambient refractive index change [20

20. J. Li, L.-P. Sun, S. Gao, Z. Quan, Y.-L. Chang, Y. Ran, L. Jin, and B.-O. Guan, “Ultrasensitive refractive-index sensors based on rectangular silica microfibers,” Opt. Lett. 36(18), 3593–3595 (2011). [CrossRef] [PubMed]

]. Hence, techniques combining the mode beating and polarimetric interference may be potential for optical sensors.

On the other hand, the use of ultra-wideband (UWB) signals for wireless communication has attracted considerable attention in the past few years [23

23. D. Porcino and W. Hirt, “Ultra-wideband radio technology: potential and challenges ahead,” IEEE Commun. Mag. 41(7), 66–74 (2003). [CrossRef]

].However, due to the low power spectral density, UWB signals can only propagate within tens of meters in wireless communication systems. Distribution of UWB signals over fiber links, i.e., UWB-over-fiber technology, provides a promising solution [24

24. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]

]. Consequently, the generation of UWB signals in the optical domain is highly demanded since no extra optical-to-electrical conversion is required. Various approaches of photonic generation of UWB signals have been reported [25

25. J. Yao, F. Zeng, and Q. Wang, “Photonic generation of ultrawideband signals,” J. Lightwave Technol. 25(11), 3219–3235 (2007). [CrossRef]

]. Specially, many schemes of generating UWB signals in the optical domain have been recently demonstrated based on nonlinear optical devices, such as semiconductor optical amplifiers (SOAs) [26

26. J. Dong, X. Zhang, J. Xu, D. Huang, S. Fu, and P. Shum, “Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier,” Opt. Lett. 32(10), 1223–1225 (2007). [CrossRef] [PubMed]

29

29. Y. Yu, J. J. Dong, X. Li, and X. L. Zhang, “UWB monocycle generation and bi-phase modulation based on Mach-Zehnder modulator and semiconductor optical amplifier,” IEEE Photon. J. 4(2), 327–339 (2012). [CrossRef]

], highly nonlinear fibers (HNLFs) [30

30. T. Huang, J. Li, J. Sun, and L. R. Chen, “All-optical UWB signal generation and multicasting using a nonlinear optical loop mirror,” Opt. Express 19(17), 15885–15890 (2011), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-17-15885. [CrossRef] [PubMed]

] and nonlinear nanowires [31

31. Y. Yue, H. Huang, L. Zhang, J. Wang, J.-Y. Yang, O. F. Yilmaz, J. S. Levy, M. Lipson, and A. E. Willner, “UWB monocycle pulse generation using two-photon absorption in a silicon waveguide,” Opt. Lett. 37(4), 551–553 (2012). [CrossRef] [PubMed]

]. However, these nonlinear optical devices as well as multiple laser diodes (LDs) result in high cost and increased system complexity. Hence, flexible and cost effective schemes of generating UWB signals based on passive optical devices are favored. Additionally, pulse shaping using interferometric systems has been widely investigated. Early in 2007, Yongwoo Park et al. reported picosecond pulse shaping based on temporal coherence synthesization [32

32. Y. Park, M. H. Asghari, T.-J. Ahn, and J. Azaña, “Transform-limited picosecond pulse shaping based on temporal coherence synthesization,” Opt. Express 15(15), 9584–9599 (2007), http://www.opticsexpress.org/abstract.cfm?URI=oe-15-15-9584. [CrossRef] [PubMed]

]. Later, reshaping of an ultrashort Gaussian-like pulse into a flat-top pulse was demonstrated using an integrated Mach-Zehnder interferometer (MZI) [33

33. M. Li, P. Dumais, R. Ashrafi, H. P. Bazargani, J.-B. Quelene, C. Callender, and J. Azana, “Ultrashort flat-top pulse generation using on-chip CMOS-compatible Mach-Zehnder interferometers,” IEEE Photon. Technol. Lett. 24(16), 1387–1389 (2012). [CrossRef]

]. However, all-fiberized devices can be more easily integrated in UWB-over-fiber systems.

In this paper, we demonstrate a novel in-line polarization-dependent microfiber interferometer (PD-MFI). The PD-MFI is fabricated from a commercial polarization-maintaining fiber with flame-heated tapering approach and has the characteristics of simple fabrication, immunity to electromagnetic interference and easy integration with fiber systems. It has comb-like transmission spectra because of the beating between different modes. These spectra are highly polarization-dependent and this property can be attributed to the mode-sensitive birefringence. The experimental results are in good accordance with the theoretical analysis. Besides, owing to vernier effect, the MFI transmission is polarization-independent for some specific wavelengths. Hence, by adjusting the working wavelength, polarization-dependent or -independent operations in the optical domain can be realized with a PD-MFI. Then, by exploiting the polarization-dependent characteristics of PD-MFIs, we implement the photonic generation of ultra-wideband (UWB) signals based on a PD-MFI, a phase modulator and a single laser diode. Polarity-switchable UWB doublet pulses with a central frequency of 6.28 GHz are successfully obtained with the method of phase-modulation-to-intensity-modulation (PM-IM) conversion. The 10-dB bandwidth of the doublets is up to 7.86 GHz, corresponding to a fraction bandwidth of 125%. Hence, the proposed PD-MFIs can be applied in microwave photonics, optical communication, optical signal processing and optical sensing.

The paper is organized as follows. In Sec. II, the fabrication of PD-MFIs is introduced and PD-MFI transmission properties are investigated. In Sec. III, we implement the principle of PD-MFIs incorporating the mode birefringence of microfibers. Then, photonic generation of UWB pulses based on a PD-MFI is demonstrated in Sec. IV, as an application of PD-MFIs in optical signal processing. At last, our conclusions are given in Sec. V.

2. Fabrication and characteristics of in-line PD-MFIs

Commercial 125μm polarization-maintaining fibers (PMFs, YOFC PM1016-A) were used to fabricate in-line PD-MFIs and Fig. 1(a)
Fig. 1 Optical microscope images of (a) the cross section of a conventional 125μm PMF and (b) a transition of a tapered PMF. The inset is a micrograph of the microfiber fabricated with a diameter of 5.8 μm.
shows the optical microscope (AxioLab A1, ZEISS) images of the PMF cross section. The in-line PD-MFIs were manufactured by tapering the PMFs into microfibers with improved flame-heated technique [8

8. S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St J Russell, and M. W. Mason, “Supercontinuum generation in submicron fibre waveguides,” Opt. Express 12(13), 2864–2869 (2004), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-13-2864. [CrossRef] [PubMed]

]. A 2-mm-wide flame generated by the burning of butane was utilized as the heat source and had a temperature of ~950°C. The PMF acts as a preform for the fabrication of micrometer-diameter microfibers and its extremities were fixed onto stages connected to a computer. Via precisely controlling the moving speeds of both stages, uniform microfibers with diameters down to 1.5 μm and lengths up to 300 mm could be obtained. The generated MFIs were well pigtailed and Fig. 1(b) shows a typical micrograph of a transition of a tapered PMF. The inset depicts a uniform smooth microfiber with a diameter of 5.8 μm. The fiber geometry can be well preserved during the tapering process and as-fabricated microfibers are effectively birefringent [34

34. Y. M. Jung, G. Brambilla, and D. J. Richardson, “Polarization-maintaining optical microfiber,” Opt. Lett. 35(12), 2034–2036 (2010). [CrossRef] [PubMed]

].

Figure 2
Fig. 2 Measured normalized transmission spectra of an as-fabricated PD-MFI. The blue and green lines are spectral curves for x- and y-polarization, respectively. The PD-MFI length was 65 mm and the diameter was 5μm. The intensity was normalized by the power at the highest peak of the transmission spectral curve.
shows typical measured normalized PD-MFI transmission spectra which were recorded by an optical spectrum analyzer (AQ6370B). The light intensity was normalized by the power at the highest peak of the transmission spectral curve. The blue line is the spectral curve for x-polarization and the green line is for y-polarization. Comb-like spectra are achieved, indicating that mode beating occurs. As can be seen in Fig. 2, the transmission spectra are highly polarization-dependent, which is distinct from those of conventional MFIs. This phenomenon is caused by the birefringence that is sensitive to guided modes. The principle of PD-MFIs will be carried out in the following section.

3. Principle of PD-MFIs

Secondly, the transmission spectra shown in Fig. 2 are nearly uniform comb and this suggests that two-mode beating is dominant. Traveling through the abrupt down taper (from the SMF to the MF), the PMF-core mode excites the fundamental and high-order modes of the MF. After the propagation, these modes are converted into the fundamental mode of PMF via the up taper (from the MF to the SMF) and mode recombination is realized. Consequently, mode interference is achieved [16

16. F. Gonthier, J. Lapierre, C. Veilleux, S. Lacroix, and J. Bures, “Investigation of power oscillations along tapered monomode fibers,” Appl. Opt. 26(3), 444–449 (1987). [CrossRef] [PubMed]

]. Assume that the length of microfiber is L. The lengths of the tapers are short enough to be neglected. The mode indices of the fundamental and higher-order modes are neff1 and neff2, respectively. The output fields Eout of PD-MFI for x- and y-polarizations can be described as
Eout,x=E1xejφ1x+E2xejφ2x
(1)
Eout,y=E1yejφ1y+E2yejφ2y
(2)
where E1 and E2 are the complex amplitudes of the two modes excited, and φix = 2πneffi,xL/λ and φiy = 2πneffi,yL/λ (i = 1,2), are the phase changes of the modes with the wavelength λ in vacuum.

As shown in Fig. 2, transmission-extinction ratios for x- and y-polarizations are not very high, around −12 dB and −17 dB, respectively. This is due to the fact that the modes with the same polarization states have different powers. However, such fringe contrast can meet most of the demands for applications of signal processing and we focus on the polarization-dependent property of the device in this paper. For simplicity, it is assumed in the simulations that the two modes have equal powers. Hence, the power transmittances of PD-MFIs for x- and y-polarizations can be expressed by Tx = cos2(πΔnxL/λ) and Ty = cos2(πΔnyL/λ), where Δnx = neff1,x - neff2,x and Δny = neff1,y - neff2,y.
ΔnxΔny=B1B2
(3)
whare Bi (i = 1,2) is the birefringence of the ith mode. According to Eq. (3), Δnx is not equal to Δny since the birefringence varies with modes. Then the free spectrum ranges (FSRs) for the two polarizations are different. Hence, it indicates that polarization-dependent operations at the same wavelength can be expected using this interferometer.

4. Photonic generation of UWB pulses based on PD-MFIs

The PD-MFI provides an idea for polarization-selective optical signal processing. Based on this method, a couple of monocycle pulses with opposite polarities can be simultaneously obtained at the same wavelength using PM-IM conversion. By incoherently combining the monocycle pulses, polarity-switchable doublet pulses can be synthesized. In this approach, diverse UWB pulses can be generated in a simple system.

4.1 Principle of the scheme

Figure 5(a)
Fig. 5 (a) Principle of UWB pulse generation based on a PD-MFI. λ0 is the carrier wavelength. φ0 is the phase of input phase-modulated signal. The green arrow denotes the polarization state of lightwave. Ix1 and Iy1 are the signal intensity on x- and y-polarizations. τ is the relative delay between the two signals. Iout is the intensity of the output signal. (b) Detuning between the PD-MFI and the carrier wavelength. The blue solid and the green dotted lines denote the PD-MFI transmission spectra for x- and y-polarizations, respectively.
shows the operation principle of UWB pulse generation based on a PD-MFI. Optical Gaussian phase-modulated signals with equal power on x- and y-polarizations are injected into the PD-MFI. λ0 is the carrier wavelength, with φ0 the phase of input phase-modulated signal. The detuning between the carrier wavelength and the PD-MFI is illustrated in Fig. 5(b). The blue solid and the green dotted lines denote the PD-MFI transmission spectra for x- and y-polarizations, respectively. The carrier wavelength is set at the intersection point of the spectral curves. At such a point, the slope of the spectral curve is positive for x-polarized lightwave while it is negative for lightwave on y-polarization. Hence, using the approach of PM-IM conversion [25

25. J. Yao, F. Zeng, and Q. Wang, “Photonic generation of ultrawideband signals,” J. Lightwave Technol. 25(11), 3219–3235 (2007). [CrossRef]

], monocycle pulses with opposite polarities at the same wavelength are obtained on x- and y-polarizations after the PD-MFI, respectively. Then by recoupling the two pulses with an appropriate time delay τ, positive or negative doublet pulses can be generated.

Figure 6
Fig. 6 Simulation results. (a) Input optical Gaussian phase-modulated signals. (b) Temporal waveforms and (c) electrical spectra of the output pulses with various τ. Red dotted line: the FCC spectrum mask.
shows the simulation results. In the simulations, the Gaussian phase-modulated signal has a 10-dB pulse-width of 100 ps, as shown in Fig. 6(a). The blue solid line and the red dotted line represent the power and the phase of signals, respectively. Figure 6(b) shows the temporal waveforms of output pulses with various τ. The blue dashed line corresponds to the waveform of τ = −125 ps. By changing τ to −25 ps, a negative UWB doublet pulse is achieved, represented by the green solid line. With τ = 0 ps, there are little “AC” components since the complementary monocycle pulses compensate for each other, shown by the shallow-green dashed line. When the time delay was adjusted to 25 ps, a positive UWB doublet pulse is obtained, as presented by the pink-dashed line. Electrical spectra of those waveforms are illustrated in Fig. 6(c). In practice, the Federal Communications Commission (FCC) regulates the electrical spectral range of UWB signals. The red dotted line is the FCC spectrum mask. The “DC” term of the electrical spectra of UWB signals were neglected to show the image of high-frequency components more clearly. It can be seen that the spectra of generated doublet pulses are within the FCC spectrum mask. Hence, the proposed method is flexible in generating diverse UWB pulses in the optical domain.

4.2 Experimental setup

In addition to the simulations, experiments were also carried out and the setup is schematically depicted in Fig. 7(a)
Fig. 7 (a) Experimental setup of the UWB pulse generation. LD, laser diode. BPG, bit pattern generator. MA, microwave amplifier. PM, phase modulator. PC, polarization-controller. PBS, polarization-beam splitter. ODL, optical delay-line. PBC, polarization-beam coupler. EDFA, Erbium-doped fiber amplifier. ATT, tunable attenuator. OC, optical coupler. DCA, digital communications analyzer. PD, photodetector. ESA, electrical spectrum analyzer. (b) Measured transmission spectra of the PD-MFI. The blue and green lines represent the spectral curves for x- and y-polarizations, respectively.
. A CW lightwave from a tunable LD was fed to a phase modulator (PM) through a polarization-controller (PC). The PC was used to make the laser polarization state parallel to the principal axis of the PM. The PM was driven by a data sequence from a bit pattern generator (BPG). The data had a repetition rate of 20 gigabits per second with a fixed pattern of one “1” per 16 bits. The return-to-zero (RZ) electrical Gaussian pulse had a 10-dB pulse-width of about 100 ps and was amplified by a microwave amplifier (MA) before feeding to the PM. The optical phase-modulated signal was then applied to the PD-MFI through another PC. After the PM-IM conversion, the signals were divided into two taps by a polarization-beam splitter (PBS). An optical delay-line (ODL) was employed to control the relative time delay between the two taps that were further recombined by a polarization-beam coupler (PBC) to generate UWB doublet pulses. Amplified by an Erbium-doped fiber amplifier (EDFA) and attenuated by a tunable attenuator (ATT), the power of generated optical signals was then split by a polarization-independent optical coupler (OC) for efficient detection. The electrical spectrum and the waveform of the UWB pulses were measured by an electrical spectrum analyzer (ESA, MS2668C) and a digital communications analyzer (DCA, 86100C), respectively.

Figure 7(b) shows the measured transmission spectra of the PD-MFI. The blue and green lines represent the spectral curves for x- and y-polarizations, respectively. As can be seen in Fig. 7(b), the free spectrum ranges (FSRs) of both polarizations are slightly different: for x-polarization, FSR is 0.83 nm and for y-polarization, it is 0.77 nm. In this investigation, the carrier wavelength of 1560.48 nm (λ0 in Fig. 7(b)) was selected. The reason is that the large linear slopes of transmission spectra can strengthen the “AC” component of UWB monocycle pulses generated by the PM-IM conversion. Furthermore, at such an intersection point, the slopes have nearly equal absolute values and opposite signs. This is useful to achieve a pair of balanced polarity-reversed monocycle pulses for the synthesis of doublet pulses.

4.3 Experimental results

Figure 8(a)
Fig. 8 Experimental results. (a-d) Temporal waveforms: negative and positive monocycles, positive and negative doublets, respectively. (e-f) corresponding electrical spectra of UWB pulses. The black dot-dashed line is the fitted envelope and the red-dotted line represents the FCC spectrum mask.
and 8(b) show the waveforms of generated negative and positive UWB monocycle pulses. The corresponding electrical spectra are shown in Fig. 8(e) and 8(f), respectively. The black dash-dotted and the red dotted lines are the fitted spectral envelope and the FCC spectrum mask. As can be seen in Fig. 8(e), negative UWB monocycle pulses (Ix1) with a central frequency (f0) of 3.64 GHz are obtained. The 10-dB bandwidth (10dB-BW) is up to 10.45 GHz. In addition, UWB monocycles pulses (Iy1) with opposite polarity are generated on y-polarization, as depicted in Fig. 8(b).

The waveforms of positive and negative UWB doublet pulses generated are presented by Fig. 8(c) and 8(d), respectively, and electrical spectra of these waveforms are shown by Fig. 8(g) and 8(h). The positive doublet signal has an f0 up to 6.36 GHz with a spectrum matching the FCC spectrum mask. The corresponding 10dB-BW is 7.86 GHz, leading to a fraction bandwidth ~125%. With respect to the negative doublet pulses, f0 is 5.00 GHz, relatively lower than that of the positive. The actual local spectral slope of this PD-MFI is slightly nonlinear, which results in that the peak of positive monocycle pulse is stronger than the dip. The resulted unbalanced monocycles bring down the peak frequency of the doublets since the doublet pulse is the superposition of monocycle ones. By optimizing the microfiber fabrication to get PD-MFIs with sign-reversed linear slopes of transmission spectra, balanced negative monocycle and doublet pulses can be obtained.

5. Conclusions

In summary, novel in-line PD-MFIs are proposed and experimentally demonstrated. Fabricated from a commercial polarization-maintaining fiber, the PD-MFIs have highly polarization-dependent transmission spectra. This is attributed to that as-fabricated microfibers are effectively birefringent and the birefringence is highly sensitive to guided modes. The experimental results are in consistent with the theoretical analysis. Besides, owing to the vernier effect, the transmission of the interferometer is polarization-independent for some specific wavelengths. Hence, polarization-dependent or -independent operations can be realized with a PD-MFI by adjusting the operating wavelength. Based on the polarization-dependent effect of PD-MFI transmission, a simple and flexible scheme of photonic generation of polarity-switchable UWB doublet pulses is proposed and experimentally implemented. The resulted doublet pulses have a central frequency of 6.28 GHz and a 10-dB bandwidth of 7.86 GHz. Hence, being advantageous for ease to fabrication and fiberized components, these PD-MFIs can find applications in a variety of areas including optical communication, optical signal processing and optical sensing.

Acknowledgments

The authors would like to show special thanks to Mr. Nai Peng for constructive discussions and providing the PMFs. This work is partially supported by the Nature Science Foundation for Distinguished Young Scholars of China (No. 61255501), the Program for New Century Excellent Talents in Ministry of Education of China (Grant No. NCET-11-0168), and the National Natural Science Foundation of China (Grant No. 60901006, and Grant No. 11174096).

References and links

1.

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2.

G. Brambilla, “Optical fibre nanowires and microwires: A review,” J. Opt. 12(4), 043001 (2010). [CrossRef]

3.

L. Tong, F. Zi, X. Guo, and J. Lou, “Optical microfibers and nanofibers: A tutorial,” Opt. Commun. 285(23), 4641–4647 (2012). [CrossRef]

4.

J. Y. Lou, L. M. Tong, and Z. Z. Ye, “Modeling of silica nanowires for optical sensing,” Opt. Express 13(6), 2135–2140 (2005), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-6-2135. [CrossRef] [PubMed]

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P. Polynkin, A. Polynkin, N. Peyghambarian, and M. Mansuripur, “Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels,” Opt. Lett. 30(11), 1273–1275 (2005). [CrossRef] [PubMed]

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F. Xu and G. Brambilla, “Demonstration of a refractometric sensor based on optical microfiber coil resonator,” Appl. Phys. Lett. 92(10), 101126 (2008). [CrossRef]

7.

Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. Wang, P. Shum, and X. Zhang, “Refractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,” Opt. Express 18(25), 26345–26350 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-25-26345. [CrossRef] [PubMed]

8.

S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St J Russell, and M. W. Mason, “Supercontinuum generation in submicron fibre waveguides,” Opt. Express 12(13), 2864–2869 (2004), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-13-2864. [CrossRef] [PubMed]

9.

F. Le Kien and K. Hakuta, “Slowing down of a guided light field along a nanofiber in a cold atomic gas,” Phys. Rev. A 79(1), 013818 (2009). [CrossRef]

10.

L. M. Tong, J. Y. Lou, R. R. Gattass, S. L. He, X. W. Chen, L. Liu, and E. Mazur, “Assembly of silica nanowires on silica aerogels for microphotonic devices,” Nano Lett. 5(2), 259–262 (2005). [CrossRef] [PubMed]

11.

P. Zhao, J. Zhang, G. Wang, M. Jiang, P. P. Shum, and X. Zhang, “Longitudinal coupling effect in microfiber Bragg gratings,” Opt. Commun. 285(23), 4655–4659 (2012). [CrossRef]

12.

P. Zhao, Y. Li, J. Zhang, L. Shi, and X. Zhang, “Nanohole induced microfiber Bragg gratings,” Opt. Express 20(27), 28625–28630 (2012), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-20-27-28625. [CrossRef] [PubMed]

13.

Y. Zhang, E. M. Xu, D. X. Huang, and X. L. Zhang, “All-optical format conversion from RZ to NRZ utilizing microfiber resonator,” IEEE Photon. Technol. Lett. 21(17), 1202–1204 (2009). [CrossRef]

14.

Y. Zhang, X. L. Zhang, G. J. Chen, E. M. Xu, and D. X. Huang, “A microwave photonic notch flter using a microfiber ring resonator,” Chin. Phys. Lett. 27(7), 074207 (2010). [CrossRef]

15.

Z. Yu, Z. Xin-Liang, C. Guo-Jie, X. En-Ming, and H. De-Xiu, “Photonic generation of millimeter-wave ultra-wideband signal using microfiber ring resonator,” Opt. Commun. 284(7), 1803–1806 (2011). [CrossRef]

16.

F. Gonthier, J. Lapierre, C. Veilleux, S. Lacroix, and J. Bures, “Investigation of power oscillations along tapered monomode fibers,” Appl. Opt. 26(3), 444–449 (1987). [CrossRef] [PubMed]

17.

Z. B. Tian, S. S. H. Yam, and H. P. Loock, “Refractive index sensor based on an abrupt taper Michelson interferometer in a single-mode fiber,” Opt. Lett. 33(10), 1105–1107 (2008). [CrossRef] [PubMed]

18.

Z. B. Tian and S. S. H. Yam, “In-line abrupt taper optical fiber Mach-Zehnder interferometric strain sensor,” IEEE Photon. Technol. Lett. 21(3), 161–163 (2009). [CrossRef]

19.

G. Salceda-Delgado, D. Monzon-Hernandez, A. Martinez-Rios, G. A. Cardenas-Sevilla, and J. Villatoro, “Optical microfiber mode interferometer for temperature-independent refractometric sensing,” Opt. Lett. 37(11), 1974–1976 (2012). [CrossRef] [PubMed]

20.

J. Li, L.-P. Sun, S. Gao, Z. Quan, Y.-L. Chang, Y. Ran, L. Jin, and B.-O. Guan, “Ultrasensitive refractive-index sensors based on rectangular silica microfibers,” Opt. Lett. 36(18), 3593–3595 (2011). [CrossRef] [PubMed]

21.

K. Kieu and M. Mansuripur, “Tuning of fiber lasers by use of a single-mode biconic fiber taper,” Opt. Lett. 31(16), 2435–2437 (2006). [CrossRef] [PubMed]

22.

Z. B. Tian, M. Nix, and S. S. H. Yam, “Laser beam shaping using a single-mode fiber abrupt taper,” Opt. Lett. 34(3), 229–231 (2009). [CrossRef] [PubMed]

23.

D. Porcino and W. Hirt, “Ultra-wideband radio technology: potential and challenges ahead,” IEEE Commun. Mag. 41(7), 66–74 (2003). [CrossRef]

24.

J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]

25.

J. Yao, F. Zeng, and Q. Wang, “Photonic generation of ultrawideband signals,” J. Lightwave Technol. 25(11), 3219–3235 (2007). [CrossRef]

26.

J. Dong, X. Zhang, J. Xu, D. Huang, S. Fu, and P. Shum, “Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier,” Opt. Lett. 32(10), 1223–1225 (2007). [CrossRef] [PubMed]

27.

Q. Wang, F. Zeng, S. Blais, and J. Yao, “Optical ultrawideband monocycle pulse generation based on cross-gain modulation in a semiconductor optical amplifier,” Opt. Lett. 31(21), 3083–3085 (2006). [CrossRef] [PubMed]

28.

B. Luo, J. Dong, Y. Yu, T. Yang, and X. Zhang, “Photonic generation of ultra-wideband doublet pulse using a semiconductor-optical-amplifier based polarization-diversified loop,” Opt. Lett. 37(12), 2217–2219 (2012). [CrossRef] [PubMed]

29.

Y. Yu, J. J. Dong, X. Li, and X. L. Zhang, “UWB monocycle generation and bi-phase modulation based on Mach-Zehnder modulator and semiconductor optical amplifier,” IEEE Photon. J. 4(2), 327–339 (2012). [CrossRef]

30.

T. Huang, J. Li, J. Sun, and L. R. Chen, “All-optical UWB signal generation and multicasting using a nonlinear optical loop mirror,” Opt. Express 19(17), 15885–15890 (2011), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-17-15885. [CrossRef] [PubMed]

31.

Y. Yue, H. Huang, L. Zhang, J. Wang, J.-Y. Yang, O. F. Yilmaz, J. S. Levy, M. Lipson, and A. E. Willner, “UWB monocycle pulse generation using two-photon absorption in a silicon waveguide,” Opt. Lett. 37(4), 551–553 (2012). [CrossRef] [PubMed]

32.

Y. Park, M. H. Asghari, T.-J. Ahn, and J. Azaña, “Transform-limited picosecond pulse shaping based on temporal coherence synthesization,” Opt. Express 15(15), 9584–9599 (2007), http://www.opticsexpress.org/abstract.cfm?URI=oe-15-15-9584. [CrossRef] [PubMed]

33.

M. Li, P. Dumais, R. Ashrafi, H. P. Bazargani, J.-B. Quelene, C. Callender, and J. Azana, “Ultrashort flat-top pulse generation using on-chip CMOS-compatible Mach-Zehnder interferometers,” IEEE Photon. Technol. Lett. 24(16), 1387–1389 (2012). [CrossRef]

34.

Y. M. Jung, G. Brambilla, and D. J. Richardson, “Polarization-maintaining optical microfiber,” Opt. Lett. 35(12), 2034–2036 (2010). [CrossRef] [PubMed]

OCIS Codes
(060.2340) Fiber optics and optical communications : Fiber optics components
(070.1170) Fourier optics and signal processing : Analog optical signal processing
(230.3990) Optical devices : Micro-optical devices
(230.5440) Optical devices : Polarization-selective devices

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: December 20, 2012
Revised Manuscript: February 27, 2013
Manuscript Accepted: February 28, 2013
Published: March 28, 2013

Citation
Ping Zhao, Jihua Zhang, Yuan Yu, Jianji Dong, Lei Shi, Yang Liu, and Xinliang Zhang, "In-line polarization-dependent microfiber interferometers and their applications in UWB signal generation," Opt. Express 21, 8231-8239 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-7-8231


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References

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  2. G. Brambilla, “Optical fibre nanowires and microwires: A review,” J. Opt.12(4), 043001 (2010). [CrossRef]
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  4. J. Y. Lou, L. M. Tong, and Z. Z. Ye, “Modeling of silica nanowires for optical sensing,” Opt. Express13(6), 2135–2140 (2005), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-13-6-2135 . [CrossRef] [PubMed]
  5. P. Polynkin, A. Polynkin, N. Peyghambarian, and M. Mansuripur, “Evanescent field-based optical fiber sensing device for measuring the refractive index of liquids in microfluidic channels,” Opt. Lett.30(11), 1273–1275 (2005). [CrossRef] [PubMed]
  6. F. Xu and G. Brambilla, “Demonstration of a refractometric sensor based on optical microfiber coil resonator,” Appl. Phys. Lett.92(10), 101126 (2008). [CrossRef]
  7. Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. Wang, P. Shum, and X. Zhang, “Refractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,” Opt. Express18(25), 26345–26350 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-25-26345 . [CrossRef] [PubMed]
  8. S. G. Leon-Saval, T. A. Birks, W. J. Wadsworth, P. St J Russell, and M. W. Mason, “Supercontinuum generation in submicron fibre waveguides,” Opt. Express12(13), 2864–2869 (2004), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-13-2864 . [CrossRef] [PubMed]
  9. F. Le Kien and K. Hakuta, “Slowing down of a guided light field along a nanofiber in a cold atomic gas,” Phys. Rev. A79(1), 013818 (2009). [CrossRef]
  10. L. M. Tong, J. Y. Lou, R. R. Gattass, S. L. He, X. W. Chen, L. Liu, and E. Mazur, “Assembly of silica nanowires on silica aerogels for microphotonic devices,” Nano Lett.5(2), 259–262 (2005). [CrossRef] [PubMed]
  11. P. Zhao, J. Zhang, G. Wang, M. Jiang, P. P. Shum, and X. Zhang, “Longitudinal coupling effect in microfiber Bragg gratings,” Opt. Commun.285(23), 4655–4659 (2012). [CrossRef]
  12. P. Zhao, Y. Li, J. Zhang, L. Shi, and X. Zhang, “Nanohole induced microfiber Bragg gratings,” Opt. Express20(27), 28625–28630 (2012), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-20-27-28625 . [CrossRef] [PubMed]
  13. Y. Zhang, E. M. Xu, D. X. Huang, and X. L. Zhang, “All-optical format conversion from RZ to NRZ utilizing microfiber resonator,” IEEE Photon. Technol. Lett.21(17), 1202–1204 (2009). [CrossRef]
  14. Y. Zhang, X. L. Zhang, G. J. Chen, E. M. Xu, and D. X. Huang, “A microwave photonic notch flter using a microfiber ring resonator,” Chin. Phys. Lett.27(7), 074207 (2010). [CrossRef]
  15. Z. Yu, Z. Xin-Liang, C. Guo-Jie, X. En-Ming, and H. De-Xiu, “Photonic generation of millimeter-wave ultra-wideband signal using microfiber ring resonator,” Opt. Commun.284(7), 1803–1806 (2011). [CrossRef]
  16. F. Gonthier, J. Lapierre, C. Veilleux, S. Lacroix, and J. Bures, “Investigation of power oscillations along tapered monomode fibers,” Appl. Opt.26(3), 444–449 (1987). [CrossRef] [PubMed]
  17. Z. B. Tian, S. S. H. Yam, and H. P. Loock, “Refractive index sensor based on an abrupt taper Michelson interferometer in a single-mode fiber,” Opt. Lett.33(10), 1105–1107 (2008). [CrossRef] [PubMed]
  18. Z. B. Tian and S. S. H. Yam, “In-line abrupt taper optical fiber Mach-Zehnder interferometric strain sensor,” IEEE Photon. Technol. Lett.21(3), 161–163 (2009). [CrossRef]
  19. G. Salceda-Delgado, D. Monzon-Hernandez, A. Martinez-Rios, G. A. Cardenas-Sevilla, and J. Villatoro, “Optical microfiber mode interferometer for temperature-independent refractometric sensing,” Opt. Lett.37(11), 1974–1976 (2012). [CrossRef] [PubMed]
  20. J. Li, L.-P. Sun, S. Gao, Z. Quan, Y.-L. Chang, Y. Ran, L. Jin, and B.-O. Guan, “Ultrasensitive refractive-index sensors based on rectangular silica microfibers,” Opt. Lett.36(18), 3593–3595 (2011). [CrossRef] [PubMed]
  21. K. Kieu and M. Mansuripur, “Tuning of fiber lasers by use of a single-mode biconic fiber taper,” Opt. Lett.31(16), 2435–2437 (2006). [CrossRef] [PubMed]
  22. Z. B. Tian, M. Nix, and S. S. H. Yam, “Laser beam shaping using a single-mode fiber abrupt taper,” Opt. Lett.34(3), 229–231 (2009). [CrossRef] [PubMed]
  23. D. Porcino and W. Hirt, “Ultra-wideband radio technology: potential and challenges ahead,” IEEE Commun. Mag.41(7), 66–74 (2003). [CrossRef]
  24. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics1(6), 319–330 (2007). [CrossRef]
  25. J. Yao, F. Zeng, and Q. Wang, “Photonic generation of ultrawideband signals,” J. Lightwave Technol.25(11), 3219–3235 (2007). [CrossRef]
  26. J. Dong, X. Zhang, J. Xu, D. Huang, S. Fu, and P. Shum, “Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier,” Opt. Lett.32(10), 1223–1225 (2007). [CrossRef] [PubMed]
  27. Q. Wang, F. Zeng, S. Blais, and J. Yao, “Optical ultrawideband monocycle pulse generation based on cross-gain modulation in a semiconductor optical amplifier,” Opt. Lett.31(21), 3083–3085 (2006). [CrossRef] [PubMed]
  28. B. Luo, J. Dong, Y. Yu, T. Yang, and X. Zhang, “Photonic generation of ultra-wideband doublet pulse using a semiconductor-optical-amplifier based polarization-diversified loop,” Opt. Lett.37(12), 2217–2219 (2012). [CrossRef] [PubMed]
  29. Y. Yu, J. J. Dong, X. Li, and X. L. Zhang, “UWB monocycle generation and bi-phase modulation based on Mach-Zehnder modulator and semiconductor optical amplifier,” IEEE Photon. J.4(2), 327–339 (2012). [CrossRef]
  30. T. Huang, J. Li, J. Sun, and L. R. Chen, “All-optical UWB signal generation and multicasting using a nonlinear optical loop mirror,” Opt. Express19(17), 15885–15890 (2011), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-17-15885 . [CrossRef] [PubMed]
  31. Y. Yue, H. Huang, L. Zhang, J. Wang, J.-Y. Yang, O. F. Yilmaz, J. S. Levy, M. Lipson, and A. E. Willner, “UWB monocycle pulse generation using two-photon absorption in a silicon waveguide,” Opt. Lett.37(4), 551–553 (2012). [CrossRef] [PubMed]
  32. Y. Park, M. H. Asghari, T.-J. Ahn, and J. Azaña, “Transform-limited picosecond pulse shaping based on temporal coherence synthesization,” Opt. Express15(15), 9584–9599 (2007), http://www.opticsexpress.org/abstract.cfm?URI=oe-15-15-9584 . [CrossRef] [PubMed]
  33. M. Li, P. Dumais, R. Ashrafi, H. P. Bazargani, J.-B. Quelene, C. Callender, and J. Azana, “Ultrashort flat-top pulse generation using on-chip CMOS-compatible Mach-Zehnder interferometers,” IEEE Photon. Technol. Lett.24(16), 1387–1389 (2012). [CrossRef]
  34. Y. M. Jung, G. Brambilla, and D. J. Richardson, “Polarization-maintaining optical microfiber,” Opt. Lett.35(12), 2034–2036 (2010). [CrossRef] [PubMed]

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