A new technique for the realization of a stable Q-switched operation in a single-frequency fiber laser based on self-injecting polarization modulation is demonstrated, for the first time to the best of our knowledge. A piezoelectric fiber stretcher was utilized to introduce periodic stress-induced polarization changes. Then the modulation of polarization state transformed into Q switching by virtue of a designed distributed Bragg reflector (DBR) resonant cavity with polarizations loss anisotropy. Finally, a stable actively Q-switched single-frequency fiber laser at 1.5 μm with Gaussian-shape pulse output was achieved. We experimentally found that, the repetition frequency (several hundred kHz) coincided with the working frequency of the polarization modulation, and the pulse width (several hundred ns) reduced with the increasing of the modulating frequency, the modulating amplitude, as well as the pump power. This stable Q-switched single-frequency fiber laser is promising for applications in optical time-domain reflectometry, coherent Doppler wind radar, and optical coherent detection. More importantly, this novel Q-switched technology may be applicable to other DBR single-frequency fiber lasers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Q-switched fiber lasers have been extensively studied owing to its advantages such as high efficiency, near diffraction-limited beam quality, compact configuration, and convenient thermal management, which can be advantageously applied in various fields, including material processing, laser weapons, nonlinear frequency conversion [1–3]. In particular, the output of a Q-switched fiber laser operating in a single-longitudinal-mode regime has a long coherent length, rendering it attracting for applications in optical time-domain reflectometry, coherent Doppler wind radar, optical coherent detection, and generating THz source [4–7].
In general, a straightforward method to obtain Q-switching in fiber laser is to insert an acoustic-optical modulator (AOM) or an electric-optical modulator (EOM) into the laser cavity [8–10]. However, owing to the employment of additional large-size modulators and resultant long cavity length, single-longitudinal-mode operation can hardly be realized in such Q-switched fiber lasers. In order to overcome this problem, small-scale piezoelectric transducers (PZT) were clamped on the laser cavity to achieve single-frequency Q-switched fiber laser operation [11–13]. But the unavoidably iterative adjusting of the bias voltage and the amplitude voltage on the PZT to eliminate any afterpulses increased operational complexity . Moreover, injection locking technique, i.e., applying an extra single-frequency laser as seed source to inject into a Q-switched fiber laser, is another approach to realize single-frequency operation of a Q-switched fiber laser [14–17]. Nevertheless, fine tuning of the external injected laser is needed to efficiently build up the relevant laser radiation in the master resonator due to the frequency-shift effect . Besides, the requirement of another single-frequency laser is also a limitation of this injection locking Q-switched system. Recently, a compact passively Q-switched single-frequency fiber laser based on a linear short distributed Bragg reflector (DBR) cavity and a semiconductor saturable absorber mirror was present by our group , but the working stability was not investigated.
In this article, a new stable single-frequency Q-switched technique based on self-injecting polarization modulation is presented. By utilizing a high-speed fiber stretcher to introduce stress-induced birefringence modulation, combining with self-injecting optical feedback loop and a DBR resonant cavity with polarizations loss anisotropy, a stable actively Q-switched 1.5 μm single-frequency fiber laser with uniform Gaussian-shape pulses output was realized. More importantly, this novel Q-switched technology may be applicable to other DBR single-frequency fiber lasers.
2. Experimental setup
The experimental setup of this actively Q-switched single-frequency fiber laser based on self-injecting polarization modulation is shown in Fig. 1. A polarization-maintaining narrow-band fiber Bragg grating (PM-NB-FBG) and a wide-band fiber Bragg grating (WB-FBG) on each ends of a home-made 1.5 cm long highly Er3+/Yb3+-codoped phosphate fiber formed a DBR laser cavity [19–22]. The PM-NB-FBG was written in a single-mode polarization-maintaining fiber with a 3 dB bandwidth of 0.06 nm and a reflectivity of 60%. Due to the birefringence of the polarization-maintaining fiber, the reflecting spectrum of the PM-NB-FBG split into two reflection peaks with two central wavelengths corresponding to the fast and slow axes of the polarization-maintaining fiber, respectively . The WB-FBG had a peak reflectivity of > 99.95% with a 3 dB bandwidth of 0.3 nm. This pair of FBGs was specially chosen so that only the reflection peak corresponding to the slow axis of the PM-FBG would fall into the center of the WB-FBG.
The laser cavity was thermally stabilized through a thermal controller with a resolution of 0.02 °C to achieve a robust single-longitudinal-mode operation. A 978 nm laser diode (LD) was used to pump the laser cavity through a wavelength division multiplexer (WDM). Then the output laser input the port 2 of an optical circulator. For ensuring a unidirectional transmission of the laser, an isolator was inserted between the port 3 of circulator and the input port of a 10/90 optical coupler. 10% of the laser power from the 10/90 optical coupler was launched into the port 1 of the circulator fusion-splicing with a high-speed piezoelectric fiber stretcher. The insertion loss of the fiber stretcher was 0.3 dB, the length of single-mode fiber in fiber stretcher was about 12.3 m, and the fiber stretching coefficient was 0.14 μm/V [24,25]. A self-injection optical feedback was realized by this fiber loop, and the fiber stretcher driven by a sinusoidal wave signal served as a polarization modulator. A polarization controller (PC) was used to pre-adjust the polarization of the self-injecting light.
3. Results and discussion
In order to elucidate the underlying Q-switching mechanism in this present scheme, an open-loop-measurement illustrated in Fig. 2(a) is utilized. Here, when the pump power Pp was equal to 225 mW, a typical sinusoidal signal with a modulating frequency fm = 250 kHz and a modulating amplitude Am = 5 V was imposed on the fiber stretcher. For the laser before polarization beam splitter (PBS), its temporal state was relatively stable, and some slight fluctuations resulted from the connatural relaxation oscillation of fiber laser [26,27]. Intriguing, instead of stable intensity performance [green curve in Fig. 2(b)] with respect to the overall light field, evident power modulation in slow/fast axis with a time period of 4 μs (in accordance with the modulating frequency) was demonstrated in Fig. 2(b). That is to say, a periodic polarization evolution was realized, and it was straightforward attributed to the stress-induced birefringence change [28,29]. To be more specific, the polarized signal intensity (defined as the output laser intensity of the slow axis) varying with the modulating frequency and the modulating amplitude of the used fiber stretcher was characterized respectively. As seen in Fig. 2(c) that measurements were taken at every 5 kHz when the modulating frequency was tuned from 180 to 400 kHz and the modulating amplitude Am = 5 V, maxima above a threshold value 6 a.u. were highlighted by red dots. On the other hand, the signal intensity responded almost linearly to the modulating amplitude at the range of 2.4 to 8 V when the modulating frequency fm = 250 kHz [see Fig. 2(d)]. Note that in our experiments, neither the polarization component (i.e., output from slow and fast axes) can be fully suppressed at any time; namely, it is unable to rotate the light field to linearly polarized state by passing through the fiber stretcher.
In what follows, we would explain how the resonator transforms the polarization evolution into the Q switching. A brief sketch is given in Fig. 3(a). As illustrated, the injecting elliptically-polarized light with signal wavelength λs travelled along the slow and fast axes. During the self-injecting framework, the fluctuation of the injecting light signal could be passed into the laser in the fiber resonant cavity [30,31]. After being reflected by the PM-NB-FBG, the fast-axis-component experienced less feedback since the wavelength λs was outside the reflectivity peak with respect to the fast axis. The origin of this phenomenon is loss anisotropy of different polarizations in the fiber resonant cavity because the reflection center of the WB-FBG was specially chosen to only match the reflection peak corresponding to the slow axis of the PM-FBG as displayed in Fig. 3(b) . Such effect grew iteratively, thereby which made the fast-axis-component off-resonant. Thus, the resonant cavity acted as an artificial polarizer and implemented intra-cavity intensity discrimination to realize Q switching. The polarization state of the laser before self-injection was investigated using a polarization analyzer, as shown in the inset of Fig. 3(c). The red dot demonstrated the polarization is clearly located on the mid-latitude zone of the Poincaré sphere, which indicates that the laser has an elliptic polarization characteristic. The polarization-extinction ratio is measured to be 0.8 dB. A sharp contrast between the signals before and after the self-injection (corresponding to the open-loop and closed-loop configurations) is illustrated in Fig. 3(c), indicating a critical role played by the designed laser cavity in producing the Q-switched pulses. Interestingly, we experimentally recognized that stable Q-switching operation required a certain threshold of the polarized signal intensity referred in Fig. 2(c). For instance, when the modulating frequencies corresponding the polarized signal intensities exceeded 6 a.u., this fiber laser with Pp = 225 mW was able to stimulate robust Q-switching. This requirement might be attributed to non-polarization-maintaining gain fiber and WB-FBG. Both elements enabled part of the energy to be exchanged between light at slow and fast axes, which somewhat weakened the intensity discrimination effect. Benefitting from the above properties, this fiber laser presented stable and uniform Gaussian-shape pulses output.
The pulse width of this actively Q-switched single-frequency fiber laser versus the repetition frequency fr, for a fixed pump power Pp of 225 mW and a fixed modulating amplitude Am of 5 V, is plotted in Fig. 4(a). The pulse width decreased almost linearly from 536 to 410 ns with the increasing of the repetition frequency fr. It needs to be explained that the repetition frequency points which could achieve stable Gaussian-shape pulses output were consistent with that (red round dot) supporting stronger polarized signal intensity response in Fig. 2(c), so the interval of the repetition frequency fr in Fig. 4(a) was asymmetrical. However, this deficiency can be solved by optimizing the property of the fiber stretcher. Moreover, the peak power Ppeak was calculated as Ppeak≈Pave/(tp × fr), where Pave was the average power and tp was the pulse width. It showed a decrease trend as the repetition frequency increased. By contrast, the average power remained basically unchanged during the increasing of the repetition frequency. Figure 4(b) demonstrates the pulse width and peak power at different modulating amplitude Am with a fixed repetition frequency fr = 250 kHz and a pump power Pp = 225 mW. The pulse width continuously reduced with a gradually slow speed, as the increasing of the signal modulating amplitude Am. And the minimum pulse width of 375 ns was acquired at the modulating amplitude Am of 8 V. Owing to the pulse narrowing and the almost invariant average power, the peak power monotonously increased to obtain a maximum value of 0.23 W.
For many actively Q-switched fiber lasers, the pump power is another important factor to affect the pulse performance [32–34]. The pulse width and the average power were measured at different pump powers from 140 to 279 mW as shown in Fig. 5(a). It was revealed that the pulse width monotonously reduced, and the narrowest pulse width of 345 ns was obtained with 279 mW pump power. The average power linearly increased along with the boost of the pump power. Due to the reducing pulse width and the increasing average power, rapid enhancement in peak power realized and the maximum peak power reached 0.32 W at the pump power of 279 mW. Figure 5(b) displays the traces of pulsing trains of this actively Q-switched fiber laser under different pump powers recorded by a high-speed InGaAs photodetector and a 1 GHz bandwidth oscilloscope. It could find that the Q-switched pulse trains were stable without obvious amplitude variation and timing jitter, which means that the Q-switched performance was not deteriorated by the enhancing of the pump power.
In order to characterize the stability of this Q-switched fiber laser, Q-switched pulse trains with a time of 400 μs was measured, as displayed in Fig. 6(a). The Q-switched pulses were stable and the pulse-to-pulse intensity fluctuation was estimated to be less than 3.8%. In addition, as a key parameter of the Q-switched pulse for the practical applications, the long-term average power stability of this Q-switched fiber laser was tested. As shown in Fig. 6(b), the laser average power instability, which is half the value yielded by dividing the maximum power fluctuation with the average power, was ± 1.06% in 8 hours.
The optical spectrum of this actively Q-switched fiber laser with the pump power of 279 mW was measured by an optical spectrum analyzer with a spectrum resolution of 0.1 nm and a span of 5 nm . A sharp unimodal with an optical signal-to-noise ratio of >72 dB is observed in Fig. 7(a). When the actively Q-switched fiber laser was working at 279 mW pump power, 250 kHz repetition frequency and 5 V modulating amplitude, the single-longitudinal-mode property was verified by a scanning Fabry-Perot interferometer (FPI) with a resolution of 7.5 MHz and a free spectral range (FSR) of 1.5 GHz. From the inset of Fig. 7(a), it was observed that this fiber laser operated in single-longitudinal-mode status without mode-hop and mode competition phenomena . The linewidth of the Q-switched laser can be estimated from the envelope of the pulse train in the scanning Fabry-Perot signal yields [11,15]. A detailed enlarged drawing of the result from the FPI is displayed in Fig. 7(b), and the linewidth of this Q-switched fiber laser was measured to be 4.2 MHz, which was limited by the resolution of the FPI. Additionally, the linewidth of CW laser (without self-injecting polarization modulation) was tested by self-heterodyne method . As shown in the inset of Fig. 7(b), the 20 dB spectrum width was estimated to be 43.1 kHz, indicating the laser linewidth was less than 2.16 kHz. By contrast, the linewidth of the Q-switched fiber laser was remarkably broadened, which can be considered from the aspect of time-bandwidth product in the transform limit case. That is, the relation ΔtΔv ≥0.441 holds for Gaussian-shape pulse, where Δt is pulse width and Δv is its linewidth . It leads to a minimum 1.28 MHz bandwidth for the 345 ns-long-pulse, accounting for a significant broadening of linewidth to MHz level.
In conclusion, we demonstrated a novel technique to implement stable single-frequency Q-switched operation by virtue of self-injecting polarization modulation, for the first time to the best of our knowledge. The Q-switched mechanisms included two aspects. Firstly, periodic stress-induced birefringence change introduced by a high-speed fiber stretcher in the feedback loop resulted in the relevant polarization evolution. Secondly, a designed DBR resonant cavity with polarizations loss anisotropy turned the evolved polarization to intensity discrimination, and finally Q-switched performance was achieved. In experiment, we found that uniform Q-switched pulses require a certain threshold of modulating amplitude. Thus, the repetition frequency (a few hundred kHz), albeit tunable, was discrete owing to discrepant frequency response of the fiber stretcher. The pulse width, in the scale of hundred ns, reduced as the increasing in the modulating frequency, the modulating amplitude, and the pump power. The pulse-to-pulse intensity fluctuation was less than 3.8%, and the laser average power instability was ± 1.06% in 8 hours. In addition, the optical signal-to-noise ratio of this Q-switched fiber laser was over 72 dB. This single-frequency Q-switched fiber laser at 1.5 μm is promising in many advanced applications such as optical time-domain reflectometry, coherent Doppler wind radar, and optical coherent detection. Most importantly, this novel Q-switched method is readily applied to other DBR single-frequency fiber lasers.
National Natural Science Foundation of China (NSFC) (11674103, 61535014, 61635004); Major Program of the National Natural Science Foundation of China (61790582); National Key Research and Development Program of China (2017YFF0104602); Fundamental Research Funds for Central Universities (2017BQ002); Guangdong Natural Science Foundation (2016A030310410, 2017A030310007); Science and Technology Project of Guangdong (2014B050505007, 2015B090926010, 2016B090925004, 2017B090911005); Science and Technology Project of Guangzhou (201804020028).
References and links
1. F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, “26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality,” Opt. Lett. 37(6), 1073–1075 (2012). [CrossRef] [PubMed]
2. H. Zhang, X. Wang, P. Zhou, Z. Gong, and X. Xu, “6 mJ, high-average-power, all-fiberized Q-switched fiber master oscillator power amplifier with low repetition rate,” Appl. Opt. 51(29), 6933–6936 (2012). [CrossRef] [PubMed]
4. S. Adachi and Y. Koyamada, “Analysis and design of Q-switched erbium-doped fiber lasers and their application to OTDR,” J. Lightwave Technol. 20(8), 1506–1511 (2002). [CrossRef]
5. Y. Liu, J. Liu, and W. Chen, “Eye-safe, single-frequency pulsed all-fiber laser for Doppler wind lidar,” Chin. Opt. Lett. 9(9), 090604 (2011). [CrossRef]
6. F. Peng, H. Wu, X.-H. Jia, Y.-J. Rao, Z.-N. Wang, and Z.-P. Peng, “Ultra-long high-sensitivity Φ-OTDR for high spatial resolution intrusion detection of pipelines,” Opt. Express 22(11), 13804–13810 (2014). [CrossRef] [PubMed]
7. W. Shi, M. Leigh, J. Zong, and S. Jiang, “Single-frequency terahertz source pumped by Q-switched fiber lasers based on difference-frequency generation in GaSe crystal,” Opt. Lett. 32(8), 949–951 (2007). [CrossRef] [PubMed]
9. F. Stutzki, F. Jansen, C. Jauregui, J. Limpert, and A. Tünnermann, “2.4 mJ, 33 W Q-switched Tm-doped fiber laser with near diffraction-limited beam quality,” Opt. Lett. 38(2), 97–99 (2013). [CrossRef] [PubMed]
10. J. Kerttula, V. Filippov, Y. Chamorovskii, K. Golant, and O. G. Okhotnikov, “Actively Q-switched 1.6-mJ tapered double-clad ytterbium-doped fiber laser,” Opt. Express 18(18), 18543–18549 (2010). [CrossRef] [PubMed]
11. Y. Kaneda, Y. Hu, C. Spiegelberg, J. Geng, and S. Jiang, “Single-frequency, all-fiber Q-switched laser at 1550 nm,” in Proc. OSA Topical Meeting Adv. Solid-State Photon. (2004), p. 126. [CrossRef]
13. Y. Zhang, Z. Feng, S. Xu, S. Mo, C. Yang, C. Li, J. Gan, D. Chen, and Z. Yang, “Compact frequency-modulation Q-switched single-frequency fiber laser at 1083 nm,” J. Opt. 17(12), 125705 (2015). [CrossRef]
14. R. Zhou, W. Shi, E. Petersen, A. Chavez-Pirson, M. Stephen, and N. Peyghambarian, “Transform-limited, injection seeded, Q-switched, ring cavity fiber laser,” J. Lightwave Technol. 30(16), 2589–2595 (2012). [CrossRef]
15. W. Li, H. Liu, J. Zhang, H. Long, S. Feng, and Q. Mao, “Q-switched fiber laser based on an acousto-optic modulator with injection seeding technique,” Appl. Opt. 55(17), 4584–4588 (2016). [CrossRef] [PubMed]
16. W. Li, H. Liu, J. Zhang, B. Yao, S. Feng, L. Wei, and Q. Mao, “Mode-hopping-free single-longitudinal-mode actively Q-switched ring cavity fiber laser with an injection seeding technique,” IEEE Photonics J. 9(1), 1–7 (2017).
17. Y. Zhang, C. Yang, C. Li, Z. Feng, S. Xu, H. Deng, and Z. Yang, “Linearly frequency-modulated pulsed single-frequency fiber laser at 1083 nm,” Opt. Express 24(4), 3162–3167 (2016). [CrossRef] [PubMed]
18. Y. Zhang, S. Wang, W. Lin, S. Mo, Q. Zhao, C. Yang, Z. Feng, H. Deng, M. Peng, Z. Yang, and S. Xu, “Compact passively Q-switched single-frequency Er3+/Yb3+ codoped phosphate fiber laser,” Appl. Phys. Express 10(5), 052502 (2017). [CrossRef]
19. S. H. Xu, Z. M. Yang, T. Liu, W. N. Zhang, Z. M. Feng, Q. Y. Zhang, and Z. H. Jiang, “An efficient compact 300 mW narrow-linewidth single frequency fiber laser at 1.5 µm,” Opt. Express 18(2), 1249–1254 (2010). [CrossRef] [PubMed]
20. S. Mo, Z. Feng, S. Xu, W. Zhang, D. Chen, T. Yang, W. Fan, C. Li, C. Yang, and Z. Yang, “Microwave signal generation from a dual-wavelength single-frequency highly Er3+/Yb3+ co-doped phosphate fiber laser,” IEEE Photonics J. 5(6), 5502306 (2013). [CrossRef]
21. Q. Zhao, Y. Zhang, W. Lin, Z. Wu, C. Li, C. Yang, Y. Zhang, Z. Feng, M. Peng, H. Deng, Z. Yang, and S. Xu, “Frequency noise of distributed Bragg reflector single-frequency fiber laser,” Opt. Express 25(11), 12601–12610 (2017). [CrossRef] [PubMed]
22. S.-H. Xu, Z.-M. Yang, Z.-M. Feng, Q.-Y. Zhang, Z.-H. Jiang, and W.-C. Xu, “Efficient fibre amplifiers based on a highly Er3+/Yb3+ codoped phosphate glass-fibre,” Chin. Phys. Lett. 26(4), 047806 (2009). [CrossRef]
23. S. Mo, S. Xu, X. Huang, W. Zhang, Z. Feng, D. Chen, T. Yang, and Z. Yang, “A 1014 nm linearly polarized low noise narrow-linewidth single-frequency fiber laser,” Opt. Express 21(10), 12419–12423 (2013). [CrossRef] [PubMed]
24. K. Zhou, Q. Zhao, X. Huang, C. Yang, C. Li, E. Zhou, X. Xu, K. K. Y. Wong, H. Cheng, J. Gan, Z. Feng, M. Peng, Z. Yang, and S. Xu, “kHz-order linewidth controllable 1550 nm single-frequency fiber laser for coherent optical communication,” Opt. Express 25(17), 19752–19759 (2017). [CrossRef] [PubMed]
25. C. Li, S. Xu, X. Huang, Z. Feng, C. Yang, K. Zhou, J. Gan, and Z. Yang, “High-speed frequency modulated low-noise single-frequency fiber laser,” IEEE Photonics Technol. Lett. 28(15), 1692–1695 (2016). [CrossRef]
26. Q. Zhao, S. Xu, K. Zhou, C. Yang, C. Li, Z. Feng, M. Peng, H. Deng, and Z. Yang, “Broad-bandwidth near-shot-noise-limited intensity noise suppression of a single-frequency fiber laser,” Opt. Lett. 41(7), 1333–1335 (2016). [CrossRef] [PubMed]
27. Q. Zhao, K. Zhou, Z. Wu, C. Yang, Z. Feng, H. Cheng, J. Gan, M. Peng, Z. Yang, and S. Xu, “Near quantum-noise limited and absolute frequency stabilized 1083 nm single-frequency fiber laser,” Opt. Lett. 43(1), 42–45 (2018). [CrossRef] [PubMed]
30. H. Wan, Z. Wu, and X. Sun, “A pulsed single-longitudinal-mode fiber laser based on gain control of pulse-injection-locked cavity,” Opt. Laser Technol. 48, 167–170 (2013). [CrossRef]
31. Q. Zhao, Z. Zhang, B. Wu, T. Tan, C. Yang, J. Gan, H. Cheng, Z. Feng, M. Peng, Z. Yang, and S. Xu, “Noise-sidebands-free and ultra-low-RIN 1.5 μm single-frequency fiber laser towards coherent optical detection,” Photon. Res. 6(4), 326–331 (2018). [CrossRef]
32. Q. Fang, W. Shi, X. Tian, B. Wang, J. Yao, and N. Peyghambarian, “978 nm single frequency actively Q-switched all fiber laser,” IEEE Photonics Technol. Lett. 26(9), 874–876 (2014). [CrossRef]
34. M. Delgado-Pinar, D. Zalvidea, A. Diez, P. Pérez-Millán, and M. Andrés, “Q-switching of an all-fiber laser by acousto-optic modulation of a fiber Bragg grating,” Opt. Express 14(3), 1106–1112 (2006). [CrossRef] [PubMed]
35. C. Yang, S. Xu, S. Mo, C. Li, Z. Feng, D. Chen, Z. Yang, and Z. Jiang, “10.9 W kHz-linewidth one-stage all-fiber linearly-polarized MOPA laser at 1560 nm,” Opt. Express 21(10), 12546–12551 (2013). [CrossRef] [PubMed]
37. M. Nakazawa, K. Suzuki, and Y. Kimura, “Transform-limited pulse generation in the gigahertz region from a gain-switched distributed-feedback laser diode using spectral windowing,” Opt. Lett. 15(12), 715–717 (1990). [CrossRef] [PubMed]