We propose a compact dual-wavelength Q-switched single-frequency fiber laser based on a 17-mm-long home-made highly Er3+/Yb3+ co-doped phosphate fiber (EYDPF) and a semiconductor saturable absorber mirror (SESAM). The short cavity length and a polarization-maintaining fiber Bragg grating (PM-FBG) ensure that only one longitudinal mode is supported by each reflection peak. The maximum pulse energy of more than 34.5 nJ was realized with the shortest pulse duration of 110.5 ns and the Q-switched fiber laser has a repetition rate reaching over 700 kHz with a temporal synchronization of pulses at two wavelengths. Besides, the optical signal-to-noise ratio (OSNR) of larger than 64.5 dB was achieved.
© 2016 Optical Society of America
Airborne differential Lidar system has been proven as a reliable and accurate apparatus to capture rupture of earthquake . To implement such a system, a dual-wavelength (DW) laser source with single-frequency operation and pulsing mode would be more robust as considering long coherent length and significant attenuation while through the detection process [2,3]. Besides, a compact size would make it ideal for integration in airborne system, where constraints on weight and bulk are often extreme.
However, to ensure a stable operation of this type of fiber laser, there are several issues need to be carefully addressed. As to pulsing operation, a straightforward method is to externally modulate continuous-wave laser by acoustic-optical modulation (AOM) or electric-optical modulation (EOM) [4–6]. Despite the merit of easily controllable to pulsing parameters, this method suffers from a significant attenuation to laser signal and several amplified stages are required for application. Q-switching is a suitable apparatus to obtain considerable output power and high-beam-quality laser, and it has attracted the research attention in the past two decades [7–11]. Compared to actively Q-switched fiber lasers which require an extra controller to generate modulating signal, passively Q-switched fiber lasers possess attractive advantages of compactness, simplicity, and flexibility in design [10, 11].
Secondly, a fundamental problems that haunts DW lasers is mode competition, which tends to destabilize the laser system. To overcome it, one of approach is to utilize polarization maintaining (PM) components, and lasers utilizing polarization-maintaining fiber Bragg gratings (PM-FBG) or PM gain fibers are believed to be stable for the divergence induced by the polarization hole burning effect [12, 13]. Besides, the phenomenon of multiple-longitudinal-mode oscillation would be caused by long cavity length for narrow longitudinal mode spacing [10, 11]. Thus, a mode-selecting mechanism should be employed to eliminate multiple longitudinal mode oscillation , but the integration of the laser turns out to be even more difficult. A laser source with a short cavity length corresponding to a large longitudinal mode spacing could ensure a stably single-longitudinal-mode (SLM) operation. The following problem is that the output power is limited with a shorter gain fiber in the cavity.
Thanks to the development of heavily rare earth (RE) ions doped multi-component glass fiber, the high gain phosphate fiber has brought about high power fiber lasers with very short cavities [15–19]. In this letter, we present a compact passively Q-switched DW-SLM fiber laser based on a self-developed 17-mm-long phosphate fiber and a semiconductor saturable absorber mirror (SESAM). A reliable DW-SLM pulsed fiber laser is achieved while the maximum pulse energy of more than 34.5 nJ was realized with the shortest pulse duration of 110.5 ns, and the wavelength spacing of the obtained dual lasing line is 0.37 nm.
2. Experimental setup
The schematic drawing of DW pulsed fiber laser is shown in Fig. 1. The two reflectors of laser cavity are composed of one PM-FBG and a SESAM. The PM-FBG as output coupler of the cavity was inscribed with a 3-dB bandwidth of 0.05 nm and a reflectivity of 50% at 1540 nm. A home-made 17-mm-long Er3+/Yb3+ co-doped phosphate fiber (EYDPF) were fusion-spliced to the PM-FBG, and this home-made gain fiber was drawn using a fiber-drawing tower through a phosphate glass preform fabricated by the rod-in-tube technique, more detail about this home-made phosphate fiber was discussed in . The SESAM, which has an absorbance of 4% at around 1540 nm and a modulation depth of 2.4%, is butt-coupled to the other end facet of EYDPF. The total resonant cavity was assembled into a copper tube, which is thermally controlled by a precision thermoelectric cooler with a resolution of 0.01°C. With a proper temperature control, a robustly single-frequency laser could be obtained without mode-hop and mode-competition phenomena. The DW-SLM fiber laser was pumped by a commercially available single-mode 980-nm laser diode (LD) through a 980/1550-nm wavelength division multiplexer (WDM). The WDM output was passed through an isolator (ISO) to prevent back reflections, which destabilized the fiber laser. The deliver fiber and optical device are polarization-maintaining components (both axis working) to maintain good stability of output laser.
3. Results and discussion
Since the different refractive index along the fast and slow axis of the fiber, the PM-FBG has two reflection peaks, one for each polarization. And the two reflection peaks are within the reflection bands of the well-design SESAM. The reflective spectrum of PM-FBG pumped by an amplitude spontaneous emission (ASE) light source in 1.5-μm region is shown in Fig. 2(a). It can be observed that the wavelength spacing of two reflective peaks of slow and fast axis is 0.37 nm. As shown in Fig. 2(b), the spectrum of DW fiber laser was measured by an optical spectrum analyzer (OSA) with the span of 5 nm and a resolution of 0.02 nm. The two lasing wavelengths are 1540.25 nm and 1540.62 nm respectively corresponding to the two reflection peak wavelengths of the PM-FBG. Besides, the optical signal-to-noise ratio (OSNR) of > 64.5 dB was obtained. The effective length of the cavity including 17-mm active fiber and a half of the 15-mm PM-FBG is only less than 25 mm, giving a longitudinal mode spacing of 4.0 GHz. Besides, the PM-FBG has a reflection bandwidth of less than 6.3 GHz, and it is clear that only one longitudinal mode is supported within the laser cavity for each reflection peak. The short cavity length and the PM-FBG ensure that only one longitudinal mode is supported by each reflection peak. Besides, the spectrums for each polarization were measured by using a polarization beam splitter (PBS) and OSA with a span of 4 nm in Fig. 2(c) and Fig. 2(d). It can be observed that only one lasing wavelength for each polarization. In addition, the single-frequency characteristics for each lasing wavelength were measured by a scanning Fabry–Pérot interferometer (FPI) with a resolution of 7.5 MHz and a free spectral range of 1.5 GHz, and the SLM operations were confirmed in the inset of Fig. 2(c) and Fig. 2(d).
Figure 3(a) demonstrates pulse duration and repetition rate as a function of pump power. It can be observed that the pulse duration was narrowed from 387 to 110.5 ns by increasing pump power. For passively Q-switched laser, pulse duration is related to cavity length and modulation depth of saturable absorber (SA) . We would expect to reduce pulse duration by using an SA with higher modulation depth because pulse duration is expected to be inversely proportional to the modulation depth of the SA . By increasing the pump power from 95.8 to 216.7 mW, the pulsing repetition rate could be varied over a wide range of frequencies from 216 to 728 kHz. The average power and pulse energy versus different pump powers are shown in Fig. 3(b). And the lasing threshold of laser pulse is around 95.8 mW as indicated in Fig. 3(b). When the pump power is above lasing threshold, the average power shows a trend of linear enhancement with increasing of pump power. And a maximum average power of more than 25.1 mW was obtained at the pump power of 216.7 mW. The single pulse energy, which could be calculated utilizing the measured average power and repetition rate, enhances linearly as the pump power increases. It is observed that the maximum pulse energy reaches 34.5 nJ at the pump power of 216.7 mW, and the calculated peak power is 0.29 W. There are some parameters that influence the peak power of passively Q-switched laser, as shown in the formula in ,Eq. (1), Ppeak is the peak power, Sp is the pulse shape factor, Ep is the pulse energy, τp is the pulse duration, ΔR is the modulation depth, A is the pumped area, TR is the round trip time of cavity, σL is emission cross section of the laser material, and ηoc is output coupling efficiency. To increase peak power, a large modulation depth is obviously important. Besides, a smaller round trip time and higher output coupling efficiency could also realize higher peak power. We expected that a higher peak power could be realized by further optimizing the cavity parameters (e.g. SESAM’s performance, cavity loss).
The pulsing shapes of the passively Q-switched laser with different pump powers are shown in Fig. 4(a). It can be observed that the single-pulse envelopes have symmetric Gaussian-like intensity profiles and no parasitic or multiple optical pulse exits. The narrowest pulse width of 110.5 ns was achieved with a pump power of 216.7 mW. As shown in Fig. 4(b), recorded traces of Q-switched pulse trains were measured by an InGaAs photodetector and a 100 MHz bandwidth oscilloscope. The stably pulsing trains with the different repetition rates were observed when the pump power was gradually increased from 132.3 to 216.7 mW. A simple express about the repetition rate is obtained for passively Q-switched laser in ,Fig. 5 with a pump power of 216.7 mW, and there is no obvious temporal delay between two wavelengths. It indicates that the two lasing wavelengths can be temporally synchronized in Q-switched process since the modulated parameters of SESAM are designed to have almost same values at two lasing wavelengths.
In conclusion, by utilizing a 17-mm-long EYDPF, a compact structure cavity with a SESAM and a PM-FBG has been presented to achieve a DW pulsed fiber laser in SLM operation. The achieved OSNR of output pulse was larger than 64.5 dB while the two lasing wavelengths are located at 1540.25 nm and 1540.62 nm. And the Q-switched pulses at two wavelengths can be temporally synchronized by using the well-designed SESAM. Moreover, the maximum pulse energy of more than 34.5 nJ was realized with the shortest pulse duration of 110.5 ns and the pulsing repetition rates ranged from 216 to 728 kHz by tuning the pump power. This type of compact DW pulsed fiber laser provides a promising candidate in airborne differential LIDAR system.
China State 863 Hi-tech Program (2014AA041902); National Natural Science Foundation of China (NSFC) (61535014,51132004, and 51302086); Fundamental Research Funds for Central Universities (2015ZP013 and 2015ZM091); Guangdong Natural Science Foundation (S2011030001349 and S20120011380); China National Funds for Distinguished Young Scientists (61325024); Science and Technology Project of Guangdong (2013B090500028, 2014B050505007); The cross and cooperative science and technology innovation team project of the CAS (2012-119), China.
References and Links
1. M. E. Oskin, J. R. Arrowsmith, A. Hinojosa Corona, A. J. Elliott, J. M. Fletcher, E. J. Fielding, P. O. Gold, J. J. Gonzalez Garcia, K. W. Hudnut, J. Liu-Zeng, and O. J. Teran, “Near-field deformation from the El Mayor-Cucapah earthquake revealed by differential LIDAR,” Science 335(6069), 702–705 (2012). [CrossRef] [PubMed]
2. X. He, X. Fang, C. Liao, D. N. Wang, and J. Sun, “A tunable and switchable single-longitudinal-mode dual-wavelength fiber laser with a simple linear cavity,” Opt. Express 17(24), 21773–21781 (2009). [CrossRef] [PubMed]
4. M. Leigh, W. Shi, J. Zong, Z. Yao, S. Jiang, and N. Peyghambarian, “High peak power single frequency pulses using a short polarization-maintaining phosphate glass fiber with a large core,” Appl. Phys. Lett. 92(18), 181108 (2008). [CrossRef]
5. R. Su, P. Zhou, H. Xiao, X. Wang, and X. Xu, “150 W high-average-power, single-frequency nanosecond fiber laser in strictly all-fiber format,” Appl. Opt. 51(16), 3655–3659 (2012). [CrossRef] [PubMed]
6. K. K. Chen, S. Alam, J. R. Hayes, H. J. Baker, D. Hall, R. Mc Bride, J. H. V. Price, D. Lin, A. Malinowski, and D. J. Richardson, “56-W frequency-doubled source at 530 nm pumped by a single-mode, single-polarization, picosecond, Yb3+-doped fiber MOPA,” IEEE Photonics Technol. Lett. 22(12), 893–895 (2010). [CrossRef]
8. A. Wang, H. Ming, J. Xie, X. Chen, L. Lv, W. Huang, and L. Xu, “Single-frequency Q-switched erbium-doped fiber ring laser by combination of a distributed Bragg reflector laser and a Mach-Zender interferometer,” Appl. Opt. 42(18), 3528–3530 (2003). [CrossRef] [PubMed]
9. 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]
11. Z. Luo, M. Zhou, J. Weng, G. Huang, H. Xu, C. Ye, and Z. Cai, “Graphene-based passively Q-switched dual-wavelength erbium-doped fiber laser,” Opt. Lett. 35(21), 3709–3711 (2010). [CrossRef] [PubMed]
12. D. Liu, N. Q. Ngo, X. Y. Dong, S. C. Tjin, and P. Shum, “A stable dual-wavelength fiber laser with tunable wavelength spacing using a polarization-maintaining linear cavity,” Appl. Phys. B 81(6), 807–811 (2005). [CrossRef]
13. S. Feng, O. Xu, S. Lu, X. Mao, T. Ning, and S. Jian, “Single-polarization, switchable dual-wavelength erbium-doped fiber laser with two polarization-maintaining fiber Bragg gratings,” Opt. Express 16(16), 11830–11835 (2008). [CrossRef] [PubMed]
14. Y. W. Song, S. A. Havstad, D. Starodubov, Y. Xie, and A. E. Feinberg, “40-nm-wide tunable fiber ring laser with single-mode operation using a highly stretchable FBG,” IEEE Photonics Technol. Lett. 13(11), 1167–1169 (2001). [CrossRef]
15. L. Xiong, P. Hofmann, A. Schülzgen, N. Peyghambarian, and J. Albert, “Short monolithic dual-wavelength single-longitudinal-mode DBR phosphate fiber laser,” Appl. Opt. 53(18), 3848–3853 (2014). [CrossRef] [PubMed]
16. 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]
17. 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 microm,” Opt. Express 18(2), 1249–1254 (2010). [CrossRef] [PubMed]
18. S. Xu, C. Li, W. Zhang, S. Mo, C. Yang, X. Wei, Z. Feng, Q. Qian, S. Shen, M. Peng, Q. Zhang, and Z. Yang, “Low noise single-frequency single-polarization ytterbium-doped phosphate fiber laser at 1083 nm,” Opt. Lett. 38(4), 501–503 (2013). [CrossRef] [PubMed]
19. 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]
20. G. J. Spühler, R. Paschotta, R. Fluck, B. Braun, M. Moser, G. Zhang, E. Gini, and U. Keller, “Experimentally confirmed design guidelines for passively Q-switched microchip lasers using semiconductor saturable absorbers,” J. Opt. Soc. Am. B 16(3), 376–388 (1999). [CrossRef]