基于光纤中SBS相位共轭的自调Q光纤激光器

A Self-Q-switched Fiber Laser Based on SBS Phase Conjugation

分析了利用光纤中的受激布里渊散射(stimulated Brillouin scattering,SBS)相位共轭效应进行激光腔内调Q,产生ns量级脉冲激光的原理,并对利用该效应产生的激光脉冲波形和脉冲形成过程进行了数值模拟,得到的脉冲波形与SBS相位共轭反射率随时间变化曲线基本一致,表明利用光纤中的SBS相位共轭作用调Q具有可行性。据此,对采用单模光纤(single-mode fiber,SMF)作为SBS池的Er3+掺杂调Q光纤激光器进行实验研究。当单模光纤长度为1.5 m时,在45 mW的光抽运功率下得到脉宽约2.6 ns,脉冲周期58.23 ns,平均功率7.35 mW的激光脉冲。进一步的研究表明:激光器中相位共轭镜的形成与SBS介质长度有关,SBS介质过长,斯托克斯线之间无固定的相位关系,不能形成相干叠加,SBS相位共轭腔不能形成;SBS介质过短,腔内正交偏振模光子寿命的改变使脉冲出现双峰现象。脉冲形成后其属性只与SBS动态属性有关。

Self-Q switched fiber lasers utilizing the mechanism of stimulated Brillouin scattering （SBS） can increase the output power by several magnitudes and the scale of high output power makes them to be very attractive laser sources for applications in many technological areas, such as telecommunication, material processing, laser surgical, range finding and remote sensing. In the presented work, the generation of pulse laser exploiting SBS phase conjugation in fiber was investigated theoretically and experimentally, and how the length of SBS medium affected the attribute of laser output was paid prior attention in our study. Numerical simulation was performed to study the waveform and the sequence of laser pulse. The waveform obtained by numerical computation was coincident with the curve described the relationship between reflectivity of SBS phase conjugation mirror and the time. The results show that changing Q value in resonant cavity by utilizing SBS phase conjugation is feasible. To investigate the attribute of laser output at different length of SBS medium, an all-fiber Er-doped Q-switched laser was designed, in which a 974 nm laser diode was used as pump source, Er-doped fiber （EDF） with length of 3 m was used as gain medium, a fiber Bragg grating （FBG） was used as fixed cavity mirror and standard single-mode fiber （SMF） G. 652 with length of 2 m, 1.5 m and 1 m were used as SBS pool individually. At the same time, an optical spectrum analyzer （ANDO6319） , a digital oscilloscope with 5 Gs/s （Tektronix TDS3052 ） and a optical power meter （ANDO AQ2140） were used to monitor the optical spectrum, temporal pulse shape and average power simultaneously. The high-speed photodetector designed by ourselves broadened the pulse duration by 20 times so as to reduce the damage to the apparatus caused by the high peak power of the laser pulses. When the length of SMF was 1.5 m, a train of pulse laser was generated with pulse duration of 2.6 ns, pulse period of 58.23 ns, average output power of 7.35 mW at pump power of 45 mW. Decreasing the length of SMF to 1 m, remarkable pulse laser with dual-peaks was obtained and the pulse period of it was 26.47 ns. Increasing the length of SMF to 2 m, only irregular stochastic pulsations were seen. Experiment data indicate that the length of SBS medium is vital to the formation of phase conjugation mirror. If it is too long, pulse laser can＇t be generated for the lack of fixed conjugation relation between Stokes lines, on the contrary, dual-peaks laser pulses occur for the change of photon lifetime in orthogonal polarized modes. Once the laser pulses are generated, the characteristics of laser pulses are only correlated with the stochastic characteristics of SBS and the pulse form maintain changelessness as the increase of the pump power. By means of these obtained experimental results, we can select proper length of SBS medium to yield perfect output pulse shapes, which enable this kind of self Q-switched fiber lasers to be more practical. In addition,enhanced stabilization of the pulsation can be achieved by modulating the pump power to be in resonance with the repetition rate.