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反向保偏光纤耦合器技术方案采用侧面熔融耦合技术[13-17],反向保偏光纤耦合器输入端由N根多模光纤反向注入抽运激光,抽运激光通过高能偏振光纤激光器光路系统的种子源放大后,由输入端N根多模光纤熔融耦合端的中心信号光纤正向输出高能偏振激光,其结构如图 1所示。研制过程中输入输出信号光纤均采用大芯径无源保偏光纤(polarization-maintaining passive(Ge) double-cald fiber,PMGDF),当光纤束熔融过程中,图 1中光纤束由位置2过渡到位置1,所形成的光纤束熔融过渡区域的大芯径保偏光纤应力区在熔融过程中很容易发生微变形,继而破坏光纤束中心信号保偏光纤应力区波导结构;当高能偏振光纤激光器输出偏振激光功率指标至3 kW以上,经反向保偏光纤耦合器输出的信号,其偏振态稳定性畸变得更加明显,高能偏振光纤激光器消光比变小,继而影响多路高能偏振激光合成效率[18-19]。
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反向保偏光纤耦合器的反向消光比是指信号光源(signal light source, SLS)顺次通过准直透镜(collimation lens, CL)、λ/2玻片、检偏镜(polarization analyzer, PA)和扩束镜(beam expander, BE),得到输出激光的最大值和最小值,测试光路如图 2所示。按(1)式计算出反向保偏光纤耦合器的反向消光比,反向消光比是正值,单位为dB。计算公式定义如下:
$ \eta=\lg \left(P_{\max } / P_{\text {min }}\right) $
(1) 式中,Pmax是反向保偏光纤耦合器输出端调整λ/2玻片和检偏镜,而扩束镜位置状态保持不动,通过激光功率计(optical power meter, OPM)获得的偏振激光最大值; Pmin是反向保偏光纤耦合器输出端调整λ/2玻片,而检偏镜和扩束镜位置状态保持不动,获得的偏振激光最小值[20]。检偏镜反射输出的激光通过功率吸收体(safty equipment, SE)进行激光安全保护处理。反向保偏光纤耦合器的反向消光比越大,其偏振特性就越好。由此在高能偏振光纤激光器产品方案设计时,应采用反向消光比大的反向保偏光纤耦合器。目前光器件制造商针对反向保偏光纤耦合器的产品非常少,同样产品偏振特性指标参数更为少见,在高能偏振光纤激光器方案设计和研制时无法了解光耦合器件的偏振特性,继而对反向保偏光纤耦合器的反向消光比概念定义研究意义重大。反向保偏光纤耦合器反向消光比的研制物理变量有:光纤束信号保偏光纤直径、信号保偏光纤应力区物理结构变化。
反向保偏光纤耦合器偏振特性研究
Research of polarization characteristics of reverse polarization-maintaining optical fiber coupler
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摘要: 为了提升高能偏振光纤激光器输出激光偏振态稳定性, 通过阐述反向保偏光纤耦合器反向消光比基本原理, 采用信号光源与(6+1)×1反向保偏光纤耦合器研制相结合, 取得了反向保偏光纤耦合器信号保偏光纤直径、信号保偏光纤应力区物理变化等因素和反向保偏光纤耦合器反向消光比的关系。结果表明, 信号保偏光纤直径越小, 输出偏振激光的偏振态越稳定, 反向消光比大于49 dB, 同时促进反向保偏光纤耦合器抽运光纤臂耦合效率提升至98%以上; 正向偏振激光输出光纤应力区物理结构变化越明显, 经反向保偏光纤耦合器反向输出偏振激光的偏振态越不稳定。该研究可为制备高消光比、高能偏振光纤激光器提供参考。Abstract: In order to improve the output laser polarization stability of high-energy polarization fiber lasers, the signal light source and (6+1)×1 reverse polarization-maintaining optical fiber coupler were combined by introducing the basic principle of reverse extinction ratio of reverse polarization-maintaining optical fiber coupler. The influence of signal polarization-maintaining optical fiber diameter and physical change of the stress area of the reverse polarization-maintaining optical fiber on the reverse extinction ratio of a reverse polarization-maintaining optical fiber coupler was studied. The results show that the smaller the diameter of the signal polarization-maintaining fiber, the more stable the polarization state of the output polarized laser, and the reverse extinction ratio is greater than 49 dB. At the same time, the coupling efficiency of the pumping fiber arm of the reverse polarization-maintaining fiber coupler is improved to more than 98%. The more serious the change of the physical structure of the stress region of the forward polarization laser output fiber, the less stable the polarization state of the output polarization laser. The research provides reference for the preparation of high extinction ratio high-energy polarization fiber lasers.
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