外腔式高功率半导体激光器的偏振相干耦合研究

The semiconductor lasers have wide range of applications including optical pumping of solid state laser, material processing and so on. To improve the pumping efficiency and the quality of the material processing, the trends in the field of the semiconductor lasers are towards developing high beam quality, high output power and narrow spectrum. The high power coherent polarization locked semiconductor laser based on VBG, in which the emissions are combined into a single beam by configuration of birefringent crystal and half wave plates, are the formation of the feedback for the combined beam by VBG to achieve the laser output with narrow spectrum and phase locking. Based on this technique, high power output can be obtained by the multi-emitters coherent combination and the high quality beam and narrow output may also be done. In this project, we propose to realize the high power coherent polarization locked semiconductor laser based on VBG and investigate the related basic theory. The study includes: 1) developing the transmission and feedback of the multi-beams combination to obtain the high degree of the coherence and couple spatialiy; 2) Analyzing the multi-cavity resonance behaviors, and studying the mechanism of the intra-cavity mode competition and the inter-cavity mode coherence to stabilize the phase locking; 3) Constructing the feedback configuration based on VBG external cavity, and optimizing the parameter of the VBG external cavity to achieve the output laser with high beam quality and narrow bandwidth.

半导体激光器在材料加工、抽运固体介质等中已经具有了广泛的应用，实现半导体激光器的高输出功率、高光束质量和窄带宽输出对提高抽运效率、提高加工质量等有着非常重要的意义。外腔式高功率半导体激光器的偏振相干耦合是基于多子腔谐振相干反馈理论，采用双折射晶体和半波片实现多路半导体激光器偏振合束，利用体布拉格栅（VBG）对合束后的光束进行反馈，以达到窄光谱相干锁相输出。该系统的最大特点在于保证增加输出功率的情况下，多路半导体激光器能够通过在外腔中进行相干合束，实现高光束质量、窄带宽输出。本项目拟深入研究外腔式高功率半导体激光器的偏振相干耦合及其所涉及的基础理论。建立适于多光路合束传输反馈的理论体系，提高多光束空间相干耦合度。建立多子腔谐振理论，研究子腔内的模式竞争和子腔间的模式相干机理，实现稳定的相位锁定。构建VBG外腔反馈系统，优化VBG外腔参数，实现高功率、高光束质量、窄带宽输出。

基于双折射晶体和半波片的空间光束偏振态控制，建立了多单管和巴条的相干偏振耦合光路系统，实现了50W输出功率~0.3nm的窄带宽输出；建立了Smile对外腔反馈的物理模型，分析了不同封装材料和工作温度下半导体激光器的Smile的变化对外腔反馈的影响，揭示了高温工作条件下模式输出不稳定的物理机理；同时建立了测试精度能够达到0.1um的自动smile表征系统，并且发现当smile小于2um时，准直透镜的取向对输出稳定性和输出耦合效率影响较大；依据半导体激光器高斯近似输出光场的相位分布，通过阶梯式的设计结构特殊结构设计了慢轴准直镜，能将慢轴压缩至~10mrad，光学准直透镜来补偿smile的影响，有利于大大提高半导体激光器的准直度，从而提高反馈输出效率，提高了输出稳定性和输出耦合效率。避免了通常采用的非球面镜结构，加工方便。建立了光纤耦合半导体激光器三维远场表征系统，解决扫描路径和半导体激光器光源的空间传播特性之间的失配问题，提高半导体激光器远场强度表征的准确性；具有较高的光学动态范围，容易探测光纤输出高阶模的产生。