高亮度窄线宽、线偏振光纤激光空频域演化机理研究与控制

High brightness narrow-linewidth, linear-polarized fiber laser is strongly required in regimes of high power fiber laser coherent beam combining, nonlinear frequency conversion, gravitational wave detection and so forth. This project is aimed to systematically study the spatial and spectral evolution mechanism and control in high brightness narrow-linewidth, linear-polarized fiber laser. In theory, the spectral, polarization, and mode evolution mechanisms will be investigated by comprehensively considering the influence of multi nonlinear effects, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM), four-wave mixing (FWM), thermal-induced Rayleigh scattering effect and so forth. The new techniques to suppress nonlinear effects such as SBS, SRS, and thermal-induced Rayleigh scattering, will be proposed. The techniques to narrow and control spectral linewidth, optimize polarization purity and stability will be proposed. In experiment, the feasibility of proposed techniques to maintain or narrow spectral linewidth, optimize polarization purity and stability, and suppress nonlinear effects will be systematically validated. The ultimate aim is to reveal the spatial and spectral evolution mechanism in high brightness narrow-linewidth, linear-polarized fiber laser, and propose feasible techniques to control the spatial and spectral properties, which will provide a reliable reference both in theory and technique for further brightness scaling, and spatial and spectral optimizing of narrow-linewidth, linear-polarized fiber laser system.

高亮度窄线宽、线偏振光纤激光在大功率相干合成、非线性频率变换、引力波探测等领域具有强烈的应用需求。本项目拟对高亮度窄线宽、线偏振光纤激光空频域演化机理与控制方法开展系统研究。理论上，将综合考虑受激布里渊散射、受激拉曼散射、自相位调制、四波混频、热致瑞利散射等多种非线性效应，研究窄线宽、线偏振光纤激光中的模式、光谱与偏振演化机理；将探索提出有效抑制受激布里渊散射、受激拉曼散射、热致瑞利散射等非线性效应的新方法；将探索提出窄化和控制光谱线宽、优化偏振纯度和偏振稳定性的新方法。实验上，将系统论证光谱保持与窄化、偏振纯度和偏振稳定性优化、非线性效应抑制等新方法用于高亮度窄线宽、线偏振光纤激光系统的可行性。最终揭示高亮度窄线宽、线偏振光纤激光空频域演化的内在机理，提出切实可行的空频域控制方法，为窄线宽、线偏振光纤激光的进一步亮度提升和空频域性能优化提供可靠的理论依据和技术支撑。

大功率窄线宽光纤激光在空间光通信、引力波探测等前沿领域和高能激光、光电对抗等国家安全领域均有重要应用需求。然而，受限于非线性效应和热效应的双重制约，大功率窄线宽光纤激光的整体性能提升面临严峻挑战。. 本项目以高亮度窄线宽、线偏振光纤激光为对象，针对空（模式、偏振）频（光谱）域开展了较系统研究，主要基础研究成果包括：（1）提出了光纤激光时频特性演化的普适性分析方法；（2）论证了种子激光时频特性和放大器结构参数对光纤放大器中受激拉曼散射、光谱展宽、四波混频、自相位调制等的影响规律和物理根源；（3）阐明了受激拉曼散射效应引起模式耦合的物理机制；（4）阐明时域特性对特殊波段窄线宽光纤激光输出特性的影响；（5）研究了不同调制信号对受激布里渊散射效应抑制的物理过程，提出适用于相干合成系统的光谱相干性评价方法。. 提出窄线宽、线偏振光纤激光亮度提升和空频域性能优化新方法，主要包括：（1）综合考虑受激拉曼散射、四波混频、自相位调制等，给出了适合高功率窄线宽线偏振放大的最佳主振荡源产生方法；（2）提出基于纯光学方法实现时频特性表征的新技术；（3）提出基于多相相位编码调制实现光谱线宽调控的新颖技术手段；验证了采用增益竞争、高增益光纤实现高效受激布里渊散射抑制和线宽压窄的可行性；（4）验证了采用锥形光纤实现受激布里渊散射、受激拉曼散射等非线性效应抑制的方法；（5）针对模式不稳定，提出增益分布优化和热效应分布调控的抑制新技术；（6）针对特殊波长窄线宽放大，提出采用时域稳定泵浦技术的线宽保持方法；（7）针对偏振纯度优化，提出基于均方根传播算法的偏振控制新方法；（8）给出了兼顾多重非线性效应和模式不稳定的主放大器设计准则；（9）实现国际公开报道的最高功率全光纤单频光纤激光器、国际公开报道的最高功率窄线宽光纤光源。. 项目整体研究结果为窄线宽、线偏振光纤激光的综合性能优化奠定了较全面的理论依据和技术支撑。