高功率光纤激光器中增益辅助四波混频效应机理与抑制方法研究

Gain-assisted four-wave mixing effect is the main power scaling limitation of high-power two-(or multi-) wavelength fiber laser systems. Assisted by other kinds of optical gain, four-wave mixing (FWM) effect can be obviously amplified, even the phase-matching condition does not meet, which is a new phenomenon in high power laser systems. However, there is still no specific theoretical study on FWM in high power multi-wavelength laser systems. And the experimental reports mainly focus on the observations of this effect. The influence factors and suppression methods have not been studied. For this reason, the present project would like to study the dynamic of FWM effect that assisted by ytterbium gain and Raman gain. The contents including the following several aspects: proposing a bi-direction propagating model based on nonlinear Schrödinger equation and rate equations, in which FWM effect, Raman gain, self-phase modulation, cross-phase modulation, and ytterbium gain are taken into account. Analyzing the influence of different kind of gain, phase-matching condition, and polarization states on the dynamic of FWM experimentally and theoretically. Exploring the suppression method of FWM effect by controlling the assisted gain, the polarization states, and the phase mismatch. By this project, we want to provide some new methods to increase the output power of multi-wavelength fiber laser systems.

增益辅助四波混频效应是高功率双(多)波长光纤激光系统功率提升的重要限制因素。在其他类型增益的辅助下，即使相位失配较大，也能产生明显的四波混频效应，这是传统问题在高功率情况下的新表现。然而，目前该现象仍缺乏针对性的理论描述，实验报道也仅停留在现象观察层面，对于影响因素、抑制方法的研究十分缺乏。鉴于此，本项目以高功率光纤系统中的四波混频效应为研究对象，针对存在拉曼增益、镱离子增益的情况，深入研究各激光成分功率演化的物理过程。具体包括：基于非线性薛定谔方程和速率方程建立综合考虑四波混频、受激拉曼散射、自相位调制、交叉相位调制、镱离子增益等因素的双向传输理论模型；从理论和实验上研究不同增益条件、相位匹配情况以及偏振特性对四波混频效应的影响；在此基础上，从增益特性调控、相位匹配条件控制、偏振态控制等方面探索抑制四波混频效应的方法，为高功率多波长光纤激光系统所遇到的实际问题提供新的解决方案。

增益辅助四波混频效应是高功率双(多)波长光纤激光系统功率提升的重要限制因素，也是影响高功率光纤激光系统中影响光束质量的重要因素。针对该物理效应该缺乏系统的研究现状，本项目对其展开详细的理论与实验研究。建立和完善了考虑增益辅助四波混频效应的物理模型，并增加了对模式的分析能力，拓展研究了在拉曼等非线性效应影响下的模式演化特性，为后续研究模式的非线性演化奠定了研究基础。研究了增益辅助四波混频的抑制方法，基于全开腔首次获得了千瓦以上的随机光纤激光器，理论和实验证实了全开放腔也能获得高功率单向输出，同时有效抑制了四波混频和增益竞争，为高功率随机光纤激光器的研究打开了新的研究方向。同时基于对非线性效应的研究，拓展了光纤激光器在低温状态下正常工作的能力、研究了受激拉曼散射对单模高峰值功率激光获得的影响等，进一步丰富了光纤激光器在非线性效应上的研究内容。