高平均功率2～5微米波段超连续相干光源技术的基础问题研究

High average power coherent supercontinuum (SC) sources spanning from 2μm to 5μm have important applications in biomedicine, infrared physics, infrared countermeasures and laser chemistry. The widely used SC is generated from highly nonlinear optical fiber or photonic crystal fiber (PCF) pumped by fiber laser. Because of the small core diameter of fibers and unmatched pump laser, it is limited to achieve higher power. Based on our study of hundreds of watts 1μm-3μm SC source, the project will adopt the way to make gain amplification process and cascaded nonlinear effects occurr simultaneously in the fiber amplifier, and then directly output high efficient mid-infrared SC. An experiment platform for SC at 2μm-5μm with the power greater than 10W will be established. Our key research contents include: designing theory and standard of gain fiber for generating SC; the interaction between gain amplification process and nonlinear effects and mechanisms of SC generation in amplifier etc. These physics problems are to be solved: pump mechanism, pumping threshold, conversion efficiency and spectral broadening limit. As the existing researches on amplifiers particularly focus on the inhibition of nonlinear effects, no complete theory has already been established to study SC generation in fiber amplifiers, so our investigations can deepen the understanding of the nonlinear physical processes in amplifiers. At the same time, it will have great engineering potential for higher power SC because large core diameter gain fibers are used in the scheme.

2-5微米高平均功率超连续相干光源在生物医学、红外物理、红外对抗、激光化学等有重要用途。被广泛采用的超连续谱产生方式是激光器泵浦高非线性光纤，因纤芯径小或泵浦激光器难匹配，实现更高功率输出的前景受限。基于我们百瓦级1-3微米超连续谱光源研究，本项目拟采用增益放大过程与级联非线性效应同时发生在光纤放大器内，直接高效率输出中红外超连续谱。建立平均功率大于10W的2-5微米超连续谱原理实验平台。重点研究内容包括：适用于超连续谱产生的增益光纤设计理论与准则；增益放大过程与非线性效应的相互作用；光纤放大器中超连续谱产生机制等。解决的物理问题主要包括：泵浦机制、泵浦阈值、转换效率、光谱宽度极限等。现有光纤放大器研究工作重点关注有害非线性效应的抑制，尚无完善理论来分析放大器中超连续谱的形成，项目研究成果可加深对放大器中非线性物理过程的理解。因采用大芯径增益光纤，方案将具有更高效率、更高功率的工程潜力。

面向中红外相干激光光源的重要应用需求，项目围绕计划书提出的预期研究目标，先后开展了多项关键基础技术攻关，理论上提出了在光纤放大器中通过功率放大和非线性光谱展宽效应获得高功率2-2.5 μm超连续谱激光，并将其用于泵浦软玻璃氟化物ZBLAN光纤和硫系玻璃As2S3光纤，最终获得了高亮度光谱可调控的2-5 μm波段超连续谱激光光源。实验上，探索了灵活多样的2μm激光脉冲产生的技术手段；搭建了双包层结构的高增益宽带2μm波段掺铥光纤放大器；掌握了软玻璃光纤的后处理技术，实现了石英光纤和软玻璃光纤的低损耗高强度熔接技术；研制了10W量级全光纤结构的2-4.5μm超连续谱激光光源；使用硫系玻璃光纤进一步获得了高亮度光谱平坦型2-5μm中红外超连续谱激光光源。理论上，研究了软玻璃光纤和光纤放大器内部2-5 μm波段超连续谱激光产生的物理机理，发现了红外孤子脉冲相关的色散和非线性效应对超连续谱激光光谱拓展带来的积极作用，同时探清了大量红外孤子脉冲的成群效应可以带来超连续谱激光光谱的平坦化，为后续中远红外超连续谱激光光源研制和应用打好了坚实基础。