基于光纤激光的DFG红外频率梳光源关键问题的研究

This research project is proposed to investigate the fundamental issues involved in the difference frequency generation (DFG) optical frequency comb laser source in the infrared region based on the femtosecond mode-locked fiber laser, including how to generate synchronized two color femtosecond pulse trains with a single color pulse train exited from a fiber laser, how to amplify them evenly, and what the noise accumulation of the DFG laser system is like, etc. By studying the spectral envelop evolution and shift behavior and the corresponding controlling technique of the femtosecond laser pulse propagating in optical fibers, we can obtain the solution of the generation of the synchronized two color femtosecond pulse trains for the requirements of fundamental lights of DFG in 5-20μm region. Overcoming the limit of the gain bandwidth of normal fiber amplifiers for the balanced amplifications of the two color femtosecond pulses can be done through optimizing the optical amplification scheme combined with controlling the spectral envelop region of the two color femtosecond pulse trains; by suppressing the influence of the fiber nonlinearity on femtosecond pulse propagation in the fiber, an efficient amplification scheme for the two color femtosecond pulse trains can be achieved. And, an effective way to suppress the noise accumulation and improve the signal to noise ratio of the DFG system can be found when investigating the additional noise behaviors and mechanism in the processes of the spectral envelop evolution of the femtosecond pulses propagating in optical fiber and the amplification of the femtosecond pulses in the fiber amplifiers. Based on the above, finally, it is reasonable to propose an optimized design principle and developing technique for infrared region DFG optical frequency comb laser source with typical nonlinear crystal based on the femtosecond mode-locked fiber laser. This project may not only open a new way to develop infrared coherent laser source, supplying solutions to the "bottlenecks" obstructs hindering the field of current infrared spectroscopy applications, but also greatly enrich research activities of the femtosecond fiber laser and the infrared laser frequency conversion technology, which is very helpful for finding new applications. Thus executing this project plan holds remarkable significance in both scientific research and technological progress.

本项目研究飞秒光纤激光DFG红外频率梳中的基频同步双色飞秒脉冲产生、放大以及系统噪声等关键科学技术问题。通过研究飞秒激光脉冲的光纤非线性频谱搬移技术，获取能满足5-20微米波段DFG要求的同步双色基频飞秒脉冲产生的新方案；结合脉冲频谱可控搬移技术，设计与优化光纤放大方案，突破光纤放大器带宽限制，克服光纤非线性效应对脉冲放大的制约，解决同步双色基频飞秒脉冲的高效均衡放大的问题；通过研究飞秒脉冲的频谱搬移和光纤放大附加噪声产生机制，寻求抑制系统噪声积累、提高DFG输出信噪比的有效途径；以此为基础，解决基于典型红外非线性晶体的光纤激光DFG红外光频率梳的结构原理与设计方案等问题，掌握关键技术。本项研究不仅为发展红外相干激光光源、解决红外光谱技术领域所面临的光源瓶颈问题开辟一条新途径，还将大大丰富飞秒光纤激光和红外激光变频技术的研究内涵、拓宽其应用，在科学研究与技术进步方面都具有极其重要的意义。

本项目研究飞秒光纤激光DFG 红外频率梳中的关键科学技术问题。项目期间，通过表象变换，建立了一种同步更新迭代的频域求解耦合广义非线性薛定谔方程（C-GNLSE）的新算法；研究了被动锁模全正色散光纤激光器的输出行为，阐明了腔内色散非线性图景和光谱滤波带宽之间的关联，揭示了腔内色散分布对光纤激光器输出特性影响的新规律，在此基础上，研制了276MHz高重复频率耗散孤子光纤激光器，并进行了压缩与光纤放大，用负色散PCF产生出了可用作10-20微米红外波段DFG基频光源的倍频程超连续谱。针对飞秒脉冲DFG应用要求，设计与加工了GaSe非线性晶体元件。项目在飞秒光纤脉冲产生、放大与超连续谱产生等方面取得的进展，为发展飞秒光纤激光DFG 红外频率梳红外相干激光光源、推动飞秒光纤激光与红外激光变频的技术进步奠定了理论技术基础。