飞秒光纤激光脉冲基频光差频产生长波红外频率梳的研究

This research project is proposed to investigate the scientific and technical issues involved in the long-wave infrared frequency comb laser source obtained by nonlinear difference frequency generation (DFG) scheme with femtosecond mode-locked fiber laser pulse fundamental lights. By using the supercontinuum generation technique in all-normal dispersion and high nonlinearity photonic crystal fiber (PCF), the time-frequency characteristics of the femtosecond mode-locked fiber laser pulse can be regulated and controlled for the generation of the two color femtosecond pulse trains for the requirements of fundamental lights of DFG in long-wave infrared region. An efficient balanced amplification approach for the two color femtosecond pulse trains can be achieved by using the cascaded self-similar fiber amplifiers with interstage shaping and filtering elements, which can overcome the optical wave breaking, prevent the breakthrough of the gain bandwidth of normal fiber amplifiers due to the pulse spectral broadening, and suppress the nonlinear chirp. And, an effective way to suppress the noise accumulation and improve the signal to noise ratio of the DFG system can be found by investigating the behaviors and mechanism of both the nonlinear noise in the processes of supercontinuum generation in all-normal dispersion and high nonlinearity PCF, and, the nonlinear chirps, stimulated Raman shifted pulses, and ASE noises in 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 crystals based on the femtosecond mode-locked fiber laser. This project may not only open a new way to develop efficient infrared coherent laser source, supplying solutions to the "bottlenecks" obstructs hindering the field of current long-wave infrared spectroscopy applications, but also greatly promote the research activity progresses in the femtosecond fiber laser and the relative infrared laser frequency conversion technology based on fiber lasers. Thus executing this project plan holds remarkable significance in both scientific research and technological progress.

本项目研究以锁模光纤激光脉冲为基频光、经DFG变频获取光纤型长波红外频率梳光源中的关键科学技术问题。通过采用全正色散高非线性PCF超连续谱产生来调控飞秒光纤激光脉冲的时频特性、获取长波红外波段DFG光频率梳的双色基频飞秒脉冲；通过级联自相似光纤放大结合级间整形滤波方案克服光波破裂、防止脉冲频谱展宽对放大器增益带宽的突破、抑制非线性啁啾，实现对双色基频飞秒脉冲的高效均衡放大；通过抑制超连续谱产生中的非线性噪声和光纤放大中产生的非线性啁啾、受激Raman频移脉冲和ASE噪声，寻求降低系统噪声积累、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。本项研究将为发展光纤型高效红外相干激光光源、解决红外光谱分析等领域面临的光源瓶颈问题开辟一条新途径，并将推动飞秒光纤激光红外非线性变频技术的进步，具有极其重要的科学意义和实用价值。

中远红外波段集中了大量分子的基带吸收线，为满足物质结构与成分分析需要，人们正在大力发展中远红外波段的光梳光源。差频产生 (DFG) 技术易于获取宽带调谐的激光变频输出，且源于同一台锁模激光器的双色基频脉冲经光整流可自动地消除载波包络偏移频率，从而可大幅简化差频光的相位锁定，使得DFG产生中远红外光梳的方案倍受人们重视，这种光梳光源在高灵敏度光谱分析、大气环境检测等领域具有极其重要的应用。 .本项目开展了基于飞秒锁模光纤激光脉冲基频光的DFG红外光梳的研究。基于飞秒光纤激光脉冲的频谱调控技术，研究了不同类型锁模光纤激光脉冲的特性及其泵浦高非线性光纤产生的超连续谱，提出了满足长波红外DFG要求的双色基频飞秒脉冲提取方案；比对了自相似和啁啾脉冲放大 (CPA) 技术的优劣，进而采用级联CPA光纤放大结合级间隔离的方案，实现了对双色基频飞秒脉冲的高效放大；解决了双色基频飞秒脉冲同步问题，并实现了对脉冲偏振态的有效调控；研究了非线性晶体相位匹配与转换效率等特性，设计了适于中远红外DFG应用的GaSe晶体元件，经优化有效拓宽了相位匹配接受带宽。.以此为基础，探明了光纤激光DFG红外光梳的主要噪声来源，设计出有效锁定重复频率的PLL电子链路，对DFG光梳主要噪声源进行了有效抑制，最终设计和构建了一种全光纤型DFG红外光梳系统，该光梳在6-10 μm范围内可任意调谐，最大光谱带宽达1.3 μm。.本项研究的成果为发展高效红外相干激光光源、解决红外光谱分析等应用领域所面临的光源瓶颈问题开辟了一条新途径，并对飞秒光纤激光脉冲红外非线性变频技术的发展起到了推动作用，所设计研制的光纤型DFG光梳可望发展成小型化分析检测仪器，在分子精密光谱与大气环境检测等领域发挥重要作用。

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