基于掺铥锁模光纤激光器的中红外光频梳

Optical frequency combs have significant applications in the areas of precision optical frequency measurement, optical arbitrary waveform generation, astronomic spectroscopy calibration, attosecond optics, and metrology. Up to now visible and near-infrared frequency combs have been matured. However, mid-infrared frequency comb is still being in the intensive research stage, which is mainly hindered by the low loss transmission spectral range of silica fiber used in frequency comb technology. Mid-infrared frequency comb has potential applications on molecular spectroscopy, atmosphere monitoring, and counter measurements, etc. Mid-infrared sources based on fluoride, chalcogenide, and tellurite fibers have been attracting much research interest internationally. Unfortunately, the research reports are only limited to the supercontinuum generation so far and there is no report on the mid-infrared frequency combs due to the lack of suitably coherent mid-infrared supercontinuum. After amplification and spectral expansion in chalcogenide fiber, the repetition rate fr and the envelop-carrier offset frequency fo of 2 micron mode-locked fiber laser will be locked to the standard microwave Rubidium clock in our project. We will study chalcogenide fiber of low positive dispersion at 2 micron and the generation of coherent octave-span supercontinuum in mid-infrared range, ~100 fs 2 micron mode-locked fiber laser and its amplification. We will realize the mid-infrared frequency comb with spectral range from 1.8 to 4.5 micron, the signal-to-noise ratio of fo larger than 30 dB at 300 kHz resolution bandwidth, and the accuracy of fr and fo in the order of 1E-11 per second.

光频梳在光频测量、光学任意波形产生、天文光谱定标、阿秒光学、计量学等领域具有重要应用，目前可见光和近红外光频梳技术已成熟，但是中红外光频梳仍处于研究阶段，这主要受限于光频梳中使用的二氧化硅扩谱介质的低损耗透光范围。中红外光频梳在分子光谱、大气监测、光电对抗等方面具有重要应用。尽管国际上对氟化物、硫化物、锑化物等红外光纤光源的研究非常活跃，但由于难以获得相干的中红外超连续谱，目前还未见中红外光频梳的报道。本项目拟采用2微米掺铥锁模光纤激光器，经放大和硫化物光纤扩谱后，实现激光器重频fr和脉冲包络与载频偏移频率fo与微波源铷钟的锁定。研究2微米正色散硫化物光纤及在中红外产生倍频程的相干的超连续谱、~100fs掺铥光纤激光器及其脉冲放大技术；利用自参考法获得信噪比优于30 dB（300 kHz分辨率）的fo信号，实现光谱从1.8微米至4.5微米，fr和fo的秒稳在1E-11量级的中红外光频梳。

中红外光频梳在分子光谱、大气监测、生物医疗和国防安全等众多领域具有重要应用。随着掺铥光纤激光器的发展，采用2微米锁模光纤激光器在中红外超连续谱光源研究中有重要地位，但是国际上对产生超连续谱的氟化物光纤的相关研究还不成熟，难以获得相干的中红外超连续谱。本项目采用2微米脉冲激光对基于光纤激光器的中红外超连续谱光源展开了全面系统的研究。研究内容主要包括： .1. 研究了基于各种锁模原理和不同腔结构的掺铥锁模光纤激光器，获得了基于石墨烯、非线性偏振旋转（NPR）、非线性放大环形镜（NALM）等不同锁模机制的2微米锁模脉冲输出。同时，为了产生基于锁模光纤激光器的稳定光频梳，需要深入研究锁模光纤激光器的非线性动力学过程以及其噪声特性，我们对掺铥锁模光纤激光器的谐波锁模、孤子分子、孤子爆炸、类噪声脉冲、光学巨波、耗散孤子共振等非线性动力学过程以及噪声的影响进行了深入研究，发表SCI期刊论文9篇。.2. 研究了2微米高峰值功率脉冲激光的产生技术。搭建了2微米混合锁模掺铥孤子光纤激光器及全光纤啁啾脉冲放大器。激光器输出中心波长1932.5 nm，重复频率66.84 MHz，输出平均功率2.7 mW。采用啁啾脉冲放大技术，将激光器输出脉冲先经过4.12 m的正色散光纤进行展宽，然后再经过双包层掺铥光纤放大系统对脉冲进行放大和压缩，获得了脉宽~515 fs，峰值功率~4.4 kW的单脉冲。.3. 采用孤子自频移的方法获得了峰值功率更高的单脉冲。搭建了一套简单可靠的产生1.6-2.2 μm可调谐飞秒光脉冲的全光纤激光系统。然后经过两级单包层掺铥光纤放大系统产生1.95 μm附近的高功率飞秒脉冲，获得了重频为25 MHz，平均功率150 mW，脉宽314 fs，脉冲能量6 nJ，峰值功率~15 kW的单脉冲。在氟化铟光纤中获得了1.8-2.3微米稳定的超连续谱输出，并展开了中红外超连续谱产生的数值研究。