宽带飞秒激光泵浦的中红外可调谐光参量放大特性研究

Ultrashort mid-infrared (mid-IR) pulsed laser has attracted numerous attentions around the world for its applications in ultrafast spectroscopy and attosecond science in the past decades. However, there are only few approaches for broadband mid-IR seed pulse generation, and the conversion efficiency is quite low, the obtained picojoule or nanojoule level pulses are not directly applicable in following researches. Following our previous theoretical proposal, a novel optical parametric amplifier (OPA) pumped by a broadband femtosecond laser is experimentally investigated in this program. A spectrally broadened Ti:sapphire laser is employed to pump the system, the initial broadband pump and seed pulses are stretched with opposite chirps in order to manipulate the relationship between their instantaneous wavelengths. As a result, broadband, CEP-stabilized, wavelength-tunable idler pulses are efficiently generated as the mid-IR seeding source even in slightly phase-mismatched conditions. Successive amplification of the broadband mid-IR pulse is realized through non-collinear phase-matching in a BIBO crystal, owing to its unique dispersion characteristics. Moreover, by further analyzing the broadband-pumped OPA process theoretically and experimentally, the approach of simultaneously optimizing the gain bandwidth and conversion efficiency, reducing the B-integral, and suppressing the quantum noise will be explored. In this way, CEP-stable few-cycle energy-scalable mid-IR laser pulses with wavelength tunability and good beam quality are generated.

中红外超短激光脉冲因其在超快光谱学及阿秒科学等领域的重要应用成为了近年来国内外的研究热点。然而，目前的宽带中红外种子脉冲产生手段较少，能量转换效率偏低，所得的皮焦或纳焦级脉冲无法直接应用于后续研究中。本项目拟在前期研究的基础上，开展宽带飞秒激光泵浦的中红外可调谐光参量放大特性研究。利用频谱展宽的钛宝石飞秒激光作为泵浦光，并向宽带泵浦光与种子光引入反方向的啁啾，通过调控泵浦光与种子光的瞬时频率对应关系，在非完美相位匹配条件下高效地产生CEP稳定的可调谐宽带闲频光作为中红外种子脉冲。在第二级中利用BIBO晶体的色散特性，实现对中红外闲频光的非共线双啁啾能量放大。通过对宽带泵浦光参量放大过程的进一步理论及实验分析，探索同时优化增益谱宽和转换效率、降低B积分以及抑制量子噪声的手段,从而提高输出脉冲的稳定性和光束质量，最终获得CEP稳定、周期量级脉宽、能量可拓展的中红外可调谐激光脉冲。

中红外飞秒激光脉冲因其超短的持续时间和极高的峰值强度在超快光谱学、阿秒科学、强激光物理等众多领域有着重要的应用，在近年来成为了国内外激光技术领域的热门研究方向之一。为了突破受到激光增益介质所限制的飞秒脉冲的波长范围，光参量放大器成为了有效地将其中心频率下转换的手段。在光参量放大过程中，如何拓宽增益带宽以及增大转换效率是提升输出脉冲性能、进一步拓展其应用领域的关键问题。本项目通过对光参量放大过程中的相位匹配条件及能量转换等特性进行深入的分析，提出了若干高效率产生周期量级中红外激光脉冲的新方案。我们在研究中采用了级联非线性晶体的结构，分别利用每块晶体不同的相位匹配角以放大种子光脉冲的不同频率范围，从而合成了几段不同的频谱；实验中产生了中心波长1.9微米、频谱范围1.6-2.1微米的宽带频谱输出，半高全宽达到280纳米；经过线性压缩，获得了持续时间为21.5飞秒的超短脉冲，对应约3.4个光周期；同时得益于级联晶体的总厚度，系统转换效率达到了31.3%。进一步地，我们理论研究了将级联晶体方案用于更高能量激光系统的可能性，仿真结果表明，通过向入射脉冲引入啁啾将其展宽，级联晶体方案可适用于更高能量的激光脉冲，能够用于产生能量高达十毫焦级的周期量级脉冲，具有很高的能量可拓展性。本项目在实施过程中，项目组在光学、物理类的国际重要期刊发表相关SCI论文4篇，参加国内会议1次，做报告1次。3年内，4名研究生、3名本科生参与到本项目中，2人获得硕士学位。

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