基于固体薄片高能量超连续的产生与压缩

Due to the supercontinuum generated from spectra-broadening currently has very low energy, the supercontinuum with high energy generated through transparent solid slices is proposed. When propagation of fs laser pulses into transparent solid medium (such as fused silica, sapphire), the laser beam spot will be easily damaged by self-focusing effect and broadened spectrum is also limited. But if the solid medium is very thin, the laser pulses can transport through the slice without any self-focusing and spot damage, the spectrum will be broadened by Kerr-effect. If there are a lot of solid slices with thin, each slice is located properly, self-focusing effect can be eliminated and beam quality with good profile will be kept. With this configuration, we can optimize self-phase modulation to obtain a broadened spectrum as broad as possible, while limiting the effect of self-focusing. Especially, a combination of the thickness and locations of the solid medium plates can control self-focusing, and the size of driving laser beam will be kept more or less the same by self-focusing through all the thin slices to maximize self-phase modulation, so pulses with high energy for example tens of mJ will be permitted. High energy few-cycle laser pulses with peak power larger than 1 TW can be obtained by compression of this high energy supercontinuum generated from transparent solid slices, and will be used in generation of high flux single-attosecond pulses, high field and coherent synthesis etc.

针对当前超连续光能量比较低的缺点，本项目采用飞秒激光在多个固体薄片中传输时将光谱展宽产生超连续光的方案来获得高能超连续光，压缩后实现周期量级太瓦激光输出。飞秒激光脉冲在固体透明介质中传播时容易自聚焦成丝将光斑撕裂，但将飞秒激光聚焦注入到固体薄片中进行光谱时，如果固体介质足够薄，飞秒激光还没来得及自聚焦成丝就离开薄片并发散到空气中，由于在空气中的自聚焦，光斑会缩小进入到第二块薄片中进一步将光谱展宽，经过多个薄片后，光谱不仅可以展宽到足够宽，而且还保持良好的光束质量。采用多个固体薄片组合的方法不仅可以避免自聚焦成丝对光斑的破坏，而且还能保持光在固体介质中有效的传播距离，获得大于倍频程的超连续光谱。采用这种方案激光的传输效率不仅大大提高，而且能承受大能量飞秒激光的注入，可望获得峰值功率大于太瓦的周期量级激光脉冲，为高能单个阿秒脉冲的产生、激光与物质相互作用等研究提供强有力的驱动光源。

针对传统充气hollow fiber光谱展宽方案由于光纤端面容易受损，不能承受高能量激光入射，造成输出能量低等缺点，本课题采用飞秒激光在多个固体薄片传输时将光谱展宽的方法，将获得大能量超连续光，进一步压缩后可获得高能量太瓦周期量级激光脉冲。主要研究的内容包括理论研究高能飞秒激光脉冲在固体透明材料中传输特性，实验研究研究光谱展宽效果与薄片材料、薄片的数量以及各薄片厚度的关系，实现载波包络相位的锁定，研究高次谐波产生与驱动激光CEP的关系，获得高能量孤立阿秒激光的产生。.本课题在研究期间，获得的重要结果和关键参数如下：.（1）采用0.8mJ/25fs/1kHz的飞秒激光聚焦注入到0.2mm熔石英薄片组中进行光谱展宽，发现当采用6片薄片可以将光谱展宽到大于一倍频程，光谱范围覆盖400~1000nm，利用啁啾镜压缩后的脉冲宽度小于7fs。采用f-2f光谱干涉技术，通过光谱干涉测量获得了拍频信号，反馈给压缩器中的色散棱镜，通过改变色散棱镜的插入量实现CEP的锁定，锁定后的CEP抖动的均方根误差仅为75.2 mrad。该结果是迄今为止的放大器CEP锁定的最好结果之一。.（2）开展了上述光源驱动产生HHG的研究。获得了HHG光谱分布随CEP变化关系，说明实现了对周期量级飞秒脉冲光场的控制，通过截止区连续分布光谱的产生获得了单个阿秒激光脉冲。.（3）开展了基于多通道（multipass cell, MPC）熔石英玻璃片的超连续光谱新方案研究。将单脉冲能量为600uJ，脉冲宽度10ps的激光脉冲，注入到MPC中，51次经过10mm熔石英玻璃片将光谱展宽，最终压缩到824fs，脉冲宽度压缩比大于10倍。我们也开展了固体薄片产生超连续光源的双波长相干合成的研究，通过控制双波段脉冲的延迟和CEP，实现了对合成后光场的分布进行调控。.通过本项目的研究，实现了高能量周期量级激光脉冲的产生，实现了光场调控，可以作为驱动产生高能量阿秒激光。为实现多太瓦周期量级激光脉冲打下了坚实的基础。