Jia Qing, Qu Kenan, Fisch Nathaniel J
Opt Lett. 2020 Sep 15;45(18):5254-5257. doi: 10.1364/OL.397321.
Compression of an intense laser pulse using backward Raman amplification (BRA) in plasma, followed by vacuum focusing to a small spot size, can produce unprecedented ultrarelativistic laser intensities. The plasma density inhomogeneity during BRA, however, causes laser phase and amplitude distortions, limiting the pulse focusability. To solve the issue of distortion, we investigate the use of optical phase conjugation as the seed pulse for BRA. We show that the phase conjugated laser pulses can retain focusability in the nonlinear pump depletion regime of BRA, but not so easily in the linear amplification regime. This somewhat counterintuitive result is because the nonlinear pump depletion regime features a shorter amplification distance, and hence less phase distortion due to wave-wave interaction, than the linear amplification regime.
在等离子体中利用背向拉曼放大（BRA）压缩强激光脉冲，然后在真空中聚焦到小光斑尺寸，可以产生前所未有的超相对论激光强度。然而，BRA过程中等离子体密度的不均匀性会导致激光相位和幅度失真，限制了脉冲的聚焦能力。为了解决失真问题，我们研究了使用光学相位共轭作为BRA的种子脉冲。我们表明，相位共轭激光脉冲在BRA的非线性泵浦耗尽 regime中可以保持聚焦能力，但在线性放大 regime中则不太容易。这个有点违反直觉的结果是因为非线性泵浦耗尽 regime的放大距离较短，因此与线性放大 regime相比，由于波-波相互作用引起的相位失真较小。
