调制滤波引起脉冲起伏研究

Analysis of pulse fluctuation caused by modulation and filtering

由于惯性约束聚变(Inertia confinement fusion,ICF)中约束激光功率较大,常会在后续光学系统中产生受激布里渊散射(SBS)。一般应用相位调制器使单纵模激光展宽,提高SBS阈值.基于计算机模拟平台进行了模拟研究,讨论经过相位及幅度调制后再通过宽带或窄带滤波器激光脉冲变化情况。结合在实验上观察到的对应于4.3 GHz调制频率的0.23 ns间隔脉冲起伏,提出滤波导致高频分量缺失是产生脉冲顶部高频起伏的根本原因,而滤波中心波长偏离激光中心波长导致在实验上观察到明显起伏。通过数值模拟给出了不同滤波波形对脉冲高频起伏的影响,并给出滤波带宽、中心波长偏差和顶部起伏大小的关系,提出减小起伏的优化方法。滤波带宽和激光中心波长存在偏差时,选择较小滤波带宽,应该优先考虑使用类高斯形的滤波器;而当滤波带宽比较大,选择类方波形滤波器较有优势。

The stimulated Brillouin scattering （SBS） effect is easy to be observed in optical system because of the huge power of laser in ICF experiment. To avoid its damage, phase modulator is used to increase the threshold of the SBS effect. Simulation is used to discuss the situation of laser pulse after amplitude modulator, phase modulator and filter. Corresponding to the experimental result of 4.3 GHz （0.23 ns in time domain） fluctuation on the top of the pulse, it is obtained that lack of high frequency part of the laser pulse causes the fluctuation with the frequency of phase modulator, while the unbalanced sidebands make this effect easy to be observed. The influence of the shape, bandwidth and mismatch of the filter on the pulse fluctuation are also discussed. Furthermore, a way to suppress the high frequency fluctuation is proposed. When the filter detunes with the center wavelength of the laser, a Gaussian filter is better for small bandwidth, whereas ideal filter is better for bigger bandwidth situation.