基于大角度非共线SBS的百皮秒激光脉冲高效放大技术研究

At present, the shock ignition is a hot point and front line subject in the field of laser fusion. However, the high peak power, high contrast and high energy hundred-picosecond pulse required in shock-ignition is challenging the present laser technique. The reason is that both CPA and MOPA are difficult to achieve an effective amplification of hundred-picosecond pulse. This project proposed a new method called non-collinear Brillouin amplifier with a large off-axis angle, which is based on the basis of stimulated Brillouin scattering (SBS) amplification. This amplification is appropriate for amplifying high-energy hundred-picosecond pulse needed in shock ignition. In addition, it is simple in construction and easy to be achieved. This project will concentrate four themes(As is shown in the following) on clarifying the mechanism of realizing high-efficiency amplification of hundred-picosecond pulse based on non-collinear SBS amplification, and realizing a high-energy output of hundred-picosecond laser pulse experimentally. The four themes are:(1) setting up a theoretical model for non-collinear SBS resonant amplification, (2) achieving an un-modulated hundred-picosecond Stokes signal pulse output, (3) investigating the change of gain characteristics and temporal waveform in the process of non-collinear SBS amplification, and (4) the SBS medium with adjustable Brillouin frequency difference. The results of this research will be of great significance in expanding the field of pulsed laser amplification, and exploring a new technology for realizing high-efficiency amplification of hundred-picosecond short laser pulses.

冲击点火是当前激光聚变领域研究的热点和前沿，其所需的高峰值功率、高对比度和高能量百皮秒激光脉冲对激光技术提出了新的挑战，现有的CPA技术和MOPA技术由于大色散高损伤阈值压缩光栅及B积分受限的制约都无法实现百皮秒激光脉冲的高效放大。本项目在受激布里渊散射（SBS）放大的基础上提出了一种大角度布里渊高效率放大的新方法，结构简单易行，非常适合冲击点火所需的高能量百皮秒激光脉冲的高效放大。项目拟围绕该方法开展非共线SBS共振放大的理论模型；无调制百皮秒Stokes信号光的产生；大角度非共线布里渊放大过程的增益特性和波形变化；布里渊频差可调谐的SBS介质等四个方面的研究，阐明基于大角度非共线SBS实现百皮秒激光脉冲高效率放大的机理，获得高能量的百皮秒激光脉冲输出。研究结果对拓展脉冲激光放大领域，探索一条百皮秒激光短脉冲高效率放大的新技术具有重要意义。

MOPA技术和CPA技术将纳秒激光脉冲的能量和飞秒激光脉冲的峰值功率不断提升至新的高度。然而，介于纳秒脉冲和超短脉冲之间的百皮秒激光脉冲放大，却处于“技术真空”和“应用荒漠”。现有的激光放大技术，都难以获得百皮秒脉冲激光的高效放大，MOPA的局限性在于：百皮秒的脉冲宽度太短，激光放大效率低，并且，百皮秒激光脉冲的高峰值功率的高B积分值，使得输出功率受限；CPA的局限性在于：百皮秒激光脉冲的啁啾量不足以获取有效的脉冲展宽和压缩，在激光的末级，目前的技术也无法加工高能高功率损伤阈值的大口径光栅。. 针对MOPA和CPA的困难，本项目提出了主动型的百皮秒激光布里渊放大（Brillouin Power Amplifier, BPA）技术，研制了主动型的Stokes频移前端，解决了高能高功率百皮秒激光布里渊放大的一系列关键问题，突破了高强度激光注入、传输和放大等难点，利用SBS能量转移技术实现了百皮秒激光脉冲的高效放大。在哈工大百焦耳激光平台上开展了系统的实验研究，优化了系统结构和参数，在神光III原型装置上成功获得了实验验证，长脉冲的大能量泵浦光在SBS能量转移过程结束后，将能量全部转移给200ps的Stokes种子脉冲，百皮秒激光脉冲的输出光强达到了7GW/cm^2，如果把激光束的口径等比例放大至驱动器的满口径，即可实现千焦耳的百皮秒激光输出。. 这种BPA放大是继MOPA技术高效放大纳秒脉冲、CPA技术高效放大飞秒脉冲之后，实现百皮秒激光脉冲高效放大，从而获得大能量高峰值功率百皮秒激光的有效技术途径，可以为相关的实验研究和应用提供百皮秒脉冲尺度的优质强光光源。. 本项目实施过程中，发表学术论文31篇，其中SCI收录29篇、EI收录1篇，会议论文1篇；项目实施的四年中共培养博士硕士毕业生8名；申请国家发明专利6项，已获得授权4项。