为了提高再生放大器输出激光脉冲的对比度，需要同时提高激光脉冲纳秒尺度的对比度和皮秒尺度的对比度。利用普克尔盒对主脉冲和预脉冲时间滤波的方法，设计了普克尔盒的双通预脉冲清洁装置，通过该装置使再生放大器输出激光纳秒尺度的预脉冲对比度提高了5个数量级；利用频谱整形方法，通过在放大器腔内适当位置插入频谱整形滤波片使得主脉冲前400 ps处的放大自发辐射对比度提高了1个数量级。最终，在双通预脉冲清洁与腔内频谱滤波共同作用下，再生放大器输出激光纳秒尺度的预脉冲对比度从4.3×10^-4提高到了6.6×10^-10，放大自发辐射对比度（主脉冲前400 ps）从5.0×10^-8提高到了5.0×10^-9。

In this paper, we propose a 36-quadrature amplitude modulation (QAM) superposition modulation technique that is featured with uneven symbol probability by nonlinear precoding, named nonlinear coded nonuniform superposition (NCNS) QAM. Its aim is to alleviate the nonlinearity effect caused by high instantaneous power in multi-input single-output (MISO) visible light communication (VLC) system, with an uneven probabilistic-shaped constellation. The transmitter includes two LEDs to send signals independently, and the receiver uses a photo detector to receive the superposed QAM signal. The experiment results show that NCNS has a better robustness against nonlinearity than pulse amplitude modulation 4, approximately gaining a 16% increase in maximum usable peak-to-peak voltage and a 33% enlargement in dynamic range area. It is a simple but effective approach to solve the bandwidth limits related to signal power and hopefully be applied in large power VLC systems such as underwater VLC, or to improve the robustness against power fluctuation.

We report a high power fiber amplifier based on nonlinear chirped-pulse amplification (NCPA). To manage the nonlinearity, pulse shaping is introduced by self-phase modulation in the fiber stretcher with the help of spectral filtering. The third-order dispersion is compensated for by the nonlinear phase shift in the NCPA. With optimization, the system can output 382 fs pulse duration with 20 W average power at 1 MHz repetition rate. The long-term average power fluctuation is measured to be 0.5% in 24 h, and the beam quality factor (M2) is 1.25.