神光-Ⅲ激光装置时标激光和任意反射面速度干涉仪探针光源产生技术

Research of time fiducial laser and probe laser of velocity interferometer system for any reflector for Shenguang-Ⅲ laser facility

在惯性约束聚变研究中,时标激光是对物理诊断数据进行分析的重要时间标尺,而任意反射面速度干涉仪(VISAR)光源则是冲击波精密诊断必不可少的探针光源.通过对物理需求的分析,提出对时标激光与VISAR光源共用脉冲产生单元,采用时分复用技术实现二者在同一台幅度调制器上的精密整形,经1×2分束后再通过声光开关进行选择输出,从而降低了系统造价,便于集中控制.采用了脉冲稳偏、高稳定空间放大、高精度温控谐波转换技术及可快速插拔精密复位的光纤耦合和传能技术,实现了时标和VISAR光源脉冲的高稳定输出.研制的时标激光系统可产生与主激光高精度同步的12路二倍频、4路三倍频时标信号,为神光-III激光装置物理实验提供了重要的时间基准.产生的VISAR光源脉冲在经过光纤系统和Nd:YAG棒状放大器后,通过温控LBO晶体倍频,然后经1 mm芯径的多模传能光纤传输至成像型VISAR系统,为物理实验提供了单纵模、高亮度、可精密整形的脉冲激光.系统已用于VISAR诊断物理实验,获得了完整的冲击加载、减速的图像,从而为冲击波调速及相关高压物理实验提供了可靠的技术手段.

Time fiducial laser is an important timing marker for different diagnostic instruments in high energy density physics experiments. The probe laser in velocity interferometer system for any reflector （VISAR） is also vitM for precise shock wave diagnosis in inertial confinement fusion （ICF） research. Here, time fiducial laser and VISAR probe laser are generated from one source in SG-III laser facility. After generated from a 1064 nm DFB laser, the continuous wave laser is modulated by an amplitude modulator driven by a 10 GS/s arbitrary waveform generator. Using time division multiplexing technology, the ten-pulse time fiducial laser and the 20 ns pulse width VISAR probe laser are split by a 1 × 2 multiplexer and then the time fiducial and VISAR pulses will be selected individually by acoustic-optic modulators. Using this technology, the cost for the system can be reduced. The technologies adopted in the system also include pulse polarization stabilization, high stable Nd：YAG amplification, high precision thermally controlled frequency conversion, fiber coupling, and energy transmission. The fiber laser system is connected to the Nd：YAG rod amplifier stage with polarizing （PZ） fibers to maintain the polarization state. The output laser of Nd：YAG amplification stage is coupled with different kinds of energy transfer fibers to propagate enough energy and maintain the pulse shape for the time fiducial and VISAR probe laser. The input and output fibers are all coupled to the rod amplifiers with high precision and being easy to plug and play for users. Since the time fiducial and imaging VISAR laser system is far from the front end room and located in the target area, the system also uses an arbitrary waveform generator （AWG） to generate the shaped ten-pulse time fiducial laser and 20 ns VISAR laser. This AWG and the other three AWGs used for the main laser pulse of SG-III laser facility will be all synchronized by 10 GHz clock inputs, realizing the smaller than 7ops （RMS） jitter between the main laser pulse, time fiducial laser and VISAR pulse. After amplification and frequency conversion, the time fiducial laser finally generates 12-beam 2ω and 4-beam 3ω laser beams, providing important reference marks for different detectors in the ICF experiments and making it convenient for the analysis of multiple diagnostic data. The VISAR laser pulse is also amplified by the Nd：YAG amplifiers and frequency-converted to 532-nm green light by a thermally controlled LBO crystal, with output energy larger than 20 mJ. Finally, the 532-nm VISAR probe laser beam is coupled with a 1-mm core diameter fused silica optical fiber, and then propagates 30 meters to the imaging VISAR system. The VISAR probe laser has been used in many high energy density physics experiments. The shock wave loading and slowdown processes are measured. Function for measuring velocity history of shock wave front movement in different kinds of materials was added to the SG-III laser facility.