Design of the photocathode drive laser system for high current electron beam operation of DC-SRF-II gun
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摘要: 为推动北京大学超导加速器实验装置不断向强流目标迈进，提出100 W红外高重频光阴极驱动激光的设计方案，主放大器采用先进的光子晶体增益光纤，保证输出光束的质量。对激光系统中的关键问题，如各部分功率指标、脉冲展宽和压缩、激光耦合等进行了设计，并且考虑了激光的非线性影响。为实现强流加速器开机运行所必备的诊断模式，也提出了对于高重频激光进行两级选频的独特设计方案。将高速的SOA光开关和低速的声光调制器相结合，产生宏脉冲结构的输出激光，从而实现加速器在诊断模式下的运行。Abstract: We present the design of a 100 W high repetition rate photocathode drive laser system for realizing high average current operation of the superconducting accelerator at Peking University. To achieve good beam quality and reliability, we choose photonic crystal fiber (PCF) as the gain medium of the main amplification unit. In addition, we address several key issues for the drive laser system, including the evaluation of the output power of each amplification unit, the design of pulse stretcher and compressor, the optimization of free space coupling setups for pump pulse and seed pulse, etc. We also combine a high-speed semi-conductor optical amplifier (SOA) optical switch with a low-speed acousto-optic modulator (AOM) to achieve the necessary diagnostic mode for the intense electron beam accelerator. This unique design is of importance for the photocathode drive laser with the repetition rate around or above 100 MHz.
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图 1 北京大学DC-SRF-II双碱光阴极驱动激光系统
Figure 1. Peking University DC-SRF-II photocathode drive laser system
① seed source and pulse stretcher, ②～⑤ amplification system, ⑥ pulse compressor and frequency doubling system, ⑦ longitudinal shaping and transverse shaping system, ⑧ pulse picking system
图 2 北京大学DC-SRF-II驱动激光系统宏脉冲诊断模式
Figure 2. Macro pulse mode of Peking University DC-SRF-II photocathode drive laser system
图 3 北京大学DC-SRF-II驱动激光系统主放大器
Figure 3. The main amplification unit of Peking University DC-SRF-II photocathode drive laser system
图 5 PCF与ROD-PCF的红外信号光耦合
Figure 5. The signal laser is coupled from PCF into ROD-PCF by four focal lenses
图 6 北京大学DC-SRF-II驱动激光系统种子源与脉冲展宽器
Figure 6. Seed pulse and pulse stretcher of Peking University DC-SRF-II photocathode drive laser system
图 7 随着可调谐滤波器的入射角度增大，输出 激光的光谱中心波长“蓝移”
Figure 7. Hypsochromic shift with the increasing angle of incidence (AOI) of tunable filter
表 1 功率放大系统输出参数表
Table 1. Parameters of DC-SRF-II laser amplifier
amplifier unit output power/mW pre-amplifier I (Yb-doped fiber) 10 pre-amplifier II (Yb-doped fiber) 200 main-amplifier I (PCF) 50 00 main-amplifier II (ROD - PCF) 100 000 
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表 2 激光耦合透镜的焦距参数表
Table 2. Focal length of coupling lenses
$ {f}_{1} $/mm $ {f}_{2} $/mm $ {f}_{3} $/mm $ {f}_{4} $/mm $ {f}_{5} $/mm $ {f}_{6} $/mm $ {f}_{7} $/mm 25 20 25 20 50 25 25
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表 3 不同入射角度所对应的输出激光光谱
Table 3. Spectral region and width of different AOI
angle of incidence/(°) spectral region/nm spectral width/nm $ 0 $ 1028～1032 4.0 $ 2.5 $ 1028～1031.5 3.5 $ 3.5 $ 1028～1031 3.0 $ 4.5 $ 1028～1030.5 2.5 $ 5.5 $ 1028～1030 2.0 $ 6.0 $ 1028～1029.5 1.5 $ 6.5 $ 1028～1029 1.0 $ 7.0 $ 1028～1028.5 0.5
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