摘要
报道了一种基于球差自补偿的高光束质量主振荡功率放大(MOPA)亚纳秒脉冲激光器,其输出波长为1064 nm,脉冲宽度为500 ps,重复频率为1 kHz,平均功率为9.7 W,峰值功率约为20 MW。系统采用Nd∶YAG/Cr∶YAG微片被动调Q亚纳秒激光器作为主振荡器,通过两级端面泵浦对YAG/Nd∶YAG/YAG键合晶体进行双通放大,通过优化放大器中激光反射镜的位置实现球差自补偿,进而满足模式匹配要求,获得了光束质量因子M2为1.5的一级放大激光输出,以及M2优于1.71的二级放大激光输出,整体放大倍数达54。该激光系统结构简单、稳定可靠,且具有高平均功率和高峰值功率的优势,在激光诱导击穿光谱、激光加工及非线性光学频率变换等领域中具有重要的应用价值。
Objective Subnanosecond pulsed lasers with pulse durations ranging from 100 ps to 1 ns have the advantages of both high peak power and high energy,which play an important role in laser processing,laser ignition,photoacoustic imaging,nonlinear optical frequency conversion,and laser-induced breakdown spectroscopy(LIBS).Improving the output power while maintaining good beam quality has always been the focus of research on high-performance lasers,especially for high-repetition-rate lasers that encounter severe thermal effects.Passively Q-switched microchip lasers are commonly used to generate sub-nanosecond laser pulses due to their compact structure,robustness,high beam quality,good spectral purity,and low cost.However,their power scaling is strictly limited by the bonded crystal structure with double-end coatings as the cavity,as thermal management and cavity design are impracticable.Fortunately,the master oscillator power amplifier(MOPA)is an ideal alternative.In this paper,a high-beam-quality subnanosecond MOPA laser system operating at a 1-kHz repetition rate with a peak power of approximately 20 MW based on spherical aberration selfcompensation is reported.Methods The pump absorption in the gain medium of the end-pumped amplification stage leads to a significant thermal lens effect with substantial optical aberrations,making it difficult to achieve high beam quality during high-power operation.As a guide for the experiment,the temperature distributions inside the gain media of the two-stage amplifiers are simulated,and the thermal focal lengths f1 and f2 are calculated using Seidel aberration theory based on the optical path difference(OPD)caused by thermally-induced refractive index changes.A passively Q-switched Nd∶YAG/Cr∶YAG microchip laser with a cavity length of 6 mm is used as the master oscillator to generate a sub-nanosecond seed pulse.The output power of the microchip laser is 180 mW at 1 kHz,corresponding to the single-pulse energy of 180μJ.Then,the seed pulse is sent to the amplifier stages through a collimating lens and isolator.Both amplifiers use an identical bonded YAG/Nd∶YAG/YAG crystal as the gain medium and a similar end-pumped dualpass structure,enabled by a polarizing beam splitter(PBS),quarter-wave plate(QWP),and reflecting mirror.The linearly polarized input and output lasers are orthogonal,while the laser inside the gain medium is circularly polarized to alleviate the impact of thermally-induced birefringence.To avoid laser-induced damage,the pump spot sizes are 0.9 mm and 1.2 mm,corresponding to pump peak powers of 170 W and 200 W,respectively.The reflecting mirrors are set at distances of f1 and f2 from the gain media,where the sign of the spherical aberration after single-pass amplification is reversed.In this way,the spherical aberration becomes selfcompensated after dual-pass amplification.In addition,the thermal lens and reflecting mirror form a 2f imaging system exactly to guarantee good mode matching.This scheme can also be applied to multi-stage amplification systems to further improve output power while maintaining good beam quality.Results and Discussions Simulation results show that the temperature distributions of two end-pumped bonded crystals in the amplifier are axisymmetric along the center of the crystal,where the high-temperature region is concentrated at the front end of Nd∶YAG owing to strong absorption,resulting in a gradient distribution of the refractive index inside the crystals,and additional OPD is introduced to the incident seed pulse.The values of OPD along the radial direction caused by thermal-induced refractive index change are presented(Fig.2).Using the Seidel aberration theory,the thermal focal lengths f1=136 mm and f2=220 mm of the two amplifier stages are calculated based on OPD within the laser beam cross-section,which are consistent with the measured values.The twostage amplifiers adopt a similar end-pumped dual-pass structure but with orthogonal output polarization to maintain the entire system within a tabletop size of 30 cm×70 cm(Fig.3).The measured seed pulse possesses an average power of 180 mW,pulse width of 517 ps,and beam quality factor of M2=1.53,whereas unstable high-order modes symmetrically surrounding the fundamental mode are observed due to a relatively large pump waist(Fig.4).The detrimental higher-order modes are effectively suppressed after amplification by optimizing the filling factor.In addition,benefiting from the self-compensation of spherical aberration and good mode matching,a high-beam-quality sub-nanosecond MOPA laser with an average power of 9.7 W and beam quality factor of M2=1.71 is obtained(Fig.6).The total magnification reaches 54 times.The pulse width is 500 ps,corresponding to a peak power near 20 MW and coefficient of variation(CV)of power of 1.6%(Fig.7).Conclusions A high-beam-quality sub-nanosecond 1064-nm MOPA pulse laser with a pulse width of 500 ps,repetition rate of 1 kHz,average power of 9.7 W,and peak power of approximately 20 MW is reported.Benefiting from self-compensated spherical aberration and good mode matching,two end-pumped dual-pass power amplifier stages based on bonded YAG/Nd∶YAG/YAG crystals boost the power of the seed laser by 54 times,with a beam quality factor of M2=1.71.Owing to its simple structure,good stability,high average power,and peak power,such a MOPA system is believed to have great potential in LIBS,laser machining,and pumping nonlinear optical frequency converters.
作者
李方杰
钟凯
池静
乔鸿展
郑逸哲
刘宇鑫
陈锴
李吉宁
徐德刚
姚建铨
Li Fangjie;Zhong Kai;Chi Jing;Qiao Hongzhan;Zheng Yizhe;Liu Yuxin;Chen Kai;Li Jining;Xu Degang;Yao Jianquan(School of Precision Instrument and Opto-Electronics Engineering,Tianjin University,Tianjin 300072,China;Key Laboratory of Optoelectronics Information Technology,Ministry of Education,Tianjin University,Tianjin 300072,China;Key Laboratory of Micro Optical Electronic Mechanical System Technology,Ministry of Education,Tianjin University,Tianjin 300072,China)
出处
《中国激光》
北大核心
2025年第10期51-59,共9页
Chinese Journal of Lasers
基金
国家自然科学基金(62175184)。
关键词
激光和激光光学
激光放大器
被动调Q
亚纳秒脉冲
球差自补偿
laser and laser optics
laser amplifier
passive Q-switching
sub-nanosecond pulse
self-compensation of spherical aberration
