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Computer simulation is always an important means for studying laser, while laser theory is the basis of simulation. Although the semi-classical laser theory can accurately describe the generation process of laser, its complexity leads to a need of huge resources and time for computation. However, in particular cases, the influence of some factors on the laser system can be neglected. If a simpler model is employed to describe the laser system, the time of simulation can be shortened significantly. In order to simulate the laser system more efficiently, a simulation model of Q-switched solid-state laser is proposed in this paper. In this model, the time-domain function of Q switch is introduced, which represents the modulation of Q switch loss over time. Because the cross section of the Nd:YAG rod is circularly shaped, the resonator eigenmodes are assumed to be a Laguerre-Gaussian beam for simplicity. Then, any other laser beam can be formed by superposition of the eigenmodes of the resonator. These series of resonator eigenmodes are coupled with the rate equations of laser crystals. Finally, the distribution of pump light field inside the laser crystal is approximated as super Gaussian distribution. Based on this physical model, the influence of pump power and pump light field distribution on the output beam of multimode Q-switched solid-state laser is investigated. The simulation results are in good agreement with the experimental data, which explains the validity of the proposed model. For instance, with the increase of pump power, the output power of the laser increases, but the overall slope efficiency decreases. This is because the diffraction loss m,n of the lower order mode is less than the diffraction loss of higher order mode. When the pumping power increases, the higher order mode that starts to oscillate has lower utilization efficiency of pump energy. Therefore, the overall slope efficiency of the laser is reduced. In order to analyze the mode competition in the multimode Q-switched solid-state laser more comprehensively, the processes of laser pulse generation, relaxation oscillation and continuous oscillation are calculated as one full cycle. The laws of pulse power and beam quality factor versus time are obtained. For example, the maximum instantaneous output power of the relaxation oscillation is about 30 times the steady continuous output power. This law has a certain reference value when analyzing the damage threshold of laser optical element. In the pulse generation stage, the beam quality factor is close to 1, which explains the fact that the pulse field composition is nearly the fundamental mode of the laser. In the relaxation oscillation, the value of the beam quality factor changes irregularly with time, because mode competition is in a non-equilibrium state at this time. When stable continuous oscillation occurs, the mode competition achieves dynamic equilibrium, which means that the proportion of each mode is no longer changed in the output light field.
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Keywords:
- numerical simulation /
- diode-pumped lasers /
- wave propagation /
- Q-switching
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[1] Yao Y H, Lu C H, Xu S W, Ding J X, Jia T Q, Zhang S A, Sun Z R 2014 Acta Phys. Sin. 63 184201(in Chinese)[姚云华, 卢晨晖, 徐淑武, 丁晶新, 贾天卿, 张诗按, 孙真荣2014 63 184201]
[2] Wang X F, Wu Z M, Xia G Q 2016 Acta Phys. Sin. 65 024204(in Chinese)[王小发, 吴正茂, 夏光琼2016 65 024204]
[3] Mao Y F, Zhang H L, Xu L, Deng B, Sang S H, He J L, Xing J C, Xin J G, Jiang Y 2015 Acta Phys. Sin. 64 014203(in Chinese)[毛叶飞, 张恒利, 徐浏, 邓波, 桑思晗, 何京良, 邢冀川, 辛建国, 江毅2015 64 014203]
[4] Zhu S S, Zhang S L, Liu W X, Niu H S 2014 Acta Phys. Sin. 63 064201(in Chinese)[朱守深, 张书练, 刘维新, 牛海莎2014 63 064201]
[5] Hou L, Han H N, Zhang L, Zhang J W, Li D H, Wei Z Y 2015 Acta Phys. Sin. 64 134205(in Chinese)[侯磊, 韩海年, 张龙, 张金伟, 李德华, 魏志义2015 64 134205]
[6] Sun Q, Yang Y, Deng Y Q, Meng F, Zhao K 2016 Acta Phys. Sin. 65 150601(in Chinese)[孙青, 杨奕, 邓玉强, 孟飞, 赵昆2016 65 150601]
[7] Dou Z Y, Tian J R, Li K X, Yu Z H, Hu M T, Huo M C, Song Y R 2015 Acta Phys. Sin. 64 064206(in Chinese)[窦志远, 田金荣, 李克轩, 于振华, 胡梦婷, 霍明超, 宋晏蓉2015 64 064206]
[8] Ge L, Chong Y, Stone A D 2010 Phys. Rev. A 82 063824
[9] Cerjan A, Chong Y D, Stone A D 2015 Opt. Express 23 6455
[10] Pick A, Cerjan A, Liu D, Rodriguez A W, Stone A D, Chong Y D, Johnson S G 2015 Phys. Rev. A 91 063806
[11] Cerjan A, Chong Y, Ge L, Stone A D 2012 Opt. Express 20 474
[12] Treci H E, Ge L, Rotter S, Stone A D 2008 Science 320 643
[13] Wohlmuth M, Pflaum C, Altmann K, Paster M, Hahn C 2009 Opt. Express 17 17303
[14] McCumber D E 1965 Bell Sys. Tech. J. 44 333
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