Ultrafast Laser Laboratory, College of Precision Instrument and Optoelectronics Engineering, Key Laboratory of Opto-Electronic Information Technology (Ministry of Education), Tianjin University, Tianjin 300072, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61805174, 61535009, 61827821, 61377041, 11527808)
Received Date:05 June 2019
Accepted Date:02 August 2019
Available Online:01 October 2019
Published Online:20 October 2019
Abstract:In recent years, picosecond laser in ultraviolet (UV) has manifested great importance for applications both in science and industry, such as biomedical research, micro machining, etc. Now, the well proven approach to generating ultra-short UV pulses is extra-cavity frequency conversion based on nonlinear optical (NLO) crystal, due to the lack of suitable laser sources directly generating UV laser. In this process of harmonic generation, the length of nonlinear crystal is an important factor affecting the conversion efficiency and beam-quality. The optimal length of the nonlinear crystal is influenced by incident laser parameters and crystal absorption coefficient. At present, for the UV 355 nm picosecond laser generated from extra-cavity sum frequency, published are few reports about detailed analysis and research on the influence of photon ratio of the incident laser beams and nonlinear crystal absorption on optimal length of sum frequency crystal. In this paper, the steady-state solutions with the highest conversion efficiency under different incident conditions are obtained by theoretical analysis and numerical simulation of the three waves coupling equations. The effects of different photon ratios and absorption effect of the sum frequency crystal on the optimum crystal length are analyzed. We propose a solution based on the fundamental frequency laser amplified to shorten crystal length and improve conversion efficiency. In this scheme, the 532 nm second harmonic laser with a high conversion efficiency over 65% can be achieved by LiB3O5 crystal. After that, the 1064 nm fundamental frequency laser is separated from the second harmonic laser, and then it is amplified by the Nd:YVO4 laser crystal pumped by an 808 nm laser diode. Finally, the ultraviolet 355 nm picosecond laser is obtained by combining the 1064 nm fundamental frequency laser with the 532 nm second harmonic laser in the LiB3O5 crystal. The simulation results show that the incident photon ratio of the sum frequency reaction can be changed by amplifying the residual fundamental frequency laser, and the optimum length of the sum-frequency crystal corresponding to the highest conversion efficiency can be shortened. Meanwhile, the absorption and walk-away effect of the sum frequency crystal can be also reduced. The final 355 nm laser output power can be increased more than 40 percent compared with the traditional scheme of early reports. In consequence, the high sum frequency conversion efficiency of the UV 355 nm picosecond laser can be obtained by changing the photo ratio of the incident laser beams through amplifying the fundamental frequency laser. Keywords:ultraviolet picosecond 355 nm laser/ sum frequency/ fundamental frequency laser amplified/ LiB3O5 crystal
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2.1.基于基频放大的紫外皮秒355 nm输出效率提高系统实验装置
腔外和频产生紫外皮秒355 nm激光的装置图如图1所示, 其中图1(a)为传统限制倍频效率的方案装置图, 图1(b)为基于基频放大的和频实验装置图. 基频光为高功率1064 nm皮秒种子光, 经过透镜组L1和L2调整光斑直径并进行光束准直. 利用LBO倍频晶体的I类相位匹配(θ = 90°, φ = 11.7°)进行倍频, 可产生532 nm二次谐波. 在传统方案中由于之后和频过程中的入射光子数配比的要求, 限制倍频光532 nm的转换效率至50%. 本文在倍频时提高了532 nm倍频光的输出功率, 使532 nm倍频光转换效率可以达到65%甚至更高[20]. 倍频后利用双色镜DM1将1064 nm基频光与532 nm倍频光分离, 外加808 nm泵浦激光由双色镜DM2输入, 通过Nd:YVO4晶体将1064 nm基频光进行放大,从而调整入射到和频LBO晶体中的光子数配比. 532 nm倍频光采用对称结构并通过改变双色镜DM1与反射镜M之间的距离实现1064 nm基频光和532 nm倍频光的时间同步. 之后, 再将放大后的1064 nm基频光和532 nm倍频光通过双色镜DM3合束并入射到和频晶体LBO中, 采取II类相位匹配(θ = 43.5°, φ = 90°)进行和频产生紫外355 nm激光. 最终, 通过分光镜DM4将三束激光分离从而得到紫外皮秒355 nm激光输出. 图 1 (a) 传统的355 nm产生装置图; (b) 基于基频放大的紫外皮秒355 nm输出效率提升系统装置图 Figure1. (a) Diagram of the traditional 355 nm generating device; (b) diagram of the UV picosecond 355 nm output efficiency improvement system based on fundamental frequency amplification