LD端面泵浦激光晶体热效应及被动锁模特性研究

【作者】 张帅一；

【导师】 李健；

【作者基本信息】 山东师范大学 ， 光学， 2008， 硕士

【摘要】 激光二级管（Laser-Diode,LD）泵浦的全固态激光器具有体积小、效率高、稳定性好和寿命长等优点,在国防、光电子产业、光通讯和医疗卫生等领域有着重要的应用。激光晶体是全固态激光器（DPSL）中最重要的核心部分,在很大程度上它决定了激光器的输出特性。随着大功率半导体激光器技术的不断完善和发展,高功率二极管泵浦固体激光器已经成为国内外学者的研究热点,这些研究主要集中在如何有效地提高激光输出功率和改善激光光束质量方面,其中最主要的障碍来自激光晶体的热效应。本文对于周边恒温冷却的圆形截面Nd:YAG和矩形截面Nd:YVO₄晶体的热效应进行了理论计算。在精确求解晶体内温度及温度场分布的基础上,求出晶体的热焦距。精确设定激光腔参数,实现低温GaAs调Q锁模Nd:GdYVO₄激光器及SBR被动锁模Nd:YVO₄激光器。其主要内容可概括如下:1.首先对全固态激光器的历史、发展进行了回顾,介绍了全固态激光器的主要特性;简单介绍了几种常用的、适于端面泵浦的激光晶体.2.分析了LD泵浦矩形截面激光晶体热效应的基本理论,计算了由端面形变引起的光程差和总的光程差,得到不同抽运功率下的热焦距,并通过实验进行了验证,实验结果与理论计算基本一致。首次提出激光晶体端面腔镜会加重激光晶体热透镜效应的结论,研究表明:对于大功率全固态激光器,由晶体端面形变引起的光程差对晶体热透镜效应有较大影响。最后介绍了LD泵浦圆形截面激光晶体Nd:YAG热效应的基本理论通过求解泊松方程,得到在不同Nd离子掺杂浓度和不同泵浦光半径情况下激光晶体内温度和温度场分布。研究结果表明:当其他条件不变时,随着Nd离子掺杂浓度增加,Nd:YAG晶体端面中心温度升高,而晶体中心轴温度衰减越快。在泵浦功率一定时,随着泵浦光斑半径减小,Nd:YAG晶体端面中心温度升高。3.介绍了端面脉冲泵浦各向异性激光晶体热透镜效应。获得激光晶体中热量分布和温度分布的动态变化。当入射的泵浦光一定的时候,温度达到稳定状态后在一定范围内来回摆动,其振幅与f成比例。4.采用低温生长GaAs晶体作为被动饱和吸收体兼输出镜,实现了Nd:GdYVO₄激光器调Q锁模运转。研究了Nd:GdYVO₄激光器的基频运转特性及调Q锁模输出特性。实验结果表明,平面镜作为输出镜时泵浦功率为10W时,获得激光的输出功率是3.5W,光-光转换效率是35%;GaAs作为输出镜时激光器调Q运转阈值是1.2W;泵浦功率是10W时,输出功率是1.72W,锁模脉冲重复频率为113MHz;泵浦功率是7W时,激光调Q锁模深度达到100%。5.设计了单端输出SBR被动锁模Nd:YVO₄激光器,得到稳定的1064 nm连续锁模激光输出,采用透射率为10%的耦合输出镜,在注入功率为10W时,得到2.86 W的平均输出功率,光光转换效率为28.6%。更多还原

【Abstract】 Diode-pumped all-solid-state lasers（DPSL） has lots of advantages, such as compact structure, high stability and optical-optical conversion efficiency. It is widely used in national defence, photoelectron industry, communication, medical treatment and sanitation. Laser crystal is the core of diode-pumped all-solid-state lasers, which decide the output characteristics of laser. In recent years, in endeavoring to obtain higher output powers from continuous-wave end-pumped solid-state lasers, one must consider thermal effects that will impact optical performance. With the development of high power diode lasers, high power DPSSL has become one of the main objectives of current research and development activities within the laser community. Research of DPSSL has mainly concentrated on scaling of output power, while remaining of a high spatial beam quality. The main problem hindering scaling of diode-pumped solid-state lasers is heat deposition within the laser medium. The thermal effect of circular-face Nd:YAG, rectangular-face Nd:YVO₄ has been calculated. The focal length of the laser crystal is obtained by calculating the temperature and temperature distribution in the crystal. GaAs mode locking Nd:GdYVO₄ laser is set up and tested in this thesis. The contents can be outlined as follows:1. The history and progress of diode-pumped solid-state lasers are reviewed; and the characters of the diode-end-pumped solid-state laser are presented; then the dissertation presents the crystals of fitting diode-end-pumped.2. The thermal effect theory of circular-face laser crystal was analysed. By using this theory, the temperature distribution in laser crystal was obstained. The thermal conduct model of laser crystal was established.Using Possion equation, the temperature distribution in laser crystal was obstained, and the OPD induced by end-face deformation and total OPD were caculated. Based on the former calculation, we obtained the focal length of laser crystal with different pumping power. The experimental result was obtained, and it almostly coincides with the caculated result.For the first time, we obtained that end-pumped resonators with one highly reflective coating directly on the laser crystal can aggravate its thermal effect. The study shows that, for the solid-state laser with high power, the OPD induced by end-face deformation has a large influence on thermal effect of laser crystal. The thermal conduct model of cylindrical laser crystal was established.Using Possion equation, the temperature distribution in laser crystal was obstained with different Nd ion concentration and pumping radius.The study shows that in the same condition when the Nd ion concentration increase, the central temperature of Nd:YAG crystal’s pumped face will rise and the temperature of crystal central axis will reduce faster. For the same ctystal, when the pumping radius decrease, the central temperature of Nd:YAG crystal will rise.3. Transient thermal profile in pulse and end-pumped anisotropic laser crystal had been firstly investigated numerically in this paper. By builting a thermal model with treatinig the repetitive thermal load as a square function of time, the dynamics of thermal buildup and temperature distribution in laser crystal was obtained. When the incidence pump power is fixed, The temperature distribution reachs the quasi-steady-state and remains the oscillatory behaviour, the repetitively oscillatory amplitude is inversely proportion to f . Adopt the means in , we can obtain the focal length of the crystal change with time, associate with the rate equations, the dynamics process of laser setup will obtained, this work is in progress. Although a Nd:YVO₄ crysal was taken for example in our calculations, the result suitable for other anisotropic laser crystal similarly.4. Passively Q-switched mode-locking Nd:GdYVO₄ laser is successfully demonstrated by using a piece of GaAs crystal grown at low temperature as the passively saturated absorber as well as the output coupler. The fundamental properties of Nd:GdYVO₄ laser are investigated. The maximum average output power of 3.5W are obtained by using plainsphere when the incident pumping power 10W, which corresponds to an optical-optical coversion efficiency of 35%; The threshold power for Q-switching mode-locked is 1.2W, the maximum average output power of 1.72W was obtained by using GaAs when the incident pumping power was 10W, mode-locked pulse train with a repetition rate of～113MHz was achieved. At the incident laser pumping power of 7W, Q-switching mode-locked with modulation depth is 100%.5. According to the principle of mode-locking and the performance of SBR, an one beam of light output laser equipment with SBR passively mode-locking Nd:YVO₄ laser is set up. A stable mode-locking laser output is obtained in experiment. The maximum average output power of 2.5 W is obtained at the pumping power of 10 W with the transmission of the output couple is 10%. The optical to optical conversion efficiencies is up to 28.6%.更多还原