【作者】 潘孙强；

【导师】 刘崇； 陈军；

【作者基本信息】 浙江大学 ， 光学工程， 2013， 博士

【摘要】 本论文的主要内容分为四个部分：端面泵浦固体激光器的热效应研究；端面泵浦固体激光振荡器中的光束质量演变以及球差对谐振腔的影响；端面泵浦固体激光放大器中的光束质量改善技术研究；侧面泵浦固体激光器及其非线性频率变换研究。光束质量是激光器的一项重要参数。激光光束质量控制及改善技术的研究一直是高功率激光器研究中的一项重要内容。影响激光光束质量的主要因素是激光晶体的热效应和增益效应。本论文通过研究激光晶体的热效应和增益效应来研究激光光束质量的控制及改善技术。首先研究了端面泵浦固体激光器的热效应。从稳态热传导方程与边界条件出发,分别采用解析方法和有限差分法计算了端面泵浦激光晶体的温度分布,进而通过计算晶体内的光程差分布来研究晶体的热透镜效应和热致畸变效应。研究了球差对热透镜焦距的影响,指出同一晶体对不同光斑半径激光的等效热透镜焦距是不一样的。将薄片上的光程差分布按Zernike多项式展开,指出球差是主要的畸变,可以用球差系数表征畸变的大小。提出了一种利用探测光的夫琅禾费衍射图样测量热透镜焦距的新方法,可在激光运行时测量热透镜焦距。实验测量了端面泵浦晶体的热透镜焦距,实验测量值与非稳腔法实验测量值符合得较好。在研究热效应的基础上,建立了端面泵浦激光振荡器的理论模型,理论模型中同时考虑了激光晶体的热效应以及增益效应,将谐振腔沿激光传播方向展开,采用分步傅立叶法仿真激光在谐振腔中振荡时的自再现模。仿真计算并实验验证了激光振荡器输出镜和全反镜端的激光光束质量是不一样的,激光从全反镜传播到输出镜时,激光光束质量被改善。提出了当激光通过晶体时,激光自身的负球差可以被晶体的正球差补偿这一光束质量改善机理。同时指出带正球差的会聚激光光束经过传播后,其球差由正球差演变为负球差,解释了激光谐振腔内激光波前负球差的由来。研究了激光晶体球差对谐振腔稳定区的影响,指出球差可导致谐振腔基模和高阶模稳定区的分离,从而可以选择合适的工作点,只有基模可以振荡,而其他高阶模由于高损耗不能振荡,实现了基模振荡输出。采用简单平平腔结构,在泵浦功率为104W时,实现了51.2W高功率基模输出,测得的激光光束质量M2为1.2。激光放大器是实现高功率激光输出的重要途径,通常情况下,由于热致畸变效应的存在,放大过程中激光光束质量会持续恶化,放大后的激光光束质量远差于激光振荡器输出的激光光束质量。通过建立端面泵浦激光放大器的理论模型,研究了端面泵浦固体激光放大器中的激光光束质量控制及改善技术。指出在放大过程中,影响放大后激光光束质量的主要因素是信号光的球差。如果信号光的球差是负球差,那么信号光通过放大晶体之后,负球差可以被激光晶体的正球差补偿,从而改善激光光束质量。在单端泵浦放大实验中,实验上实现了光束质量改善。信号光光束质量因子M2为2.2,放大光光束质量因子M2改善为1.4。在双端泵浦单级MOPA实验中,同样实现了光束质量改善,光束质量因子M2从2.71改善为1.67。实验结果和理论仿真结果符合得很好。最后研究了侧面泵浦固体激光器及非线性频率变换。研究了热致双折射效应,指出自然双折射晶体不需要考虑热致双折射效应,而光学各向同性晶体必需考虑热致双折射效应。用Nd:YAG双棒串接结构补偿热致双折射效应,同时采用基模动态稳定腔设计,实现了35W基模1064nm激光输出。分别采用腔外四倍频、三倍频的方法实现了1.85W266nm和11W355nm紫外激光输出。同时研究了BBO非线性晶体中的自热效应。更多还原

【Abstract】 This disseratation is composed of four parts:the thermal effects in a diode-end-pumped solid-state laser; the beam quality evolution of the laser field oscillating inside an oscillator&the influence of spherical aberration on the laser resonator; investigation of beam quality improvement in a diode-end-pumped amplifier; side-pumped solid-state laser&nonlinear frequency convertion.Beam quality is a critical parameter for a laser. Control and improvement of laser beam quality are necessary. Laser beam quality is affected by the thermal effects and gain effects of the laser crystal. In this dissertation, the influence of thermal effects and gain effects on the laser beam quality is investigated, in order to improve laser beam quality.Firstly, the thermal effects in a diode-end-pumped solid-state laser are studied. The crystal’s temperature distribution is solved from the heat conduction equation using analytical method and finite difference method. The optical path difference (OPD) of the laser crystal is calculated, and the thermal lens effect and thermally induced aberrations are investigated. The influence of spherical aberration on the focal length of thermal lens is studied. A laser with different beam radius passing through a thermal lens has different focal length. With the decomposition of the OPD distribution over the slices in terms of the Zernike polynomials, it finds out that the main aberration is spherical aberration. The thermally induced aberrations can be represented by spherical aberration coefficient. A new method using the diffraction pattern of a probe laser to measure the focal length of a thermal lens is proposed, which can be used when the laser is on operation.Based on the study of thermal effects, a theoretical model of end-pumped laser oscillator is developed, considering both thermal effects and gain effects. The stable transverse mode oscillating in the resonator is simulated. The discrepancy of the beam quality at two cavity mirrors is discovered. The beam quality improvement in a laser oscillator is confirmed experimentally and theoretically. The beam quality improvement is attributed to the thermally induced aberrations. A laser with negative spherical aberration is compensated by the thermally induced aberrations of the laser cystal, when it passes through the laser crystal. The influence of spherical aberration on resonator’s stable zones is also studied. The spherical aberration induced stable zones separatioin of different order’s transverse modes is pointed out. Fundmental mode operation can be achieved by choosing proper working point. An experimental setup of a diodes dual-end pumped Nd:YVO4laser is built up.51.2W fundamental mode operation is achieved with a pump power of104W, and the beam quality factor M2=1.2.Laser amplifier is an effective approach for high power laser output. However, the beam quality of the amplified laser is always worse than that of the input laser because of thermally induced aberrations. The beam quality improvement in a laser amplifier is investigated using a theoretical model of end-pumped laser amplifier. The importance of the spherical aberrtion of the input laser on the beam quality of amplified laser is discovered. An input laser with negative spherical aberration is compensated by the thermally induced aberrations of the laser cystal when it is amplified, and the laser beam quality is improved. Both in the single-end pumped and dual-end pumped laser amplifier experiments, the beam quality improvements are confirmed. The beam quality facter M2is improved from2.2to1.4in a single-end pumped laser amplifier. The beam quality facter M2is improved from2.71to1.67in a dual-end pumped laser amplifier. The experimental and numerical simulation results are in good agreement.Finally, side-pumped solid-state laser and nonlinear frequency convertion are studied. The thermally induced birefringence is investigated. A two-rod fundamental mode dynamic stable laser resonator is built up, and35W fundamental mode operation is achieved. Using external cavity nonlinear frequency conversion,1.85W266nm and11W355nm ultraviolet laser output are obtained. The thermal effect of the BBO nonlinear optical crystal is also studied.更多还原