激光二极管(LD)端面抽运固体激光器的热效应是影响激光输出特性和系统综合性能的重要因素，因此准确的热透镜焦距测量是谐振腔设计的重要前提。从热传导方程入手，计算了LD端面抽运晶体的温度梯度分布，求出探测光束经过晶体后的相位差分布，通过傅里叶变换得到探测光的夫琅禾费衍射图样。基于上述理论模型，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，可实时在线测量。由夫琅禾费衍射图样求出光程差，进而由光程差可求出激光晶体的热透镜焦距。实验测量了激光晶体的热透镜焦距，并与非稳腔法实验数据进行对比，证明了这种方法的可行性。

Thermal effects of laser diode (LD) end-pumped solid-state laser are important factors, which affect the laser output characteristics and the system performance. The accurate thermal focal length measurement is an important prerequisite for resonator design. The temperature gradient distribution of LD end-pumped solid-state laser is calculated with the heat conduction equation. The phase difference and the Fraunhofer diffraction pattern of the probe laser are obtained. Based on the above theoretical model, a new method by means of Fraunhofer diffraction pattern of probe laser to determine the thermal focal length is presented, which enables real-time on-line measurement. The optical path difference is calculated from the Fraunhofer diffraction pattern of probe laser, and then the thermal focal length is calculated from the optical path difference. The thermal focal length of laser crystal is measured experimentally, and the feasibility of the method is proved by comparing with the experimental values measured by unstable cavity method.