Abstract: This paper investigates the temperature field distribution and thermal focal length within a laser diode array (LDA) end-pumped YVO₄/Nd:YVO₄ rectangular composite crystal. A general expression of the temperature field distribution within the Nd:YVO₄ rectangular crystal was obtained by analysing the characteristics of the Nd:YVO₄ crystal and solving the Poisson equation with boundary conditions. The temperature field distributions in the Nd:YVO₄ rectangular crystal for the YVO4/Nd:YVO₄ composite crystal and the Nd:YVO₄ single crystal are researched respectively. Calculating the thermal focal length within the Nd:YVO₄ rectangular crystal was done by an analysis of the additional optical path differences (OPD) caused by heat, which was very identical with experimental results in this paper. Research results show that the maximum relative temperature on the rear face of the Nd:YVO₄ crystal in the composite crystal is 150 K and the thermal focal length is 35.7 mm when the output power of the LDA is 22 W. In the same circumstances, the experimental value of the thermal focal length is 37.4 mm. So the relative error between the theoretical analysis and the experimental result is only 4.5%. With the same conditions, the thermal focal length of the Nd:YVO₄ single crystal is 18.5 mm. So the relative rate of the thermal focal length between the YVO4/Nd:YVO₄ crystal and the Nd:YVO₄ crystal is 93%. So, the thermal stability of the output power and the beam quality of the YVO₄/Nd:YVO₄ laser is more advantageous than the laser with Nd:YVO₄ single crystal.

Key words: thermal focal length, temperature field distribution, rectangular laser composite crystal