采用具有近共焦、非稳腔特点的多程折叠光路结构，使激光光束多次通过增益介质，实现了高提取效率的激光放大器。实验中在注入22W种子激光的条件下，以43．9%的提取效率得到了63W的激光输出，斜效率超过52%，激光放大器的输出光束质量从种子激光的M(2，X)=2．08，M(2，Y)=1．92变为M(2，X)=2．84，M(2，Y)=1．79。

A Nd∶YVO₄ laser amplifier is reported,which is characterized by a near-confocal,unstable multipass structure.The laser beam from oscillator goes through the Nd∶YVO₄ several times as designed for high extraction efficiency. Based on a 22 W seed laser input,a 63 W output is obtained with extraction efficiency and slope efficiency being 43.9% and over 52% ,respectively: The M² factors of output beam quality are measured to exchange 2.84 at X direction and 1.79 at Y direction ,form 2.84 at X direction and 1.79 at Y direction for seed laser.