Construction and performance of a 20-pps unstable Nd:YAG oscillator

A positive branch Nd:YAG unstable oscillator has been constructed using a small reflecting spot centered on an antireflection coated substrate as the output mirror. The confocal condition is achieved by taking account of the lens effect of the rod as well as the back mirror curvature. Q-switched output energies of 180 mJ in 10 ns have been obtained in a diffraction limited beam with a divergence of less than 0.25 mrad, at an overall efficiency of 1.3%.