Near diffraction limited output from a 100 ns XeCl laser fitted with a phase-unifying-cavity

The performance of a long pulse XeCl laser, fitted with a negative branch unstable cavity using a phase-unifying (PU) mirror as output coupler, has been investigated. Laser beams with a divergence close to the diffraction limited value and with a relatively uniform intensity profile have been extracted by using an unstable cavity with magnification factor M identical with 2. It is shown that PU-unstable cavities are quite peculiar tools with respect to hard-edge unstable cavities, mainly for active media having a large cross section and requiring low magnification factors. Indeed, the beam divergence is increased by about 50% by replacing the phase-unifying mirror with a hard-edge aluminized mirror.