SIMPLE MODIFICATION OF POSITIVE-BRANCH UNSTABLE RESONATOR FOR DIFFRACTION-LIMITED XECL LASER-BEAM

A simple modification of the output coupler to a standard positive-bra nch confocal unstable resonator (PBUR) was proposed and the output cha racteristics were investigated and compared with those of the standard PBUR. We utilized a small convex mirror of 1.8 mm diameter as an outp ut coupler in order to reduce the beam divergence and to eliminate the subsidiary spot caused by the Fresnel reflection effect in the standa rd PBUR. We obtained a single spot in the focal plane with a nearly di ffraction-limited beam divergence of 0.09 mrad.