Extension of the self-filtering unstable resonator to a ring cavity

We present a numerical and experimental study of a confocal unstable ring r esonator that performs similarly to a self-filtering unstable resonator. Th e resonator presents two apertures: one of them is the output coupler and t he other provides spatial filtering. A simple rule for the choice of the ap erture size enables us to expand the output beam diameter without increasin g the magnification or the total length. The resonator is tested in a TEA C O2 laser in which the output beam diameter, the output energy, and the effe ctive reflectivity are measured. The energy ratio of the forward and revers e waves scales approximately as the square of the magnification. The result s show good agreement with the theoretical values. (C) 2001 Society of Phot o-Optical Instrumentation Engineers.