The beam quality of an optical parametric oscillator (OPO) within a singly resonant, confocal-positive branch unstable resonator is investigated. Resonator configurations have been found in which the beam quality of the outgoing signal exceeds the beam quality of the pump. Cavity magnification and pump-pulse duration are found to determine the signal beam quality. It is shown that signal <i>M</i><sup>2</sup> decreases with increasing pump-pulse duration for a given cavity magnification. In an experimental demonstration of a LiNbO<sub>3</sub> OPO within an unstable resonator, pumped by a multitransversal mode beam, a signal beam with an almost-single transversal mode has been generated, whereas the multitransversal mode behavior of the pump beam is projected on the idler beam.
© 2003 Optical Society of America
Shaul Pearl, Yosi Ehrlich, Shlomo Fastig, and Salman Rosenwaks, "Nearly Diffraction-Limited Signal Generated by a Lower Beam-Quality Pump in an Optical Parametric Oscillator," Appl. Opt. 42, 1048-1051 (2003)