Quasi-cw-pumped Nd:YAG laser using a negative-branch unstable resonator with a phase-unifying output coupler

Abstract
Large near-diffraction-limited beams 6 mm in diameter have been extracted from a quasi-cw-pumped Nd:YAG rod using a negative-branch unstable resonator with a phase-unifying output coupler. The Nd:YAG rod was pumped by one Kr-arc lamp driven by an ac current in a rectangular pulse shape to average output laser powers of up to 15 W, with a beam quality of approximately two times diffraction limited.