<title>Off-axis one-sided negative-branch unstable resonators compared with positive-branch unstable resonators in wave-optics characteristics</title>

Abstract

ABSTRACT Wave optics calculations for power losses and mode profiles of off-axis positive branch and negative branch unstable resonators have been investigated. The beam modes become fully one-sided with moderate power losses by shaving oneside of the front mirror by 70% to 90% for a positive branch unstable resonator, and by 33% (=(M]-1)/JM1) to 80% for anegative branch unstable. At the conference, we also discuss the advantage of negative branch unstable resonators for theapplications to high-power lasers.Keywords: Resonators, unstable resonators, beams, beam modes, positive branch, negative branch 1. INTRODUCTION To extract high quality beams from slab gain media, so called off-axis unstable resonators are attractive, and such resonatorshave r been successfully applied to solid-state lasers and carbon dioxide lasers4. In industrial field, negative-branch off-sided unstable resonators are becoming more and more dominant in spite ofthe anxious ofhot-spot problems in theresonator without enough theoretical investigations on the comparisons with positive-branch unstable resonators.Many authors have investigated misaligned unstable resonators theoretically 51 2, however, only a few 5 extendedtheir calculations to fully one-sided unstable resonators, and some of these calculations were also limited to geometricalcalculations or to special cases with Gaussian reflectors13 and could not be helpful to investigate off-axis one-sidedunstable resonators.In this paper, we show first the basic geometrical characteristics of off-axis unstable resonators and then discuss thepower losses and the beam modes by using diffraction calculations comparing positive branch (PB) and negative branch(NB) unstable resonators. Finally we discuss the advantage of negative branch unstable resonator for high-power lasersystems.