Generation of an optical beam with two-dimensional dark focus in a fractionally-degenerate cavity

Abstract

An optical beam with a dark region surrounded by light in two dimensions, formed as a coherent superposition of mainly two Hermite–Gaussian eigenmodes HG04 and HG08, has been generated in an axially end-pumped Nd:YAG laser with an astigmatic fractionally degenerate semiconfocal cavity with an intracavity obstacle (a needle). Being focused with a lens, the beam has a dark region on its axis, surrounded by high-intensity regions only in z- and y-directions while maintaining Gaussian behavior in the x direction. The influence of the number of HG0n eigenmodes and comparison of the beam with a geometric M-mode are studied numerically.