Characterising output beams of unstable laser resonators employing modal analysis

The modal analysis, based on the physical optics theory, is a strong tool to characterise a wide range of laser beams. From the simplest (fundamental) structure to the more complicated forms such as uniform cross sectional intensity profile (commonly called flat-top or top-hat beams), with or without a central shadow (usually associated with output from edge-coupled unstable resonators). It has been shown that in some cases "pure" modes, and in others the superposition of such modes, can be used to analyse laser beams of arbitrary cross sectional profile. In this paper, the so-called FGBHM (flattened Gaussian beams with a hole in the middle) model that is the superposition of standard modes is employed to characterise the output beams produced by unstable laser resonators. Using this model, the effects of the size of central hole on the beam quality will be discussed.