We present a simple mathematical model giving a possible description of a partially coherent light beam exhibiting a flat-topped transverse intensity profile. Such a model allows us to deduce the modal distribution inside a multimode stable optical cavity, assuming that the modes are of the Hermite-Gauss type. The analytical expression used to represent flat-topped profiles is of the flattened Gaussian type and leads to an exact, closed-form expression for the M-2 factor of the output beam. An analogous procedure could be used to treat the general problem of deducing the modal distribution inside a laser cavity starting from intensity measurements of the output beam. (C) 1998 Optical Society of America. [References: 18]
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