Transmittivity profile of high finesse plane parallel Fabry-Perot cavities illuminated by Gaussian beams

We report on the deviations from the standard Airy pattern of the transmittivity of a high finesse (F similar to 1000) plane parallel Fabry-Perot cavity (PPFP) illuminated by a Gaussian beam with spot size much smaller than the mirror diameter. Steady reductions of peak transmittivity and finesse have been observed when increasing the cavity length L from 10 mu m up to 6 mm. The transmittivity profile (versus the phase deviation cp from the nominal resonance condition) becomes the more asymmetric the greater the length L. These features have been elucidated by using a modal description of PPFP resonators based on the extension of the Weinstein theory to a multimode excitation, which can provide useful insights on cavities of greater finesse.