Modes and losses of the planar Fabry-Perot and confocal resonators have been computed using various kernels obtained by different approximations of diffraction theory. For Fabry-Perot resonators no practical influence was found of the shape of the kernel on the results, at least for Fresnel numbers ranging up to 42.25. For the confocal resonator large differences were found even at small Fresnel numbers N ≳ 1. In particular, a qualitative difference was found between the results obtained by the so-called parabolic approximation and the kernel of Rayleigh-Luneberg diffraction theory. These differences confirm the influence of the amplitude in the approximations.
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