The diffraction losses of the cylindrical confocal resonator having a circular coupling aperture in the center of one of the resonator mirrors have been calculated over the range of resonator Fresnel numbers

by the numerical iteration technique of Fox and Li. The specific paths by which energy is diffracted from the resonator have also been investigated and a class of equivalent resonators found that maximizes the energy diffracted through the aperture for a given value of total diffraction loss.