Diffraction losses and mode structure of equivalent TEMoo optical resonators

It is shown that to characterize the diffraction losses of any arbitrary TEM(00) laser resonator it suffices to consider only those resonators that are symmetric in their 'g' parameters, even if they are unstable. For stable resonators, a further reduction of the number of possibilities was obtained by relating the aperture diameter to the TEM(00) beam diameter by a factor 's'. It is also demonstrated that for unstable resonators, where no Gaussian beam solution (and therefore no adapted aperture) is possible, the class of resonators with diffraction losses at one mirror and none at the other is of special practical interest.