Moding Effects in Open Resonators with Aperture Coupling

It is well known
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that gaseous optical lasers show a reduction of output power as an effect of mode degeneracy for discrete separation of the end mirrors. This effect is also important when open-resonators, using two spherical mirrors, or one flat and one spherical mirror, with aperture coupling are used for measurement of such parameters as refractive index, absorption coefficient, dielectric properties and scattering cross-sections. In such a resonance system a set of modes may exist, designated by TEMp, l, q, where p is the number of radial field variations, l is the number of azimuthal field variations, and q is the number of standing wave maxima between the mirrors. For a symmetrical circular input coupling aperture l = O. The system becomes resonant for a particular mode when the mirror separation D is equal to an integral number of half-wavelengths of that mode. Different modes have slightly different wavelengths.