M ultim ode Resonators with a Small Fresnel Number ( Lowest - Order Eigenmodes

The F a b r y P e r o t Interferometer, the confocal and the spherical resonator systems are investi­ gated. The lowest-order traveling-wave type eigenmodes are calculated. Numerical values for the diffraction losses are given. The smallest diffraction losses are obtained for the general-type eigenmode of a confocal resonator system. The eigenfunctions of an open-walled resonator show a point of inflection as their characteristic feature. They are complex if the F r e s n e l Number is finite. When calculate over appropriate surfaces, their imaginary part, in the region close to the axis, decreases as F increases. In that region the waves resonate between the reflectors. Towards the rim of the system the imaginary part increases rapidly as do the diffracted waves associated with the imaginary part.