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The paraxial approximation is used to study the structure of the oscillation modes and normal frequencies of a resonator with a wavefront-reversing mirror. It is shown that when this resonator is filled with a linear and reciprocal medium, the fundamental oscillation mode has lower losses than the fundamental oscillation modes of any filled resonators formed by conventional mirrors of the same size. The absence of discrete normal frequencies in the resonator is noted.
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Reznikov et al. (1980) studied this question.
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