Calculations of fields in resonators with wavefront-reversing mirrors

It is shown rigorously that the problem of determining the oscillation modes of ring resonators with wavefront-reversing mirrors can be reduced to the determination of the oscillation modes of a certain equivalent resonator with ordinary mirrors. Methods of determining the parameters of an equivalent resonator are described and it is established that this can be both stable and unstable. The advantages of ring resonators with wavefront-reversing mirrors are noted.