Fresnel reflection from a cavity with net roundtrip gain

A planewave incident on an active etalon with net roundtrip gain may be expected to diverge in field amplitude, yet Maxwell's equations admit only a convergent solution. By examining a Gaussian beam obliquely incident on such a cavity, we find that the "side-tail" of the beam leaks into the cavity and gives rise to a field that interferes with the main portion of the beam, which is ultimately responsible for the convergence of the field. This mechanism offers perspective for many phenomena, and we specifically discuss the implications for amplified total internal reflection.