Phase conjugate oscillators

Abstract

Optical resonators containing a phase conjugate mirror (PCM) have been a subject of great interest, where the PCM is employed as an end mirror of the resonator cavity for intracavity aberration correction.1 Recent theoretical analysis indicates that the insertion of a PCM inside a ring laser cavity results in a reduction of the lock-in threshold and reduces the imbalance between the amplitudes of the oppositely directed traveling waves.2 In the extreme case of phase conjugate oscillation without conventional gain, lock-in can be completely eliminated.3 We have developed a general theory for nondegenerate oscillations in a phase conjugate oscillator (PCO) i.e., an optical resonator with a PCM as an intracavity element. The PCM consists of a nonlinear transparent medium pumped by a pair of counterpropagating laser beams so that phase conjugation of an input beam with possible gain is achieved by nondegenerate four-wave mixing (NFWM). The linear absorption/gain in the medium is also taken into account. In the absence of any conventional mirrors so that the PCO behaves like an ordinary PCM, we recover the results of phase conjugation by NFWM.4 Our study shows that the nonlinear gain required for oscillation is considerably increased/decreased due to the linear absorption/gain in the medium, while the bandwidth and the sidelobe structure of the bandpass filter are also affected. In the case when there is only one conventional mirror, the PCO reduces to a phase conjugate resonator (a resonator bounded by a conventional mirror and a PCM). Our theory shows that nondegenerate oscillation is possible in such a resonator. The theory can also be used to study the effects of linear absorption/gain on the filter operation.