An exact solution to photorefractive four-wave mixing equations with c
omplex couplings in reflection geometry is obtained. It is shown that
the efficiency of the process of phase conjugation can be enhanced by
introduction of a frequency shift between the pumps and the signal, si
milar to the case of transmission geometry. However, to obtain an impr
oved agreement with experiment, the inclusion of transverse effects is
found to be necessary. (C) 1996 Optical Society of America