Efficiency and quality of four-wave phase conjugation of a signal with a time-dependent spatial structure

It is demonstrated theoretically and experimentally that, in four-wave phase conjugation of a spatially incoherent speckle signal, the reflection coefficient and the phase conjugation fidelity fall on increase in the rate of change of the spatial structure of the speckle beam. The law of decay of both these parameters is the same for media with an exponential grating erasure rate, but in the case of double-exponential relaxation the conjugated fraction of the signal decreases much more slowly than the reflection coefficient. Experimental results show that the standard model of deep trapping levels can describe the kinetics of formation of dynamic gratings in a nominally pure crystal of strontium barium niobate, but the presence of shallow trapping levels has to be taken into account for doped crystals.