Abstract

In the past few years, several authors have reported observations of chaotic behavior in various photorefractive phase conjugators which involve some form of external feedback, either by way of mirrors or reflections from the crystal faces [1]. We have recently observed such behavior, for the first time to our knowledge, in an externally pumped phase conjugate mirror with no such feedback mechanisms. The origin of chaotic behavior in such a system is fundamentally different from that in either of the former two cases. In the case of the phase conjugate resonator, the chaos apparently arises from a competition between different cavity modes, and in the self-pumped phase conjugator from competition between different photorefractive gratings with different time scales, while in the present case where all the beams are externally supplied, the chaotic behavior is the result of time-dependent phase transfer among the four beams [2]. For the particular set of parameters (coupling constant, geometrical factors, etc.) used in our experiments, theory [3] predicts that such unstable behavior will not occur in ordinary photorefractive four-wave mixing. It will be shown, however, that the observed chaotic oscillations in the phase conjugate intensity are the result of enhanced gain which can occur when one of the pumping beams deviates slightly from the Bragg angle for the photorefractive phase grating (off-Bragg four-wave mixing, or OBFWM)[4].

© 1992 Optical Society of America