Experiments using a new photorefractive gain mechanism involving two pump beams are described. A ring resonator is used to provide a frequency-shifted signal beam. The gain with two pumps of equal intensity is found to exceed that with a single pump by about a factor of 4.
We report the observation of bistability in a unidirectional ring resonator that employs photorefractive barium titanate for gain and also for saturated loss. One photorefractive crystal provides gain via two-beam coupling in a conventional unidirectional ring configuration.1 A second crystal is arranged to couple light out of the resonator using two-beam coupling, thereby producing loss. The orientations of the two crystals are chosen so that the magnitude of the small signal loss is greater than the magnitude of the small signal gain. Thus the oscillation in the ring cannot build up from noise since there is no net gain. However, the pump intensity for the gain medium is made larger than that for the loss medium. This means that the loss saturates at a lower signal level. Hence, at large intensities, gain can exceed net loss so oscillation can occur.
We demonstrate an optical flip-flop that consists of two unidirectional ring resonators that are coupled competitively through two-beam interactions in a photorefractive BaTiO(3) crystal. This crystal provides an active loss for each ring that depends on the oscillation intensity of the other. The coupled rings exhibit flip-flop bistability when the net gain, given by the product of the passive cavity losses and the gain, is greater than unity but is less than the small-signal active loss. The state of the system can be switched with an injected signal.