Image restoration based on phase conjugation in second-order nonlinear medium

A phase conjugate replica of the complex incident signal electric field can be generated by through optical phase conjugation (OPC) based on one of the nonlinear optical processes. One of the important applications for OPC is the full recovery of an image after it transmits through a phase distorting medium. Such distortion makes the image have a poor contrast, reduces the spatial resolution, and generates additional noises. In the past, OPC was demonstrated based on photorefractive effect [1] and degenerate four wave mixing [2], which can be used to in principle to correct the blurred images due to phase distortion. However, since temporal response utilizing photorefractive effect is limited by electro-optic effect due to space-charge field, the image restoration has response times on the orders of 100 μs-1 s. For atmospheric turbulence, recovery of an image based on such a scheme is too slow to recover distorted signals and blurred images due to the dynamic turbulence. Since four wave mixing is based on third-order nonlinearity, high peak powers produced by Q-switched lasers are required. In contrast, OPC using difference frequency generation (DFG) in a second-order nonlinear crystal requires much lower powers [3]. Moreover, since such a scheme has instantaneous response time, it is perhaps the only scheme which can be used to fully recover distorted signals and blurred images after transmitting through atmosphere. In this presentation, we summary the progress made on the recovery of the blurred image caused by atmospheric turbulence to the original quality by the phase-conjugated beam generated by DFG in a second-order nonlinear KTP crystal and composite. Based on our results we conclude that the phase distortion caused by the dynamic atmospheric turbulence can be completely suppressed by imaging the phase-conjugated beam.