THERMODYNAMIC BOUNDARY ON THE PHASE CONJUGATED CORRECTION OF OPTICAL DISTORTIONS.

From the thermodynamic point of view, there is an apparent paradox in the idealized (lossless) phase conjugate beam healing process. When a beam of light passes through a phase aberrator, an object having spatially varying refractive-index gradients, the entropy of the light increases. On phase conjugate reflection the reflected (conjugate) wave retraces its path, and, on retransit through the phase aberrator, the transmitted beam is restored to its original state. If this restoration were perfect, the entropy of the reconstructed beam would decrease. Thus second-law considerations require an energy (photon) loss from the transmitted light beam sufficient to compensate for the entropy reduction. A bound on the efficiency of beam healing via phase conjugation is determined using second-law considerations alone. The first step in the analysis is to find a mechanism for entropy production during phase aberrator retrace, the second step is the calculation of the minimum entropy produced, and the final step is the determination of the loss associated with this entropy production. The determination gives some interesting insights into the wavefront reconstruction process. These are independent of system configuration or dynamic details.