Phase-Conjugate Optics,

TLDR: In this paper, the authors proposed a phase conjugation method to generate a time-reversed replica of an incident wave in real time without the need for intermediate electronics, and with amplification if desired, using nonlinear optical mixing.

Abstract: A new research area in coherent optics has emerged and has been receiving increasing attention from many scientists as its important applications are recognized. Phase conjugate optics is the name which seems to have attached itself to this new field. The main feature of phase conjugate optics is the generation of an electromagnetic wave with a phase distribution which is, at each point in space, the reversal of that of an arbitrary incoming monochromatic wave. The wavefront, after being generated, proceeds to propagate in the opposite direction, retracing in reverse the path of the incoming wave. Thus, the phase reversal or conjugation process results in what is frequently called a time-reversed replica of the incident wave. If we consider, as an example, an incoming spherical wavefront which, diverging from a point, has a radius of curvature R, its conjugate-replica will be an outgoing spherical wavefront converging toward the same point and with a radius of curvature -R. Phase conjugation techniques have been used in the past for imaging through phase distorting media; well known examples can be found in holography [1] and adaptive optical systems [2]. The new and attractive feature, which differentiates phase conjugate optics from the previous techniques, is the use of nonlinear optical mixing to generate in real time without the need for intermediate electronics, and with amplification if desired, a time-reversed replica of an incident wave.