A study is made of the characteristics of wavefront reversal in a resonantly amplifying medium. The region of the effective interaction between waves is of finite size and the phase relationships between the constant and alternating components of the resultant signal influence the signal increment.
A theoretical analysis is made of the efficiency of wavefront reversal under conditions of a degenerate form of the four-wave interaction in a resonantly absorbing (amplifying) medium allowing for changes in the pump waves as a result of their natural resonant absorption (amplification). The coefficient of reflection into a reversed wave is obtained as a function of the optical thickness of a layer of the medium, the boundary values of the pump wave intensity, and the shift of the field frequency from the line center. The optimal values of these parameters and the corresponding maximum reflection coefficient are determined.
An investigation is made of wavefront reversal of optical radiation in a nondegenerate four-wave interaction in a resonant medium. It is shown that for a pump intensity comparable with the saturation intensity of the absorption line of the resonant medium, the reversed wave spectrum has two side peaks positioned symmetrically relative to the central peak which corresponds to a degenerate interaction. These side peaks are attributable to regions of negative absorption (amplification) of weak radiation by the resonant medium situated in a high-power field of opposite pump waves. The dependences of the magnitudes and positions of these peaks on the pump intensity can be used to control the spectral and energy characteristics of the reflected radiation. The conditions for self-excitation of a reversing system based on resonant four-wave interaction are determined. In particular, the ranges of the pump intensity and the optical density of the resonant medium in which self-excitation of the reversing system may occur, are determined for a degenerate interaction.
A theoretical analysis is made of the conditions which a resonantly excited medium must have to act as a wavefront-reversing mirror. In the absence of saturation by the incident signal, the absorption cross section at the frequency of this signal should be considerably less than at the reversed-wave frequency. In the case of strong saturation it is preferable to use substances with narrow spectral lines. If a wavefrontreversing mirror is used to compensate distortions of the front of a wave during amplification, the necessary contrast of the reversed wave is a function of the gain and of the width of its profile.
An analysis is made of the theory of four-wave wavefront reversal allowing for linear absorption and reflection of all the interacting waves from a mirror which forms a second reference wave from the first incident wave. The Born approximation is used to find the optimum thickness of the nonlinear medium giving the maximum reversal coefficient. Conditions for parametric four-wave oscillation with an arbitrary frequency shift are determined and the dependence of the threshold intensity of the incident reference wave on the linear reflection coefficient of the wave-forming mirror is found for a given thickness of the medium in the absence of absorption.