Resonant wavefront reversal was observed in the active medium of a copper vapor laser employing the four-wave mixing configuration. The power reflection coefficient was 100%. The process of reversal occurred when the average power of the interacting beams was of the order of tens of milliwatts.
A theoretical and experimental investigation is made of the feasibility of wavefront reversal by four-wave interaction in a medium exhibiting Raman nonlinearity. Theoretical dependences of the reflection coefficient of the four-wave mixer on the gain increment, phase detuning, and pump asymmetry parameter are obtained for the case of plane waves. In experiments using a ruby laser and quasiplane waves the maximum reflection coefficient of the nonlinear mirror was 0.5%, which agreed with the theoretical estimate. An experimental investigation was made of the wavefront reversal quality, and possible methods of improving this are discussed.
A wavefront reversal system is proposed for high-power optical radiation. This system is based on a resonant four-wave interaction without recourse to external pump waves. Stimulated scattering of the wave being reversed in a resonant medium placed inside a resonator creates two opposite pump waves and a parametric interaction between them generates reversed radiation. The efficiency of such self-reversal is comparable with the efficiency of conventional wavefront reversal as a result of a four-wave interaction in nonresonant media with a phase nonlinearity and it may reach 10 −3 .
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.
The two-level approximation is used to make a theoretical analysis of four-wave mixing in the active medium of a laser whose radiation wavefront is to be reversed. Explicit allowance is made for changes in the amplitudes of the pump waves as a result of their amplification during propagation in the active medium. Both the completely degenerate case, when the frequencies of all four waves are the same, and the case of a small detuning of the signal wave frequency relative to the pump waves are considered. For the completely degenerate case a system of reduced equations allowing for saturation of the inversion was solved by computer using an iterative method. It was found that, under optimum conditions in the saturation regime, the power of the wave reflected from a four-wave mixer may be higher than that of the signal wave.
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.