Possible use of resonantly excited media in wavefront reversal

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.