This would happen if the output SBS beam had a wavefront which was a replica of that of the pump beam but travelling in opposite direction. As we shall see this is equivalent to saying that the output wave amplitude is proportional to the complex conjugate of the input wave amplitude. The process of such oppositely travelling replica wave fronts is called optical phase conjugation.

This amazing discovery of the method of wavefront correction did not receive any great attention immediately although Nosach et al[2], from the same group, had demonstrated that the method can be used to generate good quality amplified beams from relatively poor quality (inhomogeneous) laser amplifiers. SBS is an efficient process and has been studied a great deal as a method of optical phase conjugation. We note that the phase conjugate beam has a slightly shifted frequency compared to that of the incident one. References [3-13] provide reviews of this and other methods of nonlinear optical Phase conjugation (OPC).

A spurt in the study of optical conjugate came with the work of Hellwarth[14] who showed that the phase- conjugate beam can be obtained by degenerate four -wave -mixing (DFWM) also. Since then this process has been investigate vigorously as a means of optical phase conjugation as well as for many other applications. In the following we shall concentrate on DFWM as a method of obtaining phase- conjugation.