Phase Conjugation Mirror

Can a light wave pass through a disturbing medium, be distorted, reflect off a special mirror, and return to the source undisturbed?

Answer

Yes, the light can return undisturbed if the light wave retraces it original path as its time-reversed twin and the medium retains its previous integrity. The phase conjugate of a wave possesses exactly the same spatial properties as the original wave, but it is said to be reversed in time. This means that a phase conjugate wave exactly retraces the path of the original beam. This method has the useful property that if a light beam propagates through a distorting medium, then the phase conjugate is produced and exactly retraces the path through the distorting medium, enabling the unfavorable effects of the distorting media to be reduced or eliminated. Phase conjugation is the general term for a process in which both the direction of propagation and the overall phase factor of a wave function are reversed.

Some laser sources come with optical phaser conjugators to remove distortion in the laser beam. Optical phase conjugation occurs also when there are four waves mixing with all four waves of the same frequency. Another useful application of a phase conjugate mirror might be to put one in one reflecting path of an interferometer as a reference for detecting changes in the other path.