Introduction

Non-linear optical phase conjugation by degenerate four-wave mixing (DFWM) is an important technique with applications in many fields of science and technology including image transmission, optical image processing, optical filtering, and laser resonators[14, 29, 8]. When two counter-propagating and intense light beams interact with a non-linear medium, together with a less intense third one, a fourth beam is generated from the medium, which will be the phase conjugation of the third beam. This technique is called four-wave mixing. The unique feature of a pair of phase conjugate beams is that the aberration influence imposed on the forward (signal) beam passed through an inhomogeneous or disturbing medium can be automatically removed from the backward (phase conjugated) beam passed through the same disturbing medium[11]. The main applications of degenerate DFWM techniques are non-linear spectroscopy, real time holography, and phase conjugation. Phase conjugation by DFWM has been demonstrated in many organic and inorganic materials using pulsed or continuous-wave (CW) lasers[27, 12].

The phase conjugate light has a variety of characteristics and properties not associated with normal light and they can be enumerated as follows:

• (1) Phase compenzation effect: Waves that pass through an aberrating medium are injected into a phase conjugate mirror. If the outgoing wave is re-propagated through the medium, the waves phase distortion is compensated.

• (2) Space domain multiplicative interaction: Since phase conjugate waves are produced by interaction in a non-linear medium, the spatial multiplicative effect among the light waves appears in the phase conjugate waves.

• (3) Intensity dependant phase shift: Due to the optical Kerr effect in a non-linear medium, the phase shift of the electric field depends on the wave intensity.

• (4) Detuning dependence: The reflectance of a phase conjugate mirror strongly depends on the detuning between the pump and probe wave frequencies.

• (5) Time inversion: A phase conjugate wave can be regarded as a time inverted wave since its direction of propagation is exactly opposite that of the probe wave and the wave front is identical to that of the probe wave.

• (6) Time domain multiplicative interaction: When pulsed waves are used in the production of phase conjugate light, the resulting polarization, that is, the generalized phase conjugate wave intensity depends on the product of the interacting waves in the waves’ temporal domains.