Phase conjugate (PC) wave is generated by degenerate four-wave mixing (DFWM) where pump beam intensities are below the saturation intensity of Q-switched ruby laser beam (10 MW/㎠). Cryptocyanine dissolved in methanol is used as a nonlinear medium and the four waves involved are coherent each other. The reflectivity experimentally obtained shows that it does not have dependence on the probe beam intensity, but increases proportionaly to the square of the pump beam intensity. The reflectivity becomes to maximum when the molar concentration of the cryptocyanine solution is $8×10^{-6}$ mole/l and the thickness of the nonlinear cell is 4mm.
In absorbing medium such as cryptocyanine solution, the most dominant mechanism causing the phase conjugation is thermally induced refractive index changes, and in order to interpret the experimental results, and analytical expression of DFWM is derived by considering the generation of the transmission type and reflection type thermal volume gratings at the same time. The solution gives correct interpretation about most of the experimental results. Severe deviation between the theoretical and experimental results is observed if one attempts to explain the results by considering only of the transmission type volume grating.
In volume grating theory, the coherence among the incident waves is of utmost importance, so it is investigated by changing the optical path differences of the incident waves. In this experiment, the coherence length of the ruby laser beam is found to be 5.18±0.17 cm.
Finally the relative contribution of the transmission and reflection type thermal volume gratings to PC signal is measured by changing the polarization states of pump beams. The result obtained is that the PC signal intensity diffracted from the transmission grating is 1.53-2.14 times larger than that from the reflection grating, depending on the angle between the wave vectors of the forward pump beam and the probe beam. The experimental result is interpreted well in terms of the equations derived from the polarization dependent coupling constants. It is concluded that the contribution of reflection type thermal volume grating to PC signal is as important as that from transmission grating, and the coupled wave equations give more general and accurate expression of reflectivity ($│A_s/A_p│^2$).
