Phase conjugate waves are generated from the stimulated Brillouin scattering and from the degenerate four-wave mixing. Experiments are performed using the linearly polarized $TEM_∞$ or multitransverse mode beam from a 70 mJ, 30 nsec puls duration, Q-switched ruby laser. Due to the wavelength selection property of the resonant output etalon pair of the laser resonator, the linewidth of the laser beam is as small as 0.05Å , which corresponds to about 10cm coherence length.
In stimulated Brillouin scattering, the phase conjugation property is examined using the liquid $CS_2$ and water as the nonlinear medium. It is found that the efficiency of the phase conjugation in $CS_2$ is superior to that of water. The phase conjugation fidelity of SBS in a waveguide with 2 mm inner diameter and 110 cm long and that in the conventional cell are compared, and it is found that the waveguide is more efficient in producing phase conjugate wavefront and requires lower power than that needed in conventional cell.
Degenerate four-wave mixing (DFWM) experiment is performed in the saturable absorber of cryptocyanine dissolved in methanol. Extending the Kogelnik's coupled wave theory of volume grating to the case of degenerate four-wave mixing, the dynamic change of four waves in the nonlinear medium is investigated for the case of thermal grating. The reflectivity of the probe beam versus the ratio of the probeto-forward pump beam intensity $(q=I_p(0)/I_f(0))$ is measured for $p=I_f(0)/I_s = 0.3, 2.5$, and 20. When the pump beam intensity $I_f(0)$ is below the saturation intensity $I_s$, the reflectivity does not change even though the probe beam intensity is increased to the pump beam intensity. However, the reflectivity begins to decrease slowly for moderate pump beam intensity (p=2.5) because of the saturation of the medium, and finally it does decrease as l/q for pump beam intensity far above the saturation (p=20). The above results are explained by the extended coupled wave theory of thermal volume grating.
The importance of coherence between the incident waves is investigated by changing the relative optical path differences between the forward pump and the probe beams $(OPD_1)$, and between the backward pump and the probe beams $(OPD_2)$. The experimental results show that the thermal grating formed by the interference between the backward pump and the probe beams is not important because of the thermal diffusion. The thermal grating formed by the interference between the forward pump and the probe beams contributes mainly to the phase conjugate signal wave. Therefore the coherence between these two beams is critically important in degenerate four-wave mixing employing the thermal effect.
