Beam Amplification via Multiwave Mixing in a Kerr Medium.

This work has been focused on the study of beam amplification via multiwave mixing process in Kerr media. The analytical and the numerical solutions of coupled-wave equations of the multiwave mixing process are presented. In the analytical part, a non-depleted pump beam and a transparent medium are assumed. The analytical solutions predict that due to the existence of the diffracted beam, energy of the input beams can be exchanged even when phi = 0, where phi is the phase shift between the interference grating and the refractive index grating. They also predict that, under certain conditions, the phase mismatch can be compensated due to the nonlinear beam coupling. In the numerical part, six beams are considered and the assumptions made in the analytical part are removed. The numerical solutions show that, due to the existence of the diffracted beams, phi for the maximum gain is a function of the interaction length and the pump beam intensity. They also show that the probe beam gain is saturated as the pump to probe beam ratio is sufficiently large. Multilayer nematic liquid crystal (NLC) films with a layer thickness of 90 μm have been used in the beam amplification experiments. A multilayer NLC film can provide a longer interaction length. Probe beam gains of greater than unity have been observed for a pump to probe beam ratio of 200, phi = 0, and lambda = 514.5 nm. Experimental results indicate that high probe beam gains are limited by the scattering loss in NLC. The scattering cross section as a function of the wavelength in the NLC films has been found to be proportional to lambda^ {-2.3}. This implies that the use of lambda = 10.6 μm can reduce the scattering loss by about 400 times. The probe beam gains of the nearly degenerate multiwave mixing process in NLC films have been measured for both positive and negative phase shifts. The probe beam gains can be enhanced when the phase shift is positive and degraded when they are negative. The overall results show that beam amplification with multilayer NLC films in the infrared is promising.