Phase conjugation in ammonia

Phase conjugation, a possible technique for the removal of phase distortions during the atmospheric propagation of laser beams, is investigated for the nonlinear medium, ammonia, in the vicinity of 10.6 microns. Following a review of phase conjugation by degenerate four-wave mixing in an absorptive medium, a quantum mechanical rotation vibration model of ammonia is developed with consideration given to third-order susceptibility and the effect of Doppler broadening on the susceptibility. Model calculations are presented which indicate that phase conjugate efficiency is sensitive to model parameters, with absorption reduced and high efficiencies and lasing possible at pressures less than 1 torr and pump powers less than 1 MW and absorption playing a dominant role at pressures greater than 1 torr. Phase conjugate efficiency is also shown to decrease significantly with increasing linewidth and temperature. Finally, the theoretical efficiencies are shown to be in order-of-magnitude agreement with experimental measurements of the nonlinear susceptibility of ammonia.