Competition Between Amplified Spontaneous Emission and the Four-Wave-Mixing Process

摘要

Competition between amplified spontaneous emission (ASE) and the four wave mixing (FWM) process has been observed under conditions of two photon resonant excitation of the sodium 3d level. The nature of the competition is that the FWM process is able to prevent the occurrance of ASE, even though the gain of the ASE process calculated in the absence of competition is that of FWM. The ASE is suppressed because the fields generated by the FWN process create a new excitation pathway connecting the ground the 3d levels, and under quite general conditions this pathway interferes destructively with that due solely to the applied laser field. These effects are modeled theoretically by solving perturbatively the density matrix equations of the atomic system, thereby determining the population in the upper level and nonlinear polarization of the medium. The coupling between the various optical fields due to the nonlinear polarization is described by coupled amplitude equations. The coupling between the various optical fields due to the nonliner polarization is described by coupled amplitude equations. The solution to these equations predicts that when the wave vector mismatch is not too large the fields evolve spatially to reach steady state values, and that the population excited to the 3d level by the total steady-state optical field is much smaller than that due to the incident laser field alone.