Abstract

Previously¹ we used a weak probe Mollow sideband calculation in a three-level system to calculate thresholdless Raman gain without inversion for comparison with experimental observations. However the complexity of the result makes it difficult to extend this analysis to compute nonlinear wavemixing or phase conjugate reflectivities in the three-level Λ-system. We now show that under conditions of experimental interest, it is possible to model the three-level system by a four-level four laser field closed loop system² which results in a simple closed form expression for the gain. The advantage to this simple gain expression is that it permits straightforward computation of wavemixing and phase conjugate reflectivity. Detailed analysis of the four level system approach will be presented to clearly identify the regions where it is valid to model a three-level system by a four-level system. In particular, in the limit of low intensity or large detunings, the four level calculation is in good quantitative agreement with three level Mollow sideband gain calculations which in turn agreed qualitatively with experimental results.^3-4 We will also show preliminary results of wavemixing calculations for sodium vapor using the simple gain expression produced by this model.

© 1993 Optical Society of America