Feedback and acousto optic isolation effects on laser stability

This paper analyzes the effect of optical feedback on laser frequency stability and the acousto optic isolator concept, which was demonstrated previously. The frequency pulling effect associated with feedback of a signal at a frequency other than the oscillator frequency has been shown to require a nonlinearity, such as saturation, in the laser medium. The analysis mathematically corroborates the initial acousto optic isolator concept and the limited experimental data available. In the study of the acousto optic isolator, it was determined that an acceptable analytic expression for the imaginary part of the susceptibility of an unsaturated inhomogeneously broadened line, and the expression for the susceptibility of a saturated inhomogeneously broadened line, were not available. The expressions which were desired for the analysis in this paper were obtained for what is believed to be the first time. The results of this paper provide a foundation for the acousto optic isolator. Applicability of these findings should increase as coherent laser radars, which require isolation to insure accuracy of the Doppler information in the return signal, are developed.