Degenerate four-wave mixing and phase conjugation of pulsed 10-μm laser radiation on transient gratings inside an inverted CO 2 medium

Abstract

Degenerate four-wave mixing (DFWM) and phase conjugation (PC) of pulsed carbon-dioxide laser radiation on transient gratings inside its own laser medium have been studied both experimentally and theoretically. The operational mode of the e-beam controlled discharge carbon-dioxide laser has been chosen in such a way (CO<SUB>2</SUB>:N<SUB>2</SUB>:He equals 1:2:4, p equals 0.28 atm, t<SUB>in</SUB> equals 30 microseconds) that the laser pulse length ((tau) <SUB>out</SUB> approximately equals 10 - 20 microseconds) is comparable with an effective relaxation time of the upper laser level ((tau) <SUB>rel</SUB> approximately 15 - 30 microseconds). The time-history of the carbon-dioxide laser and PC signal pulses, and PC reflectivity have been thoroughly investigated for different cavity Q factors and specific electrical energy inputs. It has been shown, that the formation of the PC signal under the intracavity DFWM on gain (amplitude) and thermal (phase) gratings is characterized by a complicated time history that reflects the main relaxation processes taking place inside the inverted medium. A feature of the transient PC process inside the laser active medium is the dependence of the effective relaxation time of the upper laser level on the intensity of pumping waves. PC reflectivity has been obtained by numerical calculations. A comparison of the theoretical and experimental data confirms the contribution of two different mechanisms of grating formation under DFWM inside the active medium of the carbon-dioxide laser.