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Soviet Journal of Quantum Electronics

Phase conjugation of pulsed CO2laser radiation by four-wave mixing in CCl4

Alexander A. Betin, O V Mitropol'skiĭ

Published Apr 30, 1988• 1 citation
Publisher

Abstract

An experimental investigation was made of phase conjugation under conditions of degenerate four-wave interaction of CO2 laser radiation in carbon tetrachloride as a function of various parameters of the signal radiation and of the experimental geometry. It was found that the conjugation accuracy for a single-mode signal beam with a divergence of 2 mrad was 87%, but when a phase plate introducing a divergence of 70 mrad was used, the accuracy was 77%. The reflection efficiency and the phase conjugation quality depended strongly on the ratio of the energy density of the signal wave to the energy density of the pump wave. An investigation was made of the dependence of the reflection coefficient on the angular mismatch between the counterpropagating pump waves and on the matching of the optical paths of the interacting waves.

DOI
10.1070/qe1988v018n04abeh011773

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 20, 1996•0 citations
An experimental investigation was made of phase conjugation under conditions of degenerate four-wave interaction of CO2 laser radiation in carbon tetrachloride as a function of various parameters of the signal radiation and of the experimental geometry. It was found that the conjugation accuracy for a single-mode signal beam with a divergence of 2 mrad was 87%, but when a phase plate introducing a divergence of 70 mrad was used, the accuracy was 77%. The reflection efficiency and the phase conjugation quality depended strongly on the ratio of the energy density of the signal wave to the energy density of the pump wave. An investigation was made of the dependence of the reflection coefficient on the angular mismatch between the counterpropagating pump waves and on the matching of the optical paths of the interacting waves.
Phase conjugation (PC) processes at degenerate four-wave mixing (DFWM) in the active medium of a pulsed e-beam controlled discharge CO2-laser were studied both experimentally and theoretically. The experiment was carried out with the following parameters: laser gas mixture CO2:N2:He=1:2:4, gas pressure 0.3 bar, active medium length 1.2 m, resonator length 12 m (17.7 m for single line laser), interaction angle ~10 mrad at DFWM without optical delay between probe wave and co-propagating pump one. Integral PC reflectivity (PCR) strongly depends on such parameters as: specific input energy (Fig. 1); intensity ratio for probe J3 and co-propagating pump J1 wave (Fig.2); coherency of the laser radiation, that varied by rotation of mutual polarization or with optical delay between probe wave and co-propagating pump one; geometry of DFWM; output mirror reflectance Rout (8-80%) (PCR was maximum with Rout of 8% but best quality of PC signal was observed with Rout =80%); gas mixture composition and pressure, being maximum for a mixture with a low fraction of helium and at higher pressure (up to 0.6 bar). A periodical dependence of PCR on optical delay between probe and co-propagating pump wave with periods 10−3 and 10−5 m was observed. The PCR did not depend on spectral content and a degree of linear polarization of the laser radiation.
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 (CO2:N2:He equals 1:2:4, p equals 0.28 atm, tin equals 30 microseconds) that the laser pulse length (tauout approximately 10 - 20 microseconds) is comparable with an effective relaxation time of the upper laser level (taurel 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.
The results of experiments on four-wave interaction of electron-beam-controlled discharge (EBCD) pulsed CO2- and CO- lasers' radiation inside their active medium are presented. Linearly polarized probe CO2 (CO-) laser beam intersects strong electromagnetic waves counterpropagating through an inverted medium inside the laser resonator. The laser beam reflected from the active medium has been registered both in near-field zone and in far field. The experiments on recovering of optical images in near-field zone and recovering of angular divergency of laser radiation in far-field zone demonstrate that the reflected beam was a phase- conjugated one. The reflectivity of the phase-conjugated beam was up to 2% for CO2- laser and up to 0.2% for CO- laser. The time-history of phase-conjugation reflectivity and comparison with theoretical results are discussed.
An investigation was made determining the parameters affecting the phase conjugate reflectivity in a four-wave mixing scheme using a pulsed TEA CO2 laser and gaseous sulfur hexafluoride (SF6) as the nonlinear medium. With a low energy CO2 laser (60 mJ pulse energy) an optimum energy reflectivity of 20 % was achieved at a pump-probe angle of 30 mrad, a SF6 pressure of 50 hPa at the lOiim P(1O) laser transition for exactly aligned counterpropagating pump waves. In addition, the reflectivity showed a substructure when enlarging the angular mismatch between the pump waves. A simulation of the phase conjugate intensity distribution was carried out solving the Kirchhoff- Fresnel integral equation.
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 (CO2:N2:He equals 1:2:4, p equals 0.28 atm, tin equals 30 microseconds) that the laser pulse length ((tau) out approximately equals 10 - 20 microseconds) is comparable with an effective relaxation time of the upper laser level ((tau) rel 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.