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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Self phase conjugation of middle infrared radiation by four-wave mixingand stimulated scattering

Viktor I. Bespalov, Alexander A. Betin, Konstantin V. Ergakov, E. А. Zhukov, O V Mitropol'skiĭ, D. V. Osipov, S. G. Turgenev

Published Dec 11, 1992• 3 citations
Publisher

Abstract

The results are given of the latest experiments on optimization of self-PC of approximately equals 10 micrometers laser radiation by four-wave mixing in liquids. This paper reports the first realization of STS of pulsed CO2 laser radiation in SF6-Xe mixture. Possibilities are analyzed for the development of adaptive laser systems in the middle IR region for detection and amplification with PC of extremely weak optical signals (<EQ 10-15 J) on the basis of FWM and FWMF mirrors available.

DOI
10.1117/12.131847

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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.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 26, 1996•0 citations
The results are given of the latest experiments on optimization of self-PC of approximately equals 10 micrometers laser radiation by four-wave mixing in liquids. This paper reports the first realization of STS of pulsed CO2 laser radiation in SF6-Xe mixture. Possibilities are analyzed for the development of adaptive laser systems in the middle IR region for detection and amplification with PC of extremely weak optical signals (<EQ 10-15 J) on the basis of FWM and FWMF mirrors available.
As it is known, self-conjugation of the IR laser beam via Brillouin radiation stimulated scattering phenomenon is not realised because of some physical limits on radiation intensity which are the principal limitations, to the present time. That is why, one of the promising technique of phase conjugation of infrared radiation is a self-conjugation of a laser beam wavefront via four wave interaction in a nonlinear medium with feed-back loop (FWMF) [1]. Unlike conventional our wave mixing schemes, the FWMF self-conjugation is supposed to be realised without any additional beams pumping a phase conjugation mirror.
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.
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 CO2 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 CO2 laser has been chosen in such a way (CO2:N2:He equals 1:2:4, p equals 0.28 atm, tin equals 30 microsecond(s) ) that the laser pulse length ((tau) out approximately 10 - 20 microsecond(s) ) is comparable with an effective relaxation time of the upper laser level ((tau) rel approximately 15 - 30 microsecond(s) ). The time-history of the CO2 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 CO2 laser.
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.