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Quantum Electronics

Active medium of molecular CO2and CO lasers as a nonlinear component of a phase-conjugating mirror

L. A. Afanas’ev, А. А. Ионин, E A Kiselev, Yu. M. Klimachëv, А. А. Котков, Dmitrii V. Sinitsyn

Published Jun 30, 1994• 3 citations
Publisher

Abstract

An experimental study of a four-wave interaction in the active laser medium was made for long (10–10 3 μs) pulses emitted by electron-beam-controlled-discharge CO 2 and CO lasers. Linearly polarised probe radiation from a CO 2 (CO) laser was directed into the intracavity inverted medium of the laser itself. The radiation reflected back by the active medium was recorded in the near-field and far-field zones. Phase conjugation was confirmed by reconstruction of the spatial pattern of the radiation field in the near-field zone and of the angular divergence of the radiation in the far-field zone. The energy coefficient representing phase conjugation by reflection reached 2% for the CO 2 laser and 0.2% for the CO laser. The time dependence of the phase-conjugation power coefficient was analysed.

DOI
10.1070/qe1994v024n06abeh000127

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 20, 1996•0 citations
An experimental study of a four-wave interaction in the active laser medium was made for long (10–10 3 μs) pulses emitted by electron-beam-controlled-discharge CO 2 and CO lasers. Linearly polarised probe radiation from a CO 2 (CO) laser was directed into the intracavity inverted medium of the laser itself. The radiation reflected back by the active medium was recorded in the near-field and far-field zones. Phase conjugation was confirmed by reconstruction of the spatial pattern of the radiation field in the near-field zone and of the angular divergence of the radiation in the far-field zone. The energy coefficient representing phase conjugation by reflection reached 2% for the CO 2 laser and 0.2% for the CO laser. The time dependence of the phase-conjugation power coefficient was analysed.
The results of experiments on four-wave interaction of electron-beam-controlled discharge long pulsed CO2-lasers' (t-20 microsecond(s) ) and CO- lasers' (t approximately 200 microsecond(s) ) 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 one. The experiments on recovering of optical images in near-field zone and recovering of angular divergency of laser radiation in far-field zone demonstrated that reflected beam was phase-conjugated one. The reflectivity of 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.
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.
Numerical simulation is used as the basis of an analysis of nonlinear optical properties of the active medium in intracavity four-wave mixing of the radiation of a pulsed electron-beam-controlled discharge CO 2 laser on saturated-gain and refractive-index diffraction gratings. The reflection coefficient of the phase-conjugated signal is determined for various cavity Q -factors, specific input energies, and pressures of the laser-active mixture. A comparison is made of the theoretical and experimental results. It is found that the rate of formation of amplitude gratings is governed primarily by the initial population inversion and by the intensities of the interacting waves. It is shown that transient phase gratings make the dominant contribution to the phase-conjugate reflection coefficient at high pressures of the mixture.
An experimental investigation was made of the feasibility of phase conjugation by four-wave mixing in the active medium of an electron-beam-controlled CO laser when generating a backscattered wave in a loop system. The energy of a backscattered pulse was 0.3 J, representing nearly 10% of the pump energy at the entry to the amplifier. Theoretical calculations were made of the reflection coefficients of weak waves incident on gain gratings. This was done for all the transitions as a function of the angle of incidence. The strongest reflection was observed for the radiation propagating opposite to the pump 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 ((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.