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Quantum Electronics

Mechanism for phase conjugation of the beam from a flowing-gas CO laser

А. В. Бердышев, A K Kurnosov, Anatolii P. Napartovich

Published Jun 30, 1993• 1 citation
Publisher

Abstract

A mechanism for the interaction of light with a nonlinear medium which leads to an efficient phase conjugation of the beam from a flowing-gas CO laser has been studied. The interaction occurs in four-wave mixing with a feedback loop. The interaction of the light with the active gain medium results from the formation of large-scale amplitude gratings in the medium. These gratings are created by copropagating pump waves.

DOI
10.1070/qe1993v023n06abeh003098

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A mechanism for the interaction of light with a nonlinear medium which leads to an efficient phase conjugation of the beam from a flowing-gas CO laser has been studied. The interaction occurs in four-wave mixing with a feedback loop. The interaction of the light with the active gain medium results from the formation of large-scale amplitude gratings in the medium. These gratings are created by copropagating pump waves.
It is shown that, under conditions typical of the active medium of a continuous-flow CO laser, large-scale gain gratings play the main role in a phase-conjugation system based on degenerate four-wave mixing. The multifrequency nature of the emission spectrum of a CO laser increases the depth of modulation of the amplitude gratings as a result of two principal mechanisms: the dependence of the rate of relaxation of the function describing the distribution of molecules between vibrational levels on the number of such levels, when this function is disturbed; and the mutual influence of the emission and amplification processes involving adjacent vibrational transitions.
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.
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.
Nonlinear optics normally involves the change in the susceptibility of a material due to the presence of optical radiation. In many cases, the light-matter interaction produces a change in the refractive index of the medium, as occurs in photorefractive media, as well as in Kerr-like media, Brillouin-active media and absorbing media with thermallyinduced refractive index changes. Such nonlinear refractive index variation corresponds to a change in the real part of the optical susceptibility %. In resonant materials, the optical radiation induces stimulated emission (or absorption) causing transfer of population between different energy states of the medium. This can lead to saturation of the amplification (or absorption) corresponding to changes in the imaginary part of the susceptibility x= i2odk where the amplitude gain coefficient a(I) is a saturable function of the intensity /, and k is the magnitude of the wavevector of the optical radiation. The interference of two (or more) coherent beams I(x) in the laser amplifier can cause a modulation of the population inversion N(x). This modulation of the inversion is commonly known as spatial hole burning in lasers and leads to formation of a gain-grating a(x) = 1/2GN(X) where G is the stimulated emission cross-section. The gain grating can act as a very efficient diffractive optical element exhibiting high diffraction efficiency that may be even greater than unity [16] and also very high phase conjugate reflectivity using four-wave mixing (FWM) geometries [5-7].
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 phase conjugation (PC) reflectivity of the laser radiation inside its own laser medium reflects the non-linear properties of the medium. To study the properties of carbon monoxide (CO) laser medium the temporal and spectral characteristics of PC reflectivity of pulsed e-beam controlled discharge cryogenically cooled CO laser radiation under degenerate four-wave mixing (DFWM) inside its own laser medium were studied experimentally. The interaction length of DFWM was about 100 cm. The pulse length of the CO laser beam was 50 - 500 µs, the gas density was up to 0.5 Amagat, the gas temperature was about 120 K.