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Quantum Electronics

Reflection of speckle-inhomogeneous CO2laser radiation under four-wave interaction conditions with feedback

Alexander A. Betin, Konstantin V. Ergakov, O V Mitropol'skiĭ

Published Jan 31, 1994• 4 citations
Publisher

Abstract

An experimental demonstration is reported of the feasibility of phase conjugation via a four-wave interaction with feedback in the case of pulsed CO 2 laser radiation with a speckle-inhomogeneous structure. Reflection of a speckle-inhomogeneous signal with a clear phase-conjugated structure against the background of a noise component is reported. Ways of improving the quality of the phase conjugation are discussed.

DOI
10.1070/qe1994v024n01abeh000020

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An experimental demonstration is reported of the feasibility of phase conjugation via a four-wave interaction with feedback in the case of pulsed CO 2 laser radiation with a speckle-inhomogeneous structure. Reflection of a speckle-inhomogeneous signal with a clear phase-conjugated structure against the background of a noise component is reported. Ways of improving the quality of the phase conjugation are discussed.
A study is reported of a system, making use of four-wave mixing with feedback (FWMF), for high-quality phase conjugation of speckle-inhomogeneous radiation. The system includes a hologram formed on the basis of nonlinearity of gain saturation. A theoretical analysis is made of selective capabilities of a 'short' hologram, whose length is less than the longitudinal correlation lengths of the 'write' waves used to form it. The statistical-average values of the parameters of modes in an FWMF cavity are determined. The experimental results obtained for a neodymium glass system show that the fidelity of phase conjugation of speckle beams with a divergence (5 — 25) θ diffr is 80%. The maximum energy of the phase-conjugated component is 0.35 J and the maximum energy reflection coefficient of an FWMF mirror is 20.
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.
One of the efficient methods of phase conjugation (PC) is the four-wave mixing with feedback (FWMF) (Fig. 1). To select the wave conjugate to the input radiation in a FWMF resonator, it has been suggested to use either an inhomogeneous phase plate or a long (larger than the length of the longitudinal inhomogeneity of the signal) nonlinear medium.1 The diffraction efficiency of a thick hologram and its noises for a plane reference wave are calculated in Ref. 2. This paper provides the calculation of the power of the conjugate and noise waves produced by the reflection from a long phase hologram (mirror) recorded by two speckled beams. The paper also deals with experimental realization of a PC FWMF speckled beam in a long medium.
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.
To the best of our knowledge, a novel self-phase-conjugation technique for laser radiation is proposed, which is based on the self-filtration of the phase-conjugate component in a nonlinear absorbing medium, bleached by a pump wave with the intricate spatial structure in the region of location of its speckles. Similar to the stimulated Brillouin scattering (SBS) case, the mechanism of the phase-conjugate wave discrimination against the noise background is based on the coincidence of the speckles in the pump and conjugate waves throughout the bulk of the medium. To compensate for a drop in the signal power due to total absorption, the amplification in conventional laser gain cells is used. The feasibility of the phase conjugation with reflectivities from unity to tens of percent is demonstrated by analytical estimates and numerical calculations. The technique can find application in the middle and far IR spectral regions, where the implementation of phase conjugation via SBS is either hindered or impossible at all.