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Quantum Electronics

Efficiency and quality of four-wave phase conjugation of a signal with a time-dependent spatial structure

L. I. Ivleva, С. А. Корольков, O V Lyubomudrov, A. V. Mamaev, N. M. Polozkov, Vladimir V. Shkunov

Published Mar 31, 1995• 2 citations
Publisher

Abstract

It is demonstrated theoretically and experimentally that, in four-wave phase conjugation of a spatially incoherent speckle signal, the reflection coefficient and the phase conjugation fidelity fall on increase in the rate of change of the spatial structure of the speckle beam. The law of decay of both these parameters is the same for media with an exponential grating erasure rate, but in the case of double-exponential relaxation the conjugated fraction of the signal decreases much more slowly than the reflection coefficient. Experimental results show that the standard model of deep trapping levels can describe the kinetics of formation of dynamic gratings in a nominally pure crystal of strontium barium niobate, but the presence of shallow trapping levels has to be taken into account for doped crystals.

DOI
10.1070/qe1995v025n03abeh000336

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It is demonstrated theoretically and experimentally that, in four-wave phase conjugation of a spatially incoherent speckle signal, the reflection coefficient and the phase conjugation fidelity fall on increase in the rate of change of the spatial structure of the speckle beam. The law of decay of both these parameters is the same for media with an exponential grating erasure rate, but in the case of double-exponential relaxation the conjugated fraction of the signal decreases much more slowly than the reflection coefficient. Experimental results show that the standard model of deep trapping levels can describe the kinetics of formation of dynamic gratings in a nominally pure crystal of strontium barium niobate, but the presence of shallow trapping levels has to be taken into account for doped crystals.
Phase-conjugated light waves are known to reverse phase distortions in media with negligible absorption or for paraxial beam propagation.1 We examine here a different case in which a volume conjugating medium is itself imperfect, possessing phase distortions and a conjugating efficiency dependent on grating spatial frequency. We show theoretically that for multiplexed signals, cross-talk-free phase conjugation is obtained only for negligible spatial-frequency dispersion and a phase-conjugated reference wave. We further present experimental results in a severely striated sample of photorefractive strontium barium niobate; using a phase-conjugated reference wave, we demonstrate the alleviation of distortion2 and the elimination of cross talk between closely spaced angularly multiplexed signals.
Phase conjugated light waves reverse phase distortions in media with negligible absorption or for paraxial beam propagation [1]. We examine here a different case in which a volume conjugating medium is itself imperfect, possessing phase distortions and a conjugating efficiency dependent on grating spatial frequency. We show theoretically that for multiplexed signals, cross-talk-free phase-conjugation is obtained only for negligible spatial-frequency dispersion and a phase-conjugated reference or pump wave, and we present supporting experimental results.
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.
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.
The phase conjugation (PC) of a focused Gaussian laser beam with a partial spatial coherence of the wave front is studied numerically and theoretically upon SBS within the framework of a three-dimensional nonstationary SBS model, which takes into account transient processes and SBS saturation. The dependences of the PC coefficient h on the laser radiation power are obtained for different excesses of the angular divergence over the diffraction limit ξ . It is found that for the given reflectance of laser radiation from the SBS mirror, the PC coefficient monotonically decreases with increasing the divergence. For example, under the near threshold SBS conditions, h decreases from 70% to 30%, when ξ increases from 1 to 10. For the given divergence, the PC coefficient increases with increasing the reflectance and approaches the ideal one ( h > 90%) upon deep SBS saturation, when the reflectance exceeds 90%—95%.