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Soviet Journal of Quantum Electronics

Self-conjugation of pump waves with "boiling" speckle structure via backward stimulated diffusive scattering in an Fe:KNbO3crystal

Vladimir A Dyakov, С. А. Корольков, A. V. Mamaev, Vladimir V. Shkunov

Published Nov 30, 1992• 0 citations
Publisher

Abstract

We have investigated backward stimulated diffusive scattering of light with a time-varying speckle structure by photorefractive crystals in which the nonlinearity mechanism is diffusion-driven. We show that the efficiency and conjugated fraction for this process depend on the ratio of the coherence time to the dielectric relaxation time of the medium.

DOI
10.1070/qe1992v022n11abeh003657

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We have investigated backward stimulated diffusive scattering of light with a time-varying speckle structure by photorefractive crystals in which the nonlinearity mechanism is diffusion-driven. We show that the efficiency and conjugated fraction for this process depend on the ratio of the coherence time to the dielectric relaxation time of the medium.
Stationary photorefractive self-pumped phase-conjugation with one four-wave-mixing interaction region, stimulated by the photorefractive amplified backward scattering, is obtained in LiNbO 3 :Fe crystals, in which the strong backward scattered light is gotten by putting a scattering particle on the backface of the sample. Even though its reflectivities are not high (its temporal and stationary reflectivities are 10% and 3%, respectively), due to the low electrooptic coefficients of LiNbO 3 :Fe crystals, our experimental results prove that this mechanism is effective and simple comparing with the "cat" self-pumped phase-conjugator.
The threshold for self-pumped optical phase conjugation by means of backward stimulated photorefractive scattering has been dramatically lowered by inserting a retroreflecting screen into the path of the beam after the beam leaves a BaTiO3 crystal. Stable reflectivities of as much as −60% and good phase-conjugate fidelity have been demonstrated in a crystal that has too low a gain–length product to exhibit unseeded backward stimulated scattering. Precise control of the threshold for stimulated scattering is demonstrated by adjusting the seed intensity. Reflectivity and fidelity are experimentally characterized at 515 nm as a function of the following parameters: pump and seed intensities, crystal angle with respect to the incident beam direction, crystal interaction length, crystal–lens separation, and aberration strength. Operation at 633, 730–800, and 839 nm is demonstrated also.
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.
It is shown that, in photorefractive crystals with a dominating local nonlinear response, small temporal fluctuations of the space-charge field can induce strong noisy (stochastic) backscattering. For sufficiently large interaction lengths a small nonlocal component of the photorefractive response can suppress the fluctuation of the scattered-light intensity and can result in nearly complete and regular nonlinear reflection. Stochastic stimulated backscattering is detected and investigated in LiNbO3:Fe crystals. The scattering manifests itself in the optical configuration, where noiseless spatial amplification of backward waves is impossible. The main characteristics of the observed backscattering are in good qualitative agreement with the calculated ones.