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Journal of the Optical Society of America B

Investigation of stochastic photorefractive backscattering

Ekaterina Shamonina, Boris I. Sturman, S. Odoulov, Klaus H. Ringhofer

Published Oct 1, 1996• 9 citations

Abstract

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.

DOI: 10.1364/josab.13.002242

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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.

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.

It has been shown recently (see, e.g., Nature, 397, 594, 1999; 409, 490, 2001) that light pulses can be remarkably slowed down in resonant gases under very low temperatures. We show theoretically and experimentally that a similar in nature (and stronger in value) phenomenon occurs in photorefractive nonlinear media. The pulse-maximum velocity can be reduced here to the value of thickness/response time because of the two wave coupling effect. Specific properties of pulse propagation depend on the type of the photorefractive response and also duration and shape of the input pulse. Our theoretical results for the nonlocal nonlinear response are in a good agreement with experimental data obtained with BaTiO3 and Sn2P2S6 crystals.

It is shown that when approaching the spatial subharmonics generation threshold in fast photorefractive crystals (the sillenites, CdTe), a practically unlimited (singular) amplification of the nonlinear photorefractive response is possible, which results in a drastic increase in the spatial amplification of weak signals. A theory of critical spatial amplification is developed. This theory takes into account real attributes of fast photorefractive crystals such as the vectorial nature of wave coupling and nonuniform broadening of resonances owing to light absorption. The theory is applied to the analysis of the observable characteristics of critical enhancement and to optimization of the conditions of experiments aimed at the detection and investigation of this phenomenon.