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Alabama Univ., Huntsville Report

Parametric Investigation of Holographic Gratings and Optical Phase Conjugation Through Degenerate Four Wave Mixing in Saturable Absorptive/Resonant/Nonresonant Systems

Putcha Venkateswarlu

Published Nov 26, 1991• 0 citations
OpenAlex

Abstract

Parametric investigation of volume holographic gratings and optical phase conjugation are studied through coherent and incoherent beam couplings in barium titanate BaTiO(3) and through the study of bistability in phase conjugate resonators. Experiments have been done on the effect of color centers in the formation of resonant systems and holographic grating formation. A summary of the important results appears in the final report.

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Journal of the Optical Society of America B•Feb 1, 1986•0 citations
Parametric investigation of volume holographic gratings and optical phase conjugation are studied through coherent and incoherent beam couplings in barium titanate BaTiO(3) and through the study of bistability in phase conjugate resonators. Experiments have been done on the effect of color centers in the formation of resonant systems and holographic grating formation. A summary of the important results appears in the final report.
We demonstrate for the first time, to our knowledge, second harmonic optical phase conjugation, using the second spatial harmonics of a nonsinusoidal grating formed in a photorefractive material. The possibility of harmonic phase conjugation has been considered previously for artificial Kerr media.1 We have applied the same idea to barium titanate, since the existence of the higher-order gratings in this material has been known to us for some time. In our experiment, two 1.06-µm cw Nd:YAG laser beams wrote a grating in a poled photorefractive barium titanate crystal. Then a second harmonic of the YAG at 0.532 µm read out the grating in a counterpropagating direction to one of the writing beams. In this geometry, the usual Bragg condition is not satisfied. The reflectivity of the 0.532-µm signal was rather weak, ~0.01%, with ~0.1-mW reading beam power. The buildup time was around 3 s, with loosely focused writing beams with 50-mW power in each beam. The same experiment with a SBN crystal showed no significant harmonic phase conjugation. Finally, we stress that this effect is different from the previously published result.2
We describe an experiment in phase conjugation using holographic degenerate four-wave mixing with picosecond pulses. Spatial and temporal distortions are simultaneously introduced on the probe beam, respectively, by a ground glass and a Gires-Tournois interferometer. We experimentally show that after it recrosses the ground glass we obtain a time reversed pulse on the reconstructed beam. Its temporal shape is the same as that without the ground glass. We theoretically discuss this result and show that in our configuration the temporal effect depends only on the hologram dimension parallel to the direction of propagation of the beam, while the spatial phase conjugation is related to the dimensions in the perpendicular plane.
Nondegenerate four-wave mixing (FWM) has been used for wavelength conversion of optical information.1 In such experiments, two beams F-P (frequency ω1) intersect in a nonlinear medium with a small crossing angle θ (≈ 10 mrad), while a third beam B {frequency ω2) is nearly counterpro- pagating to F [Fig. 1(a)]. This can be viewed as real-time holography with frequency conversion.1 However, the physics differ considerably when the medium is transparent at frequency ω1, as there is no optically induced grating. This occurs when FWM is based on resonant two-photon, ω1 + ω2, absorption (frequencies ω1 and ω2 being resonant with a cascade three-level system and ω1 coupling unpopulated excited states). One interesting feature is that the two-photon coherence is nearly Doppler-free and does not depend much on the θ angle.
The effects of linear absorption on the degenerate and nondegenerate self-oscillation of a photorefractive phase-conjugate mirror under condition of anisotropic four-wave mixing are considered. The phase conjugation results from nonlinear mixing of two strong undepleted orthogonally polarized counterpropagating pump waves with a probe wave. We consider two types of situation for orthogonally polarized pump beams corresponding to two crystal families: one such as the Bi12SiO20 crystal family (sillenite family) and the other such as the BaTiO3 and strontium barium niobate crystal family. In both of these crystal families the degenerate and nondegenerate self-oscillations are shown to occur in the presence of linear absorption, which tends to increase the value of the coupling coefficient required for self-oscillation. Numerical results are presented that exhibit various features of degenerate and nondegenerate self-oscillation in both crystal families for different values of the phase ϕb of the backward grating.