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Ferroelectrics

Intensity-Dependent asymmetric transmission through a photorefractive etalon

Stephen Ducharme, Bao Vu

Published Dec 1, 1995• 1 citation
Publisher

Abstract

We describe observations of asymmetric transmission through a Fabry-Perot etalon which contains a photorefractive crystal of barium titanate. The etalon transmission is asymmetric due to two-beam energy coupling in the photorefractive crystal as predicted by He et al. [Q. B. He, P. Yeh and C. Gu, Opt. Lett, 17, 664 (1992)]. We have observed up to 25% asymmetry in the contrast, the ratio between the maximum and minimum transmittance, when the barium titanate crystal is reversed within the etalon. The etalon transmission is also intensity-dependent due to the intensity dependence of photorefractive two-beam energy coupling.

DOI
10.1080/00150199508216946

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Journal of the Optical Society of America B•Sep 1, 1992•16 citations
We describe observations of asymmetric transmission through a Fabry-Perot etalon which contains a photorefractive crystal of barium titanate. The etalon transmission is asymmetric due to two-beam energy coupling in the photorefractive crystal as predicted by He et al. [Q. B. He, P. Yeh and C. Gu, Opt. Lett, 17, 664 (1992)]. We have observed up to 25% asymmetry in the contrast, the ratio between the maximum and minimum transmittance, when the barium titanate crystal is reversed within the etalon. The etalon transmission is also intensity-dependent due to the intensity dependence of photorefractive two-beam energy coupling.
Intensity dependence of the two-beam coupling gain coefficient Γ, the effective trap density N eff and the intensity dependent factor η(I) is studied theoretically for the two-centre photorefractive crystals. The results show that N eff, η(I) and Γ have more complicated behaviors in two-centre crystals. When S Dγ T/S Tγ D is small, N eff increases and tends to saturate with increasing intensity for both type-A and type-B two-centre crystals. When S Dγ T/S Tγ D is large, N eff increases to a maximum and then decreases a little for type-A crystals and decreases greatly for type-B crystals.The different intensity dependence of Γ in the two types of crystals comes from their different level structures.
It is known that two-wave coupling in photorefractive crystal may result in an intensity-dependent phase shift as well as energy transfer between the two waves without phase crosstalk. In a sense, this kind of phase shift is similar to the intensity-dependent refractive-index effects in Kerr media. Consequently, optical bistability can be established in an optical cavity with a photorefractive crystal. We formulate the model and get the expected bistable transmission with numerical simulations. Several cases, including that of the codirectional coupling and the contradirectional coupling, are considered. The effects of the photorefractive crystal’s distribution absorption and the spatial phase shift between the index grating and the light interference pattern in the crystal on the nonlinear transmission characteristics have also been studied. It is interesting to note that the output of the pumping wave is dependent on the cavity intensity and thus shows bistable behavior with respect to the cavity input as well.
We report the observation of bistability in a unidirectional ring resonator that employs photorefractive barium titanate for gain and also for saturated loss. One photorefractive crystal provides gain via two-beam coupling in a conventional unidirectional ring configuration.1 A second crystal is arranged to couple light out of the resonator using two-beam coupling, thereby producing loss. The orientations of the two crystals are chosen so that the magnitude of the small signal loss is greater than the magnitude of the small signal gain. Thus the oscillation in the ring cannot build up from noise since there is no net gain. However, the pump intensity for the gain medium is made larger than that for the loss medium. This means that the loss saturates at a lower signal level. Hence, at large intensities, gain can exceed net loss so oscillation can occur.
The effect of partial spatiotemporal coherence in photorefractive two-beam coupling is studied theoretically and experimentally. Coupled-wave equations and a beam-propagation method are used to study the influence of coupling on the statistical properties of the beams. The spatial coherence is found to improve for amplified and to deteriorate for deamplified beams. A two-beam coupling experiment with light of reduced spatial coherence was performed with photorefractive barium titanate. Qualitative agreement between theory and experiment was obtained.