Ellen Ochoa

Higher-order analysis of the photorefractive effect for large modulation depths

In an extension of earlier work by Moharam et al. [ J. Appl. Phys.50, 5642 ( 1979)], we use a perturbation scheme to obtain a higher-order expression for the steady-state photorefractive effect at large modulation depths and moderate applied electric fields. The expression is applicable for paraelectric crystals for which there is no photovoltaic effect and under conditions of weak coupling. The dependence of the size of the correction term on modulation depth, concentration of donor–trap sites, and applied field is discussed. Comparison of both the first-order and the second-order theories to the Kukhtarev model, valid for small modulation depths, allows the range of validity to be quantified for certain regimes.

Real-time enhancement of defects in a periodic mask using photorefractive Bi 12 SiO 20

The first known experimental results of real-time optical defect enhancement of a periodic mask are reported. A low-intensity reference wave interferes with the Fourier transform of an object beam to form a hologram in a photorefractive crystal. The nonlinear properties of the crystal perform a filtering operation, and phase-conjugate readout results in a defect-enhanced image. Defects of size 10 microm x 100 microm have been easily detected with high signalto- noise ratio, and a discussion of performance limitations is presented.

Real-time intensity inversion using two-wave and four-wave mixing in photorefractive Bi12GeO20.

In this paper, we consider a two-wave mixing system using a photorefractive medium and compute the diffraction efficiency as a function of the writing beam ratios. In certain conditions, the diffraction efficiency is shown to be exactly proportional to the inverse of the object beam intensity. Experimental results, obtained using a Bi12GeO20 crystal, are compared to the theoretical expression. In particular, we investigate the dependence of the range of beam ratios over which inversion occurs on the experimental parameters of total writing beam intensity and applied electric field. Results of performing inversion of gray-scale objects using a degenerate four-wave mixing setup are shown.

Statistical properties of ray directions in a monochromatic speckle pattern

The statistical properties of the spatial derivatives of the phase of a monochromatic speckle pattern are studied. Initially, a one-dimensional probability density function for the derivative of the phase is obtained and compared with the solution for the analogous problem concerning instantaneous frequency of narrow-band Gaussian noise. Subsequently, a two-dimensional probability density function is derived that depends on the two first and three second spatial moments of the illumination intensity distribution of the scattering object. Some sample intensity distributions are considered for which explicit expressions for the probability density function are given.

Invited Paper Real-time Defect Enhancement using Inversion Properties of Photorefractive BSO

A real-time method to enhance defects in periodic objects is described. Using four-wave mixing techniques, a hologram is nonlinearly recorded in photorefractive BSO so that read-out results in suppression of the periodic structure and enhancement of defects. Experimental results are presented, illustrating detection of defects of size 10 μm x 100 μm. Limitations on the size of defects that can be detected are discussed.