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Photorefractive fibers of SBN provide an attractive medium for storing information. The information can be in the form of a 2-D image, called a page, and multiple pages may be stored in a single fiber by holographic recording. The multiple pages are distinguished by angular multiplexing of the reference wave. In this paper we discuss experimental results regarding the number and quality of these pages stored in SBN fibers of different diameters, lengths, and recording arrangements. The data storage capacity of these fibers is calculated taking into account crosstalk and recording resolution.

A model describing the image irradiance of two images produced by a multiple grating reflection type focusing element is developed. The gratings may be formed simultaneously or sequentially, and the recording medium can change thickness during processing. In the algorithm, the spot size and location of the image are first calculated by ray tracing from each surface grating in the multiplexed grating element. The efficiencies of these rays are then computed with a multiple grating model which allows for coupling between the fields in the grating volume. Finally, the efficiency of each ray is weighted by a factor corresponding to the intensity profile of the reconstruction beam. The sum of these weighted rays per unit area in the image plane determines the relative image irradiance. A comparison is then made between the image irradiance produced by experimental multiple focused elements and the calculated irradiance pattern. The model predicts the relative profile and the size of the irradiance patterns.

The volume holography process in a sensitized fluid is studied. The object of the research is to use forced Rayleigh scattering methods in the measurement of velocity gradients in fluids. The process is divided into three major steps: writing of the hologram; evolution of the hologram under the effect of fluid motion; and reading of the hologram by optical means. The last two steps are discussed with special emphasis on the effects of convection and diffusion as they relate to the hologram deformation and decay.

The application of volumetric diffraction gratings in photochromic materials to the measurement of velocity gradients in fluid flows is presented. The process involves the writing of a crossbeams hologram by two coherent pulsed beams that intersect inside a fluid to which a photochromic dye has been added. The hologram is continuously read at the proper Bragg angle by a third beam of lower intensity and different wavelength. The effect of flow shearing is to change the main orientation of the plane of the fringes of the hologram, which in turn reflects in a change of the angular properties of the readout beam. This is carried out until the hologram fades out by diffusion or it moves out of the Bragg condition. We show the optimization of the writing of the cross-beam grating in an optically thick material (like the photochromies used) as well as the wavelength for the most efficient reading of such gratings. Velocity gradients in a carefully controlled flow are shown and compared to alternative established methods and theoretical profiles. Finally, we discuss several alternative geometries capable of measuring different components of the velocity gradient tensor as well as multichannel arrangements capable of measuring vorticity components (e.g., ω y = ∂v z /∂x− ∂v z /∂z).

SBN is an attractive photorefractive medium, which is difficult to grow in large bulk sizes of good optical quality. Alternatively, at Stanford we grew SBN fibers of lengths exceeding 10 cm and with a diameter of μm. These fibers offer a unique environment for carrying out phase conjugation experiments, such as holographic data storage, self-phase conjugation, optical interconnects, and two-beam coupling.1 We report, to our knowledge, the first photorefractive holographic recordings in an SBN fiber. A plane wave reference and an image carrying object beam are incident on the fiber in a reflection recording geometry, where the grating vector is essentially aligned with the c axis.

Recent development of a computer-controlled Czochralski crystal growth system at Stanford University has allowed us to produce large single-crystal Sr0.6Ba0.4Nb2O6 boules. Since this material is grown in a solid solution, striations are normally present and difficult to suppress. We discuss the nature of the striations and a simple scattering measurement that provides quantitative information about the degree of compositional variation in our samples. The results show that the effect of striations on laser beam propagation is negligible over a wide range of input angles.

A method is presented to enhance defects in periodic objects in real time. 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, suppressing the high-intensity Fourier components of the periodic structure, and phase-conjugate read-out results in a defect-enhanced image. The mask to be inspected is placed in the input plane, and the defect-enhanced image appears at the output plane, in a time limited only by the time constant of the crystal. Using ESSO with a total incident light intensity of ~20 mW/cm2, the response time is ~50 ms. Defects of 10 × 100 μm2 have been easily detected with high SNR, and performance limitations are discussed.

Recent interest in neural network models for parallel distributed processing is stimulating much interest within the optics community, in particular, on the feasibility of optical associative memories.1 We illustrate how a set of spatially multiplexed volume holograms recorded in a photorefractive crystal constitutes an optical crossbar switch that globally interconnects a two-layer neural network.2 We focus on the use of the network to provide associative memory and show that the mathematical operations involved in distributed memory storage and recall can be optically implemented in a very natural fashion within the framework of photorefractive holography. These operations include vector outer products, linear combinations of outer-product matrices, and matrix-vector multiplication.

Magnetic resonance imaging techniques can be used to study fluid systems which are difficult or impossible to probe with optical methods. The problem in imaging fluid flows with magnetic resonance has been the length of time necessary to acquire an image. Recent developments in magnetic resonance imaging techniques indicate that much shorter acquisition times may be possible.

Much research has been devoted to image understanding and the automated analysis of images of 2-D scalar data, but comparatively little work has been devoted to understanding higher dimensional data. Experiments and numerical calculations of fluid flows yield large data sets consisting of vector data on 2-, 3-, and 4-D domains. These are often too complicated for manual inspection, manipulation, and display. As with scene analysis, an abstract representation of the data set would greatly facilitate these tasks. Such a representation based on the flow topology can be constructed by locating and characterizing the critical points of the velocity vector field. These points serve as a basis for building a representation of the global topology as determined by the tangent curves of the vector field. This paper describes methods for the implementing this analysis on a computer and results from their application to 2- and 3-D fluid flow data sets.

Two types of conical emission in sodium have appeared in the past. In the first, a single tightly focused laser beam is tuned to the blue side of the D1 line in conditions compatible with self-trapping. The emission is specular, has a cone half-angle of a degree, and occurs on the red side of line center. In the second type, two weakly focused counterpropagating laser beams are tuned to the red side of line center. The emission consists of radial filaments, has a cone half-angle of 8°, and is the same wavelength as the pumps. We report on yet a third type of conical emission which is distinct from the first two types by virtue of its spatial and spectral characteristics. Like the first type, the pumps are tuned to the blue side of line center, the emission has a cone half-angle of a degree and is specular. However, this phenomenon also resembles the second type of conical emission in that two counterpropagating pumps are required, the pumps are weakly focused, and the emission wavelength is the same as the pumps.

A beta switch is a 2 × 2 crossbar cell, which is the building block for many interconnection networks (i.e., Clos-Benes, Spanke). A new optical beta switch design based on the ferroelectric liquid crystal total internal reflection switch is investigated for use in dynamic optical interconnection networks. It exhibits low crosstalk and can be driven by only a few volts. Crosstalk, loss, and scaling characteristics are discussed in the context of the device cascadability.

Microelectronic chip-to-chip interconnection necessitates channeling information from a large number of independent transmitters to a large number of receiver locations. Limitations of existing electronic interconnection techniques have prompted examination of optical methods of connection. One proposal is to use reflection-type holographic optical elements to direct modulated signals from optical sources to detectors. Obtaining multiple images from a single source requires encoding a set of gratings in the same recording medium. This can be done either sequentially (incoherently) or simultaneously (coherently). In this presentation the results of an experimental and theoretical investigation of simultaneously formed multiple image reflection volume holograms are given.

Although the existence of nonlinear holographic response in phoforefractive materials has been experimentally observed and its presence treated in some theoretical detail,1 standard holographic recording techniques have yielded higher-order grating components substantially weaker than the fundamental linear component. Using the unstationary running wave technique of Huignard and Marrakchi,2 however, we show that these higher grating harmonics may be substantially increased in strength. Furthermore, this enhancement is selective in that there are distinct criteria for the optimal amplification of each grating order. In addition to a theoretical analysis of this effect, we present experimental verification using photorefractive Bi12SiO20 crystals. By measuring the diffraction efficiency of both the fundamental and second harmonic grating components, we observed substantial enhancement of this nonlinear second-order response with respect to the fundamental.

We describe a novel arrangement for obtaining enhanced nondestructive holographic readout in SBN using polarization effects and modification of the applied field between recording and readout. The enhancement is most pronounced for a certain range of spatial frequencies. The readout signal first decreases in strength but subsequently grows above the starting value, approaching 100% efficiency in some cases. Over 6 h of continuous readout have been achieved with efficiency remaining strong enough to provide one billion reads with SNR exceeding 20 dB.