We describe two novel types of optical associative memories and related experiments. A method to retrieve stored images in a hologram with the brightness of each retrieved image proportional to the overlap integral of that stored image and the input image is described. By using the bistable oscillator as a thresholding device, we demonstrate the associative features of the memories. Associative memories based on message bearing optical modes in phase conjugate resonators are also discussed.

Three types of self-pumped (passive) phase conjugators are experimentally examined, contrasting their abilities to perform aberration correction on an optically distorted laser beam. The passive ring conjugator is compared with a Sagnac interferometric retroreflector and the phase-conjugating nature of the device is qualitatively explained.

An experimental demonstration is presented of a new optically implemented analog method which generates the correlation function of a reference rf signal and a continuous signal input. A reference signal input into an acousto-optic cell produces an index-of-refraction pattern. A coherent pulse input into the cell is modulated by the index pattern. This optical pulse, phase modulated by the index pattern, is directed into a BaTiO₃ crystal and recorded there as a volume phase hologram. At this point a second input into the cell produces a second, moving, index-of-refraction pattern in the cell. A second optical input passing through the cell generates a phase-conjugate reconstruction of the original optical field, that of the pulse diffracted by the reference index pattern. The conjugate field propagates back through the cell, where it is diffracted by the moving pattern. The back-propagating cell output is collected by a spherical lens. There is a variation in intensity at the focus, as the moving index pattern shifts with respect to the stationary reconstruction. This variation, a function of the shift, is the correlation function of the two signals.

The conditions of high gain in wave mixing experiments with moving gratings in photorefractive BSO crystals are reviewed. It is shown that under optimum recording conditions the photoinduced grating is phase shifted by π/2 while the amplitude of the space charge field is increased. Efficient interactions in wave mixing experiments are thus obtained with demonstrated applications to image amplification, phase conjugation with gain and different types of oscillations. These oscillations result from self-induced moving gratings in BSO.

We present theoretical expressions and experimental measurements of the absolute phase shifts of phase conjugators. Photorefractive media, transparent media, saturable absorbers and saturable amplifiers are considered in the analysis. Experimental measurements of the absolute phase shifts are presented for barium titanate, strontium barium niobate and ruby.

Real-time 'exclusive or' operation obtained with an interferometer using a self-pumped phase conjugate mirror is reported. Also, results of image subtraction, intensity inversion and image differentiation are shown. Methods of extending the operation to higher order differentiation and of obtaining the Laplacian are discussed.

Optimal power limiting effects associated with self-focusing and self-bending of light in nematic liquid crystal films are studied using low power cw lasers and high power nanosecond pulse lasers.

Third- and fifth-order sum- and difference frequency conversion of pulsed dye laser radiation generates coherent tunable radiation in the vacuum ultraviolet at wavelengths X = 60 - 200 nm. The generated VUV light is of narrow spectral width (dE = 0.01 - 1 cm^-1) and high intensity (0.8 - 104 W; 2.109 - 3.1013 pho-tons/pulse). Because of the high spectral brightness these VUV light sources are a useful tool for VUV spec-troscopy of atoms and molecules.

We have measured the anisotropy of the mobility of photo-excited holes in nominally undoped single-crystal, poled, ferroelectric BaTiO₃ (p-BaTiO₃) at room temperature. In this crystal, a photorefractive grating results from the spatial rearrangement of trapped holes which have been excited by light-intensity beats and then drift and diffuse before recombining at similar empty traps. The resulting Coulomb field grating gives rise to a refractive index grating (the "photorefractive grating") via the electro-optic effect. In this uniaxial crystal the mobility of the holes is different parallel (μ) or perpendicular (μ) to the crystal axis (c-axis). As is well known, a uniform light beam causes an established photorefractive grating to decay exponentially in time. We have measured this decay rate with the grating wavevector oriented at various angles to the c-axis and compared these rates with the prediction of the standard model which assumes simple drift and diffusion with direct recombination time T,1 as extended by us to allow different drift and diffusion rates parallel and perpendicular to the c-axis. The data is satisfactorily fit by this model if we use μ/μ = 18 ± 7 and = 5 x 10^-1 cm²/Volt. We also require a ratio of dielectric constants el/e11 = 25 ± 15, which is consistent with the ratio measure-ments by other methods in other samples. To fit the data, we also needed values for the density of excitable traps (= 2 x 10¹⁶ cm^-3) and photoconductivity parallel to the c-axis, which we took from previously published measurements on the same crystal. 2 Our measurements were made with 8 nsec pulses at A = 532 nm. We could see no effects of finite hole recombination time, which suggests that the recombination time must be less than 1 nsec. If this is true,and recombination is a direct process, then the mobility must be larger than 0.5 cm²/sec/Volt parallel to the c-axis.

The reflection from most photorefractive, self-pumped phase conjugators differs in frequency from the incident beam by a small amount (Δω/ω ≈ 10^-15). A theory and the supporting experiments which explain such frequency shifts are presented. In our theory, four-wave mixing is responsible for the generation of the conjugated wave where a self-oscillation arising from photorefractive coupling provides the customary pumping beams. The frequency of these pumping beams is determined by a resonance cavity geometry and may be slightly different from that of the incident beam. Nondegenerate four-wave mixing using these self-oscillating pumping beams give rise to the frequency shift of the phase-conjugate reflection. Experimental results are in good agreement with theory.

The extraordinarily large thermal and orientational nonlinearities of nematic liquid crystal is studied in the context of amplified reflection in four wave mixing. Greater than 100% reflection in wavefront conjugation can be obtained.

Saturation characteristics and response times are examined for a liquid suspension of Brownian microparticles subjected to electromagnetic fields in a four-wave mixing configuration. Calculations are presented for the evolution of particle density in the field of two degenerate plane waves of arbitrary intensity and angle of incidence. The analysis is then extended to include the additional presence of weak probe and conjugate-wave fields. Nonlinear field particle interactions are shown to give rise to the formation of higher order particle gratings and to novel pump grating modulation effects which greatly enhance the phase-conjugate characteristics of the microparticle suspension.

One can construct a memory having associative characteristics using optical resonators with an internal gain medium. The device operates on the principle that an optical resonator employing a holographic grating can have user prescribed eigenmodes. Information that is to be recalled is contained in the hologram. Each information entity (e.g. an image of a cat) defines an eigenmode of the resonator. The stored information is accessed by injecting partial information (e.g. an image of the cat's ear) into the resonator. The appropriate eigenmode is selected through a competitive process in a gain medium placed inside the resonator. With a net gain greater than one, the gain amplifies the field belonging to the eigenmode that most resembles the injected field; the other eigenmodes are suppressed via the competition for the gain. One can expect this device to display several intriguing features such as recall transitions and creativity. I will discuss some of the general properties of this class of devices and present the results from a series of experiments with a simple holographic resonator employing photorefractive gain.

The dependence of stimulated Brillouin gain on the pump laser mode structure is studied both theoretically and experimentally. It is found that under conditions that are often fulfilled in practice, the gain and reflectivity are independent of the number of modes in the pump laser even for the case of strong pump depletion.

It is shown that interaction of two waves counterpropagating in Kerr nonlinear medium results in a cross-induced transformation of polarization of the waves. We show that some particular polarizations are preserved (so called nonlinear eigenpolarizations); in such cases the light-induced nonreciprocity is observed which depends on the type of eigen-polarization. We were also able to find the general solution of the spatial dynamics of the polarization of the beams in the case of arbitrary polarizations and to demonstrate that this solution suggests the existence of new multiple isolated branches ("isolas") in the "input-output" characteristic of such a system.

The steady state coupling constant of photorefractive BaTiO₃ can be significantly en-hanced by cooling it towards its tetragonal to orthorhombic phase transition at 5°C. This enhances the operation of devices such as passive and externally pumped passive phase conjugate mirrors and photorefractive optical limiters.

The results of theoretical and experimental studies of the photorefractive effect in two different classes of materials--perovskite BaTiO₃ and tungsten bronze SBN--have been examined. The interesting features of these measurements indicate that two-beam coupling coefficients, response times, and absorption coefficients enhanced significantly in doped crystals which appear to be suitable for future device concepts. A comparison of these materials showed that Ce-doped SBN:60 single crystals are superior to BaTiO₃ for use in photorefractive applications due to higher two-beam coupling, sensitivity, and overall flexibility of tungsten bronze crystals.

A two-channeled fiber optical soliton communication system is considered. It is found that the two carrier soliton propagation is in general similar to one carrier soliton propagation except the interaction of the two solitons of different carriers causes the jitters for both solitons. The jitters limit the bit rate of the communication system.

The recent observation of strong electroabsorption effects in AlGaAs/GaAs multiple quantum wells (MQWs) has raised the possibility of CCD-addressed spatial light modulators (SLMs) having much greater depth of modulation than obtainable with the Franz-Keldysh effect. In this paper, we review our work on CCD modulators employing each of these effects. The results from MQW structures are particularly promising and include optical modulation values as large as 50% at 852 nm. In addition, we have recently reported a new AlGaAs/GaAs CCD in which carriers are confined to a quantum well. This device was combined with a MQW structure, thus demonstrating the feasibility of integrating CCD and MQW structures on a common substrate.

In certain lamellar smectic liquid crystal phases, the molecules are arranged so that they are tilted with respect to the layers of the phase. When the constituent molecules of these phases are chiral, the phase exhibits ferroelectricity. The arrangement of the molecules and the symmetry of the various phases are described. These phases can be used in a variety of electro-optical devices. A number of display devices are described utilizing combinations of polarizers, reflectors, wave-plates, dyes and the ferroelectric medium.

This paper presents a description and design considerations for silicon/PLZT Spatial Light Modulators (Si/PLZT SLMs) which provide high parallel processing power, dynamic range, cellular resolution, sensitivity and the potential for implementation of "smart" optical devices. Following an overview of Si/PLZT SLMs, we discuss two current technologies related to the development of these devices to meet the highest performance goals. Finally, we predict performance in specific applications.

Organic and polymer structures exhibit unusually large, ultrafast second and third order nonlinear optical properties in a large number of structures, phases, and states. Rapid advances in the field can be achieved through further development of stable, high performance polymer structures having outstanding secondary properties as demonstrated, for example, by high performance liquid crystal polymers. For two such polymer systems, PBI and PBT, third harmonic generation measurements show that they possess large non-resonant third order optical susceptibilities whose origin resides in ultrafast, lossiess excitations of highly charge correlated 7 -electron states.

The performance of an improved Microchannel Spatial Light Modulator is discussed. In order to improve the spatial resolution, a very thin crystal (approx. 50μm thickness) is adopted and the device structure is refined. This new device has a spatial resolution of approx. 10 1p/mm at 50 % modulation. Also, the contrast ratio is improved. It is achieved by coating the surface for the charge storage of the crystal with a dielectric mirror and coating the opposite surface with an anti-reflection coating. Moreover both surfaces of the output glass window are coated with an anti-reflection coating. The improved contrast ratio is more than 3000:1.

An electron beam addressed spatial light modulator using 55 degree cut LiNb0₃ for both coherent and incoherent light modulation is reported. The device is compact and can be operated fully electronically since it incorporates a single electron gun for writing and erasing operations. With this device, a resolution of 3 1p/mm at 50 % modulation, 13 1p/mm at 5 % modulation, a cycle time of 1/30 second and contrast ratio of greater than 1000 : 1 ( coherent) and 20 : 1 (incoherent) are achieved. The device can be used in many applications such as radar and sonor processing, microwave holography, transducer for electric serial signal to optical parallel signal and large screen display.

A nematic liquid-crystal spatial light modulator based on a transverse electro-optic effect is demonstrated. The transverse field is derived from the write light intensity gradient; therefore the device operates in a differentiating or edge enhancing mode. The longitudinal component of the electric field increases the write speed and provides erasure. This drive-off capability is unusual in nematic devices. A bismuth silicon oxide crystal provides photoconductive addressing in a primitive nematic device which demonstrates the effect. Initial experimental results and theory are presented.

Electro-optic properties of thin liquid crystal cells are found to deviate significantly from those of thick cells. Results on the liquid crystal thickness dependent phase retardation, effective birefringence, threshold voltage, and decay time of thin E-7 cells are presented. A simple model which is based on the partially disordered surface layers is developed to explain the observed phenomena.

The photorefractive materials Bi₁₂Si0₂₀(BSO) and its structural isomorph Bi₁₂Ge0₂₀ (BGO) have been used as real time holographic materials in a variety of optical storage and information processing applications in the last decade [Ref 1]. Their overall response speed and comparitive ease of crystal growth - and hence availability in relatively large sizes - have frequently made BSO and BGO the materials of choice in these applications despite the supe-rior diffraction efficiencies available with the ferroelectric compounds barium titanate and strontium barium niobate (SBN) [Ref 2]. An understanding of the properties of holographic devices utilizing these materials requires an accurate description of both the holographic formation and read-out processes. The mechanics of the recording process in photorefractive materials, in which photogenerated charge carriers redistribute themselves to form a periodic electric field variation in the medium, have been studied extensively and a number of useful theoretical models now exist [Ref 3, 4]. The holographic read-out process though similarly complex, has been afforded somewhat less attention.

A quasi-static electric field can enhance laser-induced diffraction rings from a nematic liquid crystal film. This phenomenon is shown to be the combined result of the critical behavior of the sample at the Freedericksz transition and the nonlinear coupling of the op-tical and quasi-static electric fields. A novel zero-crossing effect for an inclined-incident pump beam will be shown. Potential applications for the observed effect are discussed.

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.

Dynamically stable parallel image subtraction has been achieved using phase conjugate interferometry. We use a Michelson interferometer with an internally self-pumped barium titanate crystal as the phase conjugator for both arms. Various aspects of image subtraction, including intensity inversion and "exclusive or" logic operation are demonstrated. Device operation is insensitive to alignment and optical path length perturbations.

In this paper we present a new technique for accomplishing the Fourier transformation of interferograms via total optical means. The technique involves: (1) Obtaining inter-ferograms of input optical signals through the scanning by Michelson interferometer. (2) Accomplishing the time-to-space coordinate transformation of the interferograms by linearly recording on proper medium through a temporal-spatial transducer namely KDP. (3) Completing the Fourier transformation of the spatial interferogram by using a coherent optical spectrum analyzing system and then obtaining the spectrum of the input optical signal at the output plane of the system.

In this paper, an inner-product array processor for the associative retrieval problem is presented. First, the algorithm and architecture of the array processor design are discussed. Then an optical implementation scheme is proposed. The matrix model of the associative memory is adopted. In this model, if one of the M vectors is to be reliably recalled, the dimension of the vectors, N, must be much larger than M. By taking advantage of this fact, our result offers a factor of 1/1â- 17â€"M- saving on the matrix elements. More significantly, real-time inputting and updating of the matrix elements can be potentially implemented with existing space-variant holographic elements and recently discovered liquid crystal television spatial light modulators.

This paper introduces Fe:LiNt0₃ Crystal used as a real-time recording medium which close-ly contacts with a grating to encode the black-white image. A pseudocclor image can be formed on the out-plane of white-light processor when the encoded crystal, Ithich is put on the input-plane, is illuminated with white-light.

Thermally induced distortions are of interest for many materials, components and structures used in astronomy, aerospace systems, metrology and the optics industry. Optical interference pattern analysis offers the best hope for precise information on dimensional changes of less than a micron. This paper reviews the various optical approaches to linear thermal expansion measurements. Emphasis is placed on the Michelson interferometer because of its versatility for making contactless measurements in real time and the elimination of sample shape or size constraints. The use of composite materials in optical support structures is a driving force in the utilization of improved optical systems for thermal distor-tion measurements. Auxiliary capabilities therefore include fast response, (e.g., for opto-acoustic emission analysis), long term stability (for creep effects), multi-dimensional changes (holography), automatic control of sample position and computer aided data acquisition and analysis.

This paper proposes a holospeckle interferometer with one single set-up, This is made possible by using two collimated reference beams, one at each state, in typical double exposure holographic interferometry. At reconstruction, if the same reference beams are used to reconstruct the images, then the holographic interferometry is obtained, If we put a half-wave plate into the light path of one reference beam, i.e., the polarization states of the two reference beams are orthogonal, then a speckle interferometry is obtained. In experimental mechanics, the holospecklegram provides a means to separately measure the in-plane and out-of-plane surface displacement components. In addition to this, it has some other advantages. The orthogonal polarization property of the two reference beams play a key role for determining the type of interferometry and the qualities of the bolo and speckle grams, therefore, types of orthogonal polarization have been investigated and discussed.

The Composite filter of Caulfield and Maloney, a linear combination of matched filters, is an optimum estimator in the mean-square sense. Composite filters may be made rotation invariant by using Circular harmonic filters as components. With a finite number of Circular harmonic filters, the output of the composite filter is invariant under rotation of the input only if all Circular harmonic filters have the same order.

The pocket-size liquid crystal television (LCTV) has been investigated for its potential as a two dimensional spatial light modulator. The LCTV can be addressed using both a microcomputer and a TV camera. We have measured various characteristics of this device including transmission control, bipolar modulation capabilities, and have tested its usage as an input device for an optical correlator. Optical phase distortions of the device can be compensated by a liquid gate. These results suggest that this device has great potential for optical pattern recognition and optical data processing applications.

This paper describes how the standard microchannel spatial light modulator can be (a) operated and (b) modified to generate a real-time, high-gamma readout characteristic. The paper also examines the use of such a device in a pulse-frequency modulation optical processor to achieve a wide range of real-time, nonlinear image and signal processing operations such as histogram measurement, density slicing, density-based artificial stereo and gray level image enhancement.

Rotation-invariant pattern recognition is shown to have intrinsic limitations determined by the set of patterns to be recognized and by the specific optical setup. Within these limitations, a general procedure is proposed for the generation of bipolar filters that do not require the sensitive alignment procedures involved in holographic filters and are suitable for superposition synthesis to achieve rotation invariance.

We present an analysis of multiple beam coupling and grating formation within a photorefractive crystal positioned adjacent to a phase conjugate mirror. We include the effects of beam depletion, absorption loss, and the simultaneous presence of multiple gratings.