In this paper, we describe the basic mechanisms and dynamics of photorefractivity, and holographic grating formations in dye- and fullerene C₆₀-doped nematic liquid crystal films. The effects are attributed to photo induced molecular axis reorientation in these nematic films. A nonlinear index coefficient n₂ of 10^-2 cm²/watt can be obtained in a 25 micrometer thick film, with an applied dc-voltage of approximately 1 volt. In fullerene C₆₀-doped films, these laser induced gratings can be made permanent and electronically modulated; they can be turned off and on by an ac voltage with response times of approximately 100 microsecond(s) and a few ms, respectively.

In this paper, we report a new effect -- the threshold effect of incident light intensity for the photorefractive light-induced scattering in LiNbO₃:Fe, Mg crystals, which could be used as a simple, effective technique to suppress the photorefractively light-induced scattering and is useful for us to get the noise-free photorefractive devices.

We have observed an extra fanning beam along the direction for obtaining the maximum coupling strength and it affects the reflectivity of a cat self-pumped phase conjugator. The production of the extra fanning beam for obtaining the highest coupling strength can be understood by applying Sun's method. The angle deviation of the direction of the extra fanning beam and the fanning loops affects the generation and reflectivity of the phase conjugate wave. It can be controlled by erasure. Moreover, the erasure of the crystal can be treated as another control factor of incident condition for obtaining the maximum reflectivity in a Cat SPPCM in Sun's method. We have modified Sun's method by considering this factor. We find the modified Sun's method can be use to estimate the optimal incident conditions for obtaining the relatively high reflectivity of a Cat SPPCM in a more accurate way.

Light-induced scattering in a Ce:Fe:LiNbO₃ crystal is highly temperature dependent. By considering the effect due to thermally activated ions in a photorefractive crystal, we have shown that the photo-induced space-charge field is neutralized by the activated ions. This neutralization in turn leads to scattering noise reduction in the crystal. By raising the crystal temperature, we have observed a reduction in scattering noise, which is consistent with our prediction. The signal-to-noise ratio (SNR) in a two-wave mixing amplification due to thermal activation effect is analyzed. Experimental demonstrations using a Ce:Fe:LiNbO₃ crystal are provided, in which we have shown that the SNR improves as the crystal temperature increases.

Holographic gratings recorded in BaTiO₃ with five different wavelengths have been studied. Using the knowledge of grating formation developed in this way, we have visualized infrared images in BaTiO₃ with two different two-wave mixing configurations.

The coupling kinetics of a reflection-type photorefractive (PR) hologram are studied in this paper, in which we have found that the coupling between the adjacent channels increases as the reading time prolongs. As long as the wavelength difference between the writing and reading beams is sufficiently large, there is only a small amount of increase in coupling. However, as wavelength difference between them is adequately small (but larger than the minimum wavelength separation calculated by Kogelnik's formula), the coupling between two channels increases to a significant value and then decreases. Our study shows that the reading beam is modulated by the grating in another channel for which it produces a weak back scattered light. As the backward scattered light interacts with reading beam, a new grating is formed. It in turn amplifies the back scattered light. Therefore, the coupling increases very rapidly as compared with the normal fanning process. In this paper, we have used the Kukhtarev's charge transport model to evaluate the time-evolution of coupling. We have found that the calculated results are consistent with the experimental observation.

Nonlinear properties of a crown ester-based photochromic medium and the formation of a thermal lens under laser radiation have been studied. Such a thermal lens is shown to provide the ability to increase the number of resolved deflector elements, which is important for storage systems and neural networks.

Parallel error decoding is discussed for 2D page access optical memory. Both parallel space- domain and parallel spectral-domain Reed-Solomon decoders are discussed and compared with conventional serial decoding implementations. The parallel approach is found to yield information rates of nearly 10¹² bps. A methodology is also presented for 4f optical storage system design. This approach allows the photorefractive crystal volume to be minimized while maintaining the maximum storage capacity that is consistent with a given SNR requirement.

We present our approach to optical dynamic interconnections for neural networks. In this approach we fabricate waveguide structures in photorefractive media by simply focusing and scanning a laser beam. Resultant waveguides with variable index profiles are used for the optical dynamic interconnections. We show the experimental results of waveguide fabrication in a volume of a photorefractive (PR) medium and present the optical dynamic characteristics of the PR waveguide. We propagated the green Ar ion laser light (515 nm) in the photorefractive waveguide repeatedly and observed the variation of the near-field pattern at the end face of a 2 mm-thick lithium niobate (LiNbO₃; LN) crystal. The average optical intensity of the guided beam decayed exponentially with respect to the total exposure over the waveguide. It is supposed that the guided beam redistributed or destroyed the charge density field formed by the initial exposure. A numerical analysis based on the beam propagation method and a simple PR waveguide model of this process showed reasonable agreement with the experimental results. This variation of the guided optical power can be used to implement dynamic interconnections for neural networks.

Materials for three dimensional optical memories have been the focus of significant research. The development of appropriate optical addressing technologies have been cited as critical to the practical exploitation of the potential density and throughput achievable in three dimensional optical memory architectures. Coherent optical arrays have been cited as an undeveloped but potentially groundbreaking component in the optical addressing of three dimensional memories. Designs and simulations for PLZT based single and multiple lobe optical phased arrays are presented. These PLZT based optical phased array structures implement optical scanners and scanning optical array generators. The physical structure of the devices can be varied to allow operation at a specific optical wavelength and optical beam resolution. The integrated optical scanners and scanning optical array generators can be implemented with simple three-port voltage dividers and dc current drivers. These devices could provide the geometric reconfiguration of optical pathways required of addressing schemes in three dimensional optical memory architectures.

In this paper, a four-channel associative memory based on multiplexed synchronizing data processing has been presented, whose processing speed is four times as fast as that of one- channel memory. The storage unit and the processing unit has also been separated. Photorefractive KNSBN crystals are used as storage media, and a liquid crystal photoelectric switch (LCPS) is selected to perform the thresholding feedback and gain operation. The experimental results show that some important parameters, including processing speed, output quality, etc., have been improved a lot, and the system can process natural images in real- time.

A compact, rugged and low-cost 3-D optical storage system based on space and phase-code multiplexed volume holograms is presented in this paper. Holograms are recorded at a 2-D (e.g., 16 multiplied by 16) grid of locations in a photorefractive crystal. At each location, multiple holograms are stored via phase-code multiplexing. Compared with the commonly utilized angle multiplexing, the phase-code multiplexing permits a high storage capacity without alignment problems. In contrast to wavelength multiplexing, phase-code multiplexing does not require bulky and high-cost components for wavelength tuning and can switch very quickly from one hologram to another. By combining the space and phase-code multiplexing techniques, such a 3-D optical storage system has the potential to store 1 terabit information and to retrieve data at the rate of gigabits per second. In this paper, the optical architecture is described and the system design considerations are discussed. The preliminary proof-of- concept experimental results are reported.

We report experimental results of the nonresonant third-order nonlinear properties of an acceptor-donor azo dye doped in polymethyl methacrylate (PMMA) thin films by degenerate four-wave mixing with a He-Ne laser, which have appreciable nonresonant third-order nonlinearity, (chi) ⁽³) being approximately 4 X 10^-4 esu. The OPC signal has strong sensitivity with the polarization of the three incident beams, and the self- refracted phase conjugation by the coupled two counter-propagation beams was studied. In conclusion, nonresonant third-order nonlinear mechanism is demonstrated to be a result of photoinduced birefringence of the azo molecules. The real-time polarization storage properties of the azo dye were used in the interferometry to measure the shape variation of the object.

A shift and rotation invariant photorefractive (PR) crystal based associative memory is addressed. The proposed associative memory has three layers: the feature extraction, inner- product, and output mapping layers. The feature extraction is performed by expanding an input object into a set of circular harmonic expansions (CHE) in the Fourier domain to acquire both the shift and rotation invariant properties. The inner product operation is performed by taking the advantage of Bragg diffraction of the bulky PR-crystal. The output mapping is achieved by using the massive storage capacity of the PR-crystal. In the training process, memories are stored in another PR-crystal by using the wavelength multiplexing technique. During the recall process, the output from the winner-take-all processor decides which wavelength should be used to read out the memory from the PR-crystal.

We present theoretical and experimental investigations on diffraction properties of transmission-type gratings in photorefractive crystals under the conditions of strong beam coupling effect between two writing beams. By using the coupled-recording-wave approach, the diffraction efficiency is derived as a function of intensity ratio of the two writing beams, the beam coupling constant, and the phase-shift between the interference fringes. Two configurations are discussed with (pi) /2 and -(pi) /2 phase shift between the interference fringes and the refractive index gratings. The theoretical results suggest that the case of (pi) /2 phase shift has the optimum conditions for high-fidelity holographic storage of the gray-level images. On the other hand, the case of -(pi) /2 phase shift is suitable for information processing in the Fourier plane. The experimental results are in agreement with the theoretical predictions.

Photorefractive volume holography for processing ultrashort optical pulses carrying spatial, temporal, and spatio-temporal optical information is introduced. These new holographic methods can process 4-dimensional information that in addition to the 3 spatial coordinates also include the temporal evolution of optical signals on nanosecond to femtosecond scale. Photorefractive volume holographic materials provide the medium necessary for recording and reconstruction in real-time. Applications of direct time domain and spectral domain holography for image processing, temporal matched filtering, optical pulse shaping, 3-D optical storage, and optical interconnects are discussed. Furthermore, the combined space-time holographic processing that allows the conversion between spatial and temporal optical information carrying channels is introduced. This method is used to demonstrate experimentally parallel-to-serial and serial-to-parallel data conversion for 1-D images and image-format data transmission. This holographic processor provides the advantages of self- referenced signal transmission and self-compensation for optical dispersion induced by the holographic materials, communication channel, as well as other optical components. Finally, future research directions for optical information processing with complex spatio-temporal signals are identified and discussed.

Photorefractive phenomena such as optical phase conjugation and energy coupling via wave mixing have been studied extensively in photorefractive waveguides, fibers, and bulk crystals for a wide range of applications. In many of these applications, the photorefractive device can be viewed as an adaptive spatial mode converter which converts one spatial wave-form to another in real-time with a very high conversion efficiency. In this paper, I briefly review the basic principle and the experimental investigations of adaptive spatial mode conversion using photorefractive dynamic holograms and discuss their potential applications for the coupling of laser light into and out of waveguides and/or fibers. Recent progress in the research and development of two novel techniques for efficient and fault-tolerant laser-to-fiber coupling and multimode-to-singlemode fiber-optic coupling is treated in more detail and technical issues that need to be addressed to advance these techniques towards practical applications are outlined.

Multimode fibers destroy the shape, phase, and polarization of laser beams passing through them. We show how to correct for those problems on line using devices which combine a photorefractive double phase conjugate mirror (DPCM) and a local reference beam holographic wave coupling (LRHWC).

An optical implementation of real-time truth table look-up processing is proposed that uses complementary-encoding and multi-exposure hologram with a photorefractive crystal. The processor can be used for performing the functions of full addition, half addition, subtraction, multiplication, and the calculation of complex multi-nominal with any number system.

In this paper, we propose a novel high SNR tunable filter using a photorefractive single crystal fiber with multiple holographic gratings. Since the single crystal fiber which is grown by the laser heated pedestal growth (LHPG) technique can be more than 100 mm long, volume gratings with an extremely large number of period can be obtained in the fiber. Thus, the wavelength selectivity of the grating can be very high on the order of 10⁵ so that one can achieve very narrow band filter (< 0.01 nm). To increase the signal-to-noise ratio (SNR) of the filter, a method of using multiple holographic gratings is proposed. The SNR can be improved by more than 100 times as compared with a conventional volume grating filter. Finally, the electro-optic effect can be used to change the refractive index of the crystal fiber, which in turn changes the wavelength response of the stored volume grating. Since the response time of the electro-optic effect is on the order of 1 ns, one can very rapidly tune the central-wavelength of the filter.

Planar photorefractive Fe:LiNbO₃ waveguides have been fabricated by proton-exchange and using Fe:LiNbO₃ substrates. The waveguides were characterized by scanning electron microscope, infrared spectra, and index profiles. The influence of annealing on the optical property of the waveguides was investigated.

A multimode fiber specklegram sensor using phase-conjugate mirror is presented. This sensing technique is based on the phase compensation of phase-conjugate signal, which is very sensitive to external perturbation. The preliminary experiments on displacement measurement are also provided.

The aim of the present study is to establish the nature of the stimulated photoluminescence (PL) under IR excitation in the electron trapping materials. Investigations of the PL spectra of sulfides and oxides of the alkali-earth metals at the room and liquid N temperatures were carried out. Oxide hosts were exposed to electron beam irradiation in order to create defect levels for subsequent trapping of electrons. The oxide-based materials were preferably investigated because of their high chemical stability and low hygroscopicity in comparison with sulphide host material. This fact should be taken into account when using the materials as a data recording medium.

The spectral dependencies of the optical rotatory power (ORP) and Verdet coefficient (V) of undoped Bi₁₂GeO₂₀ (BGO), Bi₁₂SiO₂₀ (BSO) and Bi₁₂TiO₂₀ (BTO), and doped with V, Fe and Cr BSO crystals are studied. It is established that Cr and Fe strongly change ORP and, respectively, V in BSO in a wide spectral range from 480 to 1180 nm. Thus an increasing of ORP by about 92.8% at 480 nm, 25.3% at 756 nm, and 5.2% at 1171 nm, is observed in BSO:Cr. The dopant Fe in BSO strongly increases the Verdet coefficient V in the whole studied range -- with about 80 -- 300%, as at the longwaves an anomalous considerable increasing of V is observed. On the contrary, Cr and Fe decrease V and ORP, respectively. Using a fiber-optic system, the local ORP and V are measured. Some differences in ORP and V are observed within and without the so-called `central core.' A good qualitative evaluation of the local impurities and dopants distributions within and without the core could be based on these differences.

Self-organization of light into a hexagonal spot array and its rotation using photorefractive KNbO₃:Fe is investigated. Furthermore two-beam coupling configurations are employed to obtain the phase conjugate image and edge extraction of an object using photorefractive KNbO₃:Fe. Finally, the self-organization is utilized to broadcast an input object to the location of each of the spots.

Baseband demodulation, lock-in detection, noise reduction, compression/expansion nonlinearities, and correlation receivers are common subjects in communication theory. In this paper we use photorefractive materials to show how real-time holography can be used to implement these temporal electronic signal processing techniques as spatio-temporal optical signal processing on images.

We analyze recording of gratings by orthogonally polarized waves-modes of birefringent crystal which are coupled by nonlinear interaction, photogalvanic current in photorefractive crystals, or by anisotropy of two-photon absorption in semiconductors. Different schemes of polarization grating recording (four-beam, two-beam, single-beam) are discussed.

In this paper, we introduce a new technology for picking-up images and one-way aberration- free image communication through a phase-disturbing medium using photorefractive four- wave mixing and theoretically analyze this photorefractive process using the photorefractive four-wave coupling equations. We also experimentally realize this operation in the LiNbO₃:Fe crystal.

A shearing interferometer with a kitty-type self-pumped phase-conjugate mirror is proposed. The measurements of the focal length of a lens are demonstrated, with a standard deviation of 1.5%. In addition, the micro-displacement of several micrometers of a lens is measured with error less than 2%.

We briefly review holographic endoscopes and propose a system using a photorefractive fiber to record hologram inside the body. We also propose a system similar to a stereomicroscope to see the 3-D image of an object inside the body in real-time.

A novel training technique for synthesizing a 3D rotation invariant bipolar filter has previously been presented (OSA Annual Meeting, October 1994). An extension of this idea is now presented. Previously, a training process using a simulated annealing algorithm for the filter was designed to yield optical correlations that were Gaussian-shaped for the preferred target and a null response for non-targeted objects. However, it is proposed that one may effectively choose other shapes for the output correlation by altering the computed filter response of the simulated annealing algorithm. A selection of basic geometric shapes may be chosen to correspond to particular classes of objects. As an illustrative example, the filter may be designed to yield a rectangular-shaped correlation in the output plane if the input object is a tank while yielding a triangular-shaped correlation for a truck input. These shaped correlations in the output plane of the optical processor may be more easily distinguished from each other and from spurious background noise through simple morphological processing of the output plane. Computer simulations of the correlation results of these filters are presented. The design of a photorefractive-based optical correlator that implements bipolar composite filters also is discussed.

A new method for implementing optically real-time wavelet-matched filtering is proposed by using nonlinear four-wave mixing in photorefractive media. In comparison with conventional matched filtering, the proposed method is able to give higher discrimination ability. Experimental results for optical character recognition are presented.

Among contemporary well-known acousto-optic rf signal processing systems, space integrating acousto-optic spectrum analyzers (SIAOSA) are most adapted for commercial applications. This is because their laboratory prototypes achieve high parameters and their elementary base is well-developed. SIAOSA can process rf signals directly at carrier in the metric and decimetric bands, has a wide band of simultaneously processed frequencies, high parameter stability, and small weight and dimensions. Due to this unique parameter combination, SIAOSA are already used in warfare, rf monitoring, and radio astronomy. A method for SIAOSA design is required in order to broaden the area where those systems can be used. In its turn, such a method requires a SIAOSA mathematical model for simulating its operation. The most general mathematical model is a statistics model. But such a model requires statistic characteristics of separate SIAOSA elements, and their connections and joint operation. The latter information can be obtained from a deterministic model based on the study of how a known signal passes through SIAOSA. Such a deterministic model is used in most works on acousto-optic spectrum analyzers. This paper is based on an experimental model and devoted to some ways of SIAOSA performance enhancement.

Volume holographic memory systems provide a compact method by which data can be stored with high density and rapid accessibility. In these systems, for example, wavelength, angle, and phase multiplexing approaches have been utilized for the addressing of data pages. We analyze, in general, practical limitations in such systems due to cross-talk effects, beam depleting and material heating absorption effects, and multiplexing approaches. For example, beam depletion during the writing and reading of holograms in a volume holographic memory will cause the envelope of the diffracted beams to not be rect-like as simple cross-talk theories rely on, and material heating will act to detune, smear, and redirect beams during readout. We also characterize the photon-limited information throughput rates during recall from these systems. Finally, we demonstrate advances in our sparse-wavelength angle-multiplexed volume holographic memory system, achieving the storage of 2,000 holographic pages, each with approximately 2.35 million bits each, utilizing 400 angles (over a 3 degree external tuning span) and five wavelengths (over a 56 nanometer span) in a 1.86 cubic centimeter volume of lithium niobate.