Effects of optical self-organization in photorefractive KNbO₃ are observed and studied by optical and electro- motive force methods. The crystal is characterized by monitoring the electric currents induced in the crystal due to applied illumination. A theoretical model of the observed effects is suggested. Spin dependent conductivity in photorefractive crystals with paramagnetic centers is discussed.

A review of physical properties of the materials used in manufacturing real acousto-optic cells for optical and RF signal processing is presented. The overall design of acousto-optic cells requires the knowledge of not only the elastic modulii of the second order, but those of higher orders also, as they determine the acoustic nonlinearity and the dynamic range of any device. The parameters of elastic anisotropy determine the near zone of cell's transducer and directly affect cell's time aperture and number of resolvable spots, and interchannel cross-talks in multichannel devices. The requirements to the properties of crystal materials used in cells with anisotropic and collinear interaction are discussed.

We investigate the use of combined optical and electrical techniques to control domain formation in ferroelectric SBN, and examine the periodic structures induced by spatially modulating the light intensity through the crystal during the electrical poling process. The role of photoexcited charges in compensating and stabilizing the induced domain structures is summarized, and the importance of thermal effects established. The process of domain re-ordering is shown to be particularly sensitive to temperature changes close to a domain freezing point of SBN, which occurs near room temperature. The resulting light-induced domain re- ordering is assessed using current monitoring during the repoling process, and photorefractive two-beam coupling of the resulting structures.

A new type of anisotropic scattering ring, accompanying strong isotropic conical scattering and isotropic scattering oscillation, is observed on a screen behind a Cu:KNSBN crystal pumped by a extraordinary beam. The latter plays a key role in the forming of the former.

Azo dye polymeric film were widely studied for real-time polarization holography and optical storage by degenerate four-wave mixing in the resonant absorption spectrum of the sample in the few past years. In this paper, we will report the experimental results of four-wave mixing based on excited-state absorption in ethyl orange doped in polyvinyl alcohol film. A He-Ne laser was used as the four-wave mixing source, and the excited light was an Ar⁺ beam. To explain the experimental results, a simplex four-level model of azo molecule polymeric system is built up. The dynamic process of phase conjugation signal by excited-state four- wave mixing is calculated through solving the rate equation of population in the excited-state. The theoretical analyses agree with the experimental results well. The excited-state optical storage mechanism of sample is discussed.

The giant volume grating diffraction efficiency of 70% in triple doped photorefractive crystal sillenite family has been observed. Measured signal to noise ratio is more than 250 in continuous hologram registration regime. A high quality interferograms of diffusely scattered objects have been obtained. These results were applied for construction a new adaptive portable sensors for engineering applications.

A photorefractive filter with 0.02 nm bandwidth was constructed. Using the Kukhtarev's charge transport model, the spectral response and electrical tuning of the photorefractive filter were analyzed. Electro-optical effect of the PR crystal was measured. Electrical tuning and thermal tuning of the filter were demonstrated.

We demonstrate the storage of 1000 holograms in LiNbO₃:Fe, recorded at 488 nm and read at 633 nm to reduce decay during read-out. We minimize Bragg mismatch problems of the two-wavelength scheme by using thin crystals, while combining peristrophic and angle multiplexing to counter the poorer angular selectivity of thin crystals. We find the reduction in read-out decay to be significant, but limited by dark conductivity.

Cross-talk noise is a fundamental limitation to storage capacity in volume holographic memory. We present a general theoretical analysis on the cross-talk noise in angle- and wavelength-multiplexed volume holographic memory systems. Results on storage capacity and its dependence on hologram separation and required signal-to-noise ratio are obtained and discussed.

For reducing the cross-talk noise and improving the storage capacity in angle-multiplexed volume holographic memories, a novel recording method is proposed. The technique employs a recording reference extending uniformly within a narrow spatial frequency bandwidth and reads the memory with a plane wave. Comparing to the conventional recording method with planar reference waves, the analytical results show that the signal-to-noise ratio obtained by using the extended reference method is about 20 dB higher. Moreover, a statistical analysis of the interpixel cross-talk noise is presented and the noise-to-signal ratio is given in a closed-form for both point reference method and extended reference method. Considering both interpage and interpixel cross-talk limited storage density. The results show that the proposed extended reference method achieves about 10 times larger storage density than the point reference method.

Multiple holograms can be storage in a volume holographic medium such as photorefractive crystal or photopolymers by using phase-code multiplexing. However, due to the limited performance of currently available spatial phase encoders, only a small number of orthogonal phase codes can be implemented. This significantly limits the capacity of holographic systems based on phase-code multiplexing scheme. In this paper, a new multiplexing method, rotation multiplexing, is presented. After a number of holograms are stored by phase-code multiplexing, the holographic medium is rotated in the incident plane by a small angle. New holograms can then be stored by the same phase-codes without cross talk. By combining the phase-code and rotation multiplexing schemes, a high capacity 3D storage system can be built with off-the-shelf devices. An additional advantage of such a holographic storage system is its burst retrieval mode that is useful for network applications. The preliminary experimental results demonstrate the feasibility of the proposed approach.

The storage capacity of the random phase-coded volume hologram multiplexing which uses a ground glass as the random phase diffuser is reported. We found that the role of the ground glass imply the storage capacity of the random phase-coded multiplexing. From the experimental results, the tolerance of the position mismatch of the decoding ground glass is with a typical value of decades of micrometers, as reading an image. On the other hand, from the computer simulation, the signal-to-noise ratio of the readout images can be obtained by the pixel size of the ground glass. After mainly considering the tolerance of the position mismatch of the ground glass and the signal-to-noise ratio of the reconstructed image, the storage capacity of the random phase-coded multiplexing can be estimated.

In this paper we demonstrated the advantage of the fanning- noise-free double doped photorefractive LiNbO₃ crystals for the 3D storage. The fanning noise can be completely suppressed. The SNR and the line resolution of the holographic reconstructed patterns in these kinds of crystal are very high. It is simple and convenient to practice using double doped LiNbO₃ crystal to suppress the fanning noise.

In this paper, we shall briefly discuss some recent works done at The Pennsylvania State University on the photorefractive (PR) fibers and their applications. First, we shall briefly review the growing process of single crystal fiber by using the laser heated pedestal growth technique. After that, the applications of PR crystal fibers to the holographic storage, fast speed wavelength tunable filter, and fiber optic true time delay line will be discussed.

Ce doped and undoped Sr_xBa_1-xNb₂O₆ (SBN) fibers grown by the laser heated pedestal growth (LHPG) technique in Stanford University were investigated by 2D scanning electron microprobe analysis. The SBN fibers grown along c [001] or a [100] axes often show radially distributed optical inhomogeneities (core effects) of varying magnitude. Ba enrichment and Sr reduction were primarily detected in the core which can be qualitatively described by a complex-segregation effect. This defect structure as a complex-congruency related phenomenon modified by the composition-control mechanism of LHPG system. Its radial dependence of effective segregation coefficient is described by the modified Burton-Prim- Slichter equation.

Of the several configurations that have been proposed and studied for the implementation of a high-density, fast- access volume holographic memory system, a 90 degree(s) angular multiplexing configuration that uses a 45 degree(s)-cut iron- doped lithium niobate (LNB:Fe) crystal has attracted much attention in the last few years. In a recent paper, Burr and Psaltis have used this configuration to show that the diffraction efficiency ((eta) ) of each hologram can be estimated by (eta) equals (M/#/N)², where N is the total number of angular-multiplexed holograms, and that the system parameter (M/#) can be determined by monitoring the recording and erasure dynamic of just one hologram. In addition, the authors have presented a mathematical expression which relates M/# to the photorefractive properties of the holographic material, the intensities of the recording beams and the recording geometry. We have applied the techniques (of Burr and Psaltis) to study a spatial and angular multiplexing configuration in the 90 degree(s) geometry. In this approach the angular multiplexing is combined with the spatial multiplexing technique by partitioning the crystal into a vertical array of `N<sub>L</sub>' layers, and each layer into a horizontal array of `N_C' cells. If each cell can accommodate `N' holograms (by angular multiplexing, for example), then the total number of holograms recorded in the sample becomes N X N_L X N_C. In this paper, we present the theoretical and experimental results on cell-to-cell variation in M/# within a layer. Our results indicate that the cell-to-cell variation in the M/# can be reduced at the cost of a lower average M/# and a longer recording time. Advantages and limitations of both the theoretical model and the experimental techniques are discussed.

Volume holographic recording using nonlinear photorefractive crystals is characterized by, among other features, nonlinear beam coupling effect, erasure-recording dynamics, and crystal anisotropy and birefringency. In this paper we first investigate the spectral diffraction properties of a reflective-type volume hologram by considering photorefractive beam coupling and recording-erasure dynamics. We then investigate the spatial diffraction properties of a PR hologram as affected by crystal refractive-index anisotropy. Its effects on the fidelity of the hologram image and on multiplexing scheme are also discussed. Finally, the combined (intrasignal) beam coupling and crystal anisotropy effect are examined for PR LiNbO₃ crystal.

Phase-conjugate external ring cavity modes of semiconductor lasers and phase-conjugate feedback effects were investigated. The phase-conjugate ring cavity is formed with a double phase-conjugate mirror. The effects of bi- and uni- directional phase-conjugate feedback on the operating state of a laser diode were studied. When the bidirectional phase- conjugate feedback is above -40 dB, the output spectrum of the laser is broadened and the operating state of the laser changes to multimode state. It was experimentally observed that in the ring cavity two opposite traveling waves, which are phase conjugates of each other, propagate.

As for our knowledge, a simplest real-time system for measuring dynamic optical phase perturbation is proposed and demonstrated. In this system a 1% weight Fe:LiNbO3 is used to record the self-interference grating by the incident light. The interferometer is a new kind of double-exposure one. The speed of the interferometer is as fast as that of the dynamic phase perturbations.

We use the bacteriorhodopsin film as a real-time large- aperture high-resolution holographic material to demonstrate its potential as a dynamic recording and imaging medium. The imaging reflectivity is found to be linearly proportional to the reading light intensity provided that the hologram is readout by short laser pulse. The bi-exponential behavior of the recording process is related to the intensity modulation of the illumination. An image resolution of 80 lines/mm and a space-bandwidth product of approximately 2 X 10⁶ are obtained with a phase conjugate reflectivity of above 30%.

A new method of 3D image recording, so-called reference-free selectogram is considered. According to this method, the object wave is split into two parts with the help of a diffraction grating. The pattern originated from the interference of these components must be recorded in a thick-layered light-sensitive material that is positioned just behind the grating. Gel-like thick-layered dichromated gelatin has been suggested for the selectogram recording. In this case, the layer represents a gel of dichromated gelatin that is sealed between two glasses. The thickness of layers varied from 1 to 3 mm. Holograms are recorded in such a material immediately during the process of exposure. The experimental data on the dependence of the diffraction efficiency on the exposure and the data on the dependence of the diffraction efficiency on the spatial frequency of the recorded grating are presented. The angular selectivity of the holograms recorded in the layers was about 10'. The experiments have shown that the lifetime of the hologram recorded in gel-like gelatin is measured by several hours. The experiments on the recording of the reference-free selectogram were carried out. To compensate the low value of the sensitivity of the material, the selectograms were recorded when concentrating the light on a small area of the layer during the exposure.

Moving grating technique is applied to dynamic holographic recording to overcome the difficulties of the fluctuation of the diffraction efficiency and the funning effect in photorefractive crystal BSO. Various nonlinear effects caused by moving gratings at large fringe modulations are experimentally investigated. It is shown in the application of optical pattern recognition that the probability of an error detection is reduced and the signal-to-noise ratio is considerably enhanced. Experimental results are presented.

The noise effect on the storage density of a holographic carrier with data file overlapping is considered. The limit storage capacity for ID-hologram recording is calculated and investigated. The methods of increasing storage density by using multilayer carriers are studied.

Photonic crystals were investigated experimentally in a scaled setup with microwaves in the form of 2D arrays of dielectric rods of high permittivity surrounded by air and are reported in the literature. These crystals render perfect mirrors for a band of wavelengths. Having an impurity built into the structure, transmission filters with a specific narrow bandwidth can be generated. The predominant feature of these structures is, that devices of high finesses are obtained with as few as 6 planes of dielectric rods. The grid constant of the rods is < (lambda) /3, with (lambda) the wavelength used in the device. The rod diameter is < (lambda) /6 and the length of the rods should exceed 2 (lambda) . Therefore photonic crystal elements extend only a few micrometers in the x-, y-, and z- dimension. In combination with monomode-waveguides and applying some areas filled with non-linear optical materials, tunable filters and switches for the routing of light can be built in a very compact way. It is the first time, that 3D additive lithography with electron-beam induced deposition is employed to generate photonic crystals. This technique allows to generate insulating or conductive structures from wires with > 80 nm diameter and micrometers dimensions with a surface roughness below 3 nm. It offers the freedom to taylor the refractive index of the material by selecting precursor materials, deposition conditions, and exposure mode. A scanning electron microscope with VIDAS-beam control system is used for this lithography. A custom designed lithography function allows to control position, dwelltime, and sequence of the pixels. Using a program generated database, which contains all pixel and time information required, 3D structures are generated with the deposition process. The devices are placed with nm precision in waveguide patterns. Macro-controlled construction of arrays of dielectric rods of high aspect ratios is presented, which resemble perfect or imperfect photonic crystals. Using specialized crystals, filters and tunable filters dense devices for routing of light or optical metrology can be fabricated.

In this paper, a multi-channel associative memory instrumentation prototype was presented, which was divided into two parts spatially. The lower part is a one-channel system using diode-pumped solid-state laser for demonstrating the holographic associative memory miniaturization, while the upper one is a four-channel system for proving the parallel processing architecture. Each channel is actually a photorefractive-based associative memory with external storage and using liquid crystal switch as threshold device in correlation domain.

An Infrared optical modulation (reticle or chopper) is used to provide directional information of infrared system for tracking and to suppress the unwanted signals from backgrounds. In this paper an infrared optical modulation using Photorefractive crystal associative memory (PRAM) technique will be provided. The proposed PRAM is used to replace the optical modulator (reticle) by using an associative hologram technique. The hologram is insert in the optical path between the optical lens of the optical system and the detector in the focal plane of the lens. When the IR fluxes illuminate the hologram, the hologram redirects the flux to the desired direction. A particular arranged detector generating the codes correspond to its position. By use of the optical signal processing and neuron network theorem, the background radiation of IR system can be suppressed effectively. Computer simulation results of PRAM based IR optical modulation are shown to be consistent with the theoretical analysis.

A rotation-invariant photorefractive correlation is reported in this paper. Matched filters synthesized with reference patterns at different rotational angles are stored in the photorefractive medium via angular and peristrophic multiplexing. During the recognition process, the input image is compared with all the reference patterns in parallel. The feasibility of the proposed correlator is demonstrated by experimental results.

With a view to understand the microscopic origin of photoinduced charge transfer in the cubic BSO crystal, we have studied the EPR spectrum of BSO:Fe³⁺ and the effect of laser illumination on the EPR signal during photorefractive grating formation at liquid nitrogen and room temperatures. The BSO:Cr crystal was grown at Alabama A&M University using Czochralski technique and the EPR spectrum was recorded at room temperature. Three EPR signals were observed. The EPR signals are due to the main dopant Cr³⁺ and the weak presence of Fe³⁺ which is inherent in BSO crystal. The effect of Ar⁺ laser illumination revealed new facts about the behavior of Cr³⁺ under laser illumination which is opposite to the behavior of Fe³⁺ in BSO:Fe³⁺ under laser illumination. Forming a grid on the face of the crystal increases the light-induced charge transfer Cr²⁺/Cr⁴⁺ is equivalent to Cr³⁺. The use of laser illumination helped to shed more understanding on the assignment of the EPR signals as well as the dynamic behavior of the charge transfer processes.

An anisotropic structure formed by the photoinduced color centers in the switching zone of the 1 X 8 fiber coupler was created and investigated. Using the property of an orientational ordering for the photowritten grating with the effective second harmonic generation the growth of an energy transfer for the probe cw He-Ne beam from the central fiber core to the selected 1 X 8 exit fibers was detected and discussed.

In order to accommodate to the available pixel numbers of spatial light modulators and to achieve the maximum possible storage capacity of a phase-code multiplexed holographic memory, it is often required to generate orthogonal phase- codes whose length (or order) is not a power of 2. This paper reviews various methods for the construction of Hadamard matrices of order n equals 4 m (with m a positive integer). It is meant to be tutorial in nature and useful in scope. No attempt is made to be exhaustive in the mathematic treatment. A complete list of Hadamard matrices for all orders n equals 4 m < 400 is given. The list can certainly be extended to even higher orders.

A photorefractive nonlinear incoherent-erasure joint- transform correlator is presented and analyzed. The nonlinearity of this correlator can be tuned from the classical-matched filter to the phase-extraction limit by increasing the erasure beam intensity. Experimental results in Bi₁₂GeO₂₀ show a transition from matched-filter correlation to phase-extraction correlation by increasing the erasure beam intensity, in agreement with our theoretical results.

Pattern formation and self phase conjugation are observed simultaneously in photorefractive potassium niobate. The effects have been modelled assuming the presence of reflection gratings. In this paper, we further the theoretical analysis by incorporating the effects of both reflection and transmission gratings. Also, we state image processing applications of self phase conjugation such as edge enhancement and contrast reversal.

Self-organization of scattering into reconfigurable hexagonal arrays has been observed in KNbO₃ by a number of researchers. Further, depending on the experimental configuration and the direction of the pump beam, these hexagonal arrays are observed to rotate about the main pump beam. In this paper we report on the results of the application of symmetry considerations to the analysis of these observations. The hexagonal array formation is postulated to develop through an interaction between secondary beams generated from the ring of beams formed from the original pump beam through a winner-take-all process. Starting with a model of the experimental configuration in which the feedback is provided by reflection from a surface which is perpendicular to the c-axis of the orthorhombic KNbO₃ crystal, we find that rotation of this hexagonal pattern is allowed by the crystal symmetry and depends on the direction of the main pump beam with respect to the crystallographic a- b- and c-axes. The direction of rotation is reversed when the beam direction is reflected in either of the two crystallographic reflection planes but will remain the same when the beam is rotated 180 degrees about the twofold crystal c-axis. The rotation of the hexagonal array should vanish when the beam lies in either of the two reflection planes, or is aligned with the c-axis. In this paper we will present the results of these symmetry considerations, and compare the results to observations.

Utilization of ultrahigh bandwidth available in optical fiber networks will require development of fast and efficient parallel-to-serial and serial-to-parallel all- optical multiplexing techniques. Such multiplexers are also useful for interfacing to optical storage devices. In this presentation we will review the application of space/time optical processing with femtosecond laser pulse to implement such multiplexers. We will focus on a novel real time optical space-time processor based on 3-wave mixing in a nonlinear optical crystal. This processor allows conversion of temporal signal sequence to a 1D spatial image, thereby realizing a serial-to-parallel multiplexer. The processor is also used to generate a wigner distribution function, which allows to determine both amplitude and phase of ultrashort temporal signals.

A new experimental method for vibration modal analysis based on all-optical photorefractive processing is presented. The method utilizes an optical lock-in approach to measure phase variations in light scattered from optically rough, continuously vibrating surfaces. In this four-wave mixing technique, all-optical processing refers to mixing the object beam containing the frequency modulation due to vibration with a single frequency modulated pump beam in the photorefractive medium that processes the modulated signals. This allows for simple detection of the conjugate wavefront image at a CCD. The conjugate intensity is shown to be a function of the first-order ordinary Bessel function and linearly dependent on the vibration displacement induced phase (delta) , for (delta) equals 4(pi) (xi) /(lambda) << 1 where (xi) is the vibration displacement and (lambda) is the source wavelength. Furthermore, the results demonstrate the unique capabilities of the optical lock-in vibration detection technique to measure vibration signals with very narrow bandwidth (< 1 Hz) and high displacement sensitivity (sub-Angstrom). This narrow bandwidth detection can be achieved over a wide frequency range from the photorefractive response limit to the reciprocal of the photoinduced carrier recombination time. The technique is applied to determine the modal characteristics of a rigidly clamped circular disc from 10 kHz to 100 kHz.