A numerical simulation model of two-beam interaction in photorefractive polymers is presented. Higher diffraction orders and beam fanning are included in the model. The simulation results were found to agree precisely with theoretical predictions and experiments. Edge enhancement of images in a two-beam coupling geometry is predicted.

Photorefractive effects in optical waveguides link two interesting research areas: First, light-induced refractive index changes in optical materials, and second waveguide structures that are the basis for integrated optical devices. In the last two decades several devices based on photorefractive waveguides have been proposed, and static and dynamic elements have been experimentally demonstrated. Two of these applications of photorefractive waveguides will be discussed in more detail. Permanent refractive index gratings in waveguide devices are of considerable interest for, e.g., integrated waveguide lasers or wavelength multiplexers that make use of the high spectral selectivity of holographic filters. Thermal fixing of holographic gratings has been investigated in channel LiNbO₃:Ti:Fe waveguides. A wavelength filter for infrared light around 1.55 micrometer has been fabricated with a peak reflectivity of over 99%, a bandwidth of 0.1 nm and nearly polarization independent properties. As another example, we have demonstrated the formation of photorefractive spatial solitons in a planar strontium-barium niobate waveguide. In such a geometry (1 + 1)-dimensional solitons are formed in a true (1 + 1)- dimensional medium, thus getting rid of the transverse instability that is inherent to soliton propagation in bulk crystals. We show that coherent collisions between two photorefractive solitons result in fusion, repelling, energy exchange, or the creation of a third soliton.

The prospect of hydrogenated amorphous silicon based photonic thin film material and devices is introduced. The hydrogen content of hydrogenated amorphous silicon controls its refractive index. Hydrogen content and therefore the refractive index patterning techniques and possibilities are described. For example, regions of a growth surface exposed to a hydrogen radical (and/or ion) flux have increased optical band gap and decreased refractive index. By careful implementation of hydrogen control the preparation of 3-D photonic crystal films on a wide variety of substrates including single crystal silicon and flexible polymer becomes possible. The size scales on which it is possible to pattern the hydrogen content are appropriate for the preparation of photonic crystal films and bulk materials designed to interact with the infrared, visible light, or micro-wave electro- magnetic spectrums. The optical band gap of amorphous silicon depends on specific hydrogenated structures. The relatively independent patterning of the band gap and refractive index makes possible an extensive array of optical devices.

Especially doubly doped LiNbO₃ single crystals were irradiated with different doses of gamma rays. The short and long-range effect of the irradiation processes on the photosensitivity of these crystals was studied. For short range effect, the photosensitivity was increased in particular the phase conjugate signal became more stronger and the rising time for the phase conjugate signal was increased except for LiNbO₃ doped Mn and Fe. For the long-range effect, the phase conjugate signal was found to increase for the crystal doped Cu and Ce then decreased after 220 days. On the other hand, the response time for the crystal doped Ni and Mn was found to decrease drastically after 150 days. Other crystals doped Fe and Mn, Fe did not show much change between short and long range effect up to 300 days. Almost all the crystals after 320 days showed oscillations on the phase conjugate signal.

The effect of the ion implantation of Fe, Mn on LiNbO₃, as well as the dispersion of Ag colloids in LiNbO₃ were studied using electron paramagnetic resonance (EPR). EPR measurements on these crystals were performed as a function of fluence at room temperature. The fluence was 1 X 10¹⁴ and 1 X 10¹⁶ ions/cm² for Fe and 2 X 10¹⁶ ions/cm² for Ag. The unpaired carrier concentration increases with increasing fluence. The photosensitivity of these crystals was determined by observing in situ the effect of the laser illumination on the EPR signal and measuring the decay and growth of the EPR signal. EPR and optical absorption showed significant differences in the dispersion of the colloids with implant temperature and implant order.

We study theoretically the properties of waveguides induced by one-dimensional steady-state biased photovoltaic spatial solitons, and show that the waveguides can be induced by both bright and dark spatial solitons in the biased photovoltaic- photorefractive crystal such as LiNbO₃. We also derive wave equations for the probe beam in the general condition and low-amplitude condition.

We present our studies on a photopolymer of poly(methyl methacrylate) (PMMA) doped with phenathrenequinone (PQ) molecules. We describe their characteristics for holographic data storage, including photo-sensitivity, material M/#, angular selectivity, dynamic exposure schedule, absorption and scattering effect. Experimental demonstrations on multiple digital data pages stored in a polymer cube are presented.

Acousto-optic deflectors (AODs) are used for data by-page recording as holograms, and for data extraction from the hologram sampled by the AOD. The possibilities and characteristics of AODs define information capacity of the holographic memory devices as well as architecture and parameters of their separate components. Usually 2 AODs are used for 2-D hologram recording, and 3 AODs -- for volume multiplex recording. The calculations of amount of the recorded holograms which can be provided by different AODs have been carried out in the present work, and the limits of information capacity increasing at expense of optimal using of both AODs and recording media, have been appointed. The specific attention have been paid to the hologram 3-D recording with angular multiplexing where information capacity can be increased by 2 orders. The difficulties to be overcome in order to achieve the biggest information capacity, have been noted. Another subject of study is the problem of the sampled data arbitrary access rate increasing which can be provided by AODs application. It has been noted that minimum access time is limited, and this limitation is connected with the duration of the signal transmission through Bragg cell and with the time of signal transmission and processing in other components of holographic memory. It has been shown that information quality of holographic memory grows along with increasing of the number of used AODs. It has also been noted that if the memory parameters are close to their limits, the cross-talks grow, and the optimum must be found.

The kinetics of the recording, compensating, and developing processes in two typical thermal fixing procedures (L-H-L and H-L procedures) are analyzed. The optimum condition (in terms of the fixing temperature, compensation time and material parameters) for each fixing procedures are obtained under the boundary condition of shot-circuit. Experimental results are shown which are in qualitative agreement with the theory in L- H-L procedure.

We demonstrate an encryption-selectable holographic multiplexing storage in LiNbO₃. The storage of multiple holograms is based on angular multiplexing and random phase encoding. Phase conjugates of the reference beams are used to read out those stored patterns. The sensitivity of the displacement of the ground glass is analyzed theoretically.

We present dynamics of the grating formation process in phenanthrenequinone- (PQ-) doped poly(methyl methacrylate) (PMMA) photo-polymer. The exposure schedule for multiplexing holograms to achieve equal-strength storage in a single location is described. The analysis method is valid for any saturated media, thus the results can be applied to other photo-polymer materials with similar kinetics.

It is well known that the conservation of energy in an optical system can be described by an invariant (intensity of the optical wave) during propagation. We identically show that reciprocity can be described by an invariant. This invariant is the overlap integral of two counterpropagating waves, that stays constant in every place of a reciprocal optical system. Applied to the double phase conjugate mirror (DPCM) this invariant can be rewritten as an equality between the ratio of the conjugation fidelity of the two ports of the DPCM and the ratio of the transmission in intensity of the two beams. If restricted to a plane wave case this relation becomes the well known equality of the diffraction efficiencies in both directions. We have implemented an experimental set-up that allows to show that the double phase conjugate mirror is reciprocal what confirms all the above discussion. We use the same set-up to measure the conjugation fidelity of the DPCM.

The limiting capabilities of 3-D holographic memory in terms of storage density and data reading rate are considered. The emphasis is on the problem of the interference noise during data reading. Various designs of 3-D holographic memory, including a multilayer system, are discussed. The possibility to effect superresolution in reading/writing holographic transparencies is shown. Under certain conditions, the storage density can be an order of magnitude higher than that determined by the diffraction limit. The paper also discusses the fields of application of holographic data storage systems with regard to the development of 'bit-by-bit' memory systems in particular systems based on the principles of near-field optics.

Recently, we report a new type of micro channel plate spatial light modulator (MSLM), in which the bulk LiNbO₃ crystal plate is replaced by an electro-optic composite material, namely, single crystal waveguide array. This array is a 2D LiNbO₃ single crystal waveguide array, which consists of single crystal waveguides with filling materials among adjacent waveguides. Since the dielectric constant of LiNbO₃ single crystal is much higher than that of gap filling materials such as polymer/epoxy, the electric field can mostly be constrained inside the crystal material. In addition, by selecting proper opaque filling material, there are also no optical crosstalks among adjacent waveguides. Thus, this special waveguide array works as a 2D set of optically and electrically isolated light guides. While the sensitivity is unchanged, the spatial resolution can be increased. In this paper, a prototype of such a MSLM is presented. The manufacturing processes of the MSLM is introduced. The evaluation on the performance of the MSLM is also reported. The advantages and limitations of the MSLM are addressed.

Theoretical calculations provided for further elaboration of sensitive elements for fiberoptic sensors of physical values are stated at present paper. Technological possibilities of such element creation and their further application are considered. Theoretical analysis shown that reflection coefficient from fiber cladding gratings is balanced with different types of gratings because of possibility of creation of considerable equivalent periodical change of refractive index, though optical wave amplitude on the fiber cladding less than on the fiber core. Fiber cladding gratings can be formed by holographic method using photosensitive medium and blue-green lasers. The case of corrugated boundary between fiber core and cladding or between fiber cladding and external medium, for e.g. water contaminated by particles of heavy metals, which have influence on refractive index and absorption of waves in fiber is possible.

We succeeded in fabricating small Bragg reflectors in glass and will present this experiment. We found that the photo- induced refractive index change caused by 1-mJ 130-fs laser pulses is elongated along the optical axis compared with the original intensity distribution near the focus of the fabrication beam. The length and radius of the index change were approximately 30 and 2 micrometers, respectively. We consider that the self-focusing due to the self-phase modulation resulted in this form. This structure is suited for the fabrication of small Bragg reflectors in glass. We fabricated 1 mm X 1 mm X 30 mm Bragg reflectors. By analyzing the diffraction efficiencies of higher order diffraction beams we estimated the refractive index distribution. The thickness and spacing of the periodic laminar structure were 1.5 and was 3.8 micrometers, respectively. The maximum index change was 0.0015. We note that we observed a quite large amount of scattering loss (approximately 15%) in the grating. We fabricated also a 1 mm X 1 mm X 60 mm grating by writing two layers of gratings and obtained a diffraction efficiency of 13%. Recent experimental results of photorefractive waveguides will also be reviewed.

We discuss the diffraction properties and the applications to optical computing and information processing based on anisotropic diffraction in a normal-cut BaTiO₃. The formula of diffraction efficiency under Bragg mismatch is derived. It fits well to the experimental results. We also discuss some applications, including optical digital logic operations, optical interconnections, matrix-matrix multiplexing, central-symmetric filters and incoherent-to- coherent optical converters.

Many researchers are now working on holographic memory. The holographic correlator is one of its different applications. The photorefractive hologram memory setup can be modified to be used as the correlator scheme. We can identify the object image by observing the direction of retrieved reference beams or correlation peaks. We have used a new scheme of using the speckle patterns as the reference beam. The speckle patterns can be made by multimode fibers or ground glasses. The hologram recorded with this speckled reference beam is less correlated with each other compared to the holograms with the unstructured reference beam. We have already proven that the hologram density could be increased with this speckled reference beam. We apply this technique to an optical correlator. The experimental result and the theoretical analysis are discussed.

The characteristics of photorefractive polymer doped with PNP in optical image processing are investigated. PNP[(S)-(-)- N-(5-nitro-2-pyridyl)prolinol] is one of the high gain chromophore in polymeric photorefractive system. In the two wave coupling experiment, the gain coefficient about 180 is observed with 25 micron thick sample at 100 V/micrometer. Higher diffraction order and phase conjugation is also observed. This photorefractive thin film is used as a holographic recording device. Fourier transform hologram is recorded by using similar arrangement of two wave coupling configuration. The reconstructed image from the photorefractive film is edge-enhanced. Degree of enhancement depends on the intensity ratio of signal to pump beam. Decreasing the ratio of signal to pump beam, degree of edge enhancement decreases. When the object pattern is defocused on the film, degree of enhancement also decreases. Optical image processing system in which degree of edge enhancement is controllable can be constructed.

A volume holographic matched filter is used for three- dimensional imaging of silicon microstructures. The filter is designed by recording a hologram that is matched to the object depth, allowing confocal imaging of the structure. Confocal imaging of micro devices provides the capability to image the performance of the device, which is not possible in scanning- electron-microscopes (SEM). A matched holographic filter can also provide more information about the device than regular confocal imaging. We present a scanned image of a silicon wafer with micro-fabricated trenches.

Photorefractive crystals offer several attractive features such as high resolution and in situ processing. As the images are erasable, these crystals are suitable for read-write applications and hence find potential use in speckle photography, image processing and holography. The BaTiO₃ crystal as recording medium has been extensively used as a novelty filter for real-time in-plane displacement measurements employing two beam coupling configuration. This paper presents new optical configurations in speckle shear photography to measure in-plan displacement and the strain in real time using BaTiO₃ crystal as recording medium. Speckle photography studies are made using a simple two-beam coupling configuration. In speckle shear photography, a diffused object illuminated with two parallel narrow laser beams is imaged inside the crystal, and a pump beam is added at this plane. The speckle patterns due to each beam and the pump beam produce index gratings. When the object is deformed, the speckle patterns shift consequently. We now have four speckle fields: two generated from the interaction of pump beam with the index gratings and two pertaining to deformed states directly transmitted through the crystal. Thus, the fields from respective points on the object interfere after passage through the crystal and produce the Young's fringe patterns. Due to strain, the fringes in each pattern are of different width and orientation, resulting in the generation of a moire pattern. The strain is obtained from the width and orientation of the fringes in the moire pattern. The experiments are conducted on a specimen with a notch, which is subjected to tensile loading. The in-plane displacement is measured separately in another experiment. Time evolution of the growth and the decay of the signal beam also is studied. The above studies are carried out at different laser wavelengths and the results are compared.

A new tuning scheme of the photorefractive filter is presented using LiNbO₃ crystal doped with 0.015Wt.% Fe. Analyzing the filter bandwidth property from the geometrical method, a new wavelength selectivity theory is derived. The center wavelength can be tuned by the real-time received beam angle control using the spatial light modulator. As tuning time depends on the response time of the spatia light modulator, it has high tuning speed. The use of thermally fixed single grating makes the uniform diffraction property over all filtering range. Designed tunable filter consists of the three channel with 4 nm bandwidth and 10 nm spacing. From the optical experiment, we get the real-time tunable filter which has 4.5 nm, 4.25 nm, 4 nm bandwidth and 1530.5 nm, 1540.5 nm, 1549.5 nm center wavelength, respectively. Demonstrated filter has a switching time of several tens milli-second and a full tuning range of 20 nm.

In this paper, we reported the electrical fixing results of photorefractive index gratings in Cu- and Mn-doped KNSBN crystals at room temperature. Due to the higher Curie temperature of KNSBN, the fixed grating may have a longer lifetime than SBN crystals, and the fixing results are different from the observation in SBN crystals. The relation between the applied field and the revealed grating is studied by analyzing the domain fixing process. We also reported the comparison of the fixing results in the single domain crystals and the multi-domain crystals. In the multi-domain crystals, the diffraction efficiency of the revealed grating is as about three times as that of the recorded grating. The probable reason is also discussed.

A scenario using cascade long-period fiber gratings to flatten the gain spectrum of the Erbium Doped Fiber Amplifier (EDFA) is presented. Long-period fiber grating can act as a kind of wavelength selective loss device, in which the guided mode is coupled with forward transmit cladding modes. Cladding mode resonance in long period fiber grating is analyzed and cladding mode fields are simplified by using coupled mode theory. The loss spectrum formula versus propagation constant and refractive index fluctuation is obtained. Four long-period fiber gratings based on standard single mode fiber are designed for the typical EDFA's gain spectrum by using more accurate transmission spectrum formula. The resulted fiber gratings chain is used to flatten the broadband gain spectrum. The computer simulation result is satisfying.

A novel system for associative memory is proposed and constructed in this paper. It is based on the theory of wavelet matched filtering and the associative characteristic of volume holographic storage in a photorefractive crystal. A primary image can be remembered and recovered from its deformed form or main features with the system. The photorefractive crystal in the system stands for both the cell of identification calculation and that of image storage. A high efficiency is obtained by a means of parallel processing in the system. The performance of the system is more accurate with the introduction of wavelet transform to the system, which is testified by the comparison between a conventional system and the novel system. Its application in fast query of human face from a face database is studied. The given experimental results are promising.

In this paper, techniques of volume holographic associative storage and statistical pattern recognition method are combined to develop a photorefractive correlator for real-time human face recognition. A set of eigenimages extracted from a large number of training images by K-L transform are stored in a photorefractive crystal by using the two-wave mixing volume holographic storage technique and used as the spatial filters. When any new image inputs the correlator, spatially separated beams with different light intensities are obtained in parallel, which represent the optical correlation results between the input and the set of eigenimages. Treating them as feature vectors, the statistical discriminant analysis will be applied to these features for classification. Since the face images in the training set can be selected to representing typical distortions, the system can deal with various distortions to a large extent.

We present, for the first time to our knowledge, a gray-level image recording method in photorefractive volume memory, in which the image to be recorded is illuminated periodically by a pulse while the reference beam interacts the medium continuously until the steady state is reached. By adjusting the duty cycle of the exposure of the image in recording, high fidelity can be maintained in image reconstruction for pixels of different gray-scale intensities. Furthermore, due to multiple beam coupling the weak input image can be greatly enhanced without the loss of fidelity.

Reliable and reproducible results on Bi₁₂GeO₂₀ (BGO) crystals depend on the thermal and optical history of the samples. It is then necessary to find well defined state of the material for which the experimental observations are reproducible. The de-excited state is reached either by an annealing at 450 - 500 K, or by illumination with photons of energies less than 2.2 eV. The corresponding photoconductivity is called the background photoconductivity. The excited state is obtained when the crystal is illuminated with photons of energies higher than 2.4 eV until reaching an optical and electrical steady state. The corresponding photoconductivity is the photoinduced photoconductivity. Comparison between these two states show that for energies higher than 2.4 eV the background and the photoinduced photoconductivity are the same, but for energies lower than 2.2 eV they are very different and the ratio of their magnitudes can reach a factor 1000. This study allows to define the conditions of writing, reading and erasing information on the BGO crystals. These effects are explained in terms of charge transfers between deep and trap levels, induced by the appropriate illuminations.

In this paper, the Taguchi Quality Engineering Method (TQEM) is applied to choose the required flight parameters in a back propagation neural algorithm for the analysis of aircraft wing loading. The screening out unwanted parameters is an important procedure in strain measurement, as using too many flight parameters increases the complexity of the network structure and also the difficulty of the learning scheme. The demonstrated results show that TQEM is easier and more reliable than traditional methods for choosing the required parameters for system identification.

We report the implementation of a low loss core-cladding waveguide structure of an a-axis Nd:MgO:LiNbO₃ single crystal fiber and the demonstration of a frequency-doubled laser made from such a cladded crystal fiber. The laser-heated pedestal grown a-axis single crystal fiber, which has an elliptical cross sectional area of about 200 micrometer X 150 micrometer, was further treated with a Mg-ion indiffusion process. An electron probe microanalysis was used to measure the Mg-ion concentration distribution of the Mg diffused layer. After a number of trials, the diffusion parameters suitable for forming a core-cladding waveguide structure have been obtained. The optical and structural properties of the cladded crystal fibers were also characterized. A diode pumped external cavity frequency doubled laser has been made from the cladded crystal fiber. At room temperature, CW green laser output with a power of 10 (mu) W at the wavelength of 0.532 micrometer has been obtained. The origins of the relatively low frequency-doubled conversion efficiency will also be discussed.

Photorefractive Fe:LiNbO₃ planar waveguides have been fabricated by proton exchange method using Fe:LiNbO₃ as substrate and benzoic acid as exchange medium. The waveguide has a much larger refractive index increment than a Ti diffused one. Through end-face coupling, a pump beam and a signal beam were injected into the waveguide and the optical amplification properties of the waveguide have been investigated experimentally. The results showed that a hundred fold amplification gain could be obtained in a wide range of grating periods and an amplification gain of 180 was achieved easily. This scheme has potential applications in optical signal processing at not very high incident intensity.

Several photorefractive crystals such as LiNbO₃ or BaTiO₃ can be used for optical information storage as well as key elements for optical signal processing due to their unique property, viz., their refractive index varies according to their incident light distribution. Thus, we can construct phase gratings inside the crystals by using interference of two light beams. The index grating amplitude (Delta) n can be computed from the equation derived from the Band Transport model for the photorefractive effect. Experimentally, the value of (Delta) n can be estimated from counting the number of peaks in the curve for the diffraction efficiency as a function of time under the phase matching condition. Its precision is about 13%. Here we report a new method improving the measurement precision to about 1% by adding the measurement for the diffraction efficiency as a function of angle mismatch. This precise measurement of (Delta) n allows us to quantify photorefractive effect with greater precision.

Using the self-pumped phase conjugation with a photorefractive BaTiO₃ crystal, we propose a device which changes the flow direction of signals. Two beams of light involve in the device and signals are transferred between them. They share the total-internal-reflection (TIR) loop in the crystal. The transferred pattern can be a positive replica of the original pattern or a negative replica of it.