Tai Chiung Hsieh

Photorefractive Fe:LiNbO 3 crystal thin plates for optical information processing

The effects of the donor acceptor concentration ratio in iron-doped lithium niobate (Fe:LiNbO3) crystal plates on photorefractive response time and on grating diffraction efficiency are studied both theoretically and experimentally. The results provide a useful guide for designing photorefractive plates for optical information-processing applications. Two devices for real-time image recognition have been demonstrated: a photorefractive joint-transform correlator and a VanderLugt correlator. The former emphasizes fast response time, and the latter emphasizes high diffraction efficiency. By appropriate adjustment of the dopant concentration and the ratio of the donor acceptor levels, photorefractive Fe:LiNbO3 crystals that facilitate specific applications have been designed and fabricated. Shift-invariant image correlations have been achieved.

One-dimensional self-focusing in photorefractive Bi12SiO20 crystal: theoretical modeling and experimental demonstration

Abstract We have experimentally demonstrated one-dimensional self-focusing of a laser beam in a photorefractive Bi 12 SiO 20 (BSO) crystal with an externally applied DC field. Fractional change in focal length larger than 50% was measured when the biased field was varied from 2 kV/cm to 8 kV/cm. Based on the band transport model, thin crystal approximation, and gradient index (GRIN) lens approximation, we derive a mathematical expression that describes the effect quantitatively.

Optimal conditions for thermal fixing of volume holograms in Fe : LiNbO3 crystals

We analyze and compare two typical recording and thermal fixing procedures of a volume hologram in a Fe:LiNbO(3) crystal (low-high-low procedure and high-low procedure). We consider the kinetics of the recording, compensating, and developing processes by taking into account the ratio of the conductivities between the protons and the electrons as a function of temperature. From the analysis the optimal environmental conditions (in terms of the fixing temperature and the compensation time) for each fixing procedure can be deduced for a crystal with given material parameters.

Real-time Image Processing using Photorefractive Crystals

We describe the combination of neural network training and volume holographic storage technologies using photorefractive crystals for real-time image processing. Experimental results on using the system for multi-channel distortion-invariant image recognition are presented.

A Hybrid Neural Network for Image Classification

Principles of the photorefractive perceptron learning algorithm are described. The influences of the finite response time and hologram erasure of the photorefractive gratings on the convergence property of the photorefractive perceptron learning are discussed. A novel neural network which could resolve these constraints is presented. It is a hybrid system which utilizes the photorefractive holographic gratings to implement the inner product between the input image and the interconnection matrix. A personal computer is used for storing the interconnection matrix and the updating procedure, and it also functions as a feedback means during the learning phase. After training the weight vectors are recorded in the volume hologram of an optical processor. This novel method combines the advantages of the massive parallelism of optical systems and the programmability of electronic computers. Experimental results of image classification are presented. It shows that the system could correctly classify the input patterns into one of the two groups after training on four examples in each group during successive iterations. The system has been extended to perform multi-category image classification.

Photorefractive crystals for real-time image processing

Heavily iron doped lithium niobate photorefractive crystals are grown and characterized. The crystals have high diffraction efficiency and fast temporal response and hence are suitable for real-time image processing. Three applications of thin Fe: LiNb03 plates are demonstrated: the joint-transform correlator, the optical perception, and the hybrid image classifier. In the first and third systems, the heavily doped (0.5% mole) crystals are used. These crystals have fast temporal response (300-400 msec) and thus provide a means for real time optical processing. In the second system, a slightly light doped (0.1% mole) crystal is used. It has relatively long (> 1 minute) response time and is used for recording the dynamic memory in a neural network. Image recognition by these three systems are presented.

Optical learning network with amplitude detection

An optical learning network using photorefractive holograms with grating amplitude detection is presented and demonstrated. The Stokes principle for light is used to realize both bipolar weight changes and bipolar activities for perceptron learning. The bipolar detection is achieved by using a double Mach-Zehnder interferometer and an electronic subtractor at the output port. Experimental results show that a zero or near to zero threshold value, the network can be trained to dichotomize the input patterns.

Thermal fixing of volume holograms in Fe:LiNbO/sub 3/ crystals

We adapt a model proposed by Yariv to analyze the two recording procedures via the kinetics of the recording, fixing, and developing processes under either short circuit or preexposed open-circuit boundary condition. From the analysis, we deduce the optimal condition (in terms of the fixing temperature and the compensation time) for each procedure. The results provide useful guides to design thermal fixing procedures for reaching the maximum fixing efficiency.

Optimum conditions for thermal fixing of volume holograms in Fe:LiNbO3

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.

The self-defocusing of a diffraction beam in a photorefractive crystal and its application in a Fourier plane multiple beam splitter

We propose using a diffraction beam to induce a self-defocusing lens and applying it in a Fourier plane multiple beam splitter (FPMBS). The self-induced lens array in the FPMBS can increase the splitting ratio, which is generated by inserting a photorefractive crystal at the Rayleigh distance of the diffraction beam from each pixel.