Long-term reading experiment on a photorefractive holographic memory with the hologram sustainment technique by optical feedback

We demonstrate a long-term continuous readout of a two-dimensional image in a photorefractive holographic memory with a BaTiO₃ crystal. A considerable extension of reading time is achieved by use of a hologram sustainment technique with an optical feedback circuit. Hologram rewritings by the simultaneous illumination of the reading beam and the feedback beam, which is incident on a crystal from the opposite direction to the reading beam, contribute to all-optical hologram sustainment without any fixing technique. In this paper, the effectiveness of the hologram sustainment technique is explained by the analysis of the temporal property of the amplitude of the index grating in a crystal. By calculating the temporal properties of the reconstructed beam intensity, we reveal the feedback rate and the coupling strength for high output efficiency. We perform an experiment on two-dimensional image reading and writing with a 45° cut BaTiO₃ crystal and show that a recorded image can be reconstructed over 20?min without critical image degradation whereas a reconstructed image fades away within 20?s in the conventional readout technique.     

Identification data reproduction in multilayered optical waveguide-type holographic memory cards

The identification (ID) data reproduction was computationally examined for the compatibility with the aperture multiplexing and the telecentric system. The mask for the multiplexing caused shade in the reproduced ID data cell. The shading tolerance was improved by modulating the phase of the original image so that the ID data holograms diffract the light in the oblique direction or with the concentrating. The telecentric system reproduced all ID data cells in the same position of the image sensor and made a distinction among the cells impossible. This was solved by reproducing the cell point symmetrically from the corresponding ID data hologram in each 3x3 ID data block.     

Information theoretic limits to the capacity of volume holographic optical memory

We derive the information theoretic limit to storage capacity in volume holographic optical memories for the limiting cases of dominant intensity noise (Gaussian noise) and dominant field noise (Rician noise). These capacity bounds are compared with the performance achievable using simple Reed-Solomon error-correcting codes.     

Pulsed digital holographic interferometry for dynamic measurement of rotating objects with an optical derotator

A method for measuring dynamic deformations of rotating objects with pulsed digital holography is described. An optical derotator is used to compensate for the rotation. A CCD camera is used to record two holograms with a short time separation (20 mus). Results of deformations between the recordings are obtained after subtraction of the phase distribution between the two digital holograms. Fringe phase maps of the phase subtraction of two holograms compensated by the derotator and recorded with a Q-switched double-pulsed ruby laser are presented. A flat disk and the blades of a fan were investigated. We used an optical arrangement that allowed us to improve laser illumination and energy efficiency. Experimental results on quantitative evaluation of dynamical out-of-plane deformations are presented.     

Double-layer networks with holographic optical switches

The double-layer networks have the advantages of being strictly nonblocking and having a simpler routing algorithm, the lowest system insertion loss, a zero differential loss, fewer drivers, fewer interconnection lines, fewer crossovers, and the best signal-to-noise-ratio characteristic compared with any nondilated network. Using holographic optical switches to construct these networks not only eliminates all interconnection lines and crossovers but also reduces the number of drivers.     

Combination of free-space and guided-wave optical interconnects for angularly multiplexed multiwavelength holographic memory

We propose and test experimentally a new scheme to implement spatially multiplexed multiwavelength holographic memory. An electro-optically modulated phase grating array on LiNbO(3) substrate is used as a guided-wave interconnect to activate the reconfigurable reference beam. The object beam is provided by free-space interconnect. An electro-optic modulation efficiency of 18 +/- 2.5% is achieved with an applied voltage of 100 V. The reference beams with different diffraction angles can implement the angle-multiplexing holographic recording. We believe this is the first report of the implementation of guided-wave electro-optic interconnect together with free-space interconnect in holographic memory applications.     

Fractional fourier domain encrypted holographic memory by use of an anamorphic optical system

We propose and demonstrate a fractional Fourier domain encrypted holographic memory using an anamorphic optical system. The encryption is done by use of two statistically independent random-phase codes in the fractional Fourier domain. If the two random-phase codes are statistically independent white sequences, the encrypted data are stationary white noise. We exploit the capability of an optical system to process information in two dimensions by using two different sets of parameters along the two orthogonal axes to encode the data. The fractional Fourier transform parameters along with the random-phase codes constitute the key to the encrypted data. The knowledge of the key is essential to the successful decryption of data. The decoding of the encoded data is done by use of phase conjugation. We present a few experimental results.     

Achromatic fourier processor with holographic optical lenses

An optical Fourier processor that allows the use of broadband light sources and colored inputs is designed, fabricated, and tested. We develop a design technique based on phase manipulation in the Fourier plane to construct an image processor that provides a chromatically corrected image making use of the good aberrations behavior of symmetrical optical systems. Only a small number of diffractive lenses and one achromatic refractive lens are required to obtain a real image. We verify our design experimentally using holographic lenses, which are presented, owing to their versatility, as a good alternative to expensive blazed diffractive elements.     

Time-domain optical coherence tomography with digital holographic microscopy

We show that digital holography can be combined easily with optical coherence tomography approach. Varying the reference path length is the means used to acquire a series of holograms at different depths, providing after reconstruction images of slices at different depths in the specimen thanks to the short-coherence length of light source. A metallic object, covered by a 150-microm-thick onion cell, is imaged with high resolution. Applications in ophthalmology are shown: structures of the anterior eye, the cornea, and the iris, are studied on enucleated porcine eyes. Tomographic images of the iris border close to the pupil were obtained 165 microm underneath the eye surface.     

Storage density of shift-multiplexed holographic memory

The storage density of shift-multiplexed holographic memory is calculated and compared with experimentally achieved densities by use of photorefractive and write-once materials. We consider holographic selectivity as well as the recording material's dynamic range (M/#) and required diffraction efficiencies in formulating the calculations of storage densities, thereby taking into account all major factors limiting the raw storage density achievable with shift-multiplexed holographic storage systems. We show that the M/# is the key factor in limiting storage densities rather than the recording material's thickness for organic materials in which the scatter is relatively high. A storage density of 100 bits/mum(2) is experimentally demonstrated by use of a 1-mm-thick LiNbO(3) crystal as the recording medium.     

Reflection-type holographic disk memory with random phase shift multiplexing

A reflection-type holographic disk memory system with random phase shift multiplexing is proposed. The experimental results show that a binary data page of 18x17 bits is recorded successfully at intervals of 4 mum in a Fe:LiNbO3 crystal with a thickness of 0.5 mm when six data pages are superimposed. Numerical results show that random phase modulation can improve the shift selectivity in shift multiplexing recording as well as in data security. Experimental and numerical results show that reflection-type holographic disk memory has a high potential for terabyte storage capacity as in transmission-type memory.     

Real-time digital holographic beam-shaping system with a genetic feedback tuning loop

A novel implementation of a real-time digital holographic system with a genetic feedback tuning loop is proposed. The proposed genetic feedback tuning loop is effective in encoding optimal phase holograms on a liquid-crystal spatial light modulator in the system. Optimal calibration of the liquid-crystal spatial light modulator can be achieved via the genetic feedback tuning loop, and the optimal phase hologram can then overcome the aberration of the internal optics of the system.     

Performance analysis of phase-code multiplexed holographic memory

For analysis of effective phase-code multiplexing in a holographic memory system four types of phase code are generated and used as a reference beam. In computer simulations the size of the address beam is fixed at 32 x 32 pixels, and 0%, 5%, 10%, 15%, 20%, and 25% phase-error rates in a pixel are purposely added to the real phase values for consideration of the nonlinear phase-modulation characteristics of a practical spatial light modulator. Cross talk and signal-to-noise ratios (SNR's) are comparatively analyzed for these phase codes by calculation of the autocorrelation and the cross correlation. The pseudorandom code (PSR) has the lowest cross-correlation mean value of 0.067 among the four types of phase code, which means that the SNR of the PSR is higher than with other phase codes. Also, the standard deviation of the PSR, indicating the degree of recalled data degradation, has the lowest value, at 0.0113.     

Intelligent Fault-tolerant Management of Electromechanical Equipment

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Reliability analysis of fault tolerant memory arrays

A model is developed to represent computer memory module reliability as a function of memory array reliability under a fault tolerant design. The fault tolerance feature of the array actually results from a revision in the use of the array so that with respect to some failure modes, the array becomes a K out of N rather than a series system. The model is used to determine array reliability under fault tolerance. The ratio of module reliability under fault tolerance to that without this feature is used as a measure of the benefits of revising array use. A key feature of the analysis is the fact that not all faults can be tolerated. The elemental memory devices examined conform to a decreasing Weibull hazard model. Consequently, evaluation of the general model for the K out of N system realized must be done numerically. However, for the special case in which K=N-1, a closed form expression for the performance measure is obtained. This special case occurs for the application of interest and it is shown that the performance measure always exceeds one and depends directly upon the proportion of faults that can be tolerated. Thus the value of fault tolerance is shown to depend upon the extent to which the array will tolerate faults. This provides a basis for deciding whether or not fault tolerance should be implemented.     

Surface profilometry with laser-diode optical feedback interferometer outside optical benches

A simple and robust interferometer with a laser diode subject to optical feedback from the interferometer is presented for surface testing of a spherical mirror. The fringe phase can be locked by the optical feedback within less than 0.2pi (peak-to-valley value) even when the interferometer is placed on a wooden table. The fringe locking is caused by the change of lasing wavelength that suppresses the net phase change to be much less than 2pi. The locked fringe pattern with spatial carriers can be analyzed by a fringe analyzer at a video rate, and the measurement results of the spherical mirror showed the same result as on an optical bench.     

Thin film light amplifier with optical feedback

A thin film light amplifier with optical feedback was prepared as a system consisting of a photoconductive element and an electroluminescent cell. The dependence of the brightness of the electroluminescent cell on the intensities of illumination of the photoconductive element was measured for such a light amplifier with a sinusoidal voltage supply. The characteristics obtained were compared with those calculated using the proposed theoretical model.     

System modeling and optimization of Fourier holographic memory

A new fast-Fourier-transform-based model of a page-oriented holographic data-storage system is presented. The model accounts for essential system and storage material features (e.g. diffraction, noises, and saturation) and provides reliable results in the form of output images, histograms, or bit-error rates. The model is built on a modular basis and provides the possibility of working with different system versions, key components, and storage materials. Applications of the method are presented through examples of optimization of the data density, reference beam size at Gaussian beam illumination, and calculation of the storage medium's positioning tolerances in accordance with the results of test measurements.