Pixelated liquid-crystal light valve for neural network application

The liquid-crystal light valve (LCLV) is a useful component for performing integration, thresholding, and gain functions in optical neural networks. Integration of the neural activation channels is implemented by pixelation of the LCLV, with use of a structured metallic layer between the photoconductor and the liquid-crystal layer. Measurements are presented for this type of valve, examples of which were prepared for two specific neural network implementations. The valve fabrication and measurement were carried out at the State Optical Institute, St. Petersburg, Russia, and the modeling and system applications were investigated at the Institute of Microtechnology, Neuchatel, Switzerland.     

Application of a wire-grid-mirror liquid-crystal light valve in a nonlinear joint transform correlator

We describe the operation of a wire-grid-mirror liquid-crystal light valve and its use for optical pattern recognition. The nonlinear characteristic of the wire-grid-mirror liquid-crystal light valve is used to implement the nonlinear joint transform correlator. Experimental results and computer simulations show that the nonlinear characteristic of the wire-grid-mirror liquid-crystal light valve can produce a well-defined correlation peak and low output background. The performance of the nonlinear joint transform correlator obtained when the wire-grid-mirror liquid-crystal light valve is used is compared with that of the binary joint transform correlator.     

Aberration production using a high-resolution liquid-crystal spatial light modulator

Phase-only liquid-crystal spatial light modulators provide a powerful means of wavefront control. With high resolution and diffractive (modulo 2pi) operation, they can accurately represent large-dynamic-range phase maps. As a result, they provide an excellent means of producing electrically controllable, dynamic, and repeatable aberrations. However, proper calibration is critical to achieving accurate phase maps. Several calibration methods from previous literature were considered. With simplicity and accuracy in mind, we selected one method for each type of necessary calibration. We augmented one of the selected methods with a new step that improves its accuracy. After calibrating our spatial light modulator with our preferred methods, we evaluated its ability to produce aberrations in the laboratory. We studied Zernike polynomial aberrations using interferometry and Fourier-transform-plane images, and atmospheric aberrations using a Shack-Hartmann wavefront sensor. These measurements show the closest agreement with theoretical expectations that we have seen to date.     

Incorporation of liquid-crystal light valve nonlinearities in optical multilayer neural networks

Sigmoidlike activation functions, as available in analog hardware, differ in various ways from the standard sigmoidal function because they are usually asymmetric, truncated, and have a nonstandard gain. We present an adaptation of the backpropagation learning rule to compensate for these nonstandard sigmoids. This method is applied to multilayer neural networks with all-optical forward propagation and liquid-crystal light valves (LCLV) as optical thresholding devices. The results of simulations of a backpropagation neural network with five different LCLV response curves as activation functions are presented. Although LCLV's perform poorly with the standard backpropagation algorithm, it is shown that our adapted learning rule performs well with these LCLV curves.     

Scale-invariant optical correlators using ferroelectric liquid-crystal spatial light modulators

New experimental results for scale-invariant implementations of the binary phase-only matched filter and the nonlinear joint transform correlator using ferroelectric liquid-crystal spatial light modulators are presented. We provide a comparative study of both architectures for real-time road-sign recognition. Signal-to-peak-noise ratios in excess of 5 dB over a scale range of 1.0 to 2.0 are achieved under realistic conditions of clutter.     

Fabrication of polymer waveguides between two optical fibers using spatially controlled light-induced polymerization

A new method for the fabrication of polymer waveguides between two optical fibers using a spatially controlled photopolymerization is reported. By taking advantage of the self-guiding effect of light through a photopolymerizable medium, polymer waveguides perfectly aligned with the fiber cores and strongly anchored to their surfaces are fabricated. The process is characterized by following in situ the coupling efficiency of a nonactinic laser source. Examples of waveguides exhibiting good coupling efficiency and high flexibility are given. By selecting the suitable monomers and adjusting the photonic parameters, the optical and mechanical waveguide properties (diameter, length, refractive index, rigidity, and flexibility) can be controlled in view of optical sensor applications.     

Erbium-activated silica-zirconia planar waveguides prepared by sol-gel route

Er³⁺ doped (100 − x)SiO₂ − xZrO₂ planar waveguides were prepared by the sol-gel route, with x ranging from 10 up to 30 mol%. Multilayer films doped with 0.3 mol% Er³⁺ ions were deposited on fused quartz substrates by the dip-coating technique. The thickness and refractive index were measured by m-line spectroscopy at different wavelengths. The fabrication protocol was optimized in order to confine one propagating mode at 1.5 μm. Photoluminescence in the near and visible region indicated a crystalline local environment for the Er³⁺ ion.     

Holographic switch with a ferroelectric liquid-crystal spatial light modulator for a large-scale switch

A holographic switch with a ferroelectric liquid-crystal spatial light modulator is proposed for large switching systems such as those used in subscriber networks. Preliminary experiments have achieved a one-input, 48-output switch. The relationship between the power of the control-light source and the number of outputs is calculated; the results agree well with the experiment. The calculation suggests that 10384-output switching can be obtained with a 25-mW control-light source. It should therefore be possible to control a large-scale switch with low-power control-light sources.     

Optical properties of a silicon-nanocrystal-based-microcavity prepared by evaporation

A silicon-based microcavity constituted by a SiO/SiO₂ multilayer and Si/SiO₂ Bragg mirrors was prepared by the evaporation technique. We give details about the microcavity realization process. The optical properties of the Si nanocrystals, obtained by the thermal dissociation of the SiO layers, were analyzed by continuous-wave and angle-resolved photoluminescence (PL) experiments. The results show a strong anisotropy of the light emitted by the silicon nanocrystals for a cavity constituted by mirrors with only two distributed Bragg reflectors (DBR).     

Polarization properties of a nematic liquid-crystal spatial light modulator for phase modulation

The polarization properties of a nematic zero-twist liquid-crystal (NLC) spatial light modulator (SLM) were studied. A large ratio between the liquid-crystal (LC) layer thickness and the pixel pitch combined with spatial variations in the applied electric field causes fringing fields between pixels. Depending on the LC alignment, the electric field components within the LC layer can result in a twist deformation. The produced inhomogeneous optical anisotropy affects the polarization of light propagating through the device. We experimentally examined polarization effects in different diffraction orders for both binary and blazed phase gratings. Simulations of the LC deformation together with finite-difference time-domain simulations for the optical propagation were used to calculate the corresponding far-field intensities. It was demonstrated how rigorous simulations of the NLC SLM properties can be used to understand the polarization features of different diffraction orders.     

Real-time phase-difference amplification with a liquid-crystal spatial light modulator

We describe a simple system for achieving real-time phase-difference amplification of interferograms. We arrange the interferogram such that it contains high-spatial-frequency carrier fringes and project it onto the write side of an optically addressed phase-only spatial light modulator. The resultant phase pattern on the modulator is read out by two readout beams, and diffraction by the carrier fringes provides the spatial heterodyning that is necessary for achieving phase-difference amplification. We present results that demonstrate real-time phase-difference amplification by as much as a factor of 10.     

Complex amplitude modulation by use of liquid-crystal spatial light modulators

We propose to study the conditions for implementation of complex amplitude modulation on standard liquid-crystal spatial light modulators when illuminated by polarized light. The spatial light modulators are used in a conventional configuration, i.e., the voltage is applied parallel to the wave-front propagation direction. The most commonly used liquid-crystal materials are compared and their usefulness in some general applications discussed. Their specificities with respect to different modulation types and application requirements are briefly described. Typical characteristics such as response time, modulation range, and wavelength insensitivity are also discussed. Finally, as an illustration, a first attempt at nomenclature is proposed for the case of a linearly polarized light.     

Full-field phase modulation characterization of liquid-crystal spatial light modulator using digital holography

A direct quantitative phase measurement method to characterize intrinsic phase modulation from an entire active area of transmissive twisted-nematic liquid-crystal spatial light modulator (TN-LCSLM) is presented using digital holography (DH). The change in birefringence of liquid crystal material with respect to addressed gray scale produces phase modulation of wavefront transmitted through TN-LCSLM. Existing methods for phase modulation characterization of LCSLM mainly provides point measurement on its total active region. In this paper, the DH method is evolved to extract quantitative phase information of an entire active area from a single digital hologram formed using the complex wavefront transmitted through TN-LCSLM.     

Optical channel waveguides by copper ion-exchange in glass

Optical channel waveguides have been obtained by electric?field?assisted diffusion of copper films on glass substrates. The mode indices of these channel waveguides were determined with the prism?coupling technique, and the refractive?index profile of the waveguide was reconstructed from measurements of the near?field intensity distribution.     

Phase calibration and applications of a liquid-crystal spatial light modulator

A simple phase-characterization method for spatial light modulators is proposed. The low-cost method permits high-precision measurement and provides data for the setting of the spatial-light-modulator operating point in the phase-modulation mode. The dynamic phase response is used to perform efficient kinoform recording. In order to record the kinoform, we modify the global iterative coding to compute phase holograms. Finally, modified phase-phase correlation is introduced. The phase-phase correlator permits sharper correlation peaks, better energy transmission, and higher discrimination than an amplitude-phase correlation. Optical experimental results are presented.     

Programmable array microscopy with a ferroelectric liquid-crystal spatial light modulator

We present a programmable array microscope that uses a ferroelectric liquid-crystal spatial light modulator (SLM) for dynamic generation of scanning apertures. A single SLM serves as both the source and the detector aperture array in a double-pass confocal system. Successive aperture frames scan the array across the viewing area for complete imaging of a sample while preserving depth discrimination. Integration of the microscope output across all aperture frames produces a confocal image.     

Optical configuration of a horizontal-switching liquid-crystal cell for improvement of the viewing angle

We propose an optical configuration of a horizontal-switching liquid-crystal cell, consisting of a splayed liquid-crystal cell and uniaxial films, to improve the viewing angle characteristics by compensating for the phase dispersion in a diagonal direction. The optical design of the proposed configuration was performed on a Poincaré sphere with geometric calculations. By fabricating in-plane switching cells with the introduced configuration, we demonstrated their optical performances. As a result, we found that the diagonal viewing angle of the proposed horizontal-switching cell could be increased by 80% compared to a symmetrical viewing cone.     

Optical pattern recognition using a unidirectional photorefractive oscillator coupled with an angular multiplexing volume hologram

Abstract

We propose and demonstrate a unidirectional photorefractive ring oscillator that couples with an external angular multiplexing volume hologram for pattern recognition. By configuring the hologram externally, a computer-generated hologram (CGH) and a spatial light modulator are utilized to generate reference beams for the hologram. Two-dimensional images are stored in the form of resonating beams in the system, whereby the nonlinear interaction between the beams allows the image that most resembles an input object to be recognized. Five images were used to illustrate the pattern recognition ability of the oscillator. It was found that the input object was successfully recognized within 10s. The fact that CGHs can generate a large number of beamlets enhances significantly the storage capacity in this system.