Complex amplitude reflectance of the liquid crystal light valve

Inhibiting zero-order light of a spatial light modulator with voltage optimization

The crucial zero-order light due to the pixelation effect of spatial light modulator (SLM) has been a serious issue in the field of light modulation, especially in applications with a high numerical aperture optical system. In this investigation, we report that by properly adjusting the high-level and low-level pixel voltages of an SLM, the zero-order light caused by the pixelation effect of an SLM can be significantly eliminated. The method is further validated under an inverted fluorescence microscope. The experimental results show that the zero-order light can be inhibited up to 91.3%, accompanied by an improvement of the modulation efficiency from 77.5% to 92.6%.

Phase-Only Optically Addressable Spatial-Light Modulator and On-Line Phase-Modulation Detection System

The influence of driving conditions on the phase-modulation ability of an optically addressable spatial-light modulator (OASLM) is investigated using an equivalent circuit method and a system for measuring wave-front modulation that uses a phase-unwrapping data-processing method, and is constructed with a charge-coupled device and wave-front sensor. 1λ peak-to-valley phase change for a 1053 nm laser beam is acquired with the home-made OASLM at the optimal driving voltage of 14 V at 200 Hz. The detection system for wave-front modulation has a spatial resolution of 200 µm for binary images and a minimum distinguishable contrast of 1 mm. On-line phase modulation with feedback control can be acquired with the OASLM and the corresponding measuring system.

Transmissive liquid crystal light-valve for near-infrared applications

An optical valve is realized by associating a nematic liquid crystal layer with a Cr-doped gallium arsenide as a photoconductive substrate. The light-valve is shown to efficiently operate in transmission at 1.06 μm optical wavelength. The optical phase shift and refractive index change are measured as a function of the incident light intensity and of the voltage applied. Additionally, the light-valve is shown to act as a self-defocusing medium. Combining transmissive properties and nonlinear features, applications for dynamic holography in the near-infrared region of the spectrum can be envisaged.

Reflection resonance switching in metamaterial twisted nematics cell

Electric switching of reflection resonances at near-IR spectral range is experimentally demonstrated in a reflective metamaterial twisted nematic liquid crystal cell. Reflective metamaterial composed of nano-sized double-split ring resonator aperture is fabricated by a focused ion beam milling. Two-fold rotational symmetry of double-split ring resonators allows for two orthogonal polarization-dependent reflection resonances in the reflective metamaterial. With an external voltage of 10V across 12μm cell gap, a full switching is achieved between two reflection resonances. Dynamic measurements show the time constants of switch-on and switch-off are in the order of 100ms and 10ms, respectively.

Image data compression based on non-negative incoherent imaging systems

Channel and memory capacities are limited and expensive resources in optical communication. In this work we propose several approaches to reduce the required capacity by using the a priori knowledge about the optical system used to capture the spatial information. The a priori knowledge is related to the triangularlike shape of the optical transfer function and to its spectral symmetry. In our work we will demonstrate a reduction in memory capacity required for the same image quality or a resolution enhancement for the same physical memory capacity. We will also present a nearly all-optical implementation of these techniques.

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, Neuchâtel, Switzerland.

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.

Intensity-only modulation for atmospheric scintillation correction by liquid-crystal spatial light modulators

We consider the use of a ferroelectric liquid-crystal spatial light modulator (FLC SLM) to control the transmittance of a telescope pupil to compensate for the effects of scintillation. Our aim here is to prove the necessary and physically interesting result that it is possible to control the intensity of light by use of FLC SLM without inducing further phase aberrations. Furthermore, we show that system errors have only a small effect on the phase of the transmitted beam.

Full-range, continuous, complex modulation by the use of two coupled-mode liquid-crystal televisions

Switchable, continuous, complex-amplitude modulation is demonstrated with two cascaded, twisted nematic liquid-crystal televisions (LCTV's), both operating in phase- and amplitude-coupled modulation modes. The condition for full-range complex modulation is that one of the LCTV's must provide a 2π-range phase modulation. A look-up table encoding method is proposed that permits the compensation of phase-amplitude coupling and nonlinearity in the two individual LCTV modulations. Experimental techniques for determining the LCTV-device parameters, for maximizing the phase-mostly modulation range and the amplitude-mostly modulation contrast, and for testing the complex-amplitude modulation are developed. Optical complex-amplitude Fresnel holograms are shown.

Color modulation properties of a liquid-crystal device

The color modulation properties of a twisted-nematic liquid-crystal device are studied based on the Jones calculus. The usefulness of Jones calculus is demonstrated for the blue mode of a supertwisted birefringent liquid-crystal device, after which some experimental results of the color modulation effect for a twisted-nematic liquid-crystal television are presented. We investigated the color dependence of the transmitted light on the applied voltage by using a chromaticity diagram. Most of the experimental results are well explained by the theory derived from the Jones calculus.

Diffraction efficiency analysis of a parallel-aligned nematic-liquid-crystal spatial light modulator

An optically addressed parallel-aligned nematic-liquid-crystal spatial light modulator is developed for applications in optical information processing and interferometry. Its performance, including diffraction efficiency, is measured, and a theoretical analysis of diffraction efficiency is performed. A comparison between the experimental results and the theoretical analysis shows good agreement. The diffraction efficiency of near-sinusoidal gratings written on the device is of the order of 30%, which is close to theoretical maximum.

Optimal realizable filters and the minimum Euclidean distance principle

Minimizing a Euclidean distance in the complex plane optimizes a wide class of correlation metrics for filters implemented on realistic devices. The algorithm searches over no more than two real scalars (gain and phase). It unifies a variety of previous solutions for special cases (e.g., a maximum signal-to-noise ratio with colored noise and a real filter and a maximum correlation intensity with no noise and a coupled filter). It extends optimal partial information filter theory to arbitrary spatial light modulators (fully complex, coupled, discrete, finite contrast ratio, and so forth), additive input noise (white or colored), spatially nonuniform filter modulators, and additive correlation detection noise (including signaldependent noise). An appendix summarizes the algorithm as it is implemented in available computer code.

Adaptive real-time architectures for phase-only correlation

A video-rate correlator can be constructed with a phase-only spatial light modulator and a CCD camera. The phase of the Fourier transform of a signal and a reference image is determined by fringe-scanning interferometry. The two measured phase images are then subtracted. The optical Fourier transform of this difference produces the phase-only correlation response. This system can update both signal and reference images with live scenery. Currently, only the joint transform correlator has demonstrated this degree of adaptivity in real time. Physically compact versions of the correlator can be built with a single spatial light modulator and a Fourier-transform lens.

Reducing Berreman's 4 x 4 formulation of liquid-crystal-display optics to 2 x 2 Jones vector equations

We use a perturbation expansion to obtain a generalized 2 x 2 Jones vector description that is equivalent to Berreman's description. We show that for typical liquid-crystal displays (LCD's) the bulk reflections are weak so that the first-order 2 x 2 Jones solution is generally sufficient and affords fast and reasonably accurate computations of the overall optical properties of LCD devices.

Image subtraction using polarization modulation of liquid-crystal television

A method of real-time image subtraction is proposed that uses successive light transmission through two similar liquid-crystal television (LCTV) panels. The polarization modulation properties of light transmitting through LCTV's are investigated theoretically and experimentally for the case of when the video signals are applied to the LCTV's. Image subtraction is accomplished by the approximate rotations of the linear polarization of light through the LCTV's.

Full complex modulation using liquid-crystal televisions

An optical architecture is proposed that uses two modified liquid-crystal televisions (LCTV's) to control amplitude and phase modulation independently. Two applications in pattern recognition are discussed.