Optimal Twist and Polarization Angles for the Reflective Liquid Crystal Light Modulator

Abstract

We determine the twist angle and the input polarization angle that optimize the efficiency, modulation sensitivity, and contrast ratio for the reflective liquid crystal modulators (especially the liquid crystal light valve). If a monochromatic light source is used, and when the input polarization is parallel to the front molecular director of the liquid crystal, the conventionally used 45° twist has a theoretical maximum reflectance of only 81%. However, a 63·6° twist angle yields the highest efficiency (theoretical maximum reflectance of 100%) as well as a higher modulation sensitivity. When the input polarization is not parallel, different options that yield a high efficiency and an increased modulation sensitivity are available. If the light source is not monochromatic but has a narrow bandwidth, the dispersion effect tends to reduce the contrast ratio, so that a tradeoff between contrast and sensitivity must be made. We show that a configuration with 65–75° twist angle has an efficiency close to 100% with continuously increasing sensitivity but decreasing contrast as the input polarization changes from 0° to ?30°.     

Contrast enhancement of underwater images with coherent optical image processors

The use of coherent optical processors for improving the quality of degraded underwater images is discussed, and a holographic filter suitable for the enhancement of contrast in underwater images is described. The filter is a modified matched spatial filter, and it performs a nonlinear local contrast enhancement. The theory of the operation of the coherent optical image processor with this new filter is presented and experimentally verified. The removal of the backscattered light from underwater images is demonstrated under laboratory conditions. The results show an improvement in the overall image contrast.     

Four-plane space-variant Fresnel-transform optical processor with a random phase encoder

We introduce a four-plane Fresnel-transform space-variant optical processor consisting of an input plane and two filter planes. One filter mask is programmable with a spatial light modulator. The second filter mask is a fixed random binary phase array with a known pseudorandom distribution of pixels. The order of the masks can be interchanged, giving different output characteristics. In one case the Horner efficiency of the correlator increases dramatically. In the other case the edge enhancement of the output image is removed. We discuss the theory for this general processor and its implementation with phase-only masks. We present experimental results when a binary magneto-optic spatial light modulator was used.     

Real-time optical image subtraction and edge enhancement using ferroelectric liquid-crystal devices based on speckle modulation

We carried out real-time optical image subtraction and edge enhancement based on a speckle modulation technique by using ferroelectric liquid-crystal polarization switches and a ferroelectric liquid-crystal spatial light modulator. A ferroelectric liquid-crystal spatial light modulator is employed as a real-time and multiple-exposure optical device, and successful results are obtained from three-exposure images modulated by speckles. Thus, image subtraction and edge enhancement are realized in real time. The whole operation is performed within several milliseconds with modest operating conditions. Because the spatial light modulator has a high resolution of greater than 100 line pairs/mm and can store fine speckle patterns, the image qualities we obtained are quite satisfactory.     

Image identification system based on an optical broadcast neural network processor

We describe the implementation of a vision system based on a hardware neural processor. The architecture of the neural network processor has been designed to exploit the computational characteristics of electronics and the communication characteristics of optics in an optimal manner, thus it is based on an optical broadcast of input signals to a dense array of processing elements. The vision system has been built by use of a prototype implementation of a neural network processor with discrete optic and optoelectronic devices. It has been adapted to work as a Hamming classifier of the images taken with a 128 x 128 complementary metal-oxide semiconductor image sensor. Its results, performance characteristics of the image classification system, and an analysis of its scalability in size and speed, with the improvement of the optoelectronic neural processor, are presented.     

Investigation of five types of switchable retroreflector films for enhanced visible and infrared conspicuity applications

We report on the physics, design, characterization, and demonstration of five viable techniques for switchable retroreflectors, including integrated electrowetting scattering, integrated and external electrowetting light valves, external liquid crystal light valve, and external liquid crystal scattering. All techniques were evaluated for use in conspicuity applications spanning wavelengths in the visible and IR (night vision). Achieved performance includes high optical efficiencies up to nearly 30% (out of a maximum 35%), visibly fast switching speeds of <100 ms, low to moderate operating voltages ranging from 5 to 60 V, more than ±45 deg of operation angle, and implementation with pressure-sensitive, adhesive-backed films of 0.7 to 1 mm thickness for flexibility and impact resistance. Each approach has unique strengths and weaknesses, which will also be discussed for applications ranging from commercial to military conspicuity.     

Wigner formulation of optical processing with light of arbitrary coherence

A unified mathematical formulation for designing and analyzing even the most general optical processor is presented. It exploits the Wigner distribution function to characterize the illumination, the input, the inherent filter, and the output results. To characterize the propagation of the light through the optical processor setup, we exploit the Wigner matrix formalism, which is appealing because it allows simple geometric analysis. The Wigner distribution function was extended to include illumination of arbitrary coherence so that processors using either coherent light or partially coherent light can be designed and analyzed with the same Wigner formalism. The basic principles, design, and analysis of the imaging and Fourier-transform operations and use of the Wigner formalism to evaluate the performance and tolerances of optical processors are presented.     

Organic semiconductor-based photonic crystals for solar cell arrays: band gap and optical properties

Photonic crystals (PCs) hold great potential for designing new optical devices because of the possibility of the manipulation of light with PCs. There has been an increase in research on tuning the optical properties of PCs to design devices. We design organic semiconductor-based PC structures and calculate optical properties using the plane wave expansion method and finite-difference time-domain method in an air background for a hexagonal lattice. We showed the possibility of the solar cell arrays for a 2D PC cavity on an organic semiconductor base infiltrated with a nematic liquid crystal. E7 type has been used as a nematic liquid crystal and 4,4′-Bis[4-(diphenylamino) styryl]biphenyl as an organic semiconductor material.     

Efficient polarization converter for projection displays

In the waveguiding limit, a twisted nematic liquid crystal cell behaves as an achromatic polarization rotator. We propose and demonstrate the application of such a polarization rotator to convert unpolarized light into linearly polarized light with almost 100% efficiency. This polarization converter has a 2:1 aspect ratio, which is close to the 16:9 ratio for modern televisions. It can be used therefore in a projection display with polarization-dependent light valves such as a liquid crystal light valve. Both transmittive and reflective light valves can be used. The temperature dependence of the achromatic polarization rotator is also studied.     

Complete polarization conversion using one crystal with dual transverse Pockels effect

Based on dual transverse Pockels effect, complete polarization conversion can be achieved by using only one electro-optic crystal and its two externally applied voltages. The electro-optic phase retardation and the azimuth angle of the field-induced principal dielectric axes of the crystal can be independently and linearly controlled by the amplitude and direction of the applied electric field, and the formulas for this correlation are deduced for arbitrary input and output polarization states. The candidate crystals mainly include the uniaxial crystals of 3 m, 62 m, and 32 symmetry groups, and the cubic crystals of 43 m and 23 symmetry groups. Theoretical analysis demonstrates that one crystal exhibiting both dual transverse Pockels effect and optical activity can also be used for complete polarization converter. The continuous polarization rotation of a linearly polarized light from 0 degrees to 180 degrees has been performed experimentally by use of single lithium niobate crystal with four lateral electrodes. In addition the light beam position-dependent polarization conversion by using a bulk electro-optic crystal is also measured in the experiment. This new type of polarization converter will have potential applications in many fields due to its simple configuration, explicit control logic of polarization conversion, and lower power consumption.     

Projection display for the generation of two orthogonal polarized images using liquid crystal on silicon panels and light emitting diodes

We present a projection system that is capable of two-dimensional and three-dimensional image display. A novel projection architecture is discussed that can simultaneously generate two linear polarized full-color images with orthogonal states of polarization using only one optical system. Both images are modulated by using two high-resolution liquid crystal on silicon panels that are illuminated with high-power light emitting diodes. The optical core and the illumination system are simulated, characterized, and optimized with nonsequential ray tracing software. A proof-of-concept demonstrator of the entire projection system is built and characterized. Important component specifications are discussed to improve the system performance.     

Carbon nanotube dispersion in nematic liquid crystals: An overview

The aim of present review article is to present a comprehensive and current overview of scientific advancement in liquid crystal and carbon nanotube suspension. Particular attention has been paid to the recent developments and fundamental understanding of carbon nanotube dispersion in nematic liquid crystals. The dispersion and interaction of carbon nanotube in liquid crystal matrices and more elaborately the effect of dispersion on the properties of liquid crystalline materials has been extensively discussed. Recent progress has shown that even a very minute concentration of carbon nanotube in liquid crystals can have a reflective impact on the electrical and optical properties of liquid crystals. Liquid crystals provide a distinctive environment for controlling the alignment of carbon nanotubes whereas carbon nanotubes are important for the enhancement and fine-tuning of liquid crystalline properties. Potential applications of liquid crystal and carbon nanotube suspension are briefly discussed. Conclusion and future perspectives of this rapidly emerging field is provided at the end.     

一种基于TN型液晶的自控光阀

对TN(扭曲向列)型液晶的透光率测量实验表明,在外加电压的作用下TN型液晶可以吸收入射光线,并且在外加电压连续改变时,对入射光的透光率将呈现连续非线性变化。根据此特性,利用数字电路来控制加到液晶电极上的电压,就得到了一种空间光调制器──自控光阀。自控光阀能控制通过光阀的光强,使之趋于设定值。     

Active modulation of nanorod plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements.     

Real-time three-dimensional object recognition with multiple perspectives imaging

A novel system for recognizing three-dimensional (3D) objects by use of multiple perspectives imaging is proposed. A 3D object under incoherent illumination is projected into an array of two-dimensional (2D) elemental images by use of a microlens array. Each elemental 2D image corresponds to a different perspective of the 3D object. Multiple perspectives imaging based on integral photography has been used for 3D display. In this way, the whole set of 2D elemental images records 3D information about the input object. After an optical incoherent-to-coherent conversion, an optical processor is employed to perform the correlation between the input and the reference 3D objects. Use of micro-optics allows us to process the 3D information in real time and with a compact optical system. To the best of our knowledge this 3D processor is the first to apply the principle of integral photography to 3D image recognition. We present experimental results obtained with both a digital and an optical implementation of the system. We also show that the system can recognize a slightly out-of-plane rotated 3D object.     

基于光谱辐射信息的高温目标图像识别

利用光学及辐射度学的基本原理，对成像过程进行理论分析，建立了照明条件下识别视场中高温目标的数学模型，揭示了影响识别效果的各物理因素。通过对这些物理因素进行分析，指出“切割波段”可有效提高信噪比，将高温目标识别出来，并通过液晶光阀予以技术实现。实验结果验证了该方案的可行性及有效性。     

Nonlinear techniques in optical synthetic aperture radar image generation and target recognition

One of the most successful optical signal-processing applications to date has been the use of optical processors to convert synthetic aperture radar (SAR) data into images of the radar reflectivity of the ground. We have demonstrated real-time input to a high-space-bandwidth optical SAR imagegeneration system by using a dynamic organic holographic recording medium and SAR phase-history data. Real-time speckle reduction in optically processed SAR imagery has been accomplished by the use of multilook averaging to achieve nonlinear modulus-squared accumulation of subaperture images. We designed and assembled an all-optical system that accomplished real-time target recognition in SAR imagery. This system employed a simple square-law nonlinearity in the form of an optically addressed spatial light modulator at the SAR image plane to remove the effects of speckle phase profiles returned from complex SAR targets. The detection stage enabled the creation of an optical SAR automatic target recognition system as a nonlinear cascade of an optical SAR image generator and an optical correlator.     

Active contour segmentation by use of a multichannel incoherent optical correlator

We describe an optoelectronic incoherent multichannel processor that is able to segment an object in a real image. The process is based on an active contour algorithm that has been transposed to optical signal processing to accelerate image processing. This implementation requires exact-valued correlations and thus opens attractive perspectives in terms of optical analog computation. Furthermore, this optical multichannel processor setup encourages incoherent processing with high-resolution images.     

GaAs-based fiber-optic pressure sensor

A new sensor developed for measurement of hydrostatic pressure up to at least 100 MPa at a standard range of ambient temperatures is described. The sensor exploits the displacement of the optical absorption edge occurring in semiconductors under the influence of hydrostatic stress as a result of pressure-induced energy shifting of conduction band extrema. The sensing element is composed of an intrinsically pure GaAs single crystal configured in the form of a microprism located at the sensor tip, and attached to two multimode (50/125 μm) optical fibers designed to deliver input light to the sensor and to output a pressure-modulated light signal to the outside of a pressure region. Characterization of the sensor has been performed for pressures up to 100 MPa and for temperatures ranging from 5 to 50°C. A procedure has been proposed involving the use of two sensors (active and compensating) to minimize temperature drift through appropriate signal processing     

Realizing optical logic with a smart-pixel spatial light modulator

A method of implementing optical logic has been realized experimentally with a novel liquid crystal on silicon spatial light modulator with an integrated lens arrays. The device allows for three optical inputs and one optical output per pixel. The different logic functions realized, OR, and, nor, nand, and xor, are discussed.