Self-adjusting dynamic binary phase holograms

An optical system constructed around a dynamic diffractive optic element, a ferroelectric liquid-crystal spatial light modulator in binary phase-only modulation mode, is investigated. The spatial light modulator is successively adjusted according to the direct binary search technique to diffract an incoming laser light beam into a predecided intensity distribution by use of feed back from the diffracted light. It was found that the feedback signal was noisy and that vibrations and limited bistability in the spatial light modulator's pixels were the main noise sources. The final diffraction efficiency depends on the degree of noise in the feedback signal, but even under fairly noisy conditions the iterations were found to converge properly.     

Real-time holographic vibrometry with a ferroelectric liquid-crystal spatial light modulator

A fast ferroelectric liquid-crystal spatial light modulator, originally developed for optical computing, has found a new application in vibrometry. A new scheme of vibration-synchronized double-exposure holographic interferometry is proposed that makes full use of the speed of the ferroelectric liquid-crystal spatial light modulator. Preliminary experiments were performed that demonstrate virtually continuous real-time vibrometric data acquisition.     

Phase-aberration correction with dual liquid-crystal spatial light modulators

A phase-aberration-correction system that uses high-resolution, twisted nematic liquid-crystal spatial light modulators in a Mach-Zehnder interferometer is presented. A correction algorithm is described and experimentally verified by use initially of one liquid-crystal panel. Phase aberrations are successfully removed by a single liquid-crystal panel, but unacceptably high amplitude variation is introduced into the wave front because of the phase-amplitude coupling of the spatial light modulator. A second panel is used to remove the amplitude modulation. The modified optical system with a multiplicative architecture is described, and results are presented that show the correction of phase aberrations with an amplitude variation of less than 10%.     

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.     

Adaptive optics with advanced phase-contrast techniques. I. High-resolution wave-front sensing

High-resolution phase-contrast wave-front sensors based on phase spatial light modulators and micromirror/ liquid-crystal arrays are introduced. Wave-front sensor performance is analyzed for atmospheric-turbulence-induced phase distortions described by the Kolmogorov and the Andrews models. A high-resolution phase-contrast wave-front sensor (nonlinear Zernike filter) based on an optically controlled liquid-crystal phase spatial light modulator is experimentally demonstrated. The results demonstrate high-resolution visualization of dynamically changing phase distortions within the sensor time response of approximately 10 ms.     

Real-time three-dimensional object reconstruction by use of a phase-encoded digital hologram

A three-dimensional (3D) object reconstruction technique that uses only phase information of a phase-shifting digital hologram and a phase-only spatial-light modulator is proposed. It is well known that a digital hologram can store both amplitude and phase information of an optical electric field and can reconstruct the original 3D object in a computer. We demonstrate that it is possible to reconstruct optically 3D objects using only phase information of the optical field calculated from phase-shifting digital holograms. The use of phase-only information enables us to reduce the amount of data in the digital hologram and reconstruct optically the 3D objects using a liquid-crystal spatial light modulator without optical power loss. Numerical evaluation of the reconstructed 3D object is presented.     

Optical rotatory-dispersion-type spatial light modulator and characteristics of the modulated light

Among known temporal-spatial light modulation methods, there is no realistic method that can precisely control a light pulse simultaneously in the temporal and spatial domains. By careful consideration of the symmetries and topological properties of electromagnetic waves, a novel spatial light modulator has been developed to create different far-field patterns for each wavelength of linearly polarized light composed of various wavelength components. The system consists of an optical rotatory dispersion device, which is like a Faraday rotator, and a spatial light modulator with parallel-alignment nematic liquid-crystal cells. Numerical simulation results show the effectiveness of this new spatial light modulation method.     

Low-coherence interferometry with synthesis of coherence function

Synthesis of a coherence function by manipulation of the spectral phase of low-coherent light with a segmented liquid-crystal phase modulator and its application in a low-coherence interferometry are described. Effects of space-time coupling caused at diffractive gratings and second-order dispersion at the spatial light modulator on the coherence function synthesis are theoretically and experimentally verified. Various coherence functions can be shaped with phase-only masks designed by simulated annealing optimization algorithm. We utilized this technique for a novel optical low-coherence reflectometry without any mechanical movement for scanning optical delay.     

Pseudorandom phase-only encoding of real-time spatial light modulators

We previously proposed a method of mapping full-complex spatial modulations into phase-only modulations. The Fourier transform of the encoded modulations approximates that of the original complex modulations. The amplitude of each pixel is encoded by the property that the amplitude of a random-phasor sum is reduced corresponding to its standard deviation. Pseudorandom encoding is designed for phase-only spatial light modulators that produce 360° phase shifts. Because such devices are rare, experiments are performed with a 326°modulator composed of two In Focus model TVT6000 liquid-crystal displays. Qualitative agreement with theory is achieved despite several nonideal properties of the modulator.     

Real-time binary phase holograms on a reflective ferroelectric liquid-crystal spatial light modulator

A ferroelectric liquid-crystal spatial light modulator with an active silicon backplane is used to implement reconfigurable reflective phase holograms. Optical results are presented for an optimized computergenerated Fourier hologram.     

Real-time velocity measurement by the use of a speckle-pattern correlation system that incorporates a ferroelectric liquid-crystal spatial light modulator

We describe a technique for noncontact velocity measurement by using double-exposure speckle-pattern techniques with optical signal processing. The two speckle patterns are recorded on a ferroelectric liquid-crystal (FLC) spatial light modulator (SLM), which is a bistable optically addressed SLM, and the composite pattern is then analyzed by an optical system similar to a joint transform correlator, in which another FLC-SLM and a position-sensitive detector are used. We show that the performance of the system can be significantly improved by adjusting the time between exposures using a real-time feedback system that is based on the position of the correlation spot in the output plane.     

New interferometric fiber-optic gyroscope with amplified optical feedback

A novel interferometric fiber-optic gyroscope with amplified optical feedback by an Er-doped fiber amplifier (EDFA) is proposed and theoretically investigated (the proposed gyroscope is named the feedback EDFA-FOG, FE-FOG in what follows). The FE-FOG functions like a resonant fiber-optic gyro (R-FOG) because of its multiple utilization of the Sagnac loop; however, it is completely different because a low-coherence light source is used. In addition, the gyro output signal is pulsed because the modulation frequency of the phase modulator placed in the Sagnac loop is selected to match the total round-trip time delay of the light, which includes the Sagnac-loop delay plus that of the feedback loop of the fiber amplifier. The sharpness of the output pulse can be adjusted by both the gain of an EDFA and the modulation depth of the phase modulator. When rotation occurs the peak position of the output pulse is shifted as a result of the Sagnac effect. The resolution of the rotation measurement depends on the sharpness of the output pulse. The techniques of both the open-loop and closed-loop methods are described in detail, which shows the great advantage of the proposed gyroscope over the to the conventional interferometric fiber-optical gyroscope (I-FOG).     

Optical image transformations for fully parallel optical analog-to-digital conversion

An optical method for a fully parallel analog-to-digital conversion is proposed. The proposed method is carried out by means of intensity transformations of an analog input image and the thresholding for the transformed images and is suitable for two-dimensional implementation based on spatial light modulators. The intensity transformations are implemented by a liquid-crystal spatial light modulator, and thresholding is simulated by computer in consideration of the optical realization.     

Learning and recall algorithm for optical associative memory using a bistable spatial light modulator

A learning and recall algorithm for optical associative memory based on the conventional correlationlearning method with three effective improvements (sparse-encoding method, constant-total-activity method, and binary memory) is proposed from a viewpoint of practical implementation. It is shown that the algorithm is suitable for implementation with a bistable spatial light modulator such as a ferroelectric liquid-crystal spatial light modulator, which has high resolution and a fast response time. The results of theoretical analysis and simulations indicate that the algorithm permits an associative-memory system with a large memory capacity to be realized. An example of an optical system for executing this algorithm is proposed. To determine the performance specifications that are required for the various optical components within the system, we simulate and evaluate the effect of noise (which is caused by nonideal components) on system performance. These results show that the system is robust in the presence of predicted noise levels.     

Speckle-noise reduction on kinoform reconstruction using a phase-only spatial light modulator

Random-phase distributions that are statistically independent individually are used for computing kinoforms. These uncorrelated kinoforms are recorded and read out sequentially by a phase-only liquid-crystal spatial light modulator, and reconstructed images with well-developed speckles are added. The fidelity of the resultant image to an original is improved as the number of additions increases. The dependence of the speckle contrast on the initial random phase and the influence of the liquid-crystal spatial light modulator's display performance on the image quality are discussed.     

Unique measurement of the parameters of a twisted-nematic liquid-crystal display

The parameters of a twisted-nematic liquid-crystal display (TN-LCD) are measured with no ambiguity when we measure the intensity transmittance of a system that simply consists of a TN-LCD, two polarizers, a quarter-wave plate, and a monochromatic beam. First we show analytically that the director angle can have a 90 degree ambiguity and the twist angle of the liquid-crystal molecules a 180 degree ambiguity. Then we uniquely measure the parameters by fitting the theoretical predictions to the intensity transmittance measured with and without the quarter-wave plate and by using the quasi local-adiabatic condition. The birefringence of the TN-LCD is measured next as a function of the applied voltage. We design a phase spatial light modulator by using the measured parameters and measure the phase delay of the output beam to excellent agreement with the theoretical prediction.     

Hadamard transforms of images by use of inexpensive liquid-crystal spatial light modulators

Incoherent light imaging by Hadamard transformations by use of an inexpensive spatial light modulator from a liquid-crystal display television is demonstrated. The transforms are implemented by means of S matrices. Good-fidelity 31 x 33 pixel images are obtained. The image distortions caused by the limitations of the spatial light modulator are discussed.     

Optimization of multichannel parallel joint transform correlator for accelerated pattern recognition

The multibeam parallel joint transform correlator for optical pattern recognition, which was recently proposed by the authors [Appl. Opt. 37, 5408 (1998)], can increase parallelism without accumulating zero-order background level at the first Fourier transform plane. To evaluate the throughput capability, an experimental trial was made, achieving a 67-ms recognition rate per face per channel, which is limited by the response of the optically addressed liquid-crystal spatial light modulator. A general design theory is developed for dense packing of the optical channels for a given spatial light modulator resolution, considering the bandwidth requirement of the target image. Then the condition for submillisecond throughput with state-of-the-art device technology is discussed.     

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.     

Multiplexing free-space optical signals using superimposed collinear orbital angular momentum states

As a proof of concept, we experimentally demonstrate multiplexing of free-space optical signals in multiple channels labeled with different states of orbital angular momentum. The multiplexing process is carried out by a dynamic liquid-crystal spatial light modulator, while the phase function is calculated by an iterative algorithm. A binary amplitude computer-generated hologram serves as a demultiplexer.