Narihiro Yoshida

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.

LCOS spatial light modulator controlled by 12-bit signals for optical phase-only modulation

We developed a liquid-crystal-on-silicon (LCOS) spatial light modulator (SLM) for phase-only modulation. The SLM was designed mainly for wavefront control in adaptive optics, optical manipulation, laser processing, etc. A dielectric multilayer mirror was incorporated into the device to enhance the reflectivity. The number of pixels was 792 x 612 and their size was 20 x 20 microns square. The range of the phase modulation exceeded one wavelength, and the light-utilization efficiency for monochromatic light was approximately 90%. The silicon backplane of the SLM was mechanically weak and its surface was not flat. The poor flatness degraded the output wavefront from the SLM. The device was driven by electronics composed of a digital-visual-interface (DVI) receiver, a field programmable gate array, and 12-bit digital-to-analog converters (DACs). The converted analog voltage signals from the DACs were transmitted to the pixels of the SLM and created phase changes. The driver had several kinds of control modes for the device, according to the level of flatness compensation. In one of the modes, the driver received 12-bit data and transferred them directly to the DACs. This 12-bit control mode enabled highly flexible control of the device characteristics. In the presentation, we report details of the device and experimental results on compensation of distortion in the output wavefront from the device.

Compact high-efficiency electrically addressable phase-only spatial light modulator

A compact electrically-addressable spatial light phase modulator module is described. The module consists of an electrically-addressed liquid crystal display (LCD), an optically-addressed phase-only spatial light modulator (SLM), one of which substrates is a fiber optic plate (FOP), a laser diode, and collimating optics for it. The module size is 95 mm long, 55 mm wide, and 90 mm high. The module had a nearly 100% reflectivity and a diffraction efficiency close to the theoretical maximum. Surplus diffraction light caused by the pixelized structure was reduced to approximately 3%, almost 50% of which was in the LCD alone. A reflection type of SLM would cause another power loss of the readout light by a half mirror, which was set up so as to separate the incident and reflected lights. An oblique incidence readout method was evaluated in the module, instead of the half mirror scheme, for the readout. We have found adequate alignments among the polarization and incident directions of the readout light, and the corresponding liquid crystal orientation. Consequently, almost no degradation in diffraction efficiency was observed for the incident angle within 45 degrees.

Fingerprint identification by an optical joint transform correlation system

An optical correlator has been used in fingerprint identification, and in order to test its properties in a practical application a compact joint transform correlation system was constructed. The system’s identification performance was qualitatively and statistically evaluated, based on fingerprint data gathered from 500 fellow workers. The power of parallelism and fast processing of optical processing enabled the system to verify an individual’s identity in less than 0.1 s on each try. The system performed a verification with a 4.1% false rejection rate and no false acceptance in “three-try.”

Frame-rate displacement measurement system utilizing an ultra-high-speed shutter camera and an optical correlator

A particle image velocimetry (PIV) system has been proposed for real-time (frame-rate) measurement and is composed of an ultra-high-speed shutter camera and an optical correlator. The ultra-high-speed shutter camera employed in this system has several functions such as optical amplification, high-speed shuttering and multishuttering for forming a double-exposure pattern of a moving object, in order to observe dimmer and high-speed phenomena. The optical correlator used in this system is a joint transform correlator (JTC) using two PAL-SLMs, which are optically addressed spatial light modulators with phase-only modulating function. By utilizing the optical correlator, a real-time fringe analysis of the double-exposure pattern was achieved. It is demonstrated that the system can measure velocity of random-dot patterns on a side of rotating disk with frame-rate response and good linearity at the rate of 10 m/s. We have studied an imaging condition such as ratio between exposure time and repetition time of double-exposure for suitable measurement. Also evaluated is effect of ion-feedback noise caused by an image intensifier in the ultra-high-speed shutter camera. The performance of the system is shown and discussed from a viewpoint of practical measurements. >

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.

High speed parallel aligned liquid crystal spatial light modulator operated at 180 Hz

We made a parallel aligned liquid crystal spatial light modulator (PAL-SLM) respond within 6 ms to meet requirements for a single-SLM color projection system. This was done by adjusting thicknesses of its optical addressing layer and its light modulating layer.

Electrically addressed spatial light phase modulator

A nonpixelized electrically addressable spatial light phase- only modulator has been developed. The device consists of an optically addressed parallel aligned nematic liquid crystal spatial light modulator (PAL-SLM), coupling optics, an XGA liquid crystal display (LCD) which serves as an accurate addressable mask for the PAL-SLM, a laser diode (LD) for illuminating the LCD and collimating optics for the LD. The device has a phase modulation capability of over 2(pi) radians and a high diffraction efficiency of greater than 35% at a spatial frequency of 10 lp/mm of binary (0,(pi) ) grating. Also when a multilevel (0, 0.5(pi) , (pi) , 1.5(pi) ) grating was written in the device, a diffraction efficiency of greater than 70% at a spatial frequency of 10 lp/mm was obtained. Moreover, surplus diffraction light (diffraction noise) caused by the pixelized structure was reduced to less than 3% by the coupling optics, compared with almost 50% occurring in the LCD by itself.

Real-time displacement measurement using a pattern correlation system incorporating an ultra-high-speed shutter camera and parallel aligned nematic liquid crystal spatial light modulators (PAL-SLMs)

PIV (Particle Image Velocimetry) system has been composed of an ultra-high-speed shutter camera and optical correlator for real-time (frame-rate) operation. The ultra-high-speed shutter camera has several functions such as optical amplification (typically /spl times/10/sup 4/), high speed shuttering (100 nsec-1 msec), and multi-shuttering (1, 2, 4, and 8 times) for forming a double-exposure pattern of an object image, in order to measure the displacement of the flow. The optical correlator is a joint transform correlator (JTC) using two PAL-SLMs, which are optically addressed spatial light modulators with phase only modulating function, for real-time analysis of the double-exposure pattern. It is demonstrated that the system can measure velocity of a random-dot pattern on a side of a rotating disk with frame-rate response. The performance of the system is shown and discussed.<<ETX>>

Displacement meter with ferroelectric liquid crystal spatial light modulator utilizing speckle photography

A speckle photographic technique was employed for the measurement of displacement of diffuse objects. In this method a double exposure is made on an FLC-SLM and the speckle correlation is detected with a position-sensitive detector. Use of this system means easy successive detection of displacement. In addition, the feedback control signals which regulate the FLC-SLM double-writing signals from PSD are expected to expand the measurable range and increase measurement accuracy. This approach has the advantage of measuring displacements of less than 1.0 ms in successive periods of several milliseconds. Experimental results are presented.