Narasimha S. Prasad

Optical alignment monitoring and stabilization using a liquid crystal television (LCTV)

Maintenance of laser beam alignment in optical systems is highly desirable for effective operation. In this paper, a LCTV-based alignment preserving scheme is discussed. We show that a laser beam can be aligned automatically to an accuracy of less than plus or minus 2 micrometer using a LCTV with an average pixel pitch of 85 micrometer. Due to simplicity and low cost, the proposed scheme is attractive for use in laser based optical systems that are used on airborne platforms.

Fuzzy-logic-based approach to color quality processing

The development of high resolution spectrophotometers and colorimeters, combined with its portability and large data processing abilities, has made the color evaluation process easier and faster. Although these instruments are very useful for rapid pass or fail color inspections in many industries such as, the automotive industry, textile industry, etc., the final decision depends primarily upon a subjective visual assessment. Besides spectral analysis, which is useful in colorant selection, the interrelationship between various environmental factors, metamerism, and texture and composition of the material (substrate), has made visual coordination an acceptable methodology to obtain a repeatable finish and color quality. Subjective assessment in color matching, especially in colors that closely resemble one another, leads to laborious and time consuming adjustments that have to be performed to obtain the right concentration of the colorants. Color evaluation and color mixing for a given material surface are interdependent. Although there are analytical methods that provide a means for colorant analysis, their application is cumbersome and involves complex calculations. In this paper we develop a fuzzy approach to obtain optimal color correlation between visual assessment, computed color differences, and colorant composition.

Optical information processing system for aiding the diagnosis of pulmonary embolism

Diagnoses of cancers and pulmonary embolism are performed by visually interpreting medical data on computer graphics displays. Interpretation aids for medical diagnosis and treatment are not available. The optical information processor system presented in this paper can be used as a second opinion in detecting cancers and classifying images; the final diagnosis is made by a physician. The optical information processing system uses a novel spatial multiplexing technique that allows several images to be processed simultaneously using the same spatial light modulator. Simulation results for liquid crystal display operated in a novel amplitude coupled with binary phase mode is described. In addition, simulation results for a phase modulating micro-mirror spatial light modulator are presented. Results using clinical data show that the optical information processing system can yield a diagnosis rate of 86%.

Simulation study of a fuzzy-logic-based controller for laser cavities

This report summarizes the Threat Object Map (TOM) handover analysis that the ODA team has performed during the past year. The areas of study include evaluating data from the STORM 4 and STORM 6 missions to determine: (1) performance of a radar to optical interceptor TOM handover, (2) sensitivities to data latency both above and within the atmosphere, (3) platform sensitivities to closely spaced objects, and (4) sensitivity to N objects handed up from the radar to M objects on the interceptor focal plane. This analysis is limited to metric only TOM handover and does not include generalized TOM evaluation. The analysis uses the OMEGA and TOMAHOC codes. OMEGA models the radar noising. TOMAHOC (Threat Object Map and Handover Code) performs the metric handover. TOMAHOC contains bias removal algorithms and a Sparse Munkres algorithm.

Laser beam diagnostics using fuzzy logic

The measurement of laser beam quality is of prime importance for various applications. M2 factor has been widely accepted as a standard for characterizing the quality of real laser beams. The inaccuracies present in the specifications of resonator elements, variations occurring due to various competing physical processes inside the lasing medium, and offsets in the cavity configuration make the beam quality deviate from the desired value. Since the beam quality can be improved by manipulating the cavity parameters, fine tuning of mirror separation distance can offer considerable modification in the beam quality. In this paper, a fuzzy logic based controller to obtain and monitor desired laser beam characteristics for stable resonators is discussed. The simulation results indicate that the proposed fuzzy logic controller will dynamically adapt to real laser beams and can offer superior performance over conventional proportional-integral-derivative (PID) controllers. The principle advantage of the present approach is that it provides a versatile means for automatic control over the beam characteristics without relying on detailed mathematical modeling techniques.

Liquid crystal television (LCTV) as an angle-tuning means for phase matching in three-wave mixing devices

In this paper we have demonstrated a wavelength tuning scheme using a twisted nematic liquid crystal based LCTV operated in an amplitude coupled binary phase mode for three wave mixing devices. A cosine chirp that is written on the LCTV by electrically addressing its pixels through a personal computer, functions as a programmable spherical lens. Deflection of the input beam is achieved by introducing tilt aberration on the lens. A spot resolution of less than 1.65 arcsec, and a steering range of 2 degrees at a wavelength of 632.8 nm were achieved by continuously varying the tilt coefficient of the lens. The potential advantages of this scheme are speed, compactness, operational ease, and power consumption.

Programmable deflector and scanner using a liquid crystal television (LCTV)

In this paper, deflection and scanning of a HeNe beam by a programmable phase Fresnel lens written on a LCTV has been demonstrated. Translation and tilt aberration respectively produces coarse and fine deflection of the beam. The rapid translation of the lens function, time dependent variation of the magnitude of the tilt aberration, and a combination of these two methods are used to demonstrate laser beam scanning. In our experiments, using the wavefront tilt aberration a measurable scan deflection accuracy of less than 8 (mu) rad is achieved. Using the lens translation scheme the beam is scanned over a 0.8 cm X 0.8 cm area at a distance of 50 cm. The scan time of the current system is limited by the standard video frame rate of the existing LCTV drive electronics. The performance limits of the LCTV-based deflection and scanning system are compared with those of acousto-optic deflectors and scanners.

Fuzzy-logic-based controller for optical resonators

The beam characteristics of a laser depend on various factors such as temperature, mechanical deficiencies of mounts, tolerance specifications, etc. As such, there is a tendency for the beam characteristics to deviate from the desired characteristics. This paper describes the development of a fuzzy-logic based controller to obtain and maintain specific output beam characteristics of an optical resonator.

Analysis of acousto-optic devices for angle tuning of optical parametric oscillators

An optical parametric oscillator (OPO) produces coherent optical radiation which is tunable over a wide range. In this paper, a novel technique for angle tuning an OPO by an acousto-optic Bragg cell is discussed. It is shown that the proposed scheme provides course as well as fine tuning of signal wavelengths with high speed, and hence is a promising alternative to the conventional tuning techniques.