Showing papers in "Optical Engineering in 1981"

Acousto-Optic Tunable Filters

Abstract: This paper reviews the theory, techniques, and applications of acousto-optic tunable filters (AOTF). Two basic types of AOTF are described, i.e., the collinear and the noncollinear configurations. The important device characteristics of the AOTF presented include the bandpass response and spectral resolution, angular aperture, and the filter tuning relation. This review concludes with a discussion of various AOTF applications.

A Sampling Theorem For The Complex Spectrogram, And Gabor's Expansion Of A Signal In Gaussian Elementary Signals

Abstract: The complex spectrogram of a signal is defined as the Fourier transform of the product of the signal and the shifted and complex conjugated version of a so-called window function; it is thus a function of time and frequency, simultaneously, from which the signal can be reconstructed uniquely. It is shown that the complex spectrogram is completely determined by its values on the points of a certain time-frequency lattice. This lattice is exactly the one suggested by Gabor in 1946; it arose in connection with Gabor's sugges-tion to expand a signal into a discrete set of Gaussian elementary signals. Such an expansion is a special case of the more general expansion of a signal into a discrete set of properly shifted and modulated window functions. It is shown that this expansion exists. Furthermore, a set of functions is constructed, which is bi-orthonormal to the set of shifted and modulated window functions. With the help of this bi-orthonormal set of functions, the expansion coefficients can be determined easily.

Laser Electro-Optic System For Rapid Three-Dimensional (3-D) Topographic Mapping Of Surfaces

Abstract: It is pointed out that the generic utility of a robot in a factory/assembly environment could be substantially enhanced by providing a vision capability to the robot. A standard videocamera for robot vision provides a two-dimensional image which contains insufficient information for a detailed three-dimensional reconstruction of an object. Approaches which supply the additional information needed for the three-dimensional mapping of objects with complex surface shapes are briefly considered and a description is presented of a laser-based system which can provide three-dimensional vision to a robot. The system consists of a laser beam array generator, an optical image recorder, and software for controlling the required operations. The projection of a laser beam array onto a surface produces a dot pattern image which is viewed from one or more suitable perspectives. Attention is given to the mathematical method employed, the space coding technique, the approaches used for obtaining the transformation parameters, the optics for laser beam array generation, the hardware for beam array coding, and aspects of image acquisition.

Tunable Fabry-Perot Filters

Abstract: The Fabry-Perot interferometer is being applied increasingly as a tunable filter. The theoretical limits to its performance are considered, including the effects of phase change at multilayer reflecting coatings. Various forms of construction are discussed as are several ways of tuning Fabry-Perot filters. The review concludes with a description of a number of practical devices.

Reduction Of Errors Of Microphotographic Reproductions By Optimal Corrections Of Original Masks

Abstract: Using image restoration techniques, we report on an algorithm for determining the optimum correction of original masks, which are to be reduced by a given microphotographic system, in order to compensate for the system's distortion. The film is assumed highly nonlinear. As an example, we compute the profile of serifs that should be added to corners to compensate for distortion in a diffraction-limited imaging system.

Tunable birefringent filters

Abstract: This article reviews the types and capabilities of birefringent filters. The general operating principles of Lyot (perfect polarizers), partial polarizing, and Solc (no internal polarizers) filters are introduced. Appropriate techniques for tuning each filter type are presented. Field of view of birefringent filters is discussed and is compared to Fabry-Perot and interference filters. The transmission and throughput advantages of birefringent filters are shown. Finally, the current state of the art in practical filters is reviewed.

Boresighting of airborne laser designation systems

Abstract: A method and apparatus is described for boresighting the sighting systems ed with a 1.06 micrometer designator laser. The method enables boresighting visual direct view optics, TV, and forward looking infrared (FLIR) sights with respect to the laser itself, without the use of external targets. The laser beam is focused onto a refractory target in the boresight device, creating very briefly an incandescent hot spot which can be "seen" by all three sensors. The radiation from this hot spot is collimated by the boresight device optics and projected back into the sights exactly anti-parallel to the laser beam. By aligning the sight reticles with this hot spot, all three types of sights are aligned relative to the laser. Since the hot flash created by the laser energy is of such short duration, the alignment can be accomplished in flight where relative motion exists between the laser designator system and the boresight module. The apparatus disclosed includes a CO 2 laser alignment fixture and a reflective collimator with a phosphor thermal imaging disk for precision alignment of the laser input and FLIR output lines of sight of the boresight module.

High-Resolution Interferometric Spectrophotopolarimetry

Abstract: Spectrophotopolarimetric capability can be added to a laboratory interferometer-spectrometer by use of a specially designed module described herein. With the instrument so augmented, high-resolution spectra can be obtained of the Stokes parameters of the reference beam and the beams diffusely reflected or transmitted by a sample medium of interest. For any such beam, the exponential Fourier transforms of the two interferograms obtained with a polarizer-analyzer oriented along the 0 deg and the 90 deg directions provide the spectra of I and Q, separately. Within experimental (and numerical) noise, this I spectrum should be the same as the one obtained with the polarizer removed. The remaining Stokes parameters U and V are obtained with a third interferogram recorded with the polarizer along the 45 deg direction. The complete theory of this instrument is described including the detailed analysis of the polarization-interferograms it provides.

Collisional Effects On Laser-Induced Fluorescence Flame Measurements

Abstract: . Laser-induced fluorescence (LIF) is a method of considerable utility for the measurement of the transient free radicals which are the keys to the chemistry of flames. Collisions experienced by the electronically excited state can alter the magnitude and the spectral form of the fluorescence signals. Recent studies on both quenching and energy transfer collisions, and their influence on LIF measurements, are treated in this review; special emphasis is given to the important and popular OH molecule. Different solutions to the problem of accounting for quenching are considered, and both effects and exploitation of energy transfer within the excited state are discussed. Although further research is needed to better quantify these collisional effects, LIF can currently provide data significant for the understanding of combustion chemistry.

Thermal Effects In Optical Systems

Abstract: Optical systems often have to operate over a wide temperature range. The effects of temperature changes on the performance of some typical optical systems are described in this paper. Methods used to counteract such variations in optical properties range from servo-controlled motion of the components and bimetal mounts with reciprocating motions to, as this paper describes, a simple choice of appropriate optical and mount materials. The optical systems considered range from single lenses to high quality multielement imaging systems.

Fast Self-Induced Refractive Index Changes In Optical Media: A Survey

Abstract: Refractive index changes in all types of optical media induced by an optical field through resonant and nonresonant mechanisms with submicrosecond responses are surveyed in this paper. Relevant data for over 100 representative media have been compiled and tabulated, and numerical examples have been worked out for several important media. For room-temperature operation, solid polydiacetylenes, CS2 liquid, and N2-buffered 12 vapor are found to be roughly comparable in sensitivity for self-induced phase shift for a given transmission loss. Nitroaniline compounds are somewhat less sensitive but also interesting. For operation in the 10 µm region, n-InSb is most sensitive. If operation at somewhat elevated temperature (100 to 200 C) is acceptable, then N2-buffered alkali-metal vapors are the best with about two orders of magnitude higher sensitivity. At liquid nitrogen temperature, a thin film of CdS could be as sensitive as a 105 times thicker sample of PTS polydiacetylene if the signal frequency is near the intrinsic exciton lines of CdS.

Techniques For Speckle Noise Removal

Abstract: In this paper, several techniques to reduce speckle noise (more generally signal independent multiplicative noise) in images are studied. The techniques include gray scale modification, frame averaging, low-pass filtering in the intensity and density domains, and application of the short space spectral subtraction image restoration technique in the density domain. Some discussions on the theoretical basis of the techniques studied are given and their performances are illustrated by way of examples.

Sampling theorem for the complex spectrogram, and Gabor's expansion in Gaussian elementary signals

Abstract: The complex spectrogram of a signal is defined as the Fourier transform of the product of the signal and the shifted and complex conjugated version of a so-called window function; it is thus a function of time and frequency, simultaneously, from which the signal can be reconstructed uniquely. It is shown that the complex spectrogram is completely determined by its values on the points of a certain time-frequency lattice. This lattice is exactly the one suggested by Gabor in 1946; it arose in connection with Gabor's suggestion to expand a signal into a discrete set of Gaussian elementary signals. Such an expansion is a special case of the more general expansion of a signal into a discrete set of properly shifted and modulated window functions. It is shown that this expansion exists. Furthermore, a set of functions is constructed, which is bi-orthonormal to the set of shifted and modulated window functions. With the help of this bi-orthonormal set of functions, the expansion coefficients can be determined easily.

Lens Barrel Optomechanical Design Principles

Abstract: It is the task of an optomechanical engineer, starting with a lens design provided by the optical designer, to devise a structure which holds various components of a lens in proper axial and radial alignment. This structure, the lens barrel, must provide a means of interfacing with the customer's system, and must be designed so as to maintain optical and mechanical integrity under a multitude of environmental conditions. This discussion encompasses the design and analysis efforts undertaken by the optomechanical engineer to comply with this requirement. Additionally, some special areas of concern to the optomechanical engineer such as various centration and assembly techniques, cementing of optical components, sealing and leak rate analyses, and reliability estimates of lens assemblies are discussed.

Stereo TV Improves Operator Performance Under Degraded Visibility Conditions

Abstract: The research reported here was initially stimulated by a contradiction in the literature that suggested there was no significant performance advantage for manipulator operators employing stereo versus conventional TV systems, while typical direct-viewed results indicated that binocular performance is always superior to monocular performance in tasks requiring depth judgement or distance estimation. This paper presents an analysis of the interaction of several important variables involved in visual display research, with particular reference to our own research comparing mono and stereo TV displays in simulated underwater environments. Three experiments were conducted in our lab to assess the impact of degraded visibility on remote manipulator operator performance using either mono or stereo TV. Subjects were required to perform tasks which differed markedly in the number and type of depth cues available. As predicted, the results indicated that stereo performance was superior to mono under most conditions tested; however, the amount of improvement was shown to be a complex function of visibility, task, and learning factors. Conclusions and recommendations for further research aimed at understanding the relative contributions of these and additional display factors are presented for discussion.

Background Suppression And Tracking With A Staring Mosaic Sensor

Abstract: This paper presents theoretical analysis for a staring mosaic infrared sensor with representative examples of data processing from a computer simulation. The analysis treats (1 ) generation of synthetic two-dimensional scenes with specified cloud geometry and desired statistical characteristics, (2) processing of frames of data from two-dimensional scenes to represent temporal, spatial, and multispectral filtering, and (3) thresholding and examination of processed scenes to implement track association. The temporal filtering includes multiple differencing, statistically optimal nonrecursive filtering, and recursive filtering. Methods are presented for reducing the computation load when calculating the optimal coefficients in spatial and multispectral filtering. The track association uses thresholding and examination to eliminate stationary objects and facilitate track assembly. For visual display, the two-dimensional scenes and the processed frames are output with a forty-eight level gray scale.

Hybrid Holographic Interferometer For Measuring Three-Dimensional Deformations

Abstract: A method for automatically measuring the three-dimensional (3-D) deformations of a diffuse surface is studied, using a real-time holographic interferometer and a television-computer system. The object surface is illuminated from three different directions. The scattered waves are recorded on a thermoplastic hologram. Real-time holographic interference patterns are observed through the hologram in a fixed direction by sequentially applying the three illuminating beams. The three fringe patterns are put into the computer using a television (TV) camera. The relative fringe order numbers are determined over the whole surface. Calculations for obtaining 3-D deformation distributions are performed, and the results are displayed on a cathode-ray tube (CRT) monitor.

Limited-Angle Three-Dimensional Reconstructions Using Fourier Transform Iterations And Radon Transform Iterations

Abstract: The principles of limited-angle reconstruction of space-limited objects using the concepts of "allowed cone" and "missing cone" in Fourier space are discussed. The distortion of a point source resulting from setting the Fourier components in the missing cone to zero has been calculated mathematically, and its bearing on the convergence of an iteration scheme involving Fourier transforms has been analyzed in detail. It was found that the convergence rate is fairly insensitive to the position of the point source within the boundary of the object, apart from an edge effect which tends to enhance some parts of the boundary in reconstructing the object. Another iteration scheme involving Radon transforms was introduced and compared to the Fourier transform method in such areas as root mean square error, stability with respect to noise, and computer reconstruction time.

Holographic Coupler For Fiber Optics

Abstract: The analysis and demonstration of a holographic coupler for single-mode optical fibers and cables is discussed. The holographic coupler is composed of two holographic windows. The holographic window is rigidly attached to the cable end. The coupling between windows proceeds by means of plane waves. The tolerances for some of the critical coupler parameters are relaxed, namely transverse alignment. Precision work is restricted to the factory produc-tion, but of simple implementation. The result is a coupler with relatively undemanding assembly operations in the field.

Luminescent Solar Concentrators

Abstract: Various planar configurations of the luminescent solar concentrator (LSC) are discussed including the uniformly doped, the stacked plate, the thin film, and the multilayered film LSC. Radiation which is lost from the luminescent plate by falling within the critical angle for total internal reflection is examined in terms of the above configurations. A ten parameter efficiency profile has been developed to better evaluate the collector performance and to allow direct comparison with other photovoltaic devices. A collector efficiency of 3.2% is reported and the feasibility of 8 to 10% efficient devices discussed. Several optical distribution factors have been defined and experimentally evaluated for the luminescent plate and associated photovoltaic cell. These factors have been used to calculate a plate-to-cell optical coupling coefficient for two commercially available solar cells. LSCs based on organic and inorganic phos-phor systems are compared and the problem of dye staility discussed.

Design criteria and recent developments of optical single particle counters for fossil fuel systems

Abstract: Optical methods for particle size distribution measurements in practical high temperature environments are approaching feasibility and offer significant advantages over conventional sampling methods. The present paper begins by summarizing user requirements for research and on-line particle measurements in fossil fuel systems. The principles of single particle counter (SPC) design are outlined followed by a discussion of practical instrument design constraints. Three instrument design concepts currently being developed at Sandia are then discussed. An overview of these current methods and other instrument designs is presented with particular emphasis on capabilities to meet user ojectives. Validation and long term testing of these new concepts is considered to be the final important step in achieving user acceptance of in situ optical counters.

Absolute Distance Interferometry

Abstract: A basis for making absolute distance measurements to an accuracy of 0.025 pm over 0-1.5 m intervals is reported. Extensions of this technology will permit distances of 50 m and greater to be measured to the same accuracy. Two-color, synthetic Michelson interferometry using a CO2 laser source capa-ble of generating four sets of R- and P-line pairs is employed. This allows reduction of the very large ambiguity exhibited by conventional Michelson interferometers as well as the resolution of difficulties which would otherwise arise due to instabilities in the measurement arm of the interferometer. This latter effect is a practical rather than a fundamental consideration, but is nonetheless important if the interferometer is to emerge from the laboratory as an effective, workable instrument. Distance is determined in terms of a denumerable number of precisely known wavelengths and fractions thereof.

Combustion Diagnosis By Coherent Anti-Stokes Raman Spectroscopy (CARS)

Abstract: Coherent anti-Stokes Raman spectroscopy (CARS) appears very promising for the remote, spatially and temporally precise probing of hostile combustion environments due to its large signal conversion efficiency and coherent signal nature. CARS is a wave mixing process in which incident laser beams at frequencies w1 and 0)2, with a frequency difference tuned to a Raman resonance in the molecular species being probed, interact to generate a coher-ent signal at frequency w3 = 2(.01 - w2. By analyzing the spectral distribution of the CARS signal, temperature measurements can be performed. Species concentration measurements derive from the intensity of the CARS radiation or, in certain cases, from its spectral shape. CARS spectra have been recorded in a variety of flames from the major flame constituents and generally show very good agreement with computer synthesized spectra. Significantly, CARS has been successfully demonstrated with both liquid and gaseous fuels in the primary zone and exhaust of practical combustors. Both thermometry and species concentration measurements have been performed. High pressure effects on CARS spectra have also been examined.

Laser Radiation-Induced Air Breakdown And Plasma Shielding

Abstract: Gas breakdown, or the ionization of the air in the path of a high power laser, is a limit on the maximum intensity which can be propagated through the atmosphere. When the threshold for breakdown is exceeded, a high density, high temperature plasma is produced which is opaque to visible and infrared wavelengths and thus absorbs the laser radiation. The threshold in the atmosphere is significantly lower than in pure gases because of laser interaction and vaporization of aerosols. This aspect of air breakdown is discussed in detail. Parametric studies have revealed the scaling laws of breakdown as to wavelength and laser pulse duration, and these will be discussed and compared with existing models. A problem closely related to breakdown is the plasma produc-tion when a high intensity laser interacts with a surface. In this case, the plasma can be beneficial for coupling laser energy into shiny surfaces. The plasma absorbs the laser radiation and reradiates the energy at shorter wavelengths; this shorter wavelength radiation is absorbed by the surface, thus increasing the coupling of energy into the surface. The conditions for the enhancement of laser coupling into surfaces will be discussed, particularly for cw laser beams, an area of recent experimen-tal investigation.

Optical Communication Through Low-Visibility Weather

Abstract: By exploiting the scattered light, it is possible to improve atmospheric optical communication through low-visibility weather. This paper summarizes the multiple-forward-scatter propagation model for a line-of-sight optical link operating in low-visibility weather. Using this model, digital communication performances for diffraction-limited and wide field-of-view receivers are compared. It is shown that background-light suppression is the key to extended link operability in the daytime. Recent work on space-distributed phase compensation techniques for achieving this suppression is described.

Theory Of Optical Propagation In The Atmosphere

Abstract: For the past few decades, numerous investigations have been reported on optical propagation in the turbulent atmosphere. In recent years, however, the theoretical study of optical propagation in fog, clouds, and other particulate matter has attracted considerable attention. Though much progress has been reported in the theoretical study of these phenomena, there are several areas where the theory is still not conclusive and further investigations are needed. This paper presents a review of the present state-of-the-art in the theory of optical propagation in the atmosphere and some discussions on the future directions of investigations in this area. Both cw and pulse propagations are considered and the quantitative discussions are given on angular broadening, pulse broadening, and coherence bandwidth of the optical wave in the turbulence and the particulate matter. It is noted that the forward scatter approximations are valid for the turbulence case, but for the case of particulate matter, it is convenient to consider three regions: first-order scattering, forward scatter, and diffusion; and it is important to take into account the effects of transmitter and receiver characteristics.

Vanadium dioxide storage material

Abstract: Vanadium dioxide undergoes a semiconductor-metal phase transi­ tion at temperature of 340 K. When prepared as a thin film on a suitable substrate, the transition exhibits hysteresis, i.e. the curve describing the state of the film versus temperature is double valued. Here the material is capable of existing in equilibrium in either of two states at the same temperature. Both high writing speed and high spatial resolution are possible, contrary to what one might normally expect from a thermal process. The writing speed is limited by the time taken to deliver the latent heat of transition, and the resolution is limited by the diffusion time during which written and unwritten adjacent areas are out of thermal equilibrium. Both of these times are connected, and they along with the latent heat dictate the exposure. Experiments thus far have produced writing times as small as 30 nanoseconds and stored spots of a few microns in size. They by no means represent limits, and writing times of a few nanoseconds and spot sizes down to a few thousand angstroms appear feasi­ ble. A discussion will be given detailing the material as a high density recording medium including laser beam writing requirements, spot sizes, stored contrast, and signal-to-noise ratio. In addition, information on optical constants, latent heat and other physical parameters will be given along with some past results concerning long term storage, cycling, lifetime, and reciprocity.

Advanced X-Ray Astrophysics Facility (AXAF)

Abstract: The Advanced X-ray Astrophysics Facility (AXAF) is to be a free-flying national x-ray observatory that is Shuttle-launched (in mid-1988), maintainable on-orbit, and retrievable. The design lifetime is > 10 years. The AXAF is conceived as an x-ray telescope with 6 nested Wolter type I mirrors (maximum aperture of 1.2 m), a focal length of 10 m, and interchangeable and replaceable focal plane instruments. The optics will provide 0.5 arcsecond imagery over a several arcminute field and somewhat reduced resolution over 1 degree in the x-ray band from 0.1 to 8 keV. The performance requirements and design of the facility will be discussed and comparisons made with an existing x-ray observatory. The scientific objectives of the AXAF will also be discussed briefly.

Quantitative Atomic And Molecular Laser Fluorescence In The Study Of Detailed Combustion Processes

Abstract: The present ability of atomic and molecular fluorescence techniques to monitor quantitatively the concentrations of species in flames is reviewed. Atomic fluorescence measurements require a saturation mode with a laser pulselength that is sufficiently short to avoid induced chemical or ionization effects but long enough to produce a steady state distribution over the accessible energy levels. Molecular fluorescence techniques are available for both low laser power and saturated modes of operation and invariably involve some level of approximation due to the complex energy level manifold that is involved. Two-photon induced fluorescence detection of atoms and molecules is of potential interest in facilitating the monitoring of various additional species that previously was only possible spectroscopically in the vacuum ultraviolet. Results of the major applications of these techniques are presented. These include temperature measurements, studies of the SOX and NOx flame chemistries, methods of obtaining energy transfer rates, laser induced chemical kinetic data, and their potential for deriving thermochemical values.

Stellar And Solar X-Ray Polarimetry

Abstract: The theory of Bragg crystal x-ray polarimeters is presented to-gether with an analysis of the statistics of photon-limited polarimeters. The focusing Bragg crystal polarimeter flown on the OSO-8 Satellite is described and a number of results obtained at 2.6 keV and 5.2 keV are presented. A polarimeter of this type with at least ten times the area is needed to answer a number of astrophysically interesting questions. A discussion is given of a Compton scattering polarimeter that is expected to be useful of energies above 40 keV. It is shown how this polarimeter may be used to test for the presence of a black hole in Cyg X-1. A solar flare x-ray polarimeter that is scheduled to be flown on an early space shuttle mission is described and an estimate is given of its sensitivity.