Optical polarimetry is a term we have recently adopted to denote any type of optical measurement where light polarization plays a key role, and where polarization itself is a carrier of information. It also refers to measurement of the state of polarization light which is emitted from various sources, over a wide range of wavelengths, or which is scattered by different objects. The sources may range from minute atomic samples to entire galaxies; and the objects may range from atoms, molecules, or micro-scopic particles to whole planets. Evidently this is a field of vast scope that would require several special issues to be represented adequately. Ellipsometry, a branch of optical polarimetry which deals with surface and thin-film characterization by polarized-light reflection, is alone the subject of a series of international conferences with proceedings published as special volumes of Surface Science.1 I n astronomy the measurement of polarization of light, both emitted and scattered, is finding broad application for deducing the microstructure of particles and the macrostructure of atmospheres.2 Other subspecialties of optical polarimetry have similarly expanded.
A variety of physical processes and conditions in the solar at-mosphere produce polarized light, a useful diagnostic tool. We describe the instruments and techniques used to measure this polarization and indicate the solar physical processes illuminated by these measure-ments. Included are descriptions of instruments for broadband observa-tions of Thomson scattering in the solar corona; for observations of resonance polarization in prominence and coronal emission lines; and for narrowband, spectrally resolved polarimetry in Fraunhofer lines formed in the photosphere and chromosphere.
A method for producing high-resolution images of the polarization information in a scene is presented, and several examples of polarization images are shown. A 35 mm camera with a polarizing filter is used to obtain a set of four slides for each image of interest. The slides are scanned with a digitizing video camera, and the resulting 512 X 512 digital images are manipulated with an image processing computer to produce separate images of the intensity, magnitude of linear polarization, and direction of polarization. Several polarization images of typical natural scenes have been processed by this method. These images show that both the magnitude and direction of polarization contain new and useful information that is not obtainable from the intensi-ty. The Multispectral Resource Sampler (MRS) experimental remote sensing satellite, to be launched in the mid 1980's, will be able to measure both intensity and polarization at several wavelengths. The results of this paper indicate that the additional information obtained from the MRS polarization data should allow previously indistinguishable ground features to be separated.
We describe a new design for an astronomical polarimeter optimized for use in the 8 to 13p spectral region based on an earlier visible wavelength instrument. The combination of a special infrared ZnSe birefringence modulator and a background-limited fast photoconductive detector permits carry-over of many of the digital signal processing advantages of the earlier instrument. Within a single measurement sequence, this instrument will measure the photometric intensity, the circular polarization, and both components of the linear polarization. Used with a 2.5 meter class infrared telescope, the device should permit a 3u detection of 1% linear or circular polarization at 10u in less than 400 seconds of any compact source brighter than 1.2 mag.
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 ° and the 90 ° 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° direction. The complete theory of this instrument is described including the detailed analysis of the polarization-interferograms it provides.
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.
Factors limiting the sensitivity of measurement obtainable with polarimetric instrumentation are discussed, with particular emphasis on modulation methods and the special problems associated with a coherent light source. Under optimum conditions the peak noise level of a Faraday-effect polarimeter with a He-Ne laser source has been found to be as low as ± 0.05 second of arc. The addition of quartz wedge units enables lateral displacements and departures from straightness to be measured and a system is described that possesses low sensitivity to laser beam direction changes and air turbulence. Profile measurements on a silica bar using this polarimetric system and measurements by precise Fizeau interferometry have agreed to better than 0.2 um over the 300 mm length of the bar.
The use of piezo-optical polarization modulation is proposed for stress analysis problems in two and three dimensions. A Stokes-meter and a new fast scattered-light technique are described. The piezo-optical effect can be advantageously used as very-small-stress sensor, when coupled with an appropriate polarimeter. In this paper, we present the theory of a polarimeter working without retardation plates, thus pro-viding low drift and accuracy. The measuring time can be as short as one millisecond. The details of realization and some performance characteristics are given. A three-dimensional stress-analysis technique is described in which the piezo-optical effect is used for polarization modulation. This method has been used successfully to measure the Mueller matrix associated with an elementary volume of a flow-birefringent liquid, thus yielding quantitative information about the velocity-gradient tensor. As another application of the piezo-optical ef-fect, we propose and describe a gravitational-wave antenna, exhibiting potentially high detectivity with a higher sensitivity than other antennas.
Photoelasticity (temporary strain birefringence) has long been utilized to transduce the stress in a material into an optical signal. It is demonstrated here that under certain conditions simple measurements (such as of area of birefringence) on optical signals resulting from stress applied to the surface of a photoelastic sheet are proportional to the magnitude of the vertical component of the applied force. These signals can be recorded photographically and consequently a photoelastic surface can be used to transduce dynamically the forces applied to that surface by natural phenomena. The polymeric material for the surface can be chosen to suit the range of stresses expected; gelatin is the most sensitive and adaptable material. Examples and applications are discussed.
Existing prototype optical current transformers are reviewed and their design as well as their functional characteristics are analyzed in the light of the practical requirements dictated by operating conditions in an extra-high-voltage (EHV) switchyard. A perspective of the development of a new magneto-optic current transformer is explored. The feasibility of such apparatus depends on technological progress in manufacturing mono-mode optical fibers which will not depolarize the transmitted light.
Perpendicular-incidence ellipsometry (PIE) is considerably simpler, both analytically and in instrumentation, than oblique-incidence ellipsometry for the characterization of anisotropic surfaces and thin films. Here we describe how perpendicular-incidence null ellipsometry (PINE) can be used to completely determine the normalized reflection matrix, R = (rHyrvy, of a surface with arbitrary anisotropy. To separate the off-diagonal elements of the reflection matrix, a non-reciprocal optical element, e.g., a Faraday rotator, must be used. Instrumental data consist of azimuth angles of a linear polarizer and a linear (quarter-wave) retarder that produce four nulls, two nulls being acquired in the presence of a specific Faraday rotation, the other two in its absence. This extension makes possible the characterization by PIE of new optical surface anisotropies heretofore not considered, such as natural or induced surface optical activity and circular dichroism, or Kerr magneto-optic effects.
Fiber optic communication generally comes to mind first when one thinks or speaks of optical communication. In fact, the two terms are often used synonymously. Indeed, the fiber optic communication technology is maturing rapidly and it offers enor-mous advantages over unguided atmospheric optical propagation systems. However, there is a variety of applications for which fiber systems are impractical and for which atmospheric optical systems may be preferred. For example, short-haul, over-the-horizon links in areas where cable real estate rights are unobtain-able, or where an optical broadcast mode is desired, or for space-to-earth optical links.
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.
Optical communication in the atmosphere, space, the marine boundary layer, and underwater is being investigated for a variety of applications. Three classes of optical communications systems will be addressed: OCULT (Optical Communications Using Laser Transceivers), ELOS (Extended Line-of-Sight) optical communications, and satellite-to-subsurface optical communica-tions. OCULT is a 10.6 bt, high rate reciprocal tracking heterodyne laser com-munications system designed for nearly all-weather duplex video bandwidth communications to horizon limited ranges. Of special interest are effects of coherent propagation through turbulent and turbid atmospheres. The ELOS system is a 1.06 µ optical aerosol scatter communications system for ranges of 30 to 300 miles. Scattering measurement at 40 to 80 miles through the marine boundary layer will be presented. The satellite-to-subsurface communication efforts deal with blue/green transmission from a satellite, through the atmo-sphere (including clouds) to a submerged receiver, exploiting the blue/green "window" in ocean water. The multiple forward scattered and diffusion trans-port of semiplane waves through clouds and ocean waters will be discussed.
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.
This paper describes a lidar system being developed for range-resolved water vapor measurements from an aircraft using the differential absorption lidar (DIAL) technique. The system uses two frequency-doubled Nd:YAG lasers to pump two independently tunable, high conversion efficiency dye lasers which operate in the near infrared between 710 and 960 nm. The "on" and "off" wavelengths which are used in the DIAL measurements are generated in sequential laser pulses with less than 100 ,us separation. The close spacing minimizes concentration errors that might result from changes in atmospheric scattering characteristics during the airborne DIAL measurement. The dye lasers have a linewidth less than 1.0 pm and an operating wavelength which is servo-controlled to less than 0.2 um using a high resolution, wave-length calibration unit. The backscattered lidar returns at the on and off wave-lengths are received by a 35 cm diameter telescope, sequentially detected by a photomultiplier tube, digitized, and stored on high speed magnetic tape. Water vapor concentration profiles are calculated for each measurement in real time by a minicomputer to permit optimum control of the experiment. A detailed description of the airborne DIAL system is given in this paper, and simulations of water vapor DIAL measurements are discussed. Experiment simulations include proposed investigations of water vapor in the boundary layer, the middle and upper troposphere, and the tropopause region. Simula-tions are also presented of the measurement of global water vapor vertical profiles with a Shuttle-borne lidar system which has operating characteristics similar to the airborne DIAL system.
The use of laser Raman measurement techniques in remote sensing applications is surveyed. A feasibility index is defined as a means to characterize the practicality of a given remote Raman measurement application. Specific applications of Raman scattering to the measurement of atmospheric water vapor profiles, methane plumes from liquid natural gas spills, and subsurface ocean temperature profiles are described. This paper will survey the use of laser Raman measurement techniques in remote sensing applications using as examples specific systems that the Computer Genetics Corporation (CGC) group has developed and engineered.
Many reflecting and refracting elements are required in the construction of a conventional optical system with a low f/number. Such a system poses problems in alignment and mechanical stabilty when operating in severe environments over a wide temerature range. A fiber bundle designed in the form of a cone with larger entrance diameter than the exit diameter transmits and reduces the uimage size. A simple lens of longer focal length and larger effective aperture can be used to produce an imageat the entrance of the cone. Since there is a gain in flux per unit area, this system intesifies as well as trasmits an image. It also enables the use of a detector of smaller size to achieve a higher signal-to-noise ratio. An effective optical systen for doing this is described and demonstrated.
With the human observer serving as the final detector in many imaging systems, the functioning of the visual system becomes an important design determinant. Here the contrast threshold is investigated in terms of underlying noise sources. Rose has shown that the threshold found at intermediate brightness levels can be accounted for on the basis of the quantum noise of the radiation absorbed by the retinal detectors. Here it is shown how reasonable and simple assumptions concerning other sources of noise would affect the threshold contrast over the full range of observable brightness levels. The resulting model may prove useful in predicting performance of observer-device systems. Results obtained with the proposed model are compared with ex-perimentally observed threshold data.
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.
We describe a simple technique, based on the method of Abeles, for routine measurements of the refractive index and the thickness of transparent planar thin films. The optical and electronic hardware required to implement the technique involves only standard equipment available in most laboratories. The results obtained with this method are compared with ellipsometric measurements.
Results of a program to experimentally evaluate existing techniques and develop new high-speed techniques for evaluation of optical distortion in aircraft windscreens are described. Present techniques for evaluation of air-craft windscreens' optical quality are based on grid board photography and point-by-point measurement of optical deviation errors. Experimental evaluation of the grid board techniques have shown them to be simple and easy to perform, but errors as large as 20 percent occur because of the associated data reduction. The approaches to point-by-point measurement of windscreen deviation errors provide high accuracy, but the time required to evaluate a single windscreen is typically 8 to 24 hours. New techniques were studied, developed, and evaluated in order to permit real-time evaluation of aircraft windscreens. Two approaches described will provide the capability for high-speed evaluation of windscreen optical distortion. These techniques utilize raster-scanned laser probe beams in conjunction with retroreflecting screens and holographic lenses. In addition to high-speed scanning techniques, a speckle photographic technique is described that can be used to evaluate binocular disparity in a transparent aircraft windscreen.
A laser scanner for reading the Universal Product Code on grocery packages in a supermarket checkout lane is described. The problems associated with reading five sides (bottom and four sides) of a package are discussed in terms of the bit-rate, spot-size, and angle of incidence variations in different orientations. The optics, detection, signal processing, and decoding are also described.
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.
Recent advances in large area charge-coupled device (CCD) imaging detectors and high-speed microprocessors have prompted the development of a new clss of electro-optical tracking instruments at the Jet Propulsion Labora-tory (JPL). These instruments are designed for standardized NASA-wide usage and are characterized by their extremely high pointing accuracy and stability and performance capabilities which are largely software defined and thus easily adapted to a variety of mission requirements. This paper presents an examination of the methods by which CCD detectors are being incorporated in star tracker instruments and the performance capabilities that can be expected from currently available devices. The multi-function sensor concept, in which a single sensor can function in a variety of guidance applications, is described. Software algorithms designed to provide efficient extraction of guidance information from both point and extended images are also presented. CCD star tracker implementations currently underway at JPL are described, and perfor-mance data obtained during laboratory testing are presented and discussed.
This paper describes techniques for the implementation of high speed, high resolution laser line and/or raster scanned recorders through use of acousto-optic deflection and modulation. A rationale for the design of scanned imaging is presented, discussing such topics as horizontal and vertical, modulation transfer function (MTF), exposure requirements when scanning with a Gaussian spot, and the effect of film characteristics (gamma). A descrip-tion of several working systems is presented. These include the LR-100/200 Laser Recorder which involves the recording of 875 line television imagery on 8 and 16 mm dry processed film, and a high resolution line scanner (4000-6000 spots) that was brassboarded solely with the use of acousto-optic components. The heart of each of these systems is a patented acousto-optic device called the chirp scanner which permits high resolution, high speed scans with literally zero flyback time. This device also forms the basis of a future system capable of direct digital recording at a 150 to 200 Mbit rate.
Scattered light which remains in an optical waveguide to be guided to the detector is an important limiting factor to the performance of integrated optical devices. In sputtered glass and niobium oxide wavegu ides the principal mechanisms for scattering are refractive index variations in the wave-guide and surface roughness of the waveguide. Theories for scattering from surface roughness, in the regime expected for these wavegu ides, predict that the wavelength variation of the scattering should be proportional to the inverse square of the wavelength. Theory for scattering from refractive index variations predicts a wavelength dependence ranging from inverse fourth power of the wavelength to no dependence upon wavelength, when the range of possible scattering diameters is varied from small to le ge with respect to the optical wavelength. Experiments on a number of relatively lossy wavegu ides indicate that there is little relation between overall waveguide loss and the magnitude of the scattering in the near forward direction. Furthermore, overall loss and in-plane scattering have different variations with wavelength suggesting that the mechanisms involved in loss and scat-tering are different. Waveguide surface preparation prior to sputtering ap-pears to be an important factor in determining overall quality, but more as it influences the bulk properties of the waveguide film than as a direct scattering mechanism. Homogeneity of the waveguide film appears to be the controlling factor in production of low scatter wavegu ides.
Reviewing the Book Review Books are a primary source of information for working scientists and engineers, so presenting helpful reviews of the latest books in optics and closely related fields has been a primary goal of mine as editor of Optical Engineering. I have sought out the leaders in the field to do the reviews and their efforts have been evident in some excellent reviews. Nevertheless, I see the need, for the sake of completeness and value of the reviews, to list certain things that I as editor would like to see in many book reviews. The following list is incomplete and in no particular order, but I hope reviewers will find it useful.