Showing papers in "Applied Optics in 1984"

Optical constants of ice from the ultraviolet to the microwave

Abstract: A compilation of the optical constants of ice Ih is made for temperatures within 60 K of the melting point. The imaginary part mim of the complex index of refraction m is obtained from measurements of spectral absorption coefficient; the real part mre is computed to be consistent with mim by use of known dispersion relations. The compilation of mim requires subjective interpolation in the near-ultraviolet and microwave, a temperature correction in the far-infrared, and a choice between two conflicting sources in the near-infrared. New measurements of the spectral absorption coefficient of pure ice are needed, at temperatures near the melting point, for 185–400-nm, 1.4–2.8-μm, 3.5–4.3-μm, 33–600-μm, and 1–100-mm wavelengths.

Analysis of atmospheric lidar observations: some comments

Abstract: There have been many discussions of solutions to the lidar equation for elastic scattering (e.g., Fernald et al.,' Klett, 2 Davis, and Collis and Russell ). Most of these are simply variations on Hitschfeld and Bordan's5 solution for meteorological radars. Klett 2 recently restated this solution in a very convenient form for the analysis of lidar observations collected in very turbid atmospheres. His paper has prompted a restatement of the more general solution of Fernald et al.l which is also applicable to mildly turbid atmospheres where both aerosol and molecular scatterers must be considered in the analysis. This has led to a simple numerical scheme for the computer analysis of lidar measurements. The lidar equation for two distinct classes of scatters (Fernald et al.') is

Automated phase-measuring profilometry of 3-D diffuse objects

Abstract: The high resolution surface profile of a 3-D diffuse object is obtained by measurement of the phase distribution across the image of a projected sinusoidal grating deformed by the surface. A shearing polarization interferometer is used for projection. Deformed grating images are detected by an array camera and processed by a microcomputer. Surface height resolutions of better than 10 μm have been attained, limited essentially by electronic quantization noise. In contrast to direct deformed grating analysis, this method, based on phase-shifting interferometric techniques, is capable of accurate measurement even with coarse projected gratings and low density image sensing arrays. Areas of application include industrial quality control, biostereometrics, robotics, and solid modeling for computer graphics.

Phase-only matched filtering

Abstract: From image processing work, we know that the phase information is significantly more important than amplitude information in preserving the features of a visual scene. Is the same true in the case of a matched filter? From previous work [ J. L. Horner , Appl. Opt.21, 4511( 1982)], we know that a pure phase correlation filter can have an optical efficiency of 100% in an optical correlation system. We examine this relationship between phase and amplitude in the case of alphanumeric characters, with and without noise, using a computer simulation. We compare the phase-only and amplitude-only filters to the classical matched filter using the criteria of discrimination, correlation peak, and optical efficiency. Three-dimensional plots of the autocorrelation and cross-correlation functions are presented and discussed.

Polarization holography. 1: A new high-efficiency organic material with reversible photoinduced birefringence.

Abstract: A new organic material for polarization holographic recording--azo-dye methyl-orange, introduced in polyvinyl alcohol (PVA)--is fabricated and investigated. It possesses all the good characteristics of the known polarization-sensitive materials--high sensitivity, reversibility, etc.--but excels them substantially in the magnitude of the photoinduced birefringence: delta n > 10(-3). This makes possible the recording of reversible polarization gratings with diffraction efficiency over 30%. Depending on the conditions of production and subsequent mechanical and thermal treatments the layers of methyl orange/PVA may also have intrinsic birefringence. In this paper the results of experimental investigations into the properties of this new material are reported.

Soot diagnostics based on laser heating

Abstract: Through numerical calculations we have investigated the possibility of developing soot diagnostics based on laser heating of the soot particles. Two strategies, one using the laser-modulated incandescence of the particles, and the other using direct detection of the evaporated C2 molecules, were examined. Both strategies can yield size distribution and volume fraction information provided the laser wavelength is near the graphite absorption band at 260 nm; otherwise, only volume fractions can be obtained.

Dispersion in GeO2-SiO2 glasses.

Abstract: Germania glass was prepared from high purity GeO2 powder. Refractive-index dispersion was used to calculate material dispersion and to provide a model for representing the dispersion of GeO2–SiO2 glasses. The wavelength of zero material dispersion is found to be in agreement with theoretical calculations. Modal propagation is modeled for a GeO2 core–silica clad fiber. Results support compositional dependence of profile dispersion in GeO2–SiO2 fibers.

Laser range finder based on synchronized scanners

Abstract: A new geometrical arrangement is proposed to improve performances of optical triangulation Two scanners in synchronization allow a linear position sensor to be used for surface topography measurement Besides a large increase in speed of measurement (approximately megahertz), the new geometry allows considerable reduction of the optical head size compared with usual geometries, so the shadow effects are reduced proportionally It also provides a means to obtain a very large field of view without compromising on resolution Geometrical analysis and experimental results are presented

Refractive properties of magnesium fluoride

Abstract: The refractive indices of a commercially available specimen of single-crystal MgF2 were determined for both the ordinary and extraordinary rays at selected wavelengths from 0.2026 to 7.04 μm. Measurements were made by means of the minimum-deviation method on a precision spectrometer near 19°C. The experimentally determined index values for each polarization were fitted to a three-term Sellmeier-type dispersion equation of the form n2-1=∑Ajλ2(λ2-λj2), where n is the refractive index, λ is the wavelength of interest, λj is the calculated wavelength of an oscillator, and Aj is the strength of the oscillator at λj. The birefringence was computed as a function of wavelength from the calculated index values obtained for the two polarizations. The dispersion coefficients were also determined for the 0 ray and for the birefringence. The results of this study are compared with previously reported work on the refractive properties of MgF2.

Unified synthetic discriminant function computational formulation.

Abstract: A most attractive approach to distortion-invariant pattern recognition uses a synthetic discriminant function (SDF) as the matched spatial filter in a correlator. In this paper, we (1) provide a general basis function and hyperspace description of SDFs, (2) advance a derivation showing the generality of the correlation matrix observation space that we use in our filter synthesis, and (3) detail a unified SDF filter synthesis technique for five different types of pattern recognition problem.

Birefringence measurements of liquid crystals

Abstract: A convenient and accurate technique for measuring the birefringence of liquid crystals at discrete wavelengths or as a continuous function of wavelength in the ultraviolet, visible, or infrared spectral regions is described. The method is based on determination of the phase differences which occur when monochromatic polarized light propagates through a medium with an anisotropic refractive index. Birefringence measurements at 0.6328 μm for two liquid crystal materials, BDH-E7 and ZLI-1132, and a continuous birefringence spectrum of ZLI-1132 from 2 to 16 μm are reported. Additionally, a liquid crystal based phase retardation plate which can be voltage tuned and calibrated to provide any degree of phase shift from 0 to 2π over a wide wavelength range (0.4–16 μm) is discussed.

Two-wavelength phase shifting interferometry.

Abstract: This paper describes a technique that combines ideas of phase shifting interferometry (PSI) and two-wavelength interferometry (TWLI) to extend the phase measurement range of conventional single-wavelength PSI. To verify theoretical predictions, experiments have been performed using a solid-state linear detector array to measure 1-D surface heights. Problems associated with TWLPSI and the experimental setup are discussed. To test the capability of the TWLPSI, a very fine fringe pattern was used to illuminate a 1024 element detector array. Without temporal averaging, the repeatability of measuring a surface having a sag of ~100 μm is better than 25-A (0.0025%) rms.

Effective reflectance of oceanic whitecaps

Abstract: The effective reflectance of the foam on the ocean surface together with the fraction of the surface covered with foam describes the optical influence of whitecaps in the solar spectral range. This effective reflectance is found to be ~22% in the visible spectral range and is presented as a function of wavelength for the solar spectral range. With the fraction of the surface covered with foam, taken from the literature, the results lead to a good agreement with satellite measured radiances and albedo values. The effective reflectance is more than a factor of 2 lower than reflectance values used to date in remote sensing and radiation budget studies. Consequently, the optical influence of whitecaps can be assumed to be much less important than formerly supposed.

Collisional narrowing effects on spectral line shapes measured at high resolution.

Abstract: The averaging effect of velocity-changing collisions reduces the Doppler broadening of isolated spectral lines and leads to one type of collisional narrowing. We present four collisionally narrowed profiles in standardized form using dimensionless parameters and estimate the quantitative effect of narrowing on the spectral line shape in terms of the magnitudes of these parameters. We show that a collisionally narrowed profile fitted by a Voigt function exhibits a characteristic signature on a plot of the residual errors in the fit. This provides a simple test for detectable narrowing effects. One of the simpler and better known models which includes collisional narrowing is the Galatry profile. We present sample plots of the residual errors resulting when theoretical profiles computed from other more elaborate models are fitted by a Galatry function.

Scattering particle characteristics and their effect on pulsed laser measurements of fluid flow: Speckle velocimetry vs particle image velocimetry

Abstract: Multiple-exposure recording of the speckle pattern trans­ lation that occurs during surface motion has been used ex­ tensively to measure in-plane displacements of solid surfaces. The application of this technique and derivative methods to the measurement of 2-D fluid velocity fields is of current in­ terest to experimentalists in fluid mechanics. Applications involving fluids are fundamentally different from those involving solids in several regards. One of the most important is that the light scattering characteristics of fluids containing small particles can be quite unlike those of solid surfaces. For example, fluids are illuminated by a pulsed sheet of laser light whose thickness is Δz. Hence scattering occurs from a volume distribution of particle scattering sites rather than a surface distribution. The particles are typically small (0.1-10 μm), and they act as discrete point sources of scattered light. Most important, from the viewpoint of this Letter, the number density of particles per unit volume and their size can vary over a very wide range of values, depending upon the fluid and its treatment. For speckle patterns to exist, the number of scattering sites per unit volume must be so high that many images overlap with random phase in the image plane. Since the number densities of scatterers in fluids can be quite low, it is possible that speckle will not be present in many fluid applications, and discrete images of particles will be photographed instead. This will change the mode of operation from laser speckle velocimetry to particle image velocimetry, as discussed in Ref. 4. Following Ref. 4, we will define the source density for a fluid volume illuminated by a laser sheet of thickness Δz to be

Fitting refractive-index data with the Sellmeier dispersion formula.

Abstract: The fitting of measured optical index data to the Sellmeier dispersion formula, using the variable projection algorithm, is described. Examples of fits obtained by this method to several Schott optical glasses and non-glass materials are given.

Laser-induced fluorescence of green plants. 1: A technique for the remote detection of plant stress and species differentiation.

Abstract: The laser-induced fluorescence (LIF) of green plants was evaluated as a means of remotely detecting plant stress and determining plant type. Corn and soybeans were used as representatives of monocots and dicots, respectively, in these studies. The fluorescence spectra of several plant pigments was excited with a nitrogen laser emitting at 337 nm. Intact leaves from corn and soybeans also fluoresced using the nitrogen laser. The two plant species exhibited fluorescence spectra which had three maxima in common at 440, 690, and 740 nm. However, the relative intensities of these maxima were distinctly different for the two species. Soybeans had an additional slight maxima at 525 nm. Potassium deficiency in corn caused an increase in fluorescence at 690 and 740 nm. Simulated water stress in soybeans resulted in increased fluorescence at 440, 525, 690, and 740 nm. The inhibition of photosynthesis in soybeans by 3-(3-4-dichlorophenyl)-1-1-dimethyl urea (DCMU) gave increased fluorescence primarily at 690 and 740 nm. Chlorosis as occurring in senescent soybean leaves caused a decrease in fluorescence at 690 and 740 nm. These studies indicate that LIF measurements of plants offer the potential for remotely detecting certain types of stress condition and also for differentiating plant species.

Mirror reflectometer based on optical cavity decay time.

Abstract: Described is a reflectometer capable of making reflectivity measurements of low-loss highly reflecting mirror coatings and transmission measurements of low-loss antireflection coatings. The technique directly measures the intensity decay time of an optical cavity comprised of low-loss elements. We develop the theoretical framework for the device and discuss in what conditions and to what extent the decay time represents a true measure of mirror reflectivity. Current apparatus provides a decay time resolution of 10 nsec and has demonstrated a cavity total loss resolution of 5 ppm.

Metallic multilayers for x rays using classical thin-film theory

Abstract: Successive evaporation of two correctly chosen elements now make possible highly efficient X–UV mirrors. However, good results depend on two points: First we must know the manufacturing tolerances; these depend strongly on the spectral range and the refractive index of materials used for the coating. The other important parameter is the roughness of each interface, which can result in significant losses in the reflectivity of the mirrors. This paper examines both points. Specular reflectance measurements at 0.159 nm are very sensitive to the roughness of each layer. To demonstrate this sensitivity we develop a method of calculation of this reflectance taking into account the roughness of each interface of the multilayer.

Multiple determination of the optical constants of thin-film coating materials

Abstract: The seven participating laboratories received films of two different thicknesses of Sc2O3 and Rh. All samples of each material were prepared in a single deposition run. Brief descriptions are given of the various methods used for determination of the optical constants of these coating materials. The measurement data are presented, and the results are compared. The mean of the variances of the Sc2O3 refractive-index determinations in the 0.40–0.75-nm spectral region was 0.03. The corresponding variances for the refractive index and absorption coefficient of Rh were 0.35 and 0.26, respectively.

Four-flux models to solve the scattering transfer equation in terms of Lorenz-Mie parameters

Abstract: The radiative transfer equation in nonemitting media is solved using a four-flux model in the case of Lorenz-Mie scatter centers embedded in a slab. The various coefficients of absorption and scattering appearing in the theory are nonphenomenological but expressed in terms of quantities available from the Lorenz-Mie framework. Formulas for the various transmittances and reflectances are established. Special cases are then discussed, and (potential or actual) applications reported.

Feature enhancement of film mammograms using fixed and adaptive neighborhoods.

Abstract: Digital techniques are presented for xerographylike enhancement of features in film mammograms. The mammographic image is first digitized using a procedure for gray scale dynamic range expansion. A pixel operator is then applied to the image, which performs contrast enhancement according to a specified function. The final transformation leads to either a positive or negative mode display as desired. We also present an adaptive neighborhood feature enhancement technique that enhances visibility of objects and details in an image. The availability of the enhanced images should aid diagnosis of breast cancer without requiring additional x-ray dose such as for xeromammography.

Modulation-transfer-function analysis for sampled image systems.

Abstract: Sampling generally causes the response of a digital imaging system to be locally shift-variant and not directly amenable to Modulation Transfer Function (MTF) analysis. However, this paper demonstrates that a meaningful system response can be calculated by averaging over an ensemble of point-source system inputs to yield an MTF which accounts for the combined effects of image formation, sampling, and image reconstruction. As an illustration, the MTF of the Landsat MSS system is analyzed to reveal an average effective instantaneous field of view which is significantly larger than the commonly accepted value, particularly in the along-track direction where undersampling contributes markedly to an MTF reduction and resultant increase in image blur.

Measurement of the Mueller matrix for ocean water

Abstract: The normalized light scattering polarization matrix has been measured for ocean water using an electrooptic light scattering polarimeter. Measurements were done on samples from the Atlantic and Pacific Oceans and the Gulf of Mexico. The polarization effects in the matrices were found to have, in general, a form which is similar to polarization effects in the Rayleigh scattering approximation; for example, all off-diagonal matrix elements except S12 and S21 were zero. Mueller matrix elements were calculated using a Mie computer code and compared to the measured matrices for ocean water. A simple one-component distribution was found to produce a reasonably good fit.

Phase and amplitude recovery from bispectra

Abstract: The bispectrum is the Fourier transform of the triple correlation, sometimes also referred to as triple product integral. We are concerned here with the bispectrum of the autotriple correlation. Bispectrum analysis can be used to solve phase problems in signal processing, since the knowledge of the bispectrum of a signal usually allows one to reconstruct both amplitude and phase of the Fourier transform signal. We present mathematical proof based on the theory of analytic functions and discuss the restrictions involved. A recursive algorithm is outlined for the reconstruction from sampled data. In addition, possibilities for noise reduction by averaging redundant information will be described. Examples are included for 1-D signals.

Polarization holography. 2: Polarization holographic gratings in photoanisotropic materials with and without intrinsic birefringence.

Abstract: Results are reported of experimental investigations on the diffraction efficiency and its polarization dependence of holographic phase polarization gratings in a photoanisotropic medium methyl orange/PVA with intrinsic birefringence. A comparative study of layers with different intrinsic birefringence was conducted. The conditions—initial birefringence and a type of polarization recording—were found to have high diffraction efficiency (>30%) and strong dependence on the direction of polarization of the linearly polarized readout beam. The results are in agreement with the theoretical considerations on such gratings.

Alignment of resonant optical cavities

Abstract: When an input Gaussian beam is improperly aligned and mode-matched to a stable optical resonator, the electric field in the resonator couples to off-axis spatial eigenmodes. We show that a translation of the input axis or a mismatch of the beam waist to the resonator waist size causes a coupling of off-axis modes which is inphase with the input field. On the other hand, a tilt of the input beam or a mismatch of the beam waist position to cavity waist position couples to these modes in quadrature phase. We also propose a method to measure these coupling coefficients and thereby provide a means to align and mode-match a resonant optical cavity in real time.

CARS temperature and species measurements in augmented jet engine exhausts.

Abstract: Over the last several years, the development of coherent anti-Stokes Raman spectroscopy (CARS) has progressed through simple flames of increasing complexity to practical application in internal combustion engines, simulations of gas turbine combustors, and a variety of other devices. Here, the first application of CARS to an actual afterburning jet engine is described. The CARS instrument constructed is capable of completely remote operation and permits instantaneous thermometry at a 20-Hz rate. Its design and operation as well as sample exhaust measurements will be presented.

Alignment of Gaussian beams.

Abstract: The design of a coupling between a semiconductor laser and a single-mode fiber, or between any two optical or acoustical elements that support Gaussian modes, is presented as a trade-off among coupling efficiency T(a), offset misalignment tolerance d(e), and angular misalignment tolerance theta(e). We show that these three parameters are subject to a trade-off limitation which takes the form 0 < T(a)(1/2)theta(e)d(e) < or = lambda/pi, and we show how to design a coupling so that the upper bound on the alignment product T(a)(1/2)theta(e)d(e) is achieved.

Focusing by electrical modulation of refraction in a liquid crystal cell

Abstract: The creation of a field-controlled variation of the index of refraction in a liquid crystal cell has been analyzed and experimentally verified. To obtain a spherical lens utilizing a simple electrode structure and capable of focusing arbitrary incoming polarizations requires four flat nematic liquid crystal cells. With electrodes fabricated well within the current capability of photolithography, near diffraction-limited performance in terms of the optical transfer function is predicted. The focusing capability of a liquid crystal lens was demonstrated using a single cell with linear transparent electrodes. A plano-convex cylindrical lens for a single incoming polarization was thus created. While the cell had a crude electrode structure, it affirmed all the major qualitative predictions. The fringing along the edge of the electrodes required for eventually obtaining near diffraction-limited performance was observed.