Showing papers in "Applied Optics in 1973"

Optical Constants of Water in the 200-nm to 200-microm Wavelength Region.

Abstract: Extinction coefficients k(lambda) for water at 25 degrees C were determined through a broad spectral region by manually smoothing a point by point graph of k(lambda) vs wavelength lambda that was plotted for data obtained from a review of the scientific literature on the optical constants of water. Absorption bands representing k(lambda) were postulated where data were not available in the vacuum uv and soft x-ray regions. A subtractive Kramers-Kronig analysis of the combined postulated and smoothed portions of the k(lambda) spectrum provided the index of refraction n(lambda) for the spectral region 200 nm

Measurement of Thin Film Parameters with a Prism Coupler

Abstract: The prism coupler, known from experiments on integrated optics, can be used to determine the refractive index and the thickness of a light-guiding thin film. Both parameters are obtained simultaneously and with good accuracy by measuring the coupling angles at the prism and fitting them by a theoretical dispersion curve. The fundamentals and limitations. of this method are discussed, its practical use, and mathematical procedures for the evaluation.

Role of cracks, pores, and absorbing inclusions on laser induced damage threshold at surfaces of transparent dielectrics.

Abstract: The concentration of the electric field strength in the neighborhood of micropores and cracks may lower the nominal external intensity for electric avalanche breakdown by a factor 2-100 depending on the geometry of the crack and the dielectric constant. The presence of absorbing inclusions at the edge of microcracks will often be the dominant mechanism giving the lowest surface damage threshold. Inclusions and cracks with characteristic dimensions less than about 10(-6) cm will not lower the breakdown threshold appreciably.

Infrared optical constants of ammonium sulfate, sahara dust, volcanic pumice, and flyash.

Abstract: Spectra of the real and imaginary parts of the refractive index m = n′ − in″ from 2.5 μm to 40 μm of ammonium sulfate, Sahara dust (as collected at Barbados), volcanic dust (pumice), and dust from a coal-fired power plant are presented. With a peak absorption index n″ = 1.0 at 10 μm Sahara dust turns out to be the strongest absorber in the atmospheric window. The dispersion of n′ is also very pronounced near 9 μm and 20 μm. Volcanic and coal-fire dust also absorb in the window region stronger than most of our earlier-investigated more normal aerosol fractions. The data for ammonium sulfate are compared with Remsberg’s measurements on an aqueous solution.

Optical Constants of Far Infrared Materials. 2: Crystalline Solids

Abstract: The far infrared optical constants of four crystalline materials at room temperature and at 1.5 K are reported. The materials are crystal quartz, sapphire, germanium, and silicon. The first two of these are birefringent (uniaxial) and both sets of optical constants are reported. The measurements extend over the range from 30 cm−1 to 350 cm−1.

Absorption coefficient of atmospheric aerosol: a method for measurement.

Abstract: Recent models predict that the effect of atmospheric aerosol particles on global temperature depends in part on b(abs,)or on n(2), where b(abs) is the aerosol absorption coefficient, and n(2) is the imaginary part of the aerosol refractive index for radiation in visible wavelengths Satisfactory techniques for measuring b(abs) have yet to be developed A method for measurement of b(abs) is described and discussed The apparatus uses a piece of opal glass to integrate light scattered by particles collected on Nuclepore filters It is quite mobile, inexpensive, and can be calibrated Analysis of the errors inherent in the measurement shows it is accurate to well within an order of magnitude for particles of radius larger than 01 microm

New thermooptical measurement method and a comparison with other methods

Abstract: A simple and sensitive method of measuring the thermally induced index changes arising from absorption of a laser beam in a low-loss material is described. The sample is placed outside the laser cavity, but at the position of minimum radius of curvature of the wavefront, which is a confocal distance behind the waist. It is estimated that the method is sensitive enough to measure absorption coefficients of the order of 5 x 10(_6) cm(_1) and it is shown experimentally to have good accuracy on low-loss materials. A detailed comparison of sensitivity and accuracy estimates is given for the various published thermal-lens methods for measuring low absorption coefficients.Although the method is illustrated for the thermal effect, it is applicable to other nonlinear index changes induced by laser beams.

Matrix operator theory of radiative transfer. 1: rayleigh scattering.

Abstract: An entirely rigorous method for the solution of the equations for radiative transfer based on the matrix operator theory is reviewed. The advantages of the present method are: (1) all orders of the reflection and transmission matrices are calculated at once; (2) layers of any thickness may be combined, so that a realistic model of the atmosphere can be developed from any arbitrary number of layers, each with different properties and thicknesses; (3) calculations can readily be made for large optical depths and with highly anisotropic phase functions; (4) results are obtained for any desired value of the surface albedo including the value unity and for a large number of polar and azimuthal angles; (5) all fundamental equations can be interpreted immediately in terms of the physical interactions appropriate to the problem; and (6) both upward and downward radiance can be calculated at interior points from relatively simple expressions.

Two-wavelength interferometry.

Abstract: Previous methods of two-wavelength variable-sensitivity interferometry are reviewed and a simplified two-wavelength technique for interferometric testing under static conditions is discussed. An extension of the static technique to real-time dynamic testing is proposed and an operational variable-sensitivity interferometer utilizing the real-time technique is described.

Optical waveguides formed by thermal migration of ions in glass.

Abstract: We present our results on optical waveguides formed by thermal diffusion of ions in glass. It was found that the peak of the ion-exchanged region can be shifted into the substrate interior by limiting the diffusion process. We also found that low loss films (<0.1 dB/cm) can be fabricated using this process and that the modal losses in these films do not agree with those losses predicted by existing theories. Also, the ion-exchange process has proved to be a simple means for fabricating tapered-edge couplers.

Evaluation of Optical Radiation Hazards

Abstract: Methodology for the evaluation of potential optical radiation hazards has been developed in response to the increasing use of high radiance optical sources, such as lasers, compact arc lamps, tungsten-halogen lamps, and electronic flash lamps. Recent biological investigations of injury from ultraviolet radiation and studies of chorioretinal injury from high radiance sources permit a realistic ocular hazard evaluation. Safe exposure criteria that may be readily applied to practical situations have been developed from the available biological data and from experience with occupational hazards. Hazard evaluation techniques, hazard data, and general control measures are provided for a number of commonly encountered ultraviolet, visible, and infrared sources.

Method for Computing Kinoforms that Reduces Image Reconstruction Error.

Abstract: An analysis of kinoform image reconstruction error is presented. This analysis considers the effects of the error introduced by the kinoform approximation and the quantization effects of plotting. The error measure developed is applied to a proposed method for computing kinoforms. Numerical results indicate that images produced by this method offer considerable reduction in the error when compared with images produced from kinoforms made with the random phase method.

Speckle Reduction Using Multiple Tones of Illumination

Abstract: The occurrence and smoothing of speckle are studied as a function of the line width for a highly collimated illuminating source. A general theory is presented for speckling in the image of a partially diffuse, phase type of object, which has a variable number of random scattering centers per resolution element. Then, an expression is derived for the wavelength spacing required to decouple the speckle patterns arising from two monochromatic tones in an imaging system, thereby establishing that it is feasible to smooth speckle using multicolor illumination. This theory is verified in a series of experiments using both laser illumination and band-limited light from a carbon arc. With highly collimated sources, we show that speckle appears laserlike for an imaged diffuser even up to line widths of 5 A. Then, smoothing of speckle is demonstrated in the imaging of a diffuser and for a section of an optic nerve when the illumination is provided by six narrow lines spread over 1500 A. Since with color-blind, panchromatic viewing the speckle smooths, a direct extension of this method to holographic microscopy, using a multitone laser, should permit one to record and reconstruct holograms of diffraction-limited resolution that are essentially speckle-free.

Reconstruction of three-dimensional refractive index fields from multidirectional interferometric data.

Abstract: Numerical and analytical techniques are presented that allow three-dimensional, asymmetric, refractive index fields to be reconstructed from optical pathlength measurements, which can be obtained using multidirectional holographic interferometry. Analytical reconstruction techniques that have been used in radioaptronomy and electron microscopy for a number of years, and recently in interferometry, are presented in the context of interferometric applications in the refractionless limit. These techniques require that optical pathlength data be collected over a 180 degrees angle of view. The required pathlength sampling rate is discussed. An efficient numerical procedure is developed for direct inversion of the data. Several numerical techniques are developed that do not require that data be collected over a full 180 degrees angle of view. All such techniques require redundant data to achieve accurate reconstructions. The required degree of redundancy increases as the angle of view decreases. Numerical simulations using six different reconstruction techniques indicate that with a 180 degrees angle of view, all are capable of providing accurate reconstructions. Four of the techniques were used to analyze simulated interferometric data recorded over an angle of view of less than 180 degrees . Examples of reasonably accurate reconstructions using data with angles of view as low as 45 degrees are presented.

Computer simulation of light pulse propagation for communication through thick clouds.

Abstract: This paper reports computer simulations of light pulse propagation through clouds. The amount and distribution of multipath time spreading was found to be independent of the detailed shape of the scattering function for sufficiently thick clouds. Moreover, the amount of multipath spreading for many scattering functions and cloud thicknesses can be predicted from a common set of data. Spatial spreading of the exit-spot diameter was found to saturate as a cloud of a given physical thickness became optically thicker and thicker. We observed that the propagation parameters for sufficiently thin clouds were dependent both on the cloud parameters and on the scattering function.

Characteristics of Optical Guided Modes in Lossy Waveguides

Abstract: A number of analysis techniques aimed at determining the characteristics of optical guided waves propagating in lossy structures are examined. The exact theory is used as a guide to assess the validity of several approximate methods based on two basic approaches: (a) geometrical optics and (b) perturbation calculations. The limitations of the conventional perturbation techniques are specified. We present a generalized procedure that permits an accurate description of metal boundaries at optical frequencies. In this case, TM modes differ from their TE counterparts by a field buildup near conducting walls and by the existence of an additional surface plasma mode. The dependence of attenuation coefficients on film thickness and mode order are discussed. The use of low-index dielectric buffers to reduce ohmic losses is considered. It is found that, with increasing buffer thickness, TM(N) modes undergo a continuous transformation to become TM(N+1).

The Optical Design of the 40-in. Telescope and of the Irenee DuPont Telescope at Las Campanas Observatory, Chile.

Abstract: The optical specifications of two astronomical telescopes designed to permit wide-field photography with critical definition are presented and compared. The 40-in. (1.016-m) and DuPont 100-in. (2.54-m) telescopes use modifications of the Ritchey-Chretien design with Gascoigne correctors. By avoiding the need for field flatteners through choice of mirrors yielding zero Petzval sum (40-in. telescope) or bending of photographic plates to a moderate field curvature (100-in. telescope), it was possible to achieve monochromatic images of ⅕ to ¼ sec of arc over fields of 3° and 2.1°, respectively.

Optical Absorption in Glass: Investigation Using an Acoustic Technique

Abstract: Experimental studies have indicated that chopped light entering a high pressure gas cell may produce sound in the gas even though the gas is not ir absorbing. Investigation into the source of this sound showed it to be due to interaction of the incident light with the cell window. The dependence of the sound pressure amplitude on cell pressure p0 indicates a p0(1/2) dependence, implying that the responsible mechanism is a diffusion process. Application of theory is successful in explaining both the quantitative and qualitative aspects of the data if the existence of an optically absorbing surface layer is assumed. The results obtained here are in general consistent with data obtained from studies of laser damage in glasses at high intensities.

Double Frequency Grating Lateral Shear Interferometer

Abstract: A simple grating lateral shear interferometer is described that can be made to give simultaneously two interferograms having shear in two orthogonal directions. The shear for the two orthogonal directions is produced in one plane by one double frequency crossed diffraction grating that can easily be produced holographically. Translating the grating sideways causes the irradiance of the interferogram to vary sinusoidally with time enabling the interferometer to be used with real-time heterodyne phase detection.

Laser induced surface damage.

Abstract: A summary of recent investigations of surface damage of transparent dielectrics is presented. Damage threshold measurements made on Owens-Illinois ED-2 laser glass at normal incidence and at Brewster’s angle are reported. For 30-nsec pulses at normal incidence, exit surface damage thresholds are typically 100 J/cm2 for ED-2 glass. The observed ratio between entrance and exit damage thresholds for the two geometries can be explained by considering the electric field strengths at the surfaces and including interference between incident and reflected light waves. A similar analysis is applied to surface damage that occurs during total internal reflection. Finally the morphology of surface damage of ED-2 laser glass is described, and a model based upon reflections from the laser induced plasma is proposed to explain the observations.

Light ray tracing through a leaf cross section.

Abstract: A light ray, incident at about 5 deg to the normal, is geometrically plotted through the drawing of the cross section of a soybean leaf using Fresnel's equations and Snell's law. The optical mediums of the leaf considered for ray tracing are: air, cell sap, chloroplast, and cell wall. The ray is also drawn through the same leaf cross section with cell wall and air as the only optical mediums. The values of the reflection and transmission found from the ray tracing tests agree closely with the experimental results obtained using a Beckman Dk-2A Spectroreflector.

Calculation of Modulation Transfer Functions of X-Ray Fluorescent Screens

Abstract: Formulas have been derived for calculating the modulation transfer function of x-ray intensifying screens. Emphasis has been placed on simplified models where results can be obtained in closed form and a physical understanding more easily realized. For selected cases, the MTF’s of transparent screens are calculated by direct integration. The MTF’s of diffusing screens are determined from suitable approximate solutions of the Boltzmann equation. The intermediate case and the transition from scattering to nonscattering are discussed. As a byproduct, formulas for light output and signal-to-noise ratio are obtained.

Fresnel zone plate imaging of gamma rays; theory

Abstract: The use of a Fresnel zone plate as a coded aperture for imaging incoherent radiation such as gamma rays has been previously reported. The coded image is in many respects similar to a hologram and can be decoded or reconstructed with a coherent optical system. In this paper, the general theory of coded-aperture imaging is presented, first for an arbitrary code and then for an on-axis zone plate, an off-axis zone plate, and a one-dimensional zone plate (or linear chirp). With the on-axis plate, a matched imaging condition is suggested as a guide to optimizing image contrast. With the off-axis zone plate and the linear chirp, it is necessary to use a half-tone screen to spatially heterodyne the object spectrum into the passband of the aperture. In all three cases, expressions for the resolution, depth of field, field of view, and relative efficiency are derived. A simplified noise analysis is presented, and some practical system constraints are discussed.

Polarization properties of lidar backscattering from clouds.

Abstract: The polarization properties of the backscattering of a lidar pulse from atmospheric clouds have been measured. A linearly polarized signal from a ruby laser at 694.3 nm is transmitted, and the scattering is simultaneously measured on a three-channel receiver that has polarizers oriented parallel, perpendicular, and at 45 degrees to the transmitted polarization. Substantial depolarizations (up to 0.5) are observed, and from the spatial variation of this depolarization the contribution of multiple scattering can be deduced.A wide variation in polarization properties is observed in different cloud types, and the results indicate that polarization signatures could be useful for cloud characterization and classification.

Gaussian light beams in inhomogeneous media.

Abstract: Vector wave solutions are obtained for the propagation of beams of light in media having slow spatial variations of the gain, loss, or index of refraction. The formalism developed here is applicable to a wide range of problems, and an example considered in detail is the propagation of off-axis beams in lenslike laser materials and optical waveguides. A procedure is also described for the diagnosis of localized dielectric inhomogeneities such as plasmas by means of Gaussian laser beams.

Loss Measurements in Thin-Film Optical Waveguides

Abstract: An apparatus for the measurement of attenuation of light in thin-film waveguides is described. It involves a prism that rides on a liquid film along the guide and couples the light out of the guide. A spatial resolution of ~0.5 mm and an accuracy of 0.02 dB/cm were achieved.

Pollutant Detection by Absorption Using Mie Scattering and Topographic Targets as Retroreflectors

Abstract: Remote pollutant measurement by absorption using topographical reflectors or atmospheric Mie scattering as a distributed reflector offers increased range and sensitivity compared to that achieved by Raman or resonance backscattering methods. The use of topographical reflectors offers the advantage of a single-ended absorption measurement for ranges up to 10 km and sensitivities to less than 0.01 ppm for a 10-mJ, 100-nsec transmitted pulse. The distributed Mie reflector permits absorption measurements over a depth cτ/2, determined by the pulse length τ, and allows ranging by time-of-flight measurement. For a 100-mJ, 100-nsec pulse sensitivities to 0.3 ppm at a 15-m depth resolution to ranges of 1–4 km are possible. This sensitivity is 104 to 105 times better than that achieved by the Raman method.

Description, performance, and wavelengths of iodine stabilized lasers.

Abstract: A description is given of lasers stabilized to components of the 129I2 spectrum in the region of the 633-nm laser lines for 3He–20Ne and 3He–22Ne. Relationships between operational characteristics such as power output, peak size, and peak width are shown, along with their relationships to some of the controllable parameters such as excitation level, iodine absorption, and iodine pressure. We found an iodine pressure broadening of about 13 MHz/Torr with a 2.6-MHz zero-pressure intercept. The frequency shift associated with iodine pressure is roughly 2 × 10−9ν/Torr to the red. Power broadening and power shifts are small, about a 10% increase in width and about 2 × 10−11ν variation in frequency for a fivefold to sixfold increase in power. These lasers exhibit a frequency stability for 10-sec sampling time of about 2 × 10−12ν and a resetability of about 1 × 10−10ν. The absolute vacuum wavelength for one iodine component has been measured against the 86Kr standard—3He–20Ne:129I2, k λ = 632 991.2670 A 0.0009 pm. The wavelengths of several other iodine components have been determined by measuring the frequency difference between them and the 129I2, k component. Among these are 3He–22Ne:129I2, B λ = 632 990.0742 ± 0.0009 pm: and 3He–20Ne:127I2, i λ = 632 991.3954 ± 0.0009 pm. These results were obtained using the Rowley-Hamon model for asymmetry in the krypton line and assume that the defined value for the standard is associated with the center of gravity of the line profile. The indicated uncertainties are statistical. No allowance has been included for imperfect realization of the krypton standard or for uncertainty in the asymmetry model.

Laser Induced Damage of Optical Elements-a Status Report.

Abstract: In the application of high-power lasers, damage to active laser materials, and to components of laser system, generally determines the limit of useful laser performance. Accordingly, there is great interest in reducing the susceptibility of optical elements ot damage. Damage in transparent dielectrics arises from three major causes, particulate inclusions or microinhomogeneities in the material, self-focusing within the materials, and surface damage due to plasma formation. The state of understanding of these phenomena, and the thresholds observed, where they have been determined, will be discussed. Dependence on pulse length will also be considered. Although most of the research accomplished to date on laser damage has concentrated on Nd-glass, the advent of very high-powered gas lasers has stimulated interest in the development of damage resistant component materials for use in the ir. Crystalline dielectrics appear to be the most likely candidate materials for ir windows. Nonlinear optics materials are particularly susceptible to damage, since they are generally exposed to high intensity radiation. Asa final item, damage in thin film dielectric coatings are considered.