bluuebunny/crossref_metadata_2025 huggingface dataset:
Generation of infrared, tunable picosecond pulses by a short-cavity dye laser
Authors: Bohnert K. , Boggess Thomas F. , Mansour Kamjou , Smirl Arthur L.
Published: February 2023

Infrared tunable picosecond pulses with a narrow spectral width are of considerable interest for the investigation of fast phenomena, e.g., in semiconductor materials with band gaps at infrared wavelengths. We describe the generation of such pulses by a short cavity dye laser (SCDL).1 The SCDL system consists of a short Fabry-Perot cavity completely filled with dye and a three-stage amplifier. The cavity length is chosen such that only one or two axial modes are lasing within the emission range of the dye. Wavelength tuning is achieved by tuning the cavity length. An optoelectronic feedback loop provides wavelength stability. The cavity and amplifiers are pumped by the second harmonic of an actively/passively mode-locked Nd:YAG laser. The emission of the SCDL is measured temporally, spectrally, and spatially resolved for various cavity lengths, dye concentrations, and pump energies using the infrared dye LDS 820. Pumping with 35-ps (FWHM) pulses, the temporal width of the dye laser pulses ranges between 7- and 18-ps (FWHM) depending on the experimental conditions. The spectral width lies between 3 and 6 A and is determined by the relatively low cavity finesse used in the present experiments. An almost Gaussian spatial beam profile is achieved.
Nonlinear effects in Bragg reflection holograms
Authors: Jannson J. , Jannson T. , Yu H.
Published: February 2023

Superhigh-efficiency (SHE) holographic technology1 can offer high reflectivities up to 99.9% (in Lippmann geometry), with regulated bandwidth and Bragg wavelength with near-IR, UV and visible spectrum. Those degrees of freedom are due to very flexible DCG processing with regulated shrinkage/swelling nonuniformity that can be analyzed by using the WKB method.2 The additional degrees of freedom, unique for holographic technology, are related to the possibility of controlling refractive-index modulation amplitude and profile. In the first case, it is possible to regulate refractive-index modulation from 0.001 to at least 0.1, while in the second case, the refractive-index modulation profile can be adjusted from sinusoidal to quasi-rectangular. The latter effect can be monitored by spectroscopic measurement of second-harmonic line reflectivity and by reflectivities of the next harmonics. In particular, by using a specific DCG processing method, the reflectivity of the second harmonic can be tuned from zero-level (equivalent to perfectly sinusoidal refractive-index modulation) to ~95 % efficiency. The possible applications of this effect are discussed, especially in the case of Lippmann holographic window films.
Image restoration with a minimum-error minimum-correlation estimator
Authors: Song Woo-Jin , Pearlman William A.
Published: February 2023

Current image estimators have the property that the estimation error or noise has positive correlation with the true object. In principle, therefore, the estimation noise can be further processed to extract more information about the object. The estimators therefore fall short of the goal of extracting as much information as possible about the object. To remove this deficiency, a new estimator is proposed which attempts simultaneously to maximize the correlation between the estimate and the object (or minimize the correlation between the error and object) and minimize the mean-square error. It turns out that this is an optimization problem with competing objectives. The solution is a compromise between the usual minimum mean-square error (Wiener) estimator (which produces an error uncorrelated with the image) and an inverse filter. The resulting new estimator (which is called MEMO for minimum-error minimum-correlation) is applied to images degraded by linear space-invariant blur and additive noise. Simulations are undertaken with different images having different extents of blur and degrees of noise degradation, where object estimates are obtained through the new MEMO estimator and, for comparison, the Wiener estimator. The MEMC estimator produced sharper and clearer restorations, especially at the lower signal-to-noise ratios.
Influence of optical fiber refractive-index profile central dip on sensor performance
Authors: May R. G. , Claus R. O. , Lee S. C. , Bennett K. D.
Published: February 2023

Improvements in graded-index multimode optical fiber sensor performance caused by an on-axis dip in the parabolic index of refraction profile of the fiber are considered. The effects of such a dip on fiber multimode propagation are first reviewed.1 Effects on both the modulation mechanisms and the resulting sensitivities of intrinsic bend loss and extrinsic coupling sensors are then analyzed. Experimental measurements of both of these observables obtained using specimens having two different degrees of such a central refractive-index profile dip are compared with this analysis. Analytical and experimental results indicate that sensor sensitivities are increased by ~10% for the specific fiber profiles considered. It is shown that by further increasing the severity of the central dip from the nominal parabolic index profile, an additional sensitivity increase of 20% is attainable.
Microcomputer program for schematic eye construction
Authors: Coile D. Caroline , O’Keefe Lawrence P.
Published: February 2023

Schematic and reduced eyes are useful models for analyzing optical components and predicting refractive states of living eyes. However, the tedious calculations necessary for their construction makes its difficult to substitute various parameters, thus limiting their practicality. We demonstrate a basic program suitable for IBM PC compatible computers that simplifies the task of constructing schematic and reduced eyes. Given input of intraocular distances, refractive indices, and radii of curvature, the computer performs the necessary calculations and displays the resulting dimensions in tabular and graphic form. The output includes the cardinal points and refractive powers of the cornea, lens, and whole eye, as well as the refractive state of the eye. Our program allows easy substitution of single parameters for comparison of the effects of individual components on the refractive state of the eye, and allows the user to specify a homogeneous or nonhomogeneous lens model. It also supports the Hewlett-Packard 7470 graphics plotter, providing publication quality drawings of the schematic eye. Calculations obtained using this program compare favorably with published schematic eyes. Comparisons with several established schematic eyes are presented, and the source code for the program is available.
Temporal asynchrony discrimination in the amblyopic fovea
Authors: Steinman Scott B. , Levi Dennis M. , McKee Suzanne P.
Published: February 2023

The human visual system is capable of discriminating incremental changes as small as 3-4 ms in the time of onset of two lines presented foveally with a temporal asynchrony of 30-40 ms.1 We have measured foveal temporal asynchrony discrimination in amblyopic observers. The amblyopic fovea, like the normal peripheral retina, exhibits elevated temporal discrimination thresholds relative to the normal fovea when short (10-60-ms) asynchronies are presented. However, the amblyopic fovea is less sensitive to temporal asynchronies than is the normal peripheral retina (20° eccentricity). In addition, the temporal deficit of the amblyopic fovea is not a consequence of reduced spatial resolution. These findings suggest that the amblyopic fovea is not comparable to the normal peripheral retina in all respects. The amblyopic fovea is limited not only in its spatial resolution but also in its ability to discriminate small temporal changes.
Context effects in magnitude estimation
Authors: Van Orden Karl , Sturr Joseph F. , Taub Harvey A.
Published: February 2023

We wished to identify the experimental conditions that might yield a steeper brightness function than had been previously reported by other investigators for briefly presented flashes. White circular stimuli of 2° or 4° and 10, 30, 100 or 1000 ms were flashed in the center of four red fixation lights. Eighty-four subjects (17-26 years) were individually tested and introduced to magnitude estimation by scaling the lengths of line stimuli, after which they were dark adapted. Then each subject made brightness judgments of the stimuli which ranged in luminance from 0.216 to 216.2 cd/m2 in 0.3 log unit steps. No modulus was used. When presented alone, flashes of 10 or 30 ms yielded slopes similar to those obtained with 1000-ms stimuli. However, when stimulus durations of 10, 100, and 1000 ms were randomly presented, a significantly steeper slope resulted for the 10-ms flashes. The steeper brightness functions found for brief flashes in this and previous experiments may be contexually based, dependent on the presence of longer stimuli.
Effect of optic nerve section on retinal light damage
Authors: Bush Ronald A. , Williams Theodore P.
Published: February 2023

Sprague-Dawley rats, age 20-24 weeks, were obtained from Zivic-Miller Laboratories with unilateral intracranial optic nerve sections which did not interrupt the blood flow or any other innervation to the eye. They were then kept in uniform illumination of 1000 lx continuously for 24 h. The outer nuclear layer (ONL) areas of the two eyes in the same animal were compared five days postexposure. Light of this intensity causes damage primarily to the photoreceptors in the retina of the albino rat. Since the ONL of the retina contains photoreceptor cell bodies, reduction in the area of this layer is proportional to the number of cells lost due to the bright light exposure. Although the ONL area was reduced in both eyes, the retina with the intact optic nerve had an ONL area which averaged ~25 % less than that in the other eye with the optic nerve cut. Controls with unilateral optic nerve sections performed in the same manner but not exposed to damaging light showed no difference in ONL area between the two eyes. A second experiment using pigmented rats gave similar results. These results suggest a role for the brain in retinal light damage.
Fundus camera stimulator
Authors: Johnson Mary A. , Sunness Janet S. , Massof Robert W. , Kays David L.
Published: February 2023

We have developed a device for projecting psychophysical stimuli onto precise retinal locations while the fundus is being viewed. The device consists of a fundus camera, modified by removing the ocular and 35-mm camera mount and replacing them with an infrared-sensitive video camera and a stimulus port. Any stimulus configuration can be used, as long as the stimulus image plane is conjugate with the retina (e.g., in the plane of the 35-mm film). Light for viewing the fundus is reduced to scotopic levels by insertion of a red cutoff filter in front of the background light. The image of the fundus is intensified using the video camera, whose output is displayed on a high-resolution video monitor. The x,y position of the stimulus is recorded using linear potentiometers and is marked by cursor on the image of the fundus displayed on the monitor. Each experimental session is videotaped. Because the deep-red viewing light reduces the resolution of fundus features, we obtain a fundus photo of each patient using standard clinical techniques, and register the fundus photo with the fundus image from one frame of a videotape obtained prior to testing, using the IS-2000 image processing system (PAR Microsystems, Inc.). In this manner, retinal locations corresponding to experimentally pertinent fundus features can be identified and electronically marked on the video monitor for later testing. If necessary, the image enhancement capabilities of the IS-2000 can be used to delineate low-contrast fundus features. Results from psychophysical measurements on clinical patients using the fundus camera stimulator are described.
Dichroic beam splitter design for parallel photothermal/photovoltaic solar energy conversion systems
Authors: Jacobson M. R. , DeSandre L. F. , Osborn D. E. , Macleod H. A.
Published: February 2023

The efficiency of hybrid photothermal/photovoltaic (PT/PV) energy conversion can be increased by separating the solar spectrum into portions matched to the PT and PV processes. Thin-film multilayer filters can implement this concept; five such filters consisting of all-dielectric or metal-dielectric layers have been designed. Three of the five are based on conventional symmetric period, induced transmission, and metal-dielectric filter designs; they transmit the most effective wavelengths, 0.68–0.95 μm, for PV conversion based on silicon cells, and reflect the remainder to absorbers in a PT loop. The other two designs, a simple high reflectance filter and an optical minus filter, perform the inverse function, reflecting the optimum PV photons and transmitting the balance. The transmission profile of each design has been computed, considering dispersion, absorption, and angle of incidence effects. These profiles are compared and evaluated with respect to the desired spectral performance. The most successful candidate design is an optical minus filter consisting of films of TiO2, ZrO2, and SiO2. Results show very sharp selection of the targeted PV spectral region and low ripple in the transmission region outside the bandstop. The main drawback to this design is its sensitivity to high incident angles; half of the bandstop moves out of the prime PV band at a 30° angle.
Spatial profiling of a laser beam using diffraction by a scanning knife-edge
Authors: Langlois P. , Lessard R. A.
Published: February 2023

A laser beam illuminating a straightedge at normal incidence produces on an observation screen a bright spot and a luminous line perpendicular to the edge. In the diffraction edge wave context, the bright spot corresponds to the geometrical wave, while the luminous line belongs to the edge wave, and diffraction fringes appear where these two waves overlap. Takenaka and Fukumitsu1 gave an analytical expression for the diffraction edge wave produced by a Gaussian beam incident on a half-plane. According to these results the edge wave is a cylindrico-Gaussian wave having its origin on the edge of the half-plane. The interesting point is that this edge wave has an intensity proportional to the intensity of the laser beam at the edge location. So, by measuring the intensity of the edge wave produced by a scanning blade it is an easy matter to determine, in real time, the spatial profile of a laser beam. The main advantage of this method is that it does not require any inversion procedure for the knife-edge data as is the case when one monitors the direct beam.2 In fact, our light detector is positioned completely outside the direct beam, thus allowing only the edge wave to come in. As far as we know, this new method, based on diffraction, is the first one which allows direct profiling of a micron-sized beam.
Psychological implications of parallel systems
Authors: Anderson James A.
Published: February 2023

A number of parallel information processing systems have been proposed which are loosely based on the architecture of the nervous system. I describe a simple model of this kind. Many of the major testable predictions of these systems fall in the realm of cognitive science: parallel, distributed, associative systems seem to have pronounced psychologies. They perform some computations well and some very poorly. It is a psychological question as to whether humans show the same pattern of errors and capabilities. I briefly describe the theory behind the models and discuss some of the major psychological predictions they generate. I present the results of large simulations of them. Specifically, I discuss psychological concept formation including formation of prototypes, the generation of semantic networks by distributed systems, and the use of the systems as distributed data bases and as simple-minded expert systems.
X-ray transmission grating calibrations
Authors: Stradling G. L. , Chmielewski J.
Published: February 2023

We have made absolute diffraction efficiency measurements of freestanding x-ray transmission gratings. The grating periods are 2000 and 3000 Å with a nominal line-to-space ratio of 1:1 and thickness from 4000 to 6000 Å. Although theoretical calculations of ideal grating performance are well established, actual physical parameters of these microstructures are not readily accessible. Accurate calibration of actual performance is critically needed to allow unfolding of multicomponent x-ray spectral data as the various dispersion orders overlap. Monochromatic line emission at C Kα, Cu Lα, and Al Kα were used. Diffraction lobes up to eighth order were observed. Significant changes in the diffraction efficiency of individual gratings occurred as the radiation wavelength was varied as a result of phase change of partially transmitted radiation. Some structural defects also caused energy dependent changes. The measurement apparatus consisted of a changeable anode Henke line source, Soller slit collimation, and an energy discriminating position sensitive proportional counter as a detector.
Soft x-ray interferometer for the measurement of material optical constants
Authors: Rockett Paul D. , Anderson Eric
Published: February 2023

We have designed a triple grating soft x-ray interferometer, which will directly measure material optical constants over the 1-10-nm range. This instrument uses a forward diffracting geometry, similar to that used in monolithic-crystal Laue x-ray interferometers. Three photolithographically produced gratings act as beam splitter, beam recombiner, and beam analyzer to measure phase shifts induced by a test foil. Each freestanding grating has a 200-nm linewidth and spacing and is 500 nm thick, fabricated in gold. In lieu of an x-ray laser, existing x-ray sources provide limited spatial and temporal coherence. To achieve high contrast fringes serious constraints must be placed both on the interferometer design and on the use of the x-ray source. Grating interferometers are notoriously insensitive to incomplete spatial coherence. In our design, a 100-μm wide source in the diffraction plane is sufficiently small to minimize grating defocus concerns and achieve high fringe visibility. Adequate temporal coherence is assured by limiting the source bandwidth to λ/Δλ = 1000 with an extended range grasshopper monochromator and a synchrotron source. Grating flatness, which could affect path length matching, is shown from existing gratings to be fully adequate.
Nonsingularity of asymmetric color matches: implications for color theory
Authors: Krantz David H.
Published: February 2023

Central mechanisms of color are defined to be the central neural processes that are equated in asymmetric color matching. For normal trichromats, asymmetric color matches are nonsingular: given any match, all sufficiently small perturbations of one of the matched colors can be matched by perturbations of the other. This fact, together with the other laws of color matching, yields a rigorous representation of normal color percepts as a 3-D manifold, and also allows the deduction that all effects of adaptation and contrast can be understood as effects on exactly three independent central mechanisms. Thus, for normal color vision, the 3-D bottleneck imposed at the receptor level by cone pigments is matched to a 3-D bottleneck at the level of central mechanisms. An experiment that I conducted with M. Alpern and D. Kirk suggests that nonsingularity holds also for the asymmetric matches of protanopes and deuteranopes. If so, the 2-D receptor space of these color defectives is accompanied by restriction to two central dimensions. This is consistent with theories in which dichromacy involves loss of an opponent mechanism but conflicts with various findings suggesting that dichromats have at least three independent central color mechanisms.
Nonlinear pulse propagation at the zerodispersion wavelength of single-mode fibers
Authors: Agrawal Govind P. , Potasek M. J.
Published: February 2023

Nonlinear pulse propagation in single-mode fibers is studied numerically using the beam-propagation method. In particular, we consider wave propagation at the zero-dispersion wavelength. Depending on the relative importance of the nonlinear and the higher-order dispersive effects, both the pulse shape and the pulse spectrum show qualitatively different behavior and new features arise when the two effects are comparable. For typical system parameters, the nonlinear and dispersive contributions are comparable for a specific pulse width τ
c
~ 1 ps. When the pulses are broader than τc, nonlinearity dominates and leads to significant pulse broadening that would limit the performance of high-data-rate optical communication systems operating at the zero-dispersion wavelength.1 For pulses shorter than τc, dispersion dominates and is responsible for the pulse broadening. Interestingly enough, the nonlinearity can reduce the amount of broadening relative to that occurring in a linear dispersive medium.
Semiconductor diode laser pumping of Nd:LiYF4
Authors: Fan T.Y. , Byer Robert L.
Published: February 2023

Semiconductor diode laser pumping of solid-state lasers has been of much recent interest because of the potential for high efficiency and good frequency stability. We report diode laser pumping of Nd:LiYF4 (YLF). The output of this laser is naturally polarized as opposed to Nd:YAG and can provide larger amplified spontaneous emission limited single-pass gains in amplifiers than Nd:YAG. In addition, it can serve as a master oscillator to injection lock for silicate or phosphate glass systems. Oscillation was demonstrated in a monolithic device. For end pumping near 791 nm with a single stripe diode laser, threshold was 860 µW, and the slope efficiency was 15% with high reflecting and 99.7% reflecting cavity mirrors. At low pump powers, only the high gain polarization oscillated at 1.047 µm in a TEM00 mode, but as the pump power was increased, the orthogonal polarization also oscillated at 1.053 µm output and increased the slope efficiency by increasing the output coupling. Further results are presented.
Allowed modes in anisotropic slab waveguides
Authors: Knoesen A. , Moharam M. G. , Gaylord T. K.
Published: February 2023

Dielectric slab waveguides are needed in the construction of essentially all integrated optics devices. In practical integrated optical circuits, the direction of propagation must change over the substrate and thus it is essential to understand the allowed modes that can propagate for a particular direction. Rigorous electromagnetic theory is applied to determine the allowed modes (both sustained modes and leaky modes) in a uniaxial crystalline waveguide such as lithium niobate. For an arbitrary direction of propagation, the directions of power flow and phase propagation differ. The sustained modes in this case may be classified as TE-like modes and TM-like modes with coupling occurring between them. For the same waveguide parameters, it is shown that a simple change of propagation direction can cause multiple allowed modes to become cut off. The conditions for cutoff are discussed. Integrated optical polarizing devices based on anisotropic cover layers are discussed.
Measurements of the nonlinear susceptibility of nematogens in the isotropic phase
Authors: Scott A. M. , Saunders F. C. , Madden P. A.
Published: February 2023

It is known that nematogenic materials such as MBBA and 5CB can have a very large nonlinear refractive index in the isotropic phase due to pretransitional local ordering. However there does not appear to have been any systematic experimental study of the factors which affect the size of the nonlinearity for different molecular structures. In this paper we report on experiments in which we have used degenerate four-wave mixing to measure the magnitude of the nonlinear susceptibility and its time constant for a number of different materials in various conditions.
Transient analysis of mode locking including mirror
Authors: Haas D. , Mclean J. , Gustafson T. K. , Tang C. L.
Published: February 2023

The Kuizenga-Siegman-Haus1 theory of mode locking is extended through three simple ordinary differential equations to treat the full transient evolution of three pulse parameters; the pulse width and chirp, amplitude and phase, and pulse position offset from the modulation peak, respectively. Application of the theory to the parametrically driven mode-locked semiconductor laser2 produces several important results. The transient buildup of the pulse from an infinite width to a final steady-state value is discussed neglecting mirror dispersion. The inclusion of facet (mirror) dispersion results in pulse widths either greater or less than the steady-state value, depending on the sign of the dispersion coefficient. This is quantitatively and intuitively understood in terms of an ABCD Gaussian pulse parameter analysis to trace the pulse width and chirp around the laser cavity. For mirrors which compress on reflection we show that the pulse widths in the cavity depend critically on the accuracy with which the mirror dispersion compensates the dispersive effects of the laser amplifying medium, a compensation error of ~10−4 limiting pulse widths to the 100-fs range for a typical semiconductor laser. Finally, we show that such compensation to shorten the pulses internally results in a threshold increase of the laser.
Band-gap engineering: from physics to optoelectronic functional devices
Authors: Capasso Federico
Published: February 2023

Band-gap engineering is a new powerful technique to design heterojunction devices. Using the rules of band-gap engineering new band diagrams with nearly arbitrary and continuous band gap variations can be constructed.1,2 This, along with the present capabilities of MBE and MOCVD growth techniques, has led to a new class of optoelectronic and electron devices and superlattice materials. These include among others multilayer avalanche photodiodes, staircase solid-state photomultipliers, phototransistors with graded gap base, and pseudoquaternary semiconductors. In addition, heterojunction band discontinuities can be artificially tuned using doping interface dipoles. This new technique has tremendous potential for novel semiconductor devices.3 Finally recent results on a new class of high detectivity, high gain, low voltage superlattice photoconductors (effective mass filters) are discussed. These devices are based on a newly discovered quantum-type photoconductivity. The high photoconductive gain is caused by the large difference between the tunneling rates of electrons and holes in a superlattice (effective mass filtering).
Microstructure electromagnetic effects
Authors: Brueck S. R. J.
Published: February 2023

Optical wavelength-scale surface microstructures, both periodic morphologies in planar geometries and higher-dimensional cavity morphologies, drastically modify material optical characteristics and lead to striking changes in the response to optical excitation. Examples are drawn from laser-material interactions and optoelectronic devices. Novel device structures, involving both thin film and heterostructure materials, are discussed.
Acoustooptic implementation of neural network models
Authors: Psaltis Demetri , Paek Eung Gl , Hong John , Farhat Nabil H.
Published: February 2023

An experimental optical vector–matrix multiplier is used for the implementation of an associative memory that is based on neural network models. An acoustooptic device in conjunction with a pulsed laser diode source is used as the input device through which an input vector is entered into the system. The interconnection matrix is stored in a computer-generated optical transparency and the product vector is detected on a CCD detector array. The output vector is serially read out, electronically processed (thresholded, for example), and then fed back to the acoustooptic device where it serves as the input for subsequent iterations. This relatively simple optical system can be used to simulate several variations of neural network models. This is done by selecting different matrices which simulate different interconnections among the neurons and also by selecting the electronic processing that is done in the feedback loop which simulates the action of the individual neurons. The storage capacity and the radius of attraction (the degree of similarity between the input and stored vectors that is required for associative recall) of such optical associative memories are characterized in detail.
Short- and middle-wavelength cone contributions to red and blue hue sensations
Authors: Drum Bruce
Published: February 2023

The hue of a short-wavelength foveal increment on a concentric 5500° white background was measured as a function of background luminance. The test stimulus was 1 s in duration and 2° in diameter. The background was 7° in diameter and was presented continuously. For each background luminance tested, the luminance of the test spot was set 0.6 log unit above increment threshold, and a quantitative hue-naming procedure was used to determine its relative redness (or greenness) and blueness. At scotopic and low photopic background levels, the test spot appeared mainly blue, with a 10–40% red component. For the next 2–3 log units of the photopic range, the hue balance shifted sharply toward red. In some cases the relative red component reached 80%. At still higher photopic levels, however, this trend reversed dramatically. For most subjects, redness disappeared entirely, leaving a mixture of blue and green. The violet-to-cyan transition coincided with a pronounced steepening of the threshold vs background intensity (tvi) function, presumably caused by selective saturation of short-wavelength cone responses.1 Test spots that appeared mainly blue had clearly visible borders, whereas spots that appeared mainly red tended to appear spatially diffuse. These results reinforce previous evidence2 that short-wavelength cone activity produces a reddish purple hue and middle-wavelength cone activity produces a bluish cyan.
Test for a correlation between Vλ and the +y opponent spectral sensitivity
Authors: Ingling C. R. , Marti-Nez-Uriegas Eugenio
Published: February 2023

Rushton and Baker1 have shown that the ratio of R to G cones varies between observers. We use this variability to ask if observers who are rich, say, in R-cone input to the Vλ or flicker channel are also likely to be rich in R-cone input to the r-g and y-b opponent-color channels. Results for the r-g channel (eight observers): Although the R/G flicker ratio varied considerably across observers, the spectral position of unique yellow varied little and did not correlate with the R/G ratio. Conclusions are similar for the y-b channel; there is no correlation between the amounts of a B primary required to cancel yellowness from long- and middle-wavelength test lights and amounts of these lights required in a flicker match.
Thresholds of monkey LGN cells to chromatic spots on a white background
Authors: Lee B. B. , Crook J. M. , Valberg A.
Published: February 2023

Measurements of psychophysical thresholds to chromatic spots presented on a white background purport to demonstrate the existence of chromatic and luminance channels. We measured thresholds to such stimuli of the phasic, magnocellular (MCL) and the tonic, wavelength-opponent parvocellular (POL), cells of the macaque lateral geniculate nucleus. Spectral sensitivity of MCL cells approximated the V function. Wtih 4° spots, blue on-center and red on-center PCL cells were the most sensitive at the spectral ends, while in midspectrum MCL cells were somewhat more sensitive than the most sensitive PCL cells; with achromatic stimuli MCL cells were substantially more sensitive than PCL cells. With small spots, sensitivity of the different opponent cells decreased substantially at the spectral ends to become close to or slightly higher than that of MCL cells. Psychophysical thresholds with human subjects at similar retinal eccentricities with similar spot sizes could be accounted for by assuming the most sensitive cells were responsible for detection, supporting the notion that different cell classes support detection in different stimulus conditions.
Recovering 3-D structure from motion using positions vs velocities
Authors: Grzywacz Norberto M. , Hildreth Ellen C.
Published: February 2023

Perceptual studies suggest that the visual system uses the rigidity assumption to recover 3-D structure from motion. Computational studies show that the rigidity assumption allows a unique 3-D interpretation from motion information integrated over a small extent in space and time. Most algorithms for recovering structure, however, are sensitive to noise and only interpret rigid motions. Ullman1 proposed the “incremental rigidity scheme” that interprets rigid and nonrigid motions and develops a 3-D model over an extended time. As new views of points appear, a new model is computed that represents the most rigid transformation from the current model. We present a formulation of Ullman’s method that uses velocity information for recovering structure. Theoretical and computer analyses show that this continuous formulation provides a quick estimate of structure but is not robust over an extended time period. The stable long-term recovery of structure requires disparate views of moving objects. For rotation about a fixed axis, the rate of convergence and quality of the 3-D solution decreases with smaller angular displacements between frames. In the limit of the continuous formulation, the solution is no longer stable. Our analysis raises important questions for the human recovery of structure from motion.
Controlled continuity for visual reconstruction
Authors: Terzopoulos Demetri
Published: February 2023

III-posed reconstruction problems are the rule rather than the exception in early vision. Image reconstruction begins with a selection of (usually intensity based) 2-D data. In visual surface reconstruction, the raw data comprise local 3-D shape estimates. Generally, the initial information may be available from multiple sources and at multiple resolutions, but it is often sparse and corrupted by noise. The goal of reconstruction is to generate the best possible interpretation of the available visual information, making explicit the continuous regions as well as the locations of bounding discontinuities. Well-posed variational-principle formulations may be obtained systematically for this broad class of visual problems by applying a controlled continuity model. Although this deterministic model provides sufficient constraint to guarantee the existence of unique solutions, it remains generic and is nonrestrictive at discontinuities. It can be expressed mathematically as functional forms involving generalized spline kernels. The constituent kernels are combined with continuity control functions which can be determined optimally according to local criteria. The resulting variational principles may be locally discretized using piecewise, finite element techniques. Fine scale solutions to the reconstruction problems are efficiently computable by multiresolution hierarchies of local-support cooperative processes under concurrent coordination.
Calculation of the number of guided transverse surface acoustic modes for Integrated acoustooptical devices
Authors: Golan G. , Griffel G. , Ruschin S. , Seidman A. , Croitoru N.
Published: February 2023

The analogy between the electromagnetic wave equation and the wave equation for the propagation of acoustic waves is used to drive the cutoff lines and the number of guided transverse surface acoustic (Love) modes in stratified piezoelectric media. The combination of these two kinds of waves occurs in integrated acoustooptic devices where the upper layer is made by diffusion of metal atoms into a dielectric substrate. The efficiency of these types of device depends strongly on the overlapping integral of the light mode and the acoustic mode which propagate in the upper layer. In many applications it is important to design the structure parameters in such a way that only a single mode of each kind exists. The present theory describes simple-design tools which enable direct calculation of the proper thicknesses, indices of refraction, the piezoelectric constants of the structure, and the rf operating frequency range. A detailed computation is given for the cases of a two-layer and a buried layer structure.
Instabilities during laser melting of semiconductors
Authors: Luscombe J. , Preston J. , Sipe J. E. , Van Driel H. M.
Published: February 2023

We have studied, experimentally and theoretically, the liquid-solid morphologies1,2 induced by the interaction of plane-polarized cw 10.6-µm laser radiation with silicon on a sapphire substrate. The experiments were performed at normal incidence, with silicon thicknesses ranging from 0.5 to 2 µm. Near the melt threshold, the large difference in optical properties of liquid and solid semiconductor drives an instability breaking the translational symmetry of the surface. The initial morphology of the nonuniform melting consists of lamellae structures, e.g., microscopic solid islands in a liquid background, which then evolve into a steady-state periodic arrangement of liquid and solid with increasing laser intensity. The structures have been studied using real-time microscopy and optical diffraction techniques. A key phenomenon in the evolution and maintenance of these surface structures is interference shielding, whereby existing liquid regions electromagnetically shield surrounding regions of solid, hindering further local melting. There is a strong competition in this process between the orientation of induced structure and the polarization of the incident field. A simple theory which exploits an analogy with the Ising model of disorder—order phase transitions is discussed.
Ultrashort compression of amplified optical pulses
Authors: Halbout J. -M. , Palfrey S. L. , Grischkowsky D. L.
Published: February 2023

We have developed an amplifier for femtosecond optical pulses that is pumped by the frequency-doubled output of a mode-locked Nd:YAG laser Q-switched at 800 Hz. We have successfully used this type of system to amplify both 100-fs output pulses from a colliding pulse mode-locked ring dye laser and 200-fs pulses obtained by compression of the output of a synchronously pumped, cavity pumped dye laser. In both cases, synchronization of the YAG laser pulses with the pulses from the dye laser is achieved by driving its acoustooptic mode-locker with a rf signal phase-locked to the optical pulse train of the dye laser. A short-term jitter of <25 ps is observed with this technique. A two-stage configuration allows amplification up to the microjoule level. We also have successfully addressed the problem of amplification of compressed optical pulses. Our primary application for this system is pulse compression in an optical fiber. The excellent mode quality of the amplified beam allows a coupling efficiency into the 4-μm fiber core of over 60%. Furthermore the very good amplitude stability enables us to obtain the highest compression ratios ever achieved on the femtosecond time scale (110 fs to 12 fs and 200 fs to 16 fs with the two systems, respectively).
Two-scale solutions of the coherence equations governing propagation in random media
Authors: Beran Mark J.
Published: February 2023

We review the two-scale method of solution of the coherence equations which govern the propagation of radiation in random media. Principal attention is paid to the solution of the fourth-order coherence function which allows us to determine the scintillation index. We also mention, briefly, recent work on the sixth-order equation. Using the two-scale method, we consider solutions for the Kolmogorov spectrum and the Gaussian and exponential correlation functions. Most of these calculations have been for an initial plane wave or point source, but there has been some work for finite beams. In two dimensions, where numerical solutions of the partial differential equations are possible, two-scale solutions are shown to give excellent agreement with the calculations.
Numerical modeling of atmospheric scintillation
Authors: Brown Wilbur
Published: February 2023

Computational techniques for modeling atmospheric propagation in conditions of saturated scintillation are discussed. Results are presented for the propagation and adaptive compensation of laser beams over such paths.
Phenomenological probability density function of turbulence-induced intensity fluctuations
Authors: Churnside James H.
Published: February 2023

The statistics of the intensity fluctuations of light that has propagated through atmospheric turbulence are described as the product of an optical field that obeys Rice-Nakagami statistics and a lognormal modulation factor. The probability density function that results contains two parameters: the coherence parameter of the Rice-Nakagami field and the variance of the logarithm of the modulation factor. If it is assumed that turbulent scales less than the Fresnel zone size contribute to the loss of coherence of the wave and that turbulent scales greater than this size contribute to the random modulation, both parameters can be calculated from physical parameters such as turbulence strength, inner scale, optical wavelength, and path length. In very weak path-integrated turbulence, the probability density function described here is very nearly lognormal, in agreement with the data. A scaling constant in the parameter calculation is chosen so that the intensity variance in this limit is that predicted by the first-order Rytov theory. In very strong turbulence, this density asymptotically approaches a negative exponential, in agreement with a number of theoretical predictions.
Induced spectral broadening of a second harmonic pulse in ZnSe crystals generated by an intense ultrashort laser pulse
Authors: Alfano R. R. , Wang Q. Z. , Jimbo T. , Ho Ping-Pei
Published: February 2023

We report on the first observation of Induced spectral broadening (ISB) of a weak nonphase matching second harmonic pulse produced from the propagation of an intense primary picosecond laser pulse through ZnSe crystals. A mode-locked Nd:glass laser with a single 8-ps pulse at the 1060-nm wavelength was weakly focused Into the sample. Output signals were sent through a spectrograph to measure the spectral distribution. Measured spectra for nonphase matching SHG and ISB generated in ZnSe crystal by a 1060-nm laser pulse indicated the extent of the spectral broadening about the second harmonic line. When the Incident laser pulse energy was 2 mJ, there was spectral broadening of ~1,100 cm−1 on the Stokes side and 770 cm−1 on the anti-Stokes side. When pulse energy was <1 mJ, the ISB was found to be linearly dependent on the pulse energy of 1060 nm. The pulse duration of the ISB measured by a streak camera at 550 nm using a 10-nm narrowband filter was ~9 ps. The Pockels term of ZnSe is responsible for the sharp line ~100 cm−1 at the SHG frequency. This weak SHG pulse travels through the crystal together with the primary pulse. Due to the large n2 of ZnSe and the Intense incident 1060-nm laser, the spectral line of the weak 530-nm laser broadens. The ISB generated around the SHG 2w0 in ZnSe crystals is attributed to the induced phase modulation by an intense picosecond laser pulse at w0.
Ionic clusters
Authors: Kennedy Richard , Kung Chung-Yi , Foster Stephen , Miller Terry A.
Published: February 2023

Details of recent experiments involving charged clusters are described. In one experiment these clusters are prepared in a supersonic expansion and their laser-induced fluorescence spectra recorded. The clusters consist of a charged Chromophore and neutral species bound to it. Results obtained include spectral shifts, cluster vibrational progressions, and resulting potential surfaces.
High-resolution IR spectroscopy of ethylene dimers
Authors: Watts R. O. , Baldwin K. G. H.
Published: February 2023

A differentially pumped free-jet expansion, together with a waveguide CO2 laser, has been used to measure the Infrared predissociation spectrum of ethylene clusters near 950 cm−1. The CO2 laser has a pressure broadened tuning range of 240 MHz. We observe sharp transitions in association with many of the laser lines which activate the monomer C2H4 molecules in their v7 mode. By varying the concentration of C2H4 in a carrier gas, by changing the carrier gas, and by using both bolometric and mass spectrometric detection systems, we are able to confirm the findings of Snels et al.1 that there is fine structure in the ethylene dimer predissociation spectrum near 950 cm−1. The linewidths of all the observed features lie in the 15-20 MHz range, giving a lower limit on the predissociation lifetime of ~10 ns.
Diode laser-array pumped cw Nd:glass laser
Authors: Lee Daniel N. , Gamo Hideya
Published: February 2023

Recent advances in high power diode lasers provide an alternative optical source for pumping solid-state lasers. There are at least three distinctive advantages to using a semiconductor laser diode: (1) The ideal match of the absorption wave-length results in a high pumping efficiency. (2) It eliminates the cooling system which causes unstable power output. (3) It reduces the physical size of the system.
High-speed coherent optical fiber systems
Authors: Linke Richard A.
Published: February 2023

The best optical detection sensitivities and longest fiber transmission distances1-3 are now obtained by techniques which employ local oscillator lasers and optical mixers to convert an optical signal to radio frequencies for detection. Such systems (generally called coherent even when phase is not detected) have proved to be the most successful route to near-quantum limited sensitivities since they provide a means for overcoming the thermal noise which limits direct photon detection schemes. For example, in a transmission experiment at 400 Mbit/s a sensitivity of 45 photons per bit was obtained4 compared with the quantum limited value of 20. This corresponds to an improvement of a factor of 10 over direct detection at this rate.5
Rigorous analysis of birefringent networks
Authors: Weis R. S. , Gaylord Thomas K.
Published: February 2023

Tunable birefringent spectral filters are used in many applications which require narrow transmitted bandwidths with high optical power throughput. A rigorous electromagnetic analysis of bulk birefringent networks will provide the basis for a more complete understanding of these filters and may also lead to new optimal network architectures. A similar rigorous analysis of the folded Sole filter has been previously presented by Yeh.1 Analyses of other network architectures, particularly multiple-pass birefringent networks, and an explicit discussion of the multiple-beam interference effects are presented.
Liquid crystal spatial light modulator with amorphous silicon photoconductor
Authors: Ashley Paul R. , Davis Jack H.
Published: February 2023

Spatial light modulators play an important role in coherent signal processing applications, primarily as an incoherent-to-coherent light converter. In this and other applications the sensitivity and spatial resolution as well as speed are important considerations which limit performance. Reported here is a liquid crystal modulator using amorphous silicon as an ohmic photoconductor material to achieve linear modulation in a reflection mode. The thin-film layer is capable of very high resolution, >95 line pairs/mm, exceeding that of most CdS photoconductors while maintaining the high speed and broadband sensitivity of crystalline silicon. Spatial resolution of over 35 line pairs/mm has been demonstrated. The white light sensitivity is better than 20 μW/cm2. A nematic liquid crystal is incorporated using both twist and birefringence mechanisms. The photoconductor consists of boron-doped amorphous silicon deposited on a glass substrate which serves as one window of the cell.