bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoionization-pumped laser geometry
Authors: Sher Mark H. , Benerofe S. J. , Young J. F. , Harris S. E.
Published: February 2023

The recently reported saturation1 of the 109-nm Auger laser in xenon III2 utilized a newly developed, oblique incidence, traveling-wave laser-produced plasma geometry. This geometry allows excitation of a variable gain length while maintaining constant pump laser intensity on target and soft x-ray conversion efficiency. Distribution of the soft x-ray pump flux over an increased length is desirable in photoionization-pumped lasers because of the pump flux, density, and gain limitations imposed by the free electrons created in the pumping process. The assumption that the gain will be highest at some optimum pump flux leads to simple scaling laws for the total small signal gain and transverse dimensions of the laser as its length is changed. The gain per unit pump energy, the brightness of the output beam, and the Fresnel number of the laser should all improve with increased length. We discuss experiments to verify these scaling laws, where we increase the length of the laser simply by increasing the angle of incidence of the pump beam. In addition, we expect to discuss the effects of varying intensity, wave-length, pulse length, and a prepulse on soft x-ray conversion efficiency and laser operation. These studies lead to a better understanding of the factors important in the design of useful photoionization lasers.
Improvements in 8 × 8-matrix x-ray optics for nonuniform layers
Authors: Berreman Dwight W.
Published: February 2023

Two improvements have been made on a recently described 8×8 transfer-matrix formulation of dynamic x-ray diffraction theory for flat crystals with uniform lateral periodicity and nonuniform periodicity normal to the surface.1 The method is particularly useful for epitaxial structures. One improvement is to break each differential transfer-matrix into a part that depends only on the average complex refractive index, which can be integrated in closed form over any thickness, plus another small part that varies sinusoidally in the oblique and normal directions over some number of cycles. A fast perturbation method can be used to combine these parts. When attenuation of the beam is very great in a thick uniform periodic substrate beneath a different or varying overlayer, we previously truncated the transfer matrix at a level where the wave amplitude is very small but still appreciable and matched boundary conditions with two transmitted and two forward-diffracted plane-polarized waves propagating into a fictitious homogeneous medium. Numerical stability is often greatly enhanced by describing the transmitted and forward-diffracted beams as four orthogonal Bloch waves in the periodic substrate, which is treated as semi-infinite, and using only the transfer matrix down to that substrate to obtain reflection parameters.
Constraints on possible visual experience
Authors: Wilson Hugh
Published: February 2023

A recent philosophic trend has recognized that empirical findings in brain research may have important implications concerning the limitations and structure of human knowledge. In keeping with this naturalization of epistemology, this paper focuses on limitations on possible visual experience implied by contemporary vision research. Discussion centers on an example from depth perception, where it can be shown mathematically that the presence of only two disparate monocular views of the world fails to provide sufficient information for the visual reconstruction of the third dimension: a vast number of possible arrangements of objects are generally compatible with the available retinal information. Current work, such as that of Marr and Poggio, suggests that the visual system reconstructs the third dimension by imposing a constraint of the form: objects are bounded by opaque surfaces that vary smoothly and minimally in depth from point to point. It will be argued that such constraints are both a priori and synthetic and that they are closely related to Kantian a priori synthetic propositions.
Anticaustics
Authors: Chastang Jean-Claude
Published: February 2023

An anticaustic is the zero optical path wavefront corresponding to a given refracting or reflecting surface and a point source. The term anticaustic was coined by Jakob I Bernoulli in the 1690s. Anticaustics were introduced as they were easier to deal with mathematically than caustics. Anticaustics represent a curious case of interesting and possibly useful knowledge that fell into oblivion only to be resurrected much later but not without misconceptions. The anticaustics associated with the useful surfaces when the point sources are at finite distances are Descartes ovals or degeneracies thereof. The algebraic derivation of this fact is given. While mentioned briefly in textbooks in the context of stigmatic images, Descartes ovals are rapidly dismissed as being no more than curiosities. Yet they have many interesting properties. They are rarely represented correctly, i.e., as two nested closed loops, and the fact (known to Descartes) that a given oval has three foci, not two, is never mentioned. These properties and their optical consequences are reviewed. It should be noted that this is an example of wavefront tracing although very limited since only one surface is involved.
Fast set of equations for generalized field tracing in optical systems
Authors: Evans Eric D. , Collins Stuart A.
Published: February 2023

This paper presents a method for the rapid computation of optical electromagnetic fields in optical systems with arbitrarily shaped, smoothly varying surfaces. The method uses the results of a generalized ray tracing routine to provide optical field polarization and wavefront curvature information. The result is a rigorously accurate approach for predicting the irradiance of scattered light fields. Equations for the approach are written in a form which removes numerical singularities, and groups of equations are written in a format ready for direct computerized lens design applications.
Ray tracing through graded-index media: concept of isoindicial surfaces, position variables, and cubic splines
Authors: Doric Sead , Zrak Ro
Published: February 2023

A general approach to the calculation of refractive index and its gradient is presented.
Photonic switching: systems considerations
Authors: Hinton H. S.
Published: February 2023

Within recent years, there has been a significant amount of interest in applying the new and developing photonics technology for telecommunications switching. Since the transmission plant has converted its facilities to fiber, there is an economic interest in completing the optical path through the switching system to the terminal facilities without requiring optical-to-electrical (o/e) conversions. Several devices have emerged within the past few years that have the capability of meeting this goal. These devices can be categorized, according to the function they perform, into two major classes. The first, called relational devices, performs the function of establishing a relation or mapping between the inputs and outputs. This relation is a function of the control signals to the device and is independent of any signal or data inputs. The second are called logic devices. In these devices, the data or information carrying signal that enters the device controls the state of the device so that a Boolean function or combination of Boolean functions is performed on the inputs.
Aliasing and contrast sensitivity in peripheral vision
Authors: Thibos Larry N. , Still D. L.
Published: February 2023

Resolution acuity is often conceived as that spatial frequency for which contrast sensitivity fails to unity. We suspect this is not the case for peripheral vision because recent experiments have shown that peripheral resolution is limited by the ambiguity introduced by aliasing, not by an inability to detect the stimulus.1,2 Since gratings beyond the resolution limit remain highly visible as aliased percepts, we predict that contrast sensitivity for detection will be well above unity at the resolution limit. Using a two-interval forced choice staircase paradigm we measured contrast sensitivity for detection of a 2.3° patch of grating (vs uniform field) displayed on an oscilloscope located 30° into the horizontal nasal visual field and having mean luminance of 80 cd/m2. The resolution limit was separately determined by the method of adjustment as the highest spatial frequency for which perception is veridical and aliasing does not occur.1 We find that peripheral contrast sensitivity has a bandpass characteristic which peaks at ~1.5 c/d and falls monotonically with spatial frequency up to the detection limit of ~20 c/d. The function passes smoothly through the resolution limit of 3.2 c/d, where sensitivity is about an order of magnitude above the absolute threshold of unity.
Generalized autocorrelation functions
Authors: Yellott John I.
Published: February 2023

The autocorrelation function (ACF) a, of a realvalued function f(x, y) (e.g., a monochromatic image) is ordinarily defined as Recently it has been proposed that in the study of spatial phase discrimination it might be useful to consider a generalized ACF.1 Suppose the nth order ACF of f is defined as Clearly af,1 (the ordinary ACF) fixes the power spectrum of f but leaves its phase spectrum unconstrained.
Equivalent noise and transduction efficiency of human vision
Authors: Pelli Denis G.
Published: February 2023

Measurement of threshold contrasts for simple patterns in the presence of dynamic visual noise, along with some simple assumptions, allows an estimation of the internal noise in the visual system. The internal noise properties may be compared with those expected for photon noise and neural noise, indicating that each plays a role in limiting visual detection. This will be related to quantum efficiency as defined by Rose1 and Barlow.2
Use of holography in velocity gradient measurement in photochromic fluids
Authors: Agui Juan C. , Hesselink Lambertus
Published: February 2023

The application of volumetric diffraction gratings in photochromic materials to the measurement of velocity gradients in fluid flows is presented. The process involves the writing of a crossbeams hologram by two coherent pulsed beams that intersect inside a fluid to which a photochromic dye has been added. The hologram is continuously read at the proper Bragg angle by a third beam of lower intensity and different wavelength. The effect of flow shearing is to change the main orientation of the plane of the fringes of the hologram, which in turn reflects in a change of the angular properties of the readout beam. This is carried out until the hologram fades out by diffusion or it moves out of the Bragg condition. We show the optimization of the writing of the cross-beam grating in an optically thick material (like the photochromies used) as well as the wavelength for the most efficient reading of such gratings. Velocity gradients in a carefully controlled flow are shown and compared to alternative established methods and theoretical profiles. Finally, we discuss several alternative geometries capable of measuring different components of the velocity gradient tensor as well as multichannel arrangements capable of measuring vorticity components (e.g., ω
y
= ∂v
z
/∂x− ∂v
z
/∂z).
Holographic grating formation in a photopolymer medium
Authors: Lanteigne David J.
Published: February 2023

Volume phase holograms of high diffraction efficiency have been produced using the Polaroid photopolymer medium, DMP-128. The modulation of the index of refraction that forms the phase grating is due to variations of the mass indensity according to the Lorentz-Lorenz relation. A model is presented for the formation of the grating, based on polymerization and diffusion of monomer during exposure, as well as percolation effects in development. Just as in silver-halide materials, the time-dependent processes in image formation may be used to characterize a holographic reciprocity failure. In situ real-time measurements of grating information are used to detemine the reaction rates.
Light scattering from spin and electronic excitations in Tc superconductors
Authors: Fleury P. A. , Lyons K. B.
Published: February 2023

The high-temperature superconductors now fall into several structural and compositional classes. However, they all are oxides of copper, show little or no isotope effect, are relatively poor metals in their normal state, have very low carrier densities, and are closely related in composition to compounds which exhibit magnetic order. All of these attributes, together of course with the very high values of Tc, strongly suggest that the heretofore very successful microscopic theory of superconductivity based on the electron-photon interaction is inadequate here. Alternative theories have now appeared which invoke excitons, plasmons, polarons, magnons, or other pairing mediators. Recently inelastic light scattering has provided direct observation of the spin and electronic excitation spectra in single crystals of the superconducting cuprates and their insulating magnetically ordered relatives. This paper reviews these observations. Particular emphasis is given to the spin fluctuations which typically are characterized by quite high (~1000-cm-1) exchange energies and their behavior with temperature and composition. Correlations between the spin fluctuation spectra and Tc are described. In addition evidence for a superconducting energy gap is presented as is the influence of electron-phonon interactions on Raman line shapes. The relationships of all these results to candidate theories for high Tc superconductivity are discussed.
Polarized Raman study of La2 −xSRxCuO4 single crystals
Authors: Weber W. H. , Peters C. R. , Wanklyn B. M. , Chen C. , Watts B. E.
Published: February 2023

We present polarized Raman measurements from oriented single crystals of La2CuO4 and superconducting La2 –
x
Sr
x
CuO4 La2CuO4 is orthorhombic at room temperature, converting to a tetragonal phase at T
ot
≃500 K. This transition has a striking effect of the Raman spectra. As T
ot
, is approached from below, the α22 spectrum shows a strong line at 105 cm−1 that broadens and softens and a weak line at 271 cm−1 that gradually disappears, both lines being unobservable in the tetragonal phase. The Sr-doped crystals, which are tetragonal at room temperature, yield an α22 spectrum similar to that of the high-T phase of La2CuO4. These changes in the spectra are explained on the basis of a factor group analysis of the modes for the two structures. For the tetragonal phases of both doped and undoped crystals lines at 429 and 228 cm−1 are assigned to the O A1g and La A1
g
modes, respectively, and a line at 230 cm−1 to the O E
g
mode. The spectra from La2CuO4 show a 2-magnon peak at ≃ 3100 cm−1, with B1
g
(tetragonal) symmetry; numerous lower-peaks due to normally forbidden phonon excitations; and evidence for strong second-order scattering. These last two features suggest a highly anisotropic excitonlike enhancement mechanism. The Sr-doped crystals do not show any of these features to the same extent as the undoped material. Instead, they show an interference between the phonon scattering and the background electronic scattering, leading to Fano-type line shapes for certain modes.
Unifying view of weakly nonlinear waveguide devices
Authors: Doran N. J.
Published: February 2023

There are two major classes of nonlinear alloptical fiber switch, namely, (1) resonant devices which display characteristic sharp response, e.g., nonlinear couplers; (2) slow (sinusoidal) response devices such as nonlinear interferometers (Mach-Zehnders, loop mirrors). There is in fact a fairly straightforward connection between these types of device. All the devices operate in the weakly nonlinear regime where only the propagation constant is modified by the nonlinear change in index. Consider a single Mach-Zehnder (MZ) with four ports (two in, two out). This slow response device can only achieve complete switching from one port to the other if both couplers are balanced (50:50) and symmetry is broken by differences in the propagating arms. The assumption in this device is that nonlinear effects occur only in the arms not in the couplers themselves. If several MZs are joined together, complete switching can be obtained with identical arms but unbalanced couplers. It is shown that the requirements are that all the MZs should be similar and that linearly the light input in one port all couples out to one of the output ports. This concatenation is formally identical to a nonlinear coupler in the limit of a large number of MZs. It is shown from numerical simulations that all the features (critical power, etc.) of a nonlinear coupler can be obtained with a model with five MZs in one coupling length. General principles and new configurations are obtained from this new way of considering nonlinear couplers and similar resonant nonlinear devices.
Switching In nonlinear directional couplers and birefringent fibers: effects of group-velocity dispersion
Authors: Trillo S. , Wabnitz S. , Wright E. M. , Stegeman G. I.
Published: February 2023

By exploiting the fast responding nonlinear refractive-index changes in optical fibers, reshaping of ultrashort optical pulses in dual core1 or dual polarization2 fibers has been recently experimentally observed. A correct modeling of the nonlinear switching of short pulses in fiber couplers requires the inclusion of group velocity dispersion. We carried out numerical simulations of the nonlinear propagation in nonlinear directional couplers, birefringent fibers, and fiber rocking rotators. The response time of detectors is longer than the pulse widths of interest. Therefore, we consider the transfer of pulse energies between two output ports, as induced by changes in the input peak power. We find that in the normal dispersion regime, the switching efficiency substantially deteriorates for pulse widths shorter than a certain critical value, which depends on the length of the coupler. Conversely, in the anomalous dispersion regime soiitonic effects may maintain the pulse shape undistorted thus preventing pulse breakup and increasing the sharpness of the switching characteristics.
Progress in noise squeezing via a Josephson parametric amplifier
Authors: Yurke B. , Kaminsky P. G. , Reid M. D. , Whittaker E. A. , Smith A. D. , Silver A. H. , Simon R. W.
Published: February 2023

We are engaged in an experimental effort to generate and detect squeezed vacuum fluctuations at microwave frequencies using a Josephson parametric amplifier. We have demonstrated 42% deamplification of 4.2-K thermal noise at 19.4 GHz. At this frequency the vacuum fluctuation noise is an order of magnitude smaller than 4.2-K thermal noise. We have installed cryogenic HEMT amplifiers in our apparatus, thereby increasing the detector sensitivity by an order of magnitude. Josephson parametric amplifiers have been notorious for exhibiting excess noise whose origin is still controversial. We have performed a systematic study of the noise performance of our device both below and above threshold in a variety of modes of operation. The experiments indicate that the excess noise in our device arises from low-frequency noise (0-20 MHz) propagating down dc bias lines. When an intense coherent oscillation is present in the signal passband, such as the pump when the amplifier is in the four-photon mode or the period doubled oscillations when the amplifier is operated in the three-photon mode above threshold, the low- frequency noise mixes with the coherent oscillation to generate noise sidebands in the amplifier's passband. Nevertheless, in suitable conditions, extremely low noise and high gains were achieved.
Pump noise in an optical parametric oscillator operated above threshold
Authors: Leong K. W. , Wong Ngai C. , Shapiro Jeffrey H.
Published: February 2023

A cw optical parametric oscillator (OPO) operated above threshold is of particular interest in current squeezing experiments. Quadrature squeezing in the signal and idler modes and in the pump mode is predicted. Intensity squeezing in the difference intensity of the signal and idler is predicted and has recently been observed.1 We have built a doubly resonant cw OPO with a magnesium oxide- doped lithium niobate crystal that is single-pass-pumped with the 530.9-nm line of a kryptonion laser. Mode hopping instability near degeneracy puts stringent requirements on the frequency stability of the pump laser and the mechanical stability of the OPO cavity. We have oberved such instability near degeneracy and determined the stability requirements for this OPO. Stable operation can be obtained as the OPO is operated away from degeneracy but at a higher threshold. Classically the single-pass pump output is predicted to be clamped at the threshold value so that the OPO in this configuration acts as a pump power limiter. Quantum mechanically, we have found that the pump intensity noise is enhanced. Measurements on the pump intensity noise spectrum in this particular configuration have been made and compared to our theoretical predictions.
Fourier transform doublet lens used with alternative laser light sources
Authors: dl Ting Liao , Dou Lin Jing
Published: February 2023

The optical Fourier transform (FT) lens is the basic part of optical information processing. Now most available FT lenses1-3 belong to those used with the monochromatic laser light source, the most commonly used wavelengths being 633 or 488 nm or with a white light source. Their aberrations are both corrected for a given wavelength of light. The image quality of those lenses will much deteriorate while they are used in a light of wave-length which is not determined during design. With the availability of laser light sources, we hope to obtain another kind of FT lens whose monochromatic aberrations are corrected for more than one wavelength at the same time but without achromatism in the sense of meeting the needs of modern optical information processing and holography. Such a lens is called a FT lens used alternatively with different light sources, with almost the same level of aberration correction for any wavelength over the given 486-656-nm spectral range. When the light source used is changed, its aberrations are almost completely invariant with wave-length, and this characteristic is called the stability of aberrations with wavelength. To our knowledge, the only reference4 in this field shows design data, but the design details are not discussed. The design of this doublet type lens with small air separation is described with emphasis on the choice of glass, the distribution of power, the amount of longitudinal chromatic aberration introduced, and the stability of aberrations with wavelength.
Apochromatic color correction using liquid optical elements: experimental verification
Authors: Sigler Robert D.
Published: February 2023

A new method of attaining apochromatic color correction in optical systems has been developed. With this technique, low-cost normally dispersive optical glasses are combined with abnormally dispersive optical liquids to form optical systems having superior chromatic correction. To verify the advantages of this approach, two telescope objectives using one and two liquids in combination with the common glasses BK7 and F2 were fabricated and tested. Design details, athermalization methods, and measured performance data are presented.
Evaluation of high-resolution lenses by measurement of the Strehl ratio
Authors: Bobroff Norman , Fadi Petra , Rosenbluth Alan E. , Goodman Douglas S.
Published: February 2023

The Strehl ratio is widely used as a theoretical measure of lens quality. It is well correlated with other traditional figures of merit, such as mean-square wavefront aberration, modulation transfer function, and Linfoot’s mean-square image-object correlation. A common misconception is that the Strehi ratio of an optical system cannot be measured directly because the unaberrated image of the system under test is unavailable. The Strehl-ratio can be determined from measurements of the peak irradiance and total power in the image of a subresolution pinhole. A map of the Strehl ratio over field is a useful and concise summary of the imaging capability of a well-corrected lens. A lens bench which measures the Strehl ratio is described. The experimental parameters are the dimension of the pinhole, the illumination coherence, and the dynamic range of the image detector. Measurement accuracy is analyzed in terms of these parameters and the wavefront aberrations present in the lens.
Design of optical systems for Gaussian laser beam transformation
Authors: Jahan S. R. , Karim M. A. , Awwal A. A. S.
Published: February 2023

Several different approaches exist for obtaining a circularly uniform laser beam. The scope of most of these approaches is limited because, while some of these waste much of the input power, the other methods waste the spatial coherency of the beam during the process of conversion. We present the design of refracting beam-transforming systems1 which are not only easy to fabricate but are also highly efficient. The design considerations used in the design process ensure that the spatial and temporal coherency of the to-be-processed laser beam are preserved by the optical system. While one of the systems is designed to convert an annular Gaussian input beam into an equivalent circularly uniform beam, the other system converts a circularly Gaussian input beam into equivalent circularly uniform beams. The proposed systems are compared to the other existing designs in terms of performance efficiency and ease of fabrication.
Simultaneous electrooptical and temperature tuning of a double resonant optical parametric oscillator
Authors: Eckardt Robert C. , Nabors C. D. , Kozlovsky W. J. , Byer R. L.
Published: February 2023

Doubly resonant optical parametric oscillators (DR OPOs) have simultaneous resonance of both signal and idler waves. This simultaneous resonance allows lower pump threshold and better frequency selection compared to singly resonant optical parametric oscillators. The stable operation of DR OPOs requires pump lasers with narrow bandwidth, good frequency stability, and good spatial mode quality. We used a semiconductor-diode-laser-pumped nonplanar-ring-oscillator miniature solid-state laser with its output converted to second harmonic at 532 nm for OPO pumping. The combined requirements of double resonance and conservation of energy can result in discontinuous wavelength tuning of DR OPOs. When the crystal temperature is changed, the output jumps from one spectral region to another. Stepwise tuning with continuous spectral coverage is possible when the OPO resonances are finely controlled with electrooptical or pump frequency tuning. An analysis of this tuning that considers temperature- dependent dispersion, thermal expansion, and the electrooptical and piezoelectric effects is presented. Results are compared with the observed performance of a monolithic doubly resonant MgO:LiNbO3 optical parametric oscillator. Pulsed parametric oscillation with single-mode signal and idler waves was observed tunable from 1.01 to 1.12 μm by temperature change and an applied electric field.
Spectral equivalence theorems
Authors: James Daniel , Wolf Emil
Published: February 2023

Equivalence theorems are known which provide conditions in which sources with different coherence properties will generate fields with the same angular distribution of the radiant intensity.1,2 Another equivalence theorem for partially coherent sources is formulated; it provides conditions which ensure that 3-D primary sources with different coherence properties generate radiation fields that have identical spectra. The results are illustrated by describing sources of different coherence lengths, all of which produce the same far zone spectrum as a spatially completely incoherent source.
Speckle-image, phase reconstruction techniques compared
Authors: Aitken G. J. M. , Johnson R. , Meng J.
Published: February 2023

The Knox-Thompson (KT), triple-correlation (TC), and phase-gradient (PG) algorithms have been shown capable of reconstructing the phase of stellar spatial spectra upto the diffraction limit of large telescopes. Combined with the modulus measurements of Labeyrie’s speckle interferometry, these techniques offer from 10 to 50 times improvement in image resolution over the 2-sec of arc limit imposed on conventional astronomical observations by atmospheric turbulence. We report results of a comparative study in which the performance of the three techniques is evaluated with a common set of photon-limited speckle-image data from both simulated and real astronomical sources. The regime of interest is < 1 photon/ speckle and <500 photons/image. In these conditions PG gives excellent reconstructed images and is the most computationally efficient of the three. KT gives slightly poorer results in severe atmospheric conditions, can be improved by image centroiding, but experiences a further distorting effect at very low photon levels. TC requires the greatest computational effort but appears to be more robust with respect to certain types of data defect. Extension of TC can give a small improvement in image quality but at a high computational cost.
Real-time deconvolution by nonlinear image processing
Authors: Javidi Bahram , caulfield H. John , Horner Joseph l.
Published: February 2023

We present a nonlinear optical processor that can perform image deconvolution in real time. This processor may be used to remove the distortion effects from an optical image, such as restoration of a smeared or out-of-focus photographic image. In this technique, both the degraded image and the distorting function are displayed side by side at the input plane of the optical processor. The Fourier transform interference intensity is obtained using a Fourier transform lens and a square law device such as a CCD array. A hard clipping nonlinearity is used to threshold the modified Fourier transform interference intensity to only two values, 1 and − 1. The effect of the nonlinearity is to remove the distorted amplitude entirely and to restore the correct phase of the original signal. An amplitude mask averaged over an ensemble of images is used at the Fourier plane to provide the amplitude spectrum. An inverse Fourier transform of the average amplitude spectrum with the recovered correct phase information can yield the original image with considerably reduced distortion.
Two mechanisms for large-scale localization
Authors: Yap Yen Lee , Burbeck Christina a.
Published: February 2023

Separation discrimination thresholds are ordinarily measured with a pair of targets that are symmetrically placed with respect to the fovea, so that eccentricity increases as separation increases. In these conditions, the separation discrimination threshold (for separations larger than some minimum value) is a constant fraction of the target separation; i.e., localization obeys Weber’s law. However, this is not generally true. If the eccentricity of the features is kept constant, localization thresholds increase with separation at a rate slightly less than predicted by Weber’s law and then become constant, varying solely with eccentricity. [Flattening has also been reported for a bisection task by Levi et al. (Vision Res., in press).] Thus there appear to be two mechanisms involved, one which is sensitive to separation and relatively insensitive to eccentricity [Burbeck, Inv. Ophthalmol. Suppl. (1988)] and one which is primarily sensitive to eccentricity. The separation discrimination thresholds, as ordinarily measured across the fovea, measure sensitivity in the flat portions of the curves where eccentricity is the main factor (because a foveally centered target presents the maximum separation achievable at a given eccentricity). However, separation itself is the primary factor in many other stimulus conditions.
Relative position comparisons between dissimilar patterns
Authors: Devalois Karen K. , Lakshminarayanan V. , Nygaard Rolf W. , Schlussel Sarah
Published: February 2023

We have previously shown1 that observers can accurately detect changes in the relative position of a vertical intersector on a horizontal line segment. Performance is not seriously compromised by varying test pattern size relative to that of the reference or by rotating the entire test figure. Here we report the effects of either reflecting the test pattern left-right around its midpoint, inverting it, or rotating it by 90°. Despite marked changes in pattern appearance, neither manipulation significantly degrades observers' ability to make the required judgment. A sufficient solution to each of these tasks would be to estimate and compare the lengths of the appropriate line segments. To determine whether this is the necessary strategy, we utilized a test figure in which the intersection is represented by a significant gap centered around the intercept. Since equal absolute amounts are removed from each side, the ratios of line segment lengths are no longer comparable. In addition, the geometric cues provided by a 2-D intersector are no longer available. Performance on this task is surprisingly good. These data imply that simple perceptual models may not be adequate to account for the results.
Role of orientation and position in Vernier acuity
Authors: Levi Dennis M.
Published: February 2023

When three-dot Vernier acuity is measured as a function of separation, thresholds are approximately proportional to the separation of the dots. One interpretation for this result is that the visual system responds to a fixed orientation cue. An alternative hypothesis is that Vernier thresholds depend on positional information, so that as the separation and, therefore, the eccentricity of the reference dots increase, positional uncertainty increases, resulting in thresholds being proportional to the eccentricity/separation of the reference dots. To distinguish between these two hypotheses, separation was pitted against eccentricity. Vernier thresholds were measured for stimuli presented on an isoeccentric arc, so that separation could be varied while holding the eccentricity of the test stimuli constant.1 Measurements were made with isoeccentric arcs at eccentricities from 0.625 to 10°. At each eccentricity, there was a range of small separations (less than half of the eccentricity) over which thresholds were proportional to separation. In this range, stimulus duration had a marked effect, as may be expected if Vernier thresholds depend on orientation tuned filters. At larger separations, Vernier thresholds were independent of separation and of stimulus duration but varied systematically with the eccentricity of the isoeccentric arc. The results suggest that both position and orientation cues are important but that they operate to limit performance over different ranges of separation.
Frequency correlated laser beam generation in sodium vapor: sodium-density and pump-intensity dependence
Authors: Kumar Prem , poelker B. Matthew
Published: February 2023

When sodium vapor is irradiated with a single-frequency dye laser (pump) tuned in the vicinity of either of the Doppler-broadened D lines and enclosed in an optical cavity, Raman shifted oscillation builds up, which is blue shifted for a blue-detuned pump and red shifted for a red-detuned pump by 1.772 GHz (sodium ground-state hyperfine splitting).1 The gain mechanism responsible for this Raman-laser action can be understood by modeling sodium as a three-level atom. The Raman laser frequency is highly correlated with that of the pump as demonstrated earlier by observing a beat frequency bandwidth between the two lasers of as low as 17 kHz.2 (The pump had rms frequency fluctuation of the order of 10 MHz.) For spectroscopic applications, for example, frequency correlated laser beams are commonly generated using acoustooptic techniques which are difficult to implement beyond the gigahertz region. The Raman laser technique is extendable, in principle, even to the millimeter range. Here we present measurements of the sodium-density and pump-intensity dependence of the Raman-laser output and compare the results with those predicted by the three-level-atom model.
Coherence properties of one-dimensional pulses in dispersive media
Authors: Wang Weijian , Wolf Emil
Published: February 2023

In recent years longitudinal coherence of electron and neutron wave packets has been investigated,1,2 and it was shown that their coherence length remains unchanged even though the length of the wave packet increases on propagation.3,4 As pointed out in Ref. 3, one may expect that a similar result will hold for light that propagates in a dispersive medium. We present some new general theorems relating to an ensemble of identical 1-D optical pulses which propagate in a linear homogeneous medium of any given dispersive properties. In particular we derive a general expression for the mutual coherence function and for the cross-spectral density function of the pulse field in the medium in terms of the Fourier spectrum of a single pulse shape. The analysis brings out clearly the distinction (already noted in Ref. 4) between the coherence length and the coherence time of fields in dispersive media. We also show that the pulse field is spatially completely coherent at each frequency, even though the shape of each pulse changes on propagation. The results have potential application to interferometry with optical pulses and with de Broglie wave packets of elementary particles.
Suppression of Raman amplification using large Stokes seeds
Authors: Heinrichs Richard M. , Winkler Irwin C.
Published: February 2023

We have made measurements of the Raman amplification of a Stokes seed for various ratios of input Stokes intensity to input pump laser intensity. The Raman medium was vibrational hydrogen with the pump and Stokes undergoing a shallow focus through the cell to stay well below the Stokes-anti-Stokes phase matching angle. With small injection ratios, independent of whether the Stokes and pump were correlated, the Stokes was amplified and the pump deleted, as normally expected. However, if the Stokes and pump were initially uncorrelated, as the injection ratio approached unity the Raman gain significantly diminished and the pump and Stokes were observed to copropagate without interaction. This effective gain suppression is predicted by the solution of the Raman gain equations for a broadband pump with negligible dispersion.1 Also predicted, and observed experimentally, is that the effect should disappear when the pump and Stokes become correlated. Measurements of the beam quality and spectral width of the input and output pump were also made. We believe that these results point to a potentially useful technique for suppressing SRS for laser propagation through various media.
Pump-induced beam steering in large aperture disk amplifiers
Authors: Murray James E. , Powell H. T. , Ross G. F. , Wintemute J. D.
Published: February 2023

Flashlamp-induced aberrations potentially limit the beam quality and fill a factor of solid state laser systems. We have measured pump-induced beam steering vs time and aperture position in the largest Nova disk amplifiers, the 31.5- and 46-cm amplifiers, with ~1-μrad sensitivity at 1052 nm. The measurements show two distinct components: a large steering near (and toward) the disk edges and a weaker steering throughout the aperture in the plane of incidence of the Brewster angle disks. The experiments suggest that the edge steering is caused by the edge cladding which is heated by amplified spontaneous emission, locally distorting the disk, while the central-aperture steering is caused by disk bending resulting from differences in flashlamp illumination of the disk faces. Calculations agree well with the amplitude and position dependence of the edge steering, and they show that face-to-face illumination differences can account for the central-steering magnitudes and directions. The implied steering of the Nova output beam (74-cm beam diam) produced by the final two amplifier stages (five 31.5-cm and four 46-cm amplifiers) is inconsequential over most of the aperture (less than ±10 μrad), but the edge steering results in a 5% loss of useful beam area.
Spontaneous oscillations in a polarization isotropic laser
Authors: Puccioni G. P. , Lippi G. L.
Published: February 2023

We report the observation of low-frequency (1–100-kHz) intensity oscillations in the output of a polarization isotropic He–Ne laser operating at 3.39 μm. Oscillations have been observed in the total intensity as well as in the intensity of orthogonally polarized (linear or circular) components of the emitted light. While the intensities of either pair of orthogonally polarized components oscillate with strictly opposite phases, the pulsations in the total intensity are not in phase with those of any polarization, suggesting that the laser output in the unstable regime does not have a definite state of polarization. Adjusting the values of the control parameters (detuning, excitation, ratio between the cavity losses for the two polarization components) we observe stable single-polarization output, nearly sinusoidal oscillations, and spiky pulsations. The results agree qualitatively with recent theoretical predictions1 for the time-dependent behavior of polarization isotropic lasers.
Fresnel lenses for infrared image forming systems
Authors: Wood A. P.
Published: February 2023

The potential of Fresnel lenses as imaging elements at IR frequencies has been studied. The advantages offered by Fresnel lens elements are particularly attractive in IR systems where weight, cost, and compactness are often at a premium. Moreover, single-point diamond turning technology has made it possible to produce sophisticated Fresnel lenses with surfaces of good optical quality. In general, for example, facets of an aspheric profile can be formed on both surfaces of a meniscus substrate. Molding from diamond machined dies introduces the possibility of cheap mass-produced plastic elements; the inherently thin construction helps to compensate for the poor transmissive characteristics of these materials at IR frequencies. The performance of these complex Fresnel lenses in image forming systems is discussed in terms of the point spread and modulation transfer functions. The aberration-free case is examined and used to define an optimum structure in terms of the number of facets and their dimensions. The effect of aberrations on the imaging characteristics is discussed, and a tolerance for these defects is suggested. Some theoretical results are presented for a set of practical designs together with measurements from a diamond turned germanium Fresnel lens.
Numerical investigation of stimulated Raman scattering
Authors: Herman Bruce J.
Published: February 2023

The theory of stimulated Raman scattering has been treated at various levels of sophistication from plane-wave steady-state models through 3-D transient quantum-electrodynamic calculations.1 Many physical effects, such as pump depletion, dispersion, level degeneracy, ac Stark shift, and parametric coupling of Stokes and anti-Stokes light, can be important in experimental setups. We describe a recently developed computer code which treats these effects in detail. The code is based on integration of the coupled Maxwell-Bloch equations of quantum optics. We discuss numerical predictions of the code relating to these effects and present applications to experimental investigations such as Raman beam cleanup and transient amplification.2
Numerical studies of stimulated Raman scattering in the transient limit
Authors: Hilfer Godehard , Menyuk Curtis R.
Published: February 2023

The following equations govern stimulated Raman scattering in the transient limit: There are two easily characterized evolution regimes.1 The first regime occurs at small z where the pump beam can be considered undepleted. In this regime the Stokes beam grows exponentially, and an integral solution has been found by Carmen et al.,2 where l Bessel functions appear. The second regime, the J regime, occurs at large z where the Stokes beam has nearly saturated. In this regime, the pump beam intensity decreases as z1/2 and an integral solution involving J Bessel functions has been found.
Defining flash psychophysically
Authors: Cogan Alexander I.
Published: February 2023

A purely temporal aspect of masking by light was studied using a modified Crawford paradigm.1 A 0.01-s pulse (the probe) was presented before, during, or after the flash, a 0.5-s pulse of intensity fixed at 25 or 100% of dc luminance (120 cd/m2). Either pulse incremented the luminance over the whole field (12° diameter). The observer’s task was to detect the probe as a change in the temporal shape of the brightness fluctuation due mainly to the flash. Probe thresholds were determined in a yes/no experiment with the probe and flash going to both eyes, the same eye, or opposite eyes. Masking was always strongest near flash onset and offset. Differences among the binocular, ipsilateral, or contralateral masking effects were small indicating the predominance of cortical on and off responses to spatially coextensive incremental pulses of luminance.2,3 Also, a binocular flash was not a stronger masker for a monocular probe, than the ipsilateral flash, although the binocular flash was brighter than a monocular flash at nominally identical luminance, area, and duration. An incremental flash is defined as a temporal interval limited by on and off responses and filled in by brightness.
Effect of contrast stretching and spatial-frequency range on reading rate
Authors: Rubin Gary S. , Turano Kathleen , Schuchard Ronald A.
Published: February 2023

Studies of image quality with photographic and TV systems suggest that visual performance depends on the effective contrast of the image (the difference between image contrast and the observer’s contrast threshold integrated across the spatial-frequency spectrum of the image). This implies that an increase in image contrast will compensate for a reduction in spatial-frequency range due to low-pass filtering. To test this hypothesis, we measured reading rates for low-pass filtered text. Letters were digitized to 256 grey levels on a 32- × 32-pixel array and low pass filtered. The filters had cutoff frequencies (at half-amplitude) of 1.34, 1.02, or 0.60 cycles/letter. Paragraphs composed of letters subtending 0.5° were displayed for 20 s on a monochrome monitor with maximum luminance of 120 cd/m2. Observers read the text aloud, and the reading rate (number of words read correctly per minute) was computed. The process of low-pass filtering also reduces the contrast of the text. We compared reading rates for the filtered text with reading rates for filtered text in which the contrast was restored (stretched) to the maximum attainable with our display (0.94). Reading rates for the filtered text decreased with reduced cutoff frequency (down 81 % for the lowest cutoff frequency, 0.60 cycle/letter). However, reading rates for the stretched filtered text were less affected by filtering (down only 20% at 0.60 cycle/letter). The results indicate that the increased image contrast can compensate for a reduction in spatial-frequency range due to low-pass filtering.
Output characteristics of a 2.03-μm He-Xe laser
Authors: Asami Shigeo
Published: February 2023

The output characteristics of a 2.03-μm He-Xe laser are studied for the purpose of developing it as a spectroscopic and interferometric light source in the IR region. The output power of the 2.03-μm line was measured with a PbS photoconductive cell as a function of discharge current for various Xe and He pressures.