bluuebunny/crossref_metadata_2025 huggingface dataset:
Three over two: a fresh look at binocular summation
Authors: Cogan Alexander I.
Published: February 2023

From the experimental evidence, at least three processes participate in the production of the binocular response to brief changes in photopic luminance. A skeletal model is proposed, which combines three processes to form two binocular channels: (i) The fused channel—the carrier of an early binocular, excitatory process, and (ii) the either-eye channel comprised of monocular ipsilateral excitation and contralateral (reciprocal) inhibition. In a late process, (i) and (ii) together produce the common binocular output. The model accounts for the usual amount of the binocular superiority at the differential luminance threshold: a factor of 1.5. The model also allows for the vagaries of binocular superiority, which can vary from less than a factor of 1 to more than a factor of 2 in different conditions and/or in different observers. In the main experiment, we measured summation at threshold, for two-pulse luminance changes (++, −−, +−, −+) presented with a variable interpulse delay (SOA) to the same eye or to opposite keda1 and Rashbass.2 The results on interocular summation were different, but only at small SOA values. At SOA > 50 ms, thresholds became independent of stimuli polarity and port of entry. Thus, rectification (squaring?) and the final binocular combination must both occur relatively late in visual processing.
Optical architectures for an adaptive associative processing module
Authors: Fisher A. D. , Lee John N.
Published: February 2023

There has been a recent resurgence of interest in associative-processing/neural-network architectures as a model for parallel processing, particularly in such difficult problem domains as image understanding and artificial intelligence. Optics offers unique capabilities for implementing the massive parallel interconnection between successive 1-D and/or 2-D information arrays required by these architectures. Furthermore, these systems are often low-precision (e.g., binary) and hence are compatible with the limited dynamic-range capabilities of optics. We are developing optical architectures consisting of multiple adaptive, associative modules1 interconnected and cascaded in particular configurations, according to a variety of organizational principles. The experimental performance of actual optical implementations of the individual adaptive associative modules will be discussed. These optical modules adapt as they are exposed to associated information pattern vectors (u,v), so that subsequent presentation of one pattern, u, results in recall of the other, v. The set of associations is dynamically learned in real time and stored as an electron charge distribution on an electrooptic crystal in a spatial light modulator. These architectures include feedback, which makes them self-correcting to a variety of optical system aberrations, and they have the capacity to store N associated pairs of N-element vectors, where N of a few hundred appear feasible.
Inference of 3-D shapes and part boundaries from occluding contours
Authors: Beusmans Jack M.H. , Hoffman Donald D. , Bennett Bruce M.
Published: February 2023

Given the human ability to recognize familiar objects from simple line drawings or silhouettes, the study of occluding contours promises insights into the way the human visual system represents objects and their shape. Hoffman and Richards1 argued that the human visual system partitions complex surfaces into parts or primitives and represents the 3-D shape of objects in terms of spatial relationships between these parts. In the present study an outline of a theory of parts is given which is based on the notion of primitive, featureless compact surfaces, which can be combined to form surfaces with bulges, or removed to leave holes. Rules based on surface curvature have been derived to decompose an arbitrary smooth surface into its constituent parts. Koenderink2 showed that the sign of curvature of the occluding contour equals the sign of Gaussian curvature along the fold locus. Knowledge of surface curvature along folds can then be extended to regions in between folds, resulting in a so-called curvature interpretation. Expressions for the number of curvature interpretations as a function of the shape of the contour have been derived and possible constraints such as genericity and locality are discussed.
Generation and amplification of femtosecond pulses using Nd:YAG technology
Authors: Mourou Gerard , Duling I. , Norris T. , Sizer T.
Published: February 2023

We report on the recent advances made in high repetition rate short pulse amplification. At the present time there are basically two different approaches for kHz amplification of femtosecond pulses. The first approach has been demonstrated by Knox et al.1 and uses a copper vapor laser as the amplifier pump source. Because of the long pump pulse (~30 ns) a multiple-pass dye amplifier is used, producing 1-μJ short pulses at 10 kHz. A fundamentally different approach has been demonstrated by our group.2,3 We use a short pump pulse of 300-μJ energy and 100-ps duration, delivered by a continuously pumped, frequency-doubled regenerative Nd:YAG amplifier. Because the latter is seeded the same 1.06-μm pulses used to drive our synchronously pumped dye oscillator, a hard synchronization is established between the amplifier pump pulse and the oscillator pulses. The pump pulse energy can therefore be efficiently transferred to the oscillator pulse by means of single-pass dye amplifiers. Five-μJ pulse energies at 1-2 kHz have been obtained, well above the threshold for white-light generation. Part of the white light can be selected and amplified by an auxiliary single-pass dye amplifier. The features of the system and its applications to ultrafast spectroscopy are highlighted.
Simultaneous operation of an array of 5 × 5 all-optical logic gates using a holographic lens array
Authors: Taghizadeh M. R. , Redmond I. R.
Published: February 2023

There has been a lot of interest in the possibility of the application of 2-D arrays of intrinsic optical bistable elements in the field of optical digital computing.
Applications of intense subpicosecond excimer lasers for atomic and molecular studies
Authors: Jara H. , Johann U. , Luk T. S. , McIntyre I. A. , McPherson A. , Schwarzenbach A. P. , Boyer K. , Rhodes C. K.
Published: February 2023

New physical phenomena are expected to occur1 with irradiation of atomic and molecular materials with subpicosecond ultraviolet radiation at intensities above ~1018 W/cm2. One such phenomenon is electron-positron pair production.
Fine structure of the second harmonic time-resolved spectrum in laser plasmas
Authors: Gu Min , Tan Weihan , Lin Zunqi , Bi Wuji , Yu Wenyan , Deng Ximing
Published: February 2023

The time-resolved and integrated spectra of the backscattering second harmonic emission were presented in previous papers.1,2 Reported here are the time-resolved spectrum of both high temporal resolution (~10 ps) and spectral resolution (~6 Å) 2 ω0 harmonic taken at 90° to the laser axis. It has been found that the intensity of the primary and the secondary peaks of the second harmonic emission is not a continuous distribution but composed of many fine and bright fringes with the interval ~1.5–3 Å in relation to atomic weight. On the basis of this observation we considered theoretically the influence of the standing electromagnetic wave and the ponderomotive force on the plasma dispersion relation and gave a unified description of the ion-acoustic frequency ω
i
for the linear conversion and the parametric decay. The results we obtained are basically consistent with the experiments. Another important point is that the second harmonic emission for P polarization lasts for a longer time than that for Spolarization, which means that a certain threshold exists to produce the linear conversion by the laser self-focusing filamentation.
Spectroscopic Investigation of ag-metal clusters
Authors: Froben F. W. , Moller K. , Schulze W. , Winter B.
Published: February 2023

The investigation of silver clusters prepared by the gas-aggregation technique or laser vaporization using mass and fluorescence spectroscopy provides some new information about the influence of the experimental parameters for the cluster size distribution.1 The observation of time-resolved emission spectra in the case of laser vaporization allows us to distinguish between different size species. This, together with mass spectroscopy and fluorescence spectroscopy, using the gas aggregation technique is useful for better understanding of how the cluster grows dynamics. The questions of fragmentation, double ionization, and Coulomb explosion are discussed as well as the influence of charged particles for the resulting size distribution. In addition to the investigations on naked metal clusters some results on molecules adsorbed on clusters are reported.2
Seeing moving parts
Authors: Rubin John M.
Published: February 2023

Uniform circular (wheel) motion and pendular motion are studied as examples of a hypothesized collection of moving-part modules. A moving-part interpretation rule1 states that given two moving points Rand B in an image, if B can be interpreted as a point on a rigid body moving at constant velocity without rotation in space, and P can be reliably interpreted as the bob of a pendulum or as a point on a wheel attached to the body at a fixed hub point, a visual system should make that interpretation.
External cavity dye laser frequency stabilizer
Authors: Boyd Tim L. , Kimble H. J.
Published: February 2023

We describe an external laser frequency stabilizer consisting of an acoustooptic frequency shifter and a fast electrooptic phase modulator. Our work follows that of Hall and Hansch, who first proposed and demonstrated the delay compensated compound frequency transducer.1 Their initial experiments produced a linewidth of ~20 kHz for the external cavity stabilization of a commercial dye laser.
Phase conjugation without spatial holes
Authors: An Sunghyuck , Sargent Murray
Published: February 2023

In the four-wave mixing scheme for phase conjugation, the pumps interfere in a nonlinear two-level medium Inducing spatial holes in the population difference. Since these holes are out of phase with the signal and conjugate waves, the projection of the induced polarization onto the waves requires averaging over the pump spatial holes. In their initial treatment of this problem, Abrams and Lind1 supposed that the squared sinusoids representing the holes in the population denominators could be approximated by1/2. Later they carried out a more realistic average by integrating the polarization over a wavelength.
Self-deflection of a light beam in a thin nonlinear film
Authors: Swartzlander G. A. , Kaplan A. E.
Published: February 2023

In the earlier work by one of the authors,1 the self-bending (or self-deflecting) effect was predicted, where a beam with an asymmetrical intensity profile follows a curvilinear trajectory in a nonlinearly refractive material. This effect was later observed in a bulk nonlinear crystal.2 With the advent of superlattices and quantum well structures with strong optical nonlinearities, very thin films may be used to replace bulk materials. Here we consider the theory and make computer simulations of the self-deflection effect induced in thin nonlinear films (where undesirable effects such as catastrophic self-focusing can be avoided). In our analysis the general integral solution for the far-field area is the Fourier transform of the incident field distribution with an intensity-dependent phase distribution. The analytical solution for a beam with a right-triangular intensity distribution results in an undistorted deflection in the far field, consistent with the geometric-optics result.1 Computer simulations were performed for many asymmetrical profiles, e.g., semi-Gaussian, all of which demonstrated an expected self-deflection. The simulation for a symmetrical 2-D Gaussian beam reveals a surprising result: instead of the expected self-focused (or self-defocused) single beam at high intensities, we observed two symmetrically self-deflected beams in the far field. The thin-film self-deflection effect may be useful for radiation protection devices and beam scanners.
All-optical measurement of the photovoltaic field in photorefractive barium titanate
Authors: MacDonald Kenneth R. , Feinberg Jack
Published: February 2023

Photorefractive materials have found many applications in nonlinear optics including image processing, phase conjugation, laser beam combining, and interferometry. Glass et al.1 proposed that uniform illumination of a photorefractive crystal could generate a dc voltage across the crystal. This photovoltaic field plays an important role in the theory of charge transport in photorefractive materials and in the performance of photorefractive devices. However, the photovoltaic field is notoriously difficult to measure using electrodes on the crystal faces, due to surface and space-charge effects. We present an all-optical technique to measure the magnitude, sign, and intensity dependence of the photovoltaic electric field in a photorefractive crystal. We use two-beam coupling in the crystal to transfer energy between two optical beams having a slight frequency shift δf(~1 Hz). In the absence of a spatially uniform electric field in the crystal, the magnitude of the energy coupling is symmetric in δf. The presence of an internal electric field causes a characteristic asymmetry in the coupling. We show that in barium titanate (BaTiO3) the variation of the coupling strength with δf is asymmetric, and the asymmetry increases with optical intensity. A fit of the data to theory yields a saturating photovoltaic field along the direction of the +c axis whose saturated value is ~80 V/cm.
Observation of harmonic phase conjugation in a photorefractive medium
Authors: Chang Tallis Y. , Yeh Pochi
Published: February 2023

We demonstrate for the first time, to our knowledge, second harmonic optical phase conjugation, using the second spatial harmonics of a nonsinusoidal grating formed in a photorefractive material. The possibility of harmonic phase conjugation has been considered previously for artificial Kerr media.1 We have applied the same idea to barium titanate, since the existence of the higher-order gratings in this material has been known to us for some time. In our experiment, two 1.06-µm cw Nd:YAG laser beams wrote a grating in a poled photorefractive barium titanate crystal. Then a second harmonic of the YAG at 0.532 µm read out the grating in a counterpropagating direction to one of the writing beams. In this geometry, the usual Bragg condition is not satisfied. The reflectivity of the 0.532-µm signal was rather weak, ~0.01%, with ~0.1-mW reading beam power. The buildup time was around 3 s, with loosely focused writing beams with 50-mW power in each beam. The same experiment with a SBN crystal showed no significant harmonic phase conjugation. Finally, we stress that this effect is different from the previously published result.2
Chaos in self-pumped four-wave mixing
Authors: Gauthier Daniel J. , Narum Paul , Boyd Robert W.
Published: February 2023

Self-pulsation and chaotic behavior have been observed in a self-pumped barium titanate phase conjugate mirror.1,2 We find that the characteristics of the attractor on which the system evolves depend critically on the following properties of the pump beam: (1) the degree of focusing, (2) the angle between the propagation direction and the crystalline caxis, (3) the location on the crystal face at which the beam enters, and (4) the input power. We have modeled our experiment by extending the theory of MacDonald and Feinberg3 of self-pumped phase conjugation due to four-wave mixing in two coupled interaction regions to include the effects of time-varying optical fields and the finite response time of the nonlinear medium. We find that this model predicts unstable operation of a self-pumped phase conjugate mirror that is similar to that observed in the experiment.
Effect of gain nonlinearities on period doubling and chaos in directly modulated semiconductor lasers
Authors: Agrawal Govind P.
Published: February 2023

Directly modulated semiconductor lasers have been predicted1,2 to exhibit period doubling and chaos in some range of the modulation parameters. These predictions are based on a simple set of two rate equations governing the dynamics of the electron and photon populations. We have found that the sequence of period-doubling bifurcations leading to chaos disappears when the rate equations are modified to include a small power-dependent reduction in the mode gain occurring due to nonlinear phenomena such as spectral hole burning. The amount of nonlinear-gain reduction required for the elimination of chaos is ~1 % for typical values of the laser parameters. Since index-guided InGaAsP lasers generally exhibit a gain reduction of 5-10%, the chaotic behavior can be ruled out for such lasers and should be of no concern for current optical communication systems. Our numerical simulations also show that gain nonlinearities affect significantly the width and the shape of picosecond pulses generated by the method of gain switching.
Elastic constants of metal superlattices measured by Brillouin scattering
Authors: Bell J. A. , Bennett W. R. , Zanoni R. , Stegeman George I. , Falco C. M. , Seaton Colin T.
Published: February 2023

The acoustic modes guided by thin-film metal superlattices have been investigated using Brillouin spectroscopy. Samples were grown on single crystal sapphire by alternately sputtering two different metals to yield a total thickness of ~0.3 μm. Thermally excited acoustic waves in the metal create a surface ripple which weakly interacts with light incident from a single-mode argon laser. The scattered light was then frequency analyzed with a tandem Fabry-Perot consisting of two synchronized three-pass cavities. The contrast ratio of this interferometer exceeds 1010 and provides sufficient stray light rejection to detect the surface Rayleigh wave and thirteen higher-order acoustic modes. We have investigated the dependence of bilayer wavelength on the elastic properties of both Cu/Nb and Mo/Ta superlattices over the 8-320-Å range. We note that the behavior of the Rayleigh velocity of our Cu/Nb samples is significantly different from a previous study of this material.1 An estimate of the elastic constants of the anisotropic superlattices was made by fitting the observed acoustic mode velocities to a parametrized acoustic model. We compare these elastic constants with those predicted from the separate bulk constituents.
Radiometry as a short-wavelength limit of phase-space representation of statistical wavefields
Authors: Foley J. T. , Agarwal Govind S. , Wolf Emil
Published: February 2023

Attempts to elucidate the foundation of radiometry have up to now met with limited success. It was found, for example, that although many different functions exhibiting some of the properties of the radiance may be defined within the framework of second-order correlation theory of statistical wavefields, none of them satisfies all the basic postulates of radiometry.1 We show that the various definitions are different phase-space representations of the cross-spectral density.2 The particular form depends on the choice of a mapping function from configuration space to phase space.3 We also show that, in the short-wavelength limit, different generalized radiance functions of a quasi-homogeneous source reduce to the same quantity, recently discussed elsewhere,4 which may be identified with the radiance of traditional radiometry.
Parallel digital adder with optical carry
Authors: McManus J. B. , Putnam R. S.
Published: February 2023

We have constructed a hybrid electronic/electrooptic digital adder based on a system suggested by Arazi1 which uses an optical-carry for fully parallel operation. The philosophy of the system is to rely on optics for operations which are difficult to do in parallel electronically. Here, electronic logic is used for summing the addend and carry bits, while the carries are generated optically. The optical-carry is based on a multistage optical path, with sources which add light to the path if a carry is generated, and with modulators which allow light to pass if a carry is propagated, at the corresponding column of the addition. The signals which set the light sources and modulators are derived from the addends simultaneously, so the addition is completed as soon as detected light signals reach the summing logic. With small changes in the driving logic, the device operates as a parallel subtractor, with optical borrow propagation. This system reduces the number of gate delays associated with electronic addition of long addends. For long addends, the system speed is limited by light loss through the carry path. Our bulk-optic breadboard system has six acoustooptic modulators as optical switches, with a single He-Ne laser source. It adds two 4-bit words at 10 MHz with a 200-ns pipeline delay.
Linear temporal response of the human cone pathways
Authors: Tyler Christopher W. , Hamer Russell D.
Published: February 2023

In the region of the CFF the visual response does not obey Weber’s law but behaves linearly in the sense that it is independent of the background adapting luminance.1 This allows measurement of the linear component of the visual response excluding nonlinear adaptation and lateral inhibitory effects. We measured this linear visual response in conditions where thresholds were determined by red-sensitive (R) or green-sensitive (G) cones alone, by using either 660- or 555-nm narrowband stimuli. The stimulus field was 5.7° in diameter at 35° eccentricity in the temporal field and had a perceptually equiluminant white surround.
Real-time computed tomography
Authors: Gmitro Arthur F. , Gindi Gene R.
Published: February 2023

An optical system for real-time reconstruction of computed tomography (CT) images was previously described.1 The system implements the filtered backprojection algorithm using a heterodyne space-integrating acoustooptic convolver for the filtering operation and an incoherent optical system for the backprojection operation. The first images reconstructed using this system are presented. These images represent the first video-rate CT reconstructions ever produced. Performance of the present system as well as a discussion of the general issue of image quality in realtime CT are presented. Alternative approaches to implementing the filtering operation are also described. The talk concludes with a discussion of the potential applications of real-time CT in medical diagnosis, industrial inspection, and as an analytic technique for the study of dynamic processes.
Phase conjugate resonator for high-resolution scanning microscopy
Authors: Cathey W. Thomas , Johnson Kristina M. , Cormack Robert H.
Published: February 2023

Previous research has shown that the resolution of an image can be increased by passing the illumination beam through the object more than once.1 This requires the use of a critically aligned mirror to reflect the wave back through the object. We analyze the effects of using a phase conjugate mirror to perform the reflection. Three advantages accrue: (1) the alignment is no longer critical; (2) the reflection does not invert the distribution that illuminates the object; and (3) more than two passes through the object can be obtained. We present descriptions of several implementations and the analyses of their operation.
Femtosecond evolution of nonthermal carrier distributions in CdSe microcrystallite doped glass
Authors: Olbright Greg R. , Fluegel B. D. , Peyghambarian N.
Published: February 2023

The femtosecond dynamics of hot carriers injected a few LO phonon energies above the bottom of the conduction band are reported. The onset of carrier screening, carrier relaxation, thermalization of the hot electron gas, and electron–hole recombination are extracted from time-resolved absorption spectra. A CPM ring-dye laser amplified to 2 μJ and 100-fs duration with a copper vapor laser at 7.3 kHz is used to generate a white light continuum probe from an ethylene glycol jet. A 750-μm thick sample of a glass doped with ZnSe is pumped at 620 nm. For a zero pump-probe delay, a well-resolved shift of the absorption edge to lower energies is observed, indicating instantaneous onset of carrier screening. At t = +300 fs, the absorption spectra are dominated by a 75-meV blue shift of the absorption edge, signature of complete carrier relaxation, and filling of the bottom of the band.1 At t = +5 ps the initial recovery of the blue shifted absorption spectra is observed, indicating the onset of electron–hole recombination. The blue shift is almost entirely recovered after 50 ps, with the exception of a small persistent shift which remains for times in excess of several hundred picoseconds. This residual persistent shift is attributed to carriers confined to traps near the bandedge.
Multiresolution model of stereopsis
Authors: Landy Michael S. , Hummel Robert A.
Published: February 2023

A model is described for stereopsis. Two stereo images are bandpass filtered at multiple scales using a Laplacian pyramid data structure.1 These bandpass image pairs are then matched in parallel at all scales and locations using an iterative, relaxation labelinglike process. At a given spatial location i and scale (or pyramid level) k, the state consists of a disparity estimate (the disparity mean M
i

k
) and a confidence in that estimate (the disparity variance V
i

k
). Disparity matches are only made near the disparity currently indicated, M
i

k
. The update of M
i

k
includes a data-driven term (from the matches), a local averaging term (from M
j

k
, where j is a location near i in the image at scale k), a top-down term (from M
i

k
+1), and a bottom-up term (from M
i

k−
1). These terms are gated by the corresponding confidence terms (V
i

k
, V
i

k
+1, and V
i

k−
1), which reflect the quality of the match. Simulations of the model are discussed both in terms of the model’s efficacy in computing stereoscopic depth, and in general concerning how computation may take place cooperatively across multiple scales.
Scattering and absorption in thin-film waveguides
Authors: Wlodarczyk Marek T.
Published: February 2023

This paper reports a novel ray optic analysis of scattering and absorption in thin-film waveguides. The ray optic approach, similar to the one used for grating couplers,1 enables us to investigate the propagation of a guided mode in a waveguide with random surface roughness, random inhomogeneity of the film permittivity, and absorbing film and cladding regions in a relatively simple way. The extinction coefficient is obtained by following a zigzagging ray in the film. Over the path between the interfaces a film absorption coefficient and a lumped volume scattering coefficient are multiplied by the distance traveled between the interfaces. At the interfaces, the power amplitude is multiplied by an exponential power reflection coefficient. This coefficient is calculated by adding perturbation contributions due to different loss mechanisms to the reflection coefficients of an unperturbed waveguide. The extinction coefficient is calculated by dividing the cumulative attenuation by the bounce distance. Scattering losses show much stronger wavelength dependence than the absorption losses. The surface scattering results in strongest attenuation. Both absorption and scattering are maximum for TM higher-order modes.
Absolute two-photon absorption and three-photon ionization cross sections for atomic oxygen at 201 nm and 193 nm
Authors: Bamford D. J. , Jusinski L. E. , Bischel W. K.
Published: February 2023

Quantitative detection of atomic oxygen, an important species in combustion and atmospheric chemistry, can be accomplished using multiphoton techniques if the relevant cross sections are known. We have recently reported1 absolute measurements of the two-photon absorption and excited state photoionization cross sections for O(3P) at 226 nm, the wavelength of the first allowed two-photon transition. Although two-photon transitions at shorter wavelengths are expected to have smaller cross sections,2 coincidences with excimer laser lines could lead to enhanced detection sensitivity. To explore this possibility, we have used the techniques described In Ref. 1 to measure the corresponding cross sections for the 4p3P2,1,0 ← 2p3P2 transition at 201 nm and for the 5p3P2,1,0←2p3P2 transition at 193 nm. Results are discussed critically and compared with ab initio calculations.2 The application of similar techniques to the detection of fluorine atoms3 is also described.
Analyses of Wiener spectra, radiant intensity, and radiance functions using Schell and quasi-homogeneous source models
Authors: Jannson T.
Published: February 2023

It is well known that two statistical optics topics, physical radiometry and random screen (diffuser) optics, can be analyzed using the formalism of (monochromatic) random processes. In the first case, however, ergodic statistical ensemble averaging is physically realized through detector temporal integration in the parametric approximation1 while, in the second case, the statistical averaging has a purely mathematical character. This important difference does not prevent us, however, from applying some definitions of basic quantities of physical radiometry to diffuser theory or vice versa. In this paper, several aspects of such comparative analyses are discussed. This study is basically related to such physical optic quantities as radiant intensity, the Wiener spectrum, and radiance. Two theoretical models, the quasi-homogeneous approximation2 and the more general Schell source formalism are discussed.
Generation of ultrahigh peak power pulses with the technique of chirped pulse amplification
Authors: Strickland D. , Maine P. , Mourou G.
Published: February 2023

Nd:glass amplifiers have very good energy storage capabilities, but the energy extraction is extremely inefficient for short pulse amplification because of peak power limitations. These limitations can, however, be circumvented by frequency chirping and stretching the pulse prior to amplification. By amplifying the longer pulses, much higher energies can be achieved. After amplification, the pulse width is compressed to a value approximately equal to 1/Δf, where Δf is the total frequency chirp. To demonstrate this technique, we have produced picosecond pulses with energies in the tens of millijoules range.1 A cw mode-locked Nd:YAG laser produces 100-ps pulses which are focused into a 2.4-km long single-mode optical fiber. At the output of the fiber, the pulses have been frequency chirped over a 3.5-nm bandwidth by self-phase modulation and stretched to 330 ps in duration by group velocity dispersion. The pulses are then amplified in a silicate glass regenerative amplifier to an energy of 2 mJ and further amplified to 50 mJ in a multipass system. The pulse width is then compressed to <2 ps by a double grating compressor which has an overall energy efficiency of 50 %.2
Method for measuring the optical properties of slightly absorbing, inhomogeneous dielectric thin films
Authors: Carniglia Charles K.
Published: February 2023

The optical constants of thin-film materials can be determined from measurements of the reflectance R and the transmittance T of a single layer film deposited on a transparent substrate. If the film is homogeneous and absorbing and is thick enough to have several maxima and minima, the index n and extinction coefficient k can be determined from T alone using the envelope method. If the film is inhomogeneous but nonabsorbing, the degree of inhomogeneity and index can be determined from R alone. This determination is based on the assumption that the inhomogeneous index of the film varies smoothly from a value of n1 at the outer surface of the film to a value of n2 at the substrate surface of the film. When the film is both inhomogeneous and absorbing, both R and T are necessary for the determination of the optical properties. The solution is most easily obtained in terms of the degree of inhomogeneity α = √(n2/n1. If the absorption in the film is <5 %, α can be determined from the reflectance scan independently of n and k. Then, n and k can be determined from the transmittance scan using the measured value of α. For more highly absorbing films, the values of n and k can be used to obtain a more accurate measure of α and the procedure can be iterated to find the best solution. The algorithm is applied to films of Sc2O3 which were measured as part of a comprehensive study presented at the 1983 OSA Annual Meeting.1
Ultranarrow x-ray lines radiated by an electron beam in a solid-state superlattice
Authors: Kaplan A. E. , Law C. T.
Published: February 2023

We show that anomalous dispersion in the vicinity of absorption edges of the material can greatly enhance x-ray transition radiation of a low-energy electron beam passing through the multilayer solid-state structure. We showed earlier1 that the critical energy of the electron beam required to obtain soft x-ray radiation in such a structure with a 50-100-Å spatial period can be as low as a few hundred keV. We demonstrate here that the anomalous dispersion near the absorption edge of one of the solid-state materials forming the multilayer structure yields radiation with very narrow spectrum in the vicinity of that edge. In addition to the resonant condition which determines the spatial period of the system, one must choose one of the materials (radiator) to have a frequency of the absorption edge (K, L, M … series) close to the required frequency (usually, for short wavelength, it must be a heavy element), whereas another one (spacer) is a light material, e.g., B, Be, and C. The analysis of the theoretical and experimental data on atomic scattering factors2 available to date reveals that the proposed technique may produce a universal method for generation of very narrow x-ray lines with Δω/ω ≃ 10-4-10-5 in the range between 1 and 200 Å with most of the elements of the periodic table. For example, Ba/Be multilayer structure with 1000 individual layers, each of which is 100 Å thick, irradiated by 1-MeV electron beam, yields 10-6 photons/eV/electron at a wave length of 16 Å with the linewidth Δω/ω ≃ 10-4.
Ultraprecise determination of thermal expansion differences with the optically wrung sandwich seal
Authors: Hagy Henry E.
Published: February 2023

Ultraprecise thermal expansion differentials are determined through photoelastic analysis of a symmetrical sandwich seal.1 Seal fabrication normally employs fusion or fritting, requiring temperature cycles that can modify initial expansions of interest. Optical wringing of flat, polished surfaces at room temperature gives strong, elastic bonds for joining seal components. Observation of optical retardation in the central seal member with temperature leads to the determination of expansion differentials with a precision of ±2 ppb/°C over the −195–100°C range. Test advantages are (a) inexpensive test specimens and experimental apparatus, (b) easy, quick data collection and reduction, and (c) seals may be disassembled and components used for other combinations. If the expansion of one component is known, the absolute expansion of the other member is obtained. Test results are presented for Corning code 7971 ULE glass seals in which component parts are given different thermal histories. The absolute expansion of a low-expansion transparent glass ceramic is derived from a seal made against a sample of ULE glass with known expansion.
Multiphoton ionization of a model H− ion
Authors: Greenwood W. G. , Eberly Joseph H.
Published: February 2023

We present the results of computer modeling of above threshold, multiphoton ionization of H−, the hydrogen negative ion, by an intenser laser. This system provides an uncomplicated structure for the study of ionization since the extra electron has only one bound state. Experimental results have recently become available for this system.1
Restoration of linear motion blurred images using a joint transform processor
Authors: Cheng Feng , Xu Xin , Wu Shudong , Yu Francis T. S.
Published: February 2023

Although a joint Fourier transform processor is effective for the correlation operation,1 it can also be used as a generalized coherent image processor.2 In this paper we show that blurred photographic images due to linear motion can be restored with a joint transform processor. The major advantage of using the joint transform processor (JTP) for image deblurring must be the avoidance of spatial filter synthesis. In other words, the deconvolution function3 for image deblurring can be directly implemented at the input plane of a JTP, and the difficulty caused by the singularities of an inverse filter can be avoided. To generate a bipolar deconvolution function, we have encoded two sets of π phase shifted sinusoidal gratings of the same spatial frequency.
Spatial solitary wave optical memory
Authors: McDonald Graham S. , Firth William J.
Published: February 2023

We present simulations of an optical memory array based on a unidirectional ring cavity partially filled with a fast nonlinear medium. The spontaneous occurrence of spatial solitary waves in such resonators (pumped by Gaussian beams) has already been predicted by numerical simulations1 and analytic studies.2 We have shown the feasibility of using these robust attractors as a basis for binary memory through a comparative study of the two signs of nonlinearity in a two-pixel (whole- beam switching) configuration and with a 19-bit (part-beam switching) solitary array. Gaussian beams can be encoded in their asymptotic solitary basis in just a few cavity transits and subsequently retain this information perfectly over many thousand round trips suggesting a strong candidate for nonlinear memory elements. No material pixellation has been included and, instead, straightforward all-optical nonlinear channeling defines each pixel site.
Self-pulsations and chaos in asymmetric external cavity semiconductor lasers
Authors: Park J. D. , Seo D. S. , McInerney J. G.
Published: February 2023

Semiconductor lasers, by virtue of their unique physical properties and their extensive use in optical communication and data storage, are particularly appropriate vehicles for studying complex dynamic phenomena such as frequency locking, mode beating, instabilities, and chaos. Previous experiments using external cavity semiconductor lasers have demonstrated self-pulsations and chaos, especially when small tilt asymmetries are introduced into the feedback from the external reflector,1 with subharmonic bifurcation and successive frequency locking being reported separately by different groups. Detailed experimental and theoretical studies of these systems have led us to develop a theoretical model which uses coherent rate equations modified for strong feedback systems. Using this model, we have predicted nearly sinusoidal self-pulsations and subharmonic bifurcations, in substantial agreement with our experiments. We have also observed period doubling and quasiperiodicity with increasing drive current in numerical simulations, depending on the values of various external parameters such as external mirror reflectivity and tilt angle. Here we compare theoretical and experimental data and discuss the physical mechanisms responsible for the observed phenomena. Particular attention will be paid to the significance of small tilt asymmetries, and the interaction between the resulting self-pulsations and various characteristic damping frequencies in determining the route to chaos.
Single-frequency emission at 532 nm from a unidirectional, internally doubled, diode laser array pumped, Nd:YAG ring laser
Authors: Dixon G. J.
Published: February 2023

Single-frequency operation of an internally doubled Nd:YAG laser is one method of eliminating the high frequency amplitude fluctuations that are often observed in the output of such a device when multilon-gitudinal-mode oscillation occurs. A single-frequency output is also desirable for many holographic and spectroscopic applications. Single-longitudinal-mode operation of a Nd:YAG laser can be accomplished through the elimination of spatial hole burning in the gain medium or through the insertion of a spectrally selective loss element in the laser cavity.1 The first method is preferable in an internally doubled device since it adds less intracavity loss and maximizes the harmonic conversion efficiency. The traveling wave intracavity field in a unidirectional ring laser has been shown to effectively eliminate spatial hole burning in Nd:YAG.2 By inserting a crystal of KTP in the cavity of a unidirectional, laser diode array pumped Nd:YAG ring laser operating at 1064 nm, we have been successful in producing 0.75 mW of single frequency output at 532 nm. The overall conversion efficiency of pump radiation to harmonic output is 0.2-0.3%. It is anticipated that overall pump conversion efficiencies of a few percent are possible with optimization of the cavity design and the use of lower loss optics.
High repetition rate amplifiers for femtosecond pulses
Authors: Nighan W. L. , Young D. A. , Gong T. , Gzara K. , Fauchet P. M.
Published: February 2023

Copper vapor lasers (CVL) allow high repetition rate amplification of femtosecond pulses from colliding pulse mode-locked lasers (CPM).1 Sources of focusable femtosecond pulses in the microjoule energy range are of great importance for ultrafast spectroscopy.
Passive waveguide couplers by ion exchange
Authors: Hussell C. P. , Zhenguang H. , Srivastava R. , Ramaswamy R.
Published: February 2023

We present results on the design and fabrication of 3-dB single-mode cross-couplers in Ag+ – Na+ ion-exchanged BK7 glass. The normal mode propagation method was used to determine the characteristics of a graded-index slab waveguide structure which simulates the effective index profile of our channel waveguide devices. The results indicate that the adiabatic condition

(

Δ
β
≫
 
0.43
θ

γ
¯

)

originally stated by Burns and Milton1 for step index slab waveguide devices is practically unchanged for graded-index devices such as ours. This criterion was used to design a cross-coupler incorporating an angle of ~0.1°. Several couplers were fabricated on each BK7 substrate using asymmetric-symmetric 2×2 couplers by performing ion exchange for 12 h at 340°C in an AgNO3 + NaNO3 melt. The measured results demonstrate a 50:50 power split ratio when either input waveguide is excited. Furthermore, the device is fiber compatible, polarization independent, and relatively wavelength insensitive. Losses as low as 2.5 dB in 50-mm long devices are easily achieved. We are currently investigating how to minimize the device length by incorporating bends. This consideration is necessary if these devices are to be incorporated in larger integrated optic devices such as the 4×4 coupler.
Long distance multiwavelength interferometry
Authors: Walsh C. J. , Brown N. , Fairman P. S.
Published: February 2023

The use of multiple wavelengths in an interferometer to measure absolute distance is well established.1 We previously reported2 on such measurements over distances up to 25 m using a CO2 laser source. At longer distances, the systematic variations in the laser frequency, which produce fluctuations in the interferometer phase, become large enough to cause ambiguities in the algorithms used to calculate the distance in terms of synthetic wavelengths.1
Broken symmetries and traveling waves in the spatial patterns of lasers
Authors: Green C. , Narducci L. M. , Tredicce J. R.
Published: February 2023

We showed recently1 that the coupling between different transverse cavity modes yields spatiotemporal instabilities in a CO2 laser system using the radius of curvature of the mirrors as the control parameter to change the strength of such a coupling. Our results were in good agreement with theoretical calculations based on a modal expansion of the Maxwell-Bloch equations. However, the model was based on an intrinsic radial symmetry of the transverse configuration of the field, while in the experiment that assumption was shown to fail when the separation in frequency of the empty cavity modes was reduced below a certain critical value.1
Femtosecond multiple pulse impulsive simulated Raman scattering in α-perylene molecular crystals
Authors: Weiner A. M. , Leaird D. E. , Wiederrecht G. P. , Nelson K. A.
Published: February 2023

Generation of coherent optic phonons through impulsive stimulated Raman scattering (ISRS) can occur whenever a sufficiently short optical pulse passes through a Raman active medium. The first experiments demonstrating ISRS were performed with α-perylene molecular crystals1; subsequently, similar observations were reported for a variety of other materials. One difficulty which can be associated with the impulsive scattering technique is an inherent lack of mode selectivity. In this paper we demonstrate multiple pulse impulsive stimulated Raman scattering, a powerful new technique for achieving mode selectivity in ISRS experiments. By using phase-only spectral filtering techniques, we efficiently generate terahertz repetition-rate bursts of femtosecond pulses,2 which we use for repetitive impulsive excitation in our experiments. Individual phonon modes are selected by tuning the pulse repetition rate to the vibrational period. We demonstrate this concept by selectively and individually exciting the 104-cm™1 vibrational mode and the 80-cm™1 librational mode in α-perylene molecular crystals. This technique permits direct observation of the individual cycles of the vibrational and librational oscillations, without the mode beating which occurred in previous experiments,1 and makes possible accurate measurements of the dephasing times.
Infrared phase modulators with graded multiple quantum wells
Authors: Holm David A. , Taylor Henry F.
Published: February 2023

We investigate the feasibility of a graded multiple quantum well as an infrared phase modulator. The existence of infrared intraband transitions in multiple quantum wells is well known.1 By applying an external voltage to the well, the wavelength of the transition can be altered. This shifting absorption center induces a corresponding refractive index change of the material, which provides the basis of the electrooptic phase modulator. The introduction of a linear composition gradient in the well causes the well potential to vary across its width. The external voltage also introduces a linear variation in the potential, but now both inside and outside the well. In the absence of a material gradient, the refractive index change is a quadratic function of the voltage. We show that including a built-in composition gradient changes a weak quadratic electrooptic effect into a strong linear one. In particular, for a Ga1_x-AlxAs well with AlAs barriers we find the voltage needed for a 180° phase change to be of the order of 150 V, 2 orders of magnitude less than a conventional electrooptic material. The combination of index change linearity and low power consumption makes this structure an excellent candidate as a mid-infrared index modulator.
Nonlinear optical properties of corona-onset poled films of coumarin-substituted copolymers
Authors: Knoesen A. , Mortazavi M. A. , Kowel S. T. , Hoover J. M. , Henry R. A. , Lindsay G. A.
Published: February 2023

Previously, corona-onset poling at elevated temperatures (COPET) was demonstrated to produce large orientational order by aligning nonlinear chromophores in a polymeric host.1,2 The film system consisted of nonlinear azo dye chromophores dissolved in polymethyl methacrylate (PMMA). The films exhibited large and stable second harmonic intensities, but the films were strongly absorbing in the visible. For this investigation, new materials were developed that consisted of a coumarin-based chromophore attached to a PMMA backbone through an alkyl spacer with various dye concentrations. Spincast films on BK7 substrates were poled with the COPET technique. The poled nonlinear copolymer films were characterized by spectroscopic absorption and second harmonic generation. The second harmonic properties are presented as a function of concentration, thickness, poling temperature, and poling current. Large, stable second harmonic coefficients of the order of 19 pm/V are observed. Additionally, the films exhibit low absorption at wavelengths exceeding 450 nm.