bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser spectroscopy studies of Cr4+ centers
Authors: Jacobsen S. M. , Eilers H. , Hoffman K. R. , Hömmerich U. , Yen W. M.
Published: February 2023

We discuss the nature of the excited states of the NIR laser center in Cr:YAG. Recent results show that this particular center behaves in a similar manner to that of a Cr:alexandrite laser, inasmuch as the first excited state can be considered to be a "storage level" during room temperature laser operation. This follows from NIR laser excitation studies that show the observed low temperature sharp lines at 1277 and 1280 nm are true zero-phonon lines. The first excited state is found to be a doublet, with a splitting of 30 cm-1. These new measurements also lead to a new estimation of the absorption cross-section at 1.064 μm (2 × 10-17 cm2). This relatively high cross-section may affect the choice of the optimum pumping geometries and crystal dimensions as the understanding of this laser system matures. We will also present some laser performance measurements for this system. Finally, we will compare the energy levels of several d2 transition metal ions and attempt to show the relationships and correlations between them.
Properties of the Cr:LiSAF laser crystal.
Authors: DeLoach L. D. , Payne S. A. , Smith L. K. , Krupke W. F. , Morris R. C.
Published: February 2023

The properties of Cr-doped LiSrAIF6, or Cr:LiSAF, that significantly affect laser design considerations have been investigated. We examined LiSrA1–
x
Cr
x
F6 samples having doping levels ranging from x = 0.01 to x = 1 and found that the cw laser properties are not adversely affected by the Cr-doping level. In essence, the optimum Cr-doping level can be chosen to match the requirements of each application. In fact, the fully substituted system, LiSrCrF6, or LiChrom, can be pumped at 790 nm, far out in the absorption wing, and nevertheless efficiently generate laser light. This is possible, in part, because the emission lifetime of LiChrom is nearly the same as that of a lightly doped sample. It is noteworthy that the highly doped version of Cr:LiSAF permits the use of AlGaAs laser diodes operating in the 750-780 nm range to serve as useful pump sources.
Fiber optic sensors
Authors: Kersey Alan D.
Published: February 2023

This tutorial describes the basic principles of the various optical-modulation techniques that can be utilized in sensing applications using fiber optics. Intensity-based and interferometric sensing techniques are covered, and a wide range of sensing systems are discussed. In particular, intensity-based sensors for engineering-type applications, such as temperature, pressure, and flow, and high performance interferometric systems for acoustic, magnetic field, and rotation (gyroscope) sensing are described in detail. Details of the techniques used with interferometric sensors for demodulation and multiplexing of sensors are also covered.
Perception of 3D structure from static form and/or motion cues in a patient with visual motion deficits
Authors: Sinha Pawan , Vaina Lucia M.
Published: February 2023

Structure-from-motion studies typically employ motion sequences in which any single frame is devoid of any 3D structure information. Real world sequences, however, present the observer with both dynamic and static form cues. To investigate how these concurrently active sources of 3D information interact, we designed stimuli that, upon moving, provide inconsistent structure cues, i.e., the form cues suggest a 3D organization different from that indicated by motion cues. We showed (Sinha, 1992) that in such stimuli form cues can overwhelm the motion cues and that the rigidity assumption is extremely fragile in such situations (rigid objects are perceived as deforming non-rigidly). Here we bring further evidence to these hypotheses in a psychophysical study of motion perception in a patient, AMG, with an occipital-parietal lesion, whose form perception was normal. AMG could not extract 3D structure from motion cues alone, but when both static form and dynamic cues were available, her interpretation of moving stimuli (in terms of perceiving rigidity or non-rigidity) was similar to that of control subjects.
Design, fabrication, and test of Schwarzschild soft x-ray microscopes
Authors: Tanaka Kazuo A.
Published: February 2023

Schwarzschild microscopes have been designed, fabricated, and tested. The distance from an object to film is 1500 mm with a magnification of 15 and f/3.6. These values were optimized with use of ray-tracing program. W/B4C, Ni/C, and Mo/Si multilayered mirrors were used for λ = 42, 73, 160 Å, respectively. The surface roughness of fused silica substrate is from 1 to 3 Å. The resolution test was conducted by using a test mesh backlit with laser produced gold X rays as an object to obtain the images on a Kodak 101–07 film. Results indicate 0.5 μm resolution. A zooming tube with a Csl photo cathode was also tested as a high spatial resolution detector. The resolution on the photocathode is less than 1 μm, corresponding to 0.1 μm resolution for a system with a magnification of 10.
Contact x-ray microscopy of in vitro human chromosome fibers using pulsed laser-plasmas
Authors: Richardson M. , Shinohara K. , Kinjo Y. , Tanaka K. A.
Published: February 2023

We describe the use of intense pulsed (<1 ns) x- rays from a laser-plasma for making highresolution (100 Å), high-contrast, contact images of hydrated human chromosome fibers. The fibers are suspended in a 10 μm thick H2O cell having a thin 100 nm SiN entrance window anda rear recording panel of PMMA resist. Single-shot irradiation of the cell with x-rays generated from an Au laser-plasma produced by a 5 J, 1 ns duration, laser pulse at 526 nm is sufficient to expose the resist for a cell-target separation of 20 mm. The detailed structure of the image created in three-dimensional relief in the PMMA is faithfully transferred to a thin, positive polymer replica and analyzed with high-resolution electron microscopy. We discuss the implications of this work and describe refinements to this approach that will enhance its usefulness in highresolution imaging of in vitro specimens in the biological and life sciences.
Optical fabrication at Rockwell Power Systems
Authors: Lowrey W. H. , Bender J. W. , Tonnessen T. W.
Published: February 2023

Rockwell Power Systems is currently involved in the fabrication of advanced optics for a number of applications. An overview of our fabrication and metrology capability will be given. Much of our current activity is in the fabrication of optics for synchrotron beamlines. These include optics for use with x-rays or UV radiation. Cooled and uncooled optics have been fabricated, including flats, spheres, and toroids. Results of these fabrication activities will be discussed, including the metrology methods used. In addition to beamline optics fabrication, we will discuss the fabrication of more conventional optical components, including high-accuracy aspheric mirrors for advanced imaging systems.
Multi-resolution pyramidal image compression via perfect convergent neural associative memory
Authors: Liu Hua-Kuang
Published: February 2023

A multiresolution pyramidal image/data compression technique via the perfect convergent unipolar terminal attractor based associative memory is described. By properly choosing adaptive thresholds for dynamic iteration of the unipolar binary neuron states with an inner-product terminalattractor based neural associative memory,1 it has been demonstrated via computer simulation that perfect convergence and correct retrieval can be achieved.2 The perfect convergence feature of the neural associative memory can be used to reduce a 2D image/data into an image of much smaller dimensions, i.e., if n × n neurons are used as one unit cell, an image of N × N unipolar binary pixels can first be divided into (N/n) × (N/n) cells. For example, each cell can be associatively retrieved into two pre-determined memory vectors that may be considered as belonging to two bit planes. If one of the memory states is assigned a value of 1 and the other is assigned 0, then the combination of the two bit planes yields an image of a dimension being that of the original reduced by n × n. Continuation of the process will yield a pyramidal architecture of reduced image/data of different resolutions. Since in each layer the data features and format are dependent on the memorized vectors chosen, an abundance of choices can be made to tailor for specific needs of image processing applications.
3D grating optical interpretation of human vision allows a joint explanation of the Stiles–Crawford effects I and II
Authors: Lauinger Norbert
Published: February 2023

According to this explanation, the SCE I is based on a chromatic shift of the diffraction order triplet in the multilayer phase grating of the outer nuclear layer of the retina toward blue. This means that, on chromatic adaptation of the eye to perpendicularly incident light, the blue color channel loses brightness and the red gains relatively, which manifests itself as an SCE II red shift. In addition to the SCEs for white test lights and a constant or variable pupillary opening, the wavelength-specific decreases and increases in brightness (SCE I) and the red shift or reversal hue shift toward blue (SCE II), as occur for monochromatic test stimuli against a trichromatic background, become understandable. The two SCEs prove to be combined effects in three chromatic retinal chip diffraction orders. They can no longer be interpreted independently as separate effects at L, M, and S cones (interpreted as waveguides). The 3D grating optical explanation of the Purkinje shift and the SCEs also provides information on the interaction of three variables (the pupillary diameter, the third grating spacing in the retinal chip, and the α0β0δ0 angle relative position of light cones to the 3D grating) during bright/dark and chromatic adaptation of the eye, i.e., the interaction of stimulus-related variables, which are all relevant in front of the photoreceptor plane.
The anisotopy of χ(3)(−ω; ω, −ω,ω) in zincblende semiconductors
Authors: Schroeder W. A. , Dvorak M. D. , Andersen D. R. , Smirl A. L.
Published: February 2023

We investigate the anisotropy of the degenerate, instantaneous, third-order, bound-electronic nonlinearity χ(3)
ijkl
in zincblende semiconductors by monitoring the rotated polarization component of the transmitted radiation in both single-and two-beam (pump–probe) experiments as a function of the crystal orientation and the incident pulse intensity. The measurements were conducted on a variety of semiconductors (GaAs, InP, CdTe, ZnSe, and GaP) by using an amplified, 1-ps Styryl 13 dye laser system operating at 950 nm. The results showed that the induced nonlinear polarization rotation of the transmitted signal due to the anisotropy σ of χ(3), defined as σ = χ1111 − (χ1122 + χ1221 + χ1212), has the expected periodic dependence on crystal rotation for light propagating along the (001) and (110) crystallographic axes. For propagation along the (111) axis no polarization rotation was observed—a result that concurs with the fact that χ(3) is isotropic in this direction. A time-delayed two-beam pump-probe measurement showed that the nonlinearity is instantaneous to within the 1-ps experimental resolution. In addition, the third-order of this nonlinear effect was confirmed in the pump-probe experiment by the dependence of the signal on the incident fluence. Determination of the components of the χ(3) tensor by using this experimental technique will be discussed.
Adaptive optical tiling network
Authors: Qiao Yong , Psaltis Demetri
Published: February 2023

We report an experimental two-layer optical tiling network,1 the hidden layer of which grows during training. The hidden neurons are added like tiles only when they are needed. The first layer of interconnections is implemented by a dynamically refreshed photorefractive (BaTiO3) volume hologram. The hidden layer starts with a single unit, which is trained with the perceptron algorithm to classify the 2-D training inputs as well as possible. Then a second hidden neuron is added and trained to correct as well as possible classification errors that the first hidden neuron makes. This procedure is repeated until a satisfactory hidden representation of the inputs is obtained. The second layer, implemented with an optical matrix-vector multiplier, is then trained to solve the problem based on the hidden representation. A key feature of this system is the use of dynamic copying2 to control the weight decay in the photorefractive crystal as new holograms are formed.
Theory and simulation of the formation and erasure of space-charge gratings in photoconductive polymers
Authors: Schildkraut J. S. , Buettner A. V.
Published: February 2023

Equations that describe the formation of space charge in a photoconducting polymer are presented. The equations are written in a non-dimensional form that includes the electric field dependence of charge generation efficiency, charge mobility, recombination rate, and trapping rate. The equations are solved numerically for the condition of illumination with an intensity distribution that varies sinusoidally with position. The effect of traps on the formation of a space-charge grating is modeled. Finally the erasure of the space-charge grating by uniform illumination is considered.
Observation of vibronic quantum beats in semiconductor microcrystallites
Authors: Machol Janet L. , Wise Frank W. , Patel Ramesh
Published: February 2023

The transient absorption of excitons in PbS microcrystallites exhibits a terahertz-frequency damped sinusoidal response attributed to quantum beats between vibrational eiqenstates of the microcrystallite.
Double charge layer and ablation pressure of the nanosecond laser-produced plasmas at the metal target under the first and fourth harmonics of Nd:glass laser
Authors: Bufetov I. A. , Fomin V. K. , Fedorov V. B. , Kravtsov Yu. A. , Prokhorov A.M.
Published: February 2023

The experimental comparison of laser-produced, plasmas characteristics under the influence of radiation of wavelength λ=1.06 μ and 0.265 μ(laser intensity I ~ (109 – 1013) W/cm2, pulse duration 10 ns, focusing spot diameter d ~ (0.1 -3)mm) on the Al metal target Is carried out. The dependence of dense plasma ablation pressure P and Its electron temperature Te (which is measured by the double charge layer potential) versus I are found out to have various behavior at different λ. Whereas the pressure depends only on the Intensity of the radiation P(kBar)=0.1*(I(MW/cm2))1/2, the electron temperature value Te and the character of the Te(I) increase depend not only on I and λ, but also on d. It is obtained, that, when d is constant Te(λ = 1.06μ) > Te(λ = 0.265μ). The saturation of the dependence Te(I) with the increase of I at the largest value of d = 3 mm is found out. And at the smallest d = 0.1mm the relation Te(I) ~ I1/2 takes place, If λ = 1.06 μ. The study of X-ray emission from plasma enabled to evaluate Te ~ 300 eV at the experimental conditions I ~ 1013 W/cm2, λ = 1.06 μ and double charge layer potential ≈ I keV. The possible explanation of the phenomena under Investigation is suggested.
Fourier plane filtering by a thick grating: space band-width limitations
Authors: Athale R. A. , Raj K.
Published: February 2023

The space-bandwidth requirements on a thick grating in the Fourier plane of a shift invariant coherent optical image processor are analyzed. Based on Kogelnik's relationship for the diffraction efficiency, expressions for the space-bandwidth products (SBWs) of the input image (N) and the point spread function (M) are obtained in terms of grating parameters. It is found that thick ness of the thick grating limits the field of view (FOV) of the input and hence limits the maximum SBW image that it can handle. A thin grating filter scales more readily to larger values of N and M. By angular multiplexing of multiple grating sets the thick grating FOV can be improved. Though the thick grating possesses the extra degree of freedom, higher information storage capacity, and higher diffraction efficiency, the finite FOV limitation makes it less suitable for shift invariant operations via Fourier plane filtering.
An image processing technique for spatial filtering and pattern recognition
Authors: Joseph J. , Singh K. , Pillai P. K. C.
Published: February 2023

We describe an image processing scheme that employs the principles of two-beam coupling in BaTiO3 and the selective erasure of the FT hologram. The use of the FT of an erase beam makes the system more versatile in performing different operations with a single setup. An experimental demonstration has been performed by using two schemes, one with a counterpropagating erasure and the other with a copropagating erasure, to perform edge enhancement, bandpass filtering, and character recognition. The present system is real-time operational, energy efficient, and cascadable.
Circular harmonic filters for an incoherent correlator
Authors: Lohmann Adolf W. , Mendlovic David
Published: February 2023

Optical correlation can be done in spatially incoherent light. Such correlators are advantageous compared with coherent correlators because diffuse or self-luminous objects can be used directly as inputs without converting them to coherent light inputs. Moreover, incoherent light systems are less sensitive to the lateral positioning of the filter. For the coherent correlator, it has been shown1 that by using a single circular harmonic (CH) function as a reference object, rotation invariant pattern recognition can be achieved. Unfortunately, these functions are usually complex (having phase and amplitude information) and cannot be used directly with the incoherent correlator. In this work2 the real-positive implementation of a CH function in an in-coherent correlator has been investigated. We propose to use a conventional incoherent correlator scheme with two filters simultaneously in order to implement the real and imaginary parts of information contained in a single CH. By using these two filters followed by some post-processing steps, rotation invariant pattern recognition was achieved. In addition, partial information about the angular orientation of the input object can be also obtained. Computer simulations and experiments were performed.
Bispectral imaging through unknown deterministic aberrations
Authors: Miller Donald T. , Morris G. Michael , Williams David R.
Published: February 2023

When the deterministic aberrations are known in an optical system, traditional de-blurring methods, such as the Wiener filter and aberration compensating filters, are effective. However, when the aberrations are difficult to quantify, as are telescope aberrations or the aberrations in the human eye, more practical methods are needed. One potential method for de-blurring a system with unknown aberrations is the bispectral imaging method. This image recovery technique reconstructs the object's Fourier phase by using a sequence of statistically independent aberrated images of the spatially incoherent object. It has been found to be promising in removing the effects of deterministic aberrations when random aberrations are present or are artificially introduced into the system.1, 2 Through computer simulations, we have found the correct amount of random aberrations to have present in a system containing deterministic aberrations to optimize the image quality of the reconstruction at high light levels. Defocus and several third order aberrations were considered in the isoplanatic case. The performance of this method was characterized by reconstructing a point source and computing its Strehl ratio. These results will be used to incorporate the bispectral imaging method as part of a noninvasive technique to reconstruct high resolution images of the eye's fundus in human subjects. Computer simulated images will be presented.
Application of flip-chip bonding techniques for Si based smart pixels
Authors: Mansoorian B. , Shih D. , Fan C. , Ozguz V. , Esener S.
Published: February 2023

Material and operational compatibility among processing elements, detectors, and modulators is critical issue in the implementation of smart pixels. This paper describes the hybrid integration efforts for MQW and PLZT modulator arrays with Si detectors and Si VLSI processing elements. Detectors and driver circuits compatible with the MOSIS process have been realized. An amplifier circuit has been designed to drive PLZT with a 35 V swing at 5 MHz. This is accomplished by using series connected transistors and a current-mirror-like structure to increase the breakdown voltage of the circuit. The combination of the driver and a reflective PLZT modulator can produce light modulation with a dynamic range of 600:1. Studies of the speed response of PLZT 9/65/35 show rise and fall times of 10 ns, limited by the drive circuitry. MQW modulators have been implemented by using InAlGaAs/InGaAs operating at 1.06 μm on GaAs substrates. This structure produces modulators that are compatible with Si detectors and flip-chip bonding with Si VLSI circuitry. Flip-chip bonded smart pixel arrays incorporating PLZT and MQW modulators and characteristics of simple optical links between the smart pixels have been studied.
Water adsorption on fluoride crystal surfaces studied by optical second harmonic generation
Authors: Zink J. C. , Reif J. , Matthias E.
Published: February 2023

Second harmonic generation (SHG) with p-p polarization has been used for studying the symmetry of the crystal-water interface during water adsorption on polished surfaces of alkali halide and alkaline-earth halide crystals in UHV. The adsorption dynamics can be explained by Langmuir kinetics, by assuming several different adsorption sites. The azimuthal anisotropy of the SHG yield, reflecting 3m symmetry for (111) surfaces, shows different adsorption dependencies for BaF2 and CaF2. We conclude that water grows epitaxially on BaF2(111). In contrast, for CaE2 the adsorbate symmetry differs from the one of the substrate, with increasing coverages. Unexpectedly, LiF(100) shows a pronounced m symmetry caused by water adsorption, which is not the case for NaF(100). The m symmetry on LiF is stable and indicates a preferred direction of the water dipoles in the adsorbate layer.
Second harmonic generation with multimode laser beams
Authors: Qu Y. , Singh Surendra
Published: February 2023

Second harmonic generation (SHG) with multimode fundamental optical beams is investigated experimentally. The fundamental beam is derived from a laser oscillating simultaneously in many longitudinal modes that have the same transverse profile (TEM00). The fundamental beam from the laser is optimally focussed into a nonlinear crystal. Noncritical Type-I phase matching is utilized to generate a second harmonic signal. The number of longitudinal modes in the laser is varied systematically from one to seven, and the dependence of the second harmonic signal on the number of modes in the fundamental beam is studied. In all cases the total fundamental intensity is held constant. Experimental results will be discussed and compared with theoretical models.
Two-color time-resolved measurements of optical nonlinearities
Authors: Wang J. , Said A. A. , Hagan D. J. , Sheik-Bahae M. , Van Stryland E. W.
Published: February 2023

We show that the simple Z-scan technique1 can be extended to study the dynamics of optical nonlinearities by using a nondegenerate time-delay excite-probe geometry. In this scheme, we obtain information regarding the signs, magnitudes, lifetimes, and dispersions of both absorptive and refractive components of optical nonlinearities. Nondegenerate two-photon absorption in ZnSe and cross-phase modulation in ZnS and ZnSe are measured, and the results are shown to be in good agreement with those of the recent two-band model, which utilizes the nonlinear Kramers-Krönig transformation to relate nonlinear absorption to nonlinear refraction.2 The results for ZnSe, for example, show the dominance of the ultrafast bound electronic Kerr effect at small delays, while the long-lived and cumulative free carrier refraction dominates at longer delays. Using exact nonlinear wave propagation analysis, we can identify and measure degenerate and nondegenerate coefficients of two-photon absorption and n2. In addition, the picosecond dynamics of the free-carrier nonlinearities (absorption, band filling) can also be investigated.
Measurement of the dispersion of the nonlinear refractive index in wide-gap dielectrics from the UV to the IR
Authors: DeSalvo R. , Said A. A. , Sheik-Bahae M. , Hagan D. J. , Van Stryland E. W.
Published: February 2023

The n2 of BaF2, MgF2, LiF, sapphire (Al2O3), and diamond are measured with picosecond pulses from a Q-switched, mode-locked Nd:YAG laser at wavelengths of 1064 nm, 532 nm, 355 nm, and 266 nm with the corresponding pulsewidths of 30 ps (FWHM), 21 ps (FWHM), 17 ps (FWHM), and 15 ps (FWHM), respectively. The experimental data is compared to the recently published two-band model,1 which predicts the nonlinear refractive index as a function of the incident photon energy to the material's energy gap, ħω/E
g
. The two-photon absorption coefficient is measured in diamond at 355 nm and 266 nm and yields values of β = 0.3 cm/GW and β = 0.7 cm/GW, respectively. The dispersion of n2 is measured by the Z-Scan technique,2 where the transmittance of a focused Gaussian beam with constant energy is monitored through an aperture in the far-field as a function of the sample's position with respect to focus. This technique gives a direct measurement of the sign and magnitude of the nonlinear refractive index. By removal of the far-field aperture, the two-photon absorption (2PA) coefficient of the material can be measured. For photon energies in the range E
g
/2<ħω < E
g
, the material will exhibit 2PA.
Noncontact temperature measurement using grating expansion
Authors: Lang M. , Zaidi Saleem H. , Brueck S. R. J.
Published: February 2023

Precise, real-time noncontact temperature measurement of semiconductor wafers during processing steps is of critical importance in semiconductor manufacturing. This is particularly true for new processes, such as rapid thermal processing (RTP), which are being developed in response to the reduced thermal budgets associated with extreme submicrometer features. Present noncontact temperature measurement schemes based on pyrometry, infrared transmission, and Raman scattering each suffer from some combination of high cost, measurement complexity, process sensitivity, and insufficient resolution. Recently, we have proposed and demonstrated a technique based on thermal expansion of a grating etched into the Si.1 A highly precise temperature readout (±0.5°C) is achieved in a projection moire configuration in which the grating on the wafer is referenced to a second grating formed by interference of two laser beams at the wafer surface. Various geometric configurations have been investigated. A major benefit of this technique is that it is insensitive to the inevitable tilt and warpage of the wafer under the extreme thermal conditions of RTP. An optical Fourier transform technique is demonstrated that allows for a direct temperature measurement. Signal processing techniques have been developed both for optimizing the signal-to-noise ratio, and hence the resolution, and for measurement automation.
Quantum optics of a laser-cooled atomic point sample
Authors: Heinzen D. J. , Miller J. D. , Matthews M. R. , Cline R. A.
Published: February 2023

A small number of atoms that are confined to dimensions on the order of an optical wavelength are expected to exhibit novel collective behavior, including superradiance, subradiance, and unusual photon statistics and spatial correlations. We have identified a technique by which such a wavelength-sized atomic point sample may be realized in the laboratory. Atoms that are strongly confined in an optical dipole-force potential may be laser-cooled via stimulated and spontaneous Raman transitions between ground state atomic sublevels. Because the ac Stark shift of the two lower state sublevels is the same, the atoms may be cooled and trapped simultaneously. Also, interatomic radiation pressure and collisional effects can be kept to a minimum in a dipole force trap. We are currently pursuing such experiments using laser-cooled Rb atoms. We have successfully loaded laser-cooled Rb into a dipole force trap and have also driven the stimulated Raman transitions between the Rb ground state sublevels. We will report on our experimental progress and discuss their application to fundamental investigations in quantum optics.
InP photonic integrated circuits for local area networks
Authors: Renaud Monique
Published: February 2023

The very high speed optical loop architectures that are now emerging for LANs would benefit from access nodes that preserve the transparency of the network and that would be capable of multifunctional operation at high bit rates (1,2.4,… Gb/s). The transparency implies that all functions are based on optical signal processing such as, for example, optical header interpretation or optical synchronization. Achieving all functions in a single monolithically integrated photonic circuit is obviously very attractive but necessitates combining several different devices, such as optical switches, detectors, modulators, and optical amplifiers, on the same substrate. Also, closely spaced high speed electronics are desirable. Monolithically integrated InP photonic circuits specially designed for such access nodes to an optical ring operating at 1 Gb/s will be presented. The integration schemes and the electrical and optical characteristics of the integrated devices will be reviewed and dis cussed in view of the system application.
Using an acousto-optic image correlator to measure real-time tissue motion in ultrasound images
Authors: Giles Michael K. , Clark Natalie , Harrison Sarah H. , Kast Brian A. , Stalker Terry
Published: February 2023

Angle independent ultrasonic flow imaging has been accomplished by tracking speckle patterns produced by moving tissue in ultrasound images. Scientists at Duke University have developed digital hardware to measure local displacement vectors of speckle patterns in sequential digitized 2-D ultrasound images at video rates.1 Recent experiments demonstrate that Sandia's acousto-optic image correlator can be used to measure these 2-D speckle flow vectors in real time. The accuracy of the acousto-optic system appears to be as good as that of the digital system for the limited data set tested; however, the acousto-optic correlator has much greater potential for ultra-high-speed computation of flow images because of the inherent parallelism of the optical architecture.
Ultrafast optical switching in a dense medium of two-level atoms
Authors: C. Carlos Lizárraga , Aceves Alejandro B.
Published: February 2023

In this work the authors will present analytical results that explain the numerical simulations of M. E. Crenshaw et al.1 on the extended optical Bloch equations that include near dipole–dipole interactions. These simulations have shown the possibility of an ultrafast switch, caused by an incident unaltered pulse in a thin dense medium of two-level atoms. The switching parameter, defined as

Ω
0

ε

, is a ratio of the Rabi frequency to the strength of the near dipole–dipole interaction Our analysis will show an exact solution for the first switching condition,

Ω
0

ε

≈
1

. Furthermore, an asymptotic analysis will show that to leading order in the asymptotic expansion, only three possible final states of the medium occur. These states, if characterized by the population inversion w, are total inversion (w
f
= 1), no inversion (w
f
= -1), or half inversion (w
f
= 0). For the case w
f
= 0, a reduced system in a phase plane is derived. This reduced system will help us understand this self-transparent behavior. We will conclude by presenting the results on the stability analysis in parameter space.
Observation of all-optical switching in nonlinear AlGaAs, X-junctions
Authors: Villeneuve A. , Stegeman G. I. , Aitchison J. S.
Published: February 2023

Recent theoretical results have predicted digital all-optical switching in nonlinear symmetric X-junctions (NLXJ's).1, 2 These devices use the optically induced index change to break the symmetry of the structure, thereby changing the relative proportions of the zero and first order modes present in the double mode center section. The output then routes the signal, depending on the magnitude of the first order component. We have fabricated a range of NLXJ's in AlGaAs; the devices were designed for operation in the half band gap region, where the detrimental effects of two-photon absorption are minimized and purely refractive nonlinearities were observed. We will present experimental switching characteristics for symmetric and asymmetric NLXJ's and compare them with those expected from theory. Autocorrelation measurements were performed on the separated outputs and suggest that the response is instantaneous and digital-like.
Non-cavity-length-matched additive pulse mode-locking laser
Authors: Wang Ching-yue , Shen Jia-qiang , Xiang Wang-hua , Zhang Ruo-bing
Published: February 2023

A new theoretical and experimental model for a non-cavity-length-matched additive pulse mode-locking laser is presented. The equations describing the new kind of laser are also given in this paper. Numerical calculations were completed, and the results show the effect of parameters on the operation of the non-cavity-length-matched additive pulse mode-locking laser. The relations between the compression rate of pulse width and the cavity parameters were obtained, as were the optimum operation conditions for this new kind of laser. In the experiments, a nonlinear antiresonant ring was used to shorten the pulse, instead of a nonlinear coupled cavity. It is very easy to change the amount of the nonlinear phase shift by changing the reflectivity of the beam splitter in the nonlinear antiresonant ring. Stable operation of the non-cavity-length-matched additive pulse mode-locking laser with Nd:YAG as the gain medium was realized. A stable mode-locking pulse train with a pulse width of 88 ps and power of 0.5 W for a 200 MHz repetition rate has been obtained.
Development of a translation-invariant image representation
Authors: Ahumada A. , Tabernero A.
Published: February 2023

The simple cells of primary visual cortex have been proposed to result from Hebbian associative learning. A recent model of this theory1 develops a "column" of units supported by a small patch of visual field. The receptive fields of these units converge to the eigenvectors of the covariance matrix of the retinal inputs. Since many eigenvalues are effectively the same, columns associated with different patches would not in general have units with similar receptive fields in corresponding positions in the columns. A translation-invariance learning algorithm2 can ensure that a layer of linear receptive fields is uniform. It can force the corresponding column elements to have the same receptive field. Simulations of these two processes on regularly sampled arrays show that they can cooperatively find eigenvectors and force corresponding column elements to have the same receptive fields. When the image is sampled at slightly disordered positions, the eigenvectors of different columns no longer are identical, but the two processes still cooperate to give an appropriate output network.
A real-time motion detection system using BSLMs
Authors: Suzuki Hideo , Fukushima Seiji , Nagatsu Akihito
Published: February 2023

Moving objects are optically detected with real-time processing speed by comparing two time-continuous images. In the system, bipolar-operational spatial light modulators (BSLMs)1 are the optical key devices, which are reflective-type ferroelectric SLMs developed at NTT Opto-Electronics Laboratories. First, one input image at time t−Δt was binarized as b(t−Δt) by using the thresholding property of the BSLM1, and b(t−Δt) was kept during the time period Δt. Next, another input image at the time t was binarized as b(t) by using the BSLM2. Background image elimination can be performed by subtracting b(t−Δt) from b(t), and the Exclusive-OR (XOR) operation between them is used for the subtraction. A He–Ne laser beam incident on the BSLM2 is reflected from it and illuminated onto BSLM1. The observed light from BSLM1 is the XOR image between b(t) and b(t−Δt). All of the BSLMs are controlled by one personal computer. The time period Δt is determined only by the processing speed of the BSLMs (a few milliseconds). Therefore, moving objects faster than the TV frame rate can be detected by using this system. Experimental results and system evaluations are presented.
Concentric-circle gratings for surface-emitting, distributed-feedback lasers
Authors: King O. , Erdogan T. , Hall D. G. , Anderson E. H. , Rooks M. J.
Published: February 2023

A new class of distributed-feedback semiconductor lasers termed "concentric-circle-grating, surface-emitting" (CCGSE) lasers has recently been demonstrated.1 Fabricating gratings of the quality needed to produce lasing in well behaved circularly symmetric modes is a challenging problem. We compare here two methods of pattern definition by electron-beam lithography and examine their effects on the performance of CCGSE lasers. We explore issues such as uniformity of both the grating linewidth and periodicity. These features are found to have the greatest effect on the spatial character of the lasing mode.
Monte Carlo simulation of quantized atomic motion in one-dimensional optical molasses
Authors: Marte Peter , Dum R. , Taieb A. R. , Zoller P.
Published: February 2023

A Monte Carlo simulation method is developed to model quantized atomic motion in one-dimensional optical molasses.1 We calculate atomic absorption and emission spectra and energy distribution functions for various J → J′ transitions coupled to linear and circularly polarized light for standing wave configurations.2 We are particularly interested in generation of sidebands in atomic spectra corresponding to quantized atomic motion.3 Our Monte Carlo method is based on propagating an atomic center-of-mass wave packet and simulating a sequence of spontaneous emissions between families of Bloch states in the periodic laser-induced potentials. Propagation of the (periodic part of the) atomic wave function in time is performed by using a variant of the split-operator FFT algorithm. Band structure calculations for these atomic configurations are presented.
Effect of background radiation noise on heterodyne detection efficiency in the use of a diffraction-free beam
Authors: Iftekharuddin K. M. , Karim M. A.
Published: February 2023

Several experiments have been carried out to maximize optical heterodyne efficiency when the Gaussian laser beam is used as the signal field.1 Further, it has already been shown that heterodyne efficiency is significantly improved with the use of the diffraction-free Bessel beam in the absence of noise.2 However, for most of the small signal detection schemes, background radiation noise is known to limit the achievable SNR. In our present work we consider background noise effect on the maximum heterodyne efficiency when the Bessel beam is used as the signal, and we accordingly investigate the tilt and offset effect of the incoming signal.
Methane flux measurement with a near IR diode laser
Authors: Hovde Chris , Stanton Alan C.
Published: February 2023

Methane plays an important role in the chemistry and radiative transport of the atmosphere. Its sources in the biosphere are widely distributed, making it difficult to estimate the net annual flux of methane into the atmosphere or the reasons behind its increasing concentration. We describe eddy correlation methane flux instrumentation that uses a 1650 nm diode laser operating at 22°C to measure absorption in an open atmospheric path. The measurement region is formed by two mirrors, separated by 0.5 m, which are used for multipassing the laser beam (optical path length 53 m). A room temperature photodiode detects the transmitted laser power. Wavelength modulation techniques are used to achieve sensitivity to the 0.2% ambient methane absorbance. Results are presented from a prototype sensor that was deployed 3 m above a capped landfill in Oak Ridge, Tennessee, in November 1991. The measurements were conducted in collaboration with scientists at the National Oceanic and Atmospheric Administration Atmospheric Turbulence and Diffusion Division. We observed high fluxes of methane, 6.4 g m−2 d−1. More recent results also will be discussed.
Single-mode emission from a broad-area diode laser with an external cavity
Authors: Mailhot Sylvain , McCarthy Nathalie
Published: February 2023

Broad-area (BA) semiconductor lasers can provide high power coherent laser beams. However, the optical output of these lasers has a short coherence length and contains several transverse modes, which can cause degradation of the far field pattern. We report near-single-mode oscillation of a cw, 76 μm wide, BA double-heterostructure diode laser with output power up to 100 mW. Mode selection was achieved by using a self-injection-locking technique. The external cavity was formed by using a mirror in front of the BA laser, and an étalon was positioned in between. The results were obtained at feedback levels of about 5% of the output power. By slightly tilting the étalon, it behaved like a scanning Fabry-Perot resonator and selection of the different longitudinal modes of the laser was possible. This technique is applicable to other types of semiconductor lasers. Indeed, we obtained similar results with a cw multimode visible diode laser. In that case, feedback was provided by phase conjugation with a BaTiO3 crystal. Coherence properties and far field patterns will also be presented.
In situ diode laser diagnostics of a plasma etching reactor
Authors: Stanton Alan C. , Oh Daniel B.
Published: February 2023

A major objective of the microelectronics industry is to perfect methods for controlling dimensional accuracy during plasma etching of submicrometer features in wafer substrates. Optical sensors as monitors of gas phase etch chemistry are attractive because they offer the potential of nonintrusive in situ measurements for use as real time inputs to process control. In this study, lead salt infrared diode lasers were used to measure CF4 feed gas dissociation, neutral CF2 reactive intermediates, and CF2O etch products in fluorocarbon-based plasmas used for etching of silicon and silicon dioxide. These measurements were conducted in situ in a GEC reference cell reactor by using high frequency diode laser wavelength modulation techniques and a single-pass optical configuration. Correlations between measured species concentrations and process characteristics, such as etch rates, end point, and selectivity, were identified. For example, diode laser measurement of CF2 concentration was found to be useful as an indicator of etch selectivity. End point detection by diode laser monitoring of CF2O during etching of SiO2 was also demonstrated. Further research and development based on these results are expected to lead to new process control strategies for improved reliability and product yield in plasma etching.
Fundamental and second harmonic dependence of the optical encoding of χ(2) gratings in borosilicate glasses
Authors: Driscoll T. J. , Lawandy N. M.
Published: February 2023

An explanation for the ability to optically encode χ(2) gratings in centrosymmetric materials has eluded experimentalists and theorists for nearly a decade. Central to the problem is the creation of a large internal dc field (~106 V/m) that alternates with the periodicity necessary for quasi-phase matching. These fields are believed to couple to χ(3) of the media to create the χ(2) grating responsible for efficient frequency doubling. Q-switched mode-locked pulses at 1.064 μm and 532 nm were used to create χ(2) gratings in 1 cm thick samples of a high-barium low-alkali borosilicate glass (SK5, Schott Corp.). Preparation studies revealed an IR intensity threshold dependence of the order of gigiawatts per square centimeter. With the preparing IR intensity held constant at a value sufficiently above the IR threshold, we observed an exponential dependence in the optical encoding of effective χ(2) values versus the second harmonic intensity for writing values above 100 W/cm2. As the preparing green intensity is lowered, however, a sharp resonance is seen where the amplitude of the encoded grating is over 2 orders of magnitude above the value of the extrapolated exponential fit. This behavior has been observed at both room and liquid nitrogen temperatures.
Electro-optic probing of integrated circuits by using poled polymers
Authors: Nagatsuma Tadao , Yaita Makoto , Shinagawa Mitsuru
Published: February 2023

External electro-optic probing of high-speed integrated circuits has been conducted by using a semiconductor laser diode and an organic poled polymer as both a sampling pulse source and a superstrate electric-field sensor. Even though polymeric sensors are completely in contact with ICs to increase sensitivity and spatial resolution, the capacitive loading effect is negligible because of a low dielectric constant (ε ~ 3). The poled polymers used are state-of-the-art methacrylate side-chain copolymers having an electro-optic coefficient r33 of more than 10 pm/V at a wavelength of 1.3 μm. Poled polymeric films attached to substrates such as polyimides and glasses are diced into tiny chips of 1–2 mm square and directly bonded to the ICs tested. The laser pulse is incident upon the back side of the polymeric film at an angle of 50° and reflected off at the surface of the interconnect metallization. The longitudinal component of the electric field induces a change in polarization of the laser beam. Measurement at frequencies of over 20 GHz with near-shot-noise limited voltage sensitivity of less than 10-20 mV and internal node measurement of silicon submicrometer LSIs as well as GaAs ICs have been demonstrated.
Femtosecond excitonic photocurrent transients in quantum well striplines
Authors: Knox W. H. , t'Hooft G. W. , Nuss M. C. , Miller D. A. B. , Doran G. E. , Cunningham J. E. , Chiu T. H.
Published: February 2023

Electro-optic sampling and tunable infrared femtosecond optical pulses are used to measure photocurrent transients generated in GaAs quantum well coplanar microstrip samples at 77 K. This combination allows us to investigate the dynamics of photocurrent with 300 fs time resolution while exciting near the exciton resonance. We find no evidence for ac–dc Stark effect (below-bandgap field-induced optical rectification) signals at small detunings with the electric field applied either parallel or perpendicular to the quantum wells; however, other photoconductive overshoot transients are observed.
The perceived sharpness of blue computer displays at video rate presentations
Authors: Prasad K. Venkatesh , Julesz Bela
Published: February 2023

In order to quantify the perceived sharpness of blue color, a Landolt-C (a ring with a gap of 1.25 minutes of arc in visual angle) was rendered in blue at randomly chosen orientations. The observers' perception of the Landolt-C was termed as "highly sharp" if they reported its orientation correctly for at least 80% of the presentations. All displays contained a saturated yellow background, so as to isolate the blue cone system. When the Landolt-C was preceded by either no field or by a red or green filled circle, at least 100 ms were needed for a highly sharp perception. Contrary to our expectations, when a blue field preceded the blue Landolt-C, a highly sharp perception resulted, even at display durations as brief as 50 to 67 ms. We believe that this novel finding has implications for high definition television (HDTV) and graphic visualization methods, which have incorrectly assumed, until now, that blue color always yields a poor spatio-temporal resolution.