bluuebunny/crossref_metadata_2025 huggingface dataset:
Achieving high gain (65 cm-1) in photorefractive BaTiO3
Authors: Ewbank M. D. , Vazquez R. A. , Cudney R.S. , Bacher G. D. , Feinberg Jack
Published: February 2023

Because of their large optical nonlinearities, photorefractive materials are candidates for use in optical devices. Typically, gains of a few thousand are achieved (e.g., in image amplification) with intensity two-beam-coupling coefficients in the range of ~10-20-cm-1.1,2 In this paper, we demonstrate two-beam-coupling coefficients as high as Γ = 65 cm-1 for extraordinary polarized light in a wedged crystal of BaTiO3 at a near-optimal grating spacing and orientation. This coupling coefficient for extraordinary polarization is, to our knowledge, by far the largest measured to date in any bulk photorefractive sample, and it causes the intensity of a beam to double after traversing only 100 μm of the crystal.
Encoding for error correction in optical computing
Authors: Ellett Scott A. , Walkup John F. , Krile Thomas F.
Published: February 2023

Optical processors can perform complex calculations in parallel at high speeds. They commonly suffer, however, from low accuracy, which currently hinders their widespread use. Various models for the noise in optical processors have been developed.1 In addition, various techniques have been developed to improve their accuracy. One technique involves the use of error-detection and correction codes to improve the accuracy of optical linear algebra processors.2 This allows some of the errors that occur during computation to be detected and possibly corrected. These codes, developed earlier for data transmission and fault tolerance in electronic processors, can thus be used to improve accuracy. Many of the required encoding and decoding operations can be performed optically. Computer simulations of optical matrix-vector multipliers employing various error codes will be presented. They indicate that a significant improvement in accuracy can be obtained when they are compared with processors not employing error codes.
Optically addressed CMOS VLSI liquid-crystal spatial light modulators
Authors: Jared David A. , Slagle Timothy M. , Johnson Kristina M. , Wagner Kelvin
Published: February 2023

A new class of optically addressed spatial light modulators (SLMs) based on complementary metal-oxide semiconductor (CMOS) very-large-scale integration (VLSI) CMOS VLSI and liquid-crystal technology is discussed. The SLMs consist of a liquid-crystal modulator on a CMOS VLSI backplane containing photodetectors and analog electronics. In this manner, by combining the flexibility of semiconductor electronics and the parallelism of liquid-crystal modulator, SLMs can be designed to perform a wider variety of computational functions. New optoelectronic computing architectures are possible by providing a tighter coupling between the electronic and the optical processing.
Optical measurements of soot and droplet size in the near-tip region of a high-temperature and -pressure diesel spray
Authors: Parker Terence E. , Morency Joseph R. , Foutter Richard R. , Rawlins W. Terry
Published: February 2023

Spectrally resolved emission measurements and multiwavelength extinction measurements are used to probe diesel fuel sprays at temperatures of 700-950 K and pressures of 20-30 atm. The emission measurements are performed by using a 12-element infrared spectrometer that operates in the 6-12 μm wavelength range and is sampled at 125 kHz. These measurements are performed in the near-tip region of a diesel injector, where the optical thickness in visible and ultraviolet wavelengths is a major barrier to traditional measurements. The spectrally resolved emission measurements can be used to identify sooting zones within the fuel spray by means of the characteristic spectral signature of soot at flame temperatures. In addition, molecular radiators are clearly present in both oxidizing and pyrolyzing sprays. The spatial resolution of these measurements is 1.2 mm at the spray centerline. A multiwavelength, coaxial extinction measurement in the near-tip region of the spray has also been demonstrated. The outputs from a CO2 laser (9.2μm), an Nd:Yag laser (1.06μm), and a He-Ne laser (0.633μm) were aligned and focused to a beam size of 0.25 mm at the spray centerline. The ratio of these extinction measurements is a function of droplet size and is relatively insensitive to optical depth (transmissivity of the spray in the visible region was typically below 50%). Droplet diameters were observed to be approximately 6μm in the near-tip region of the spray.
Two-photon photodissociation of H2O in flames by a KrF excimer laser
Authors: Pitz R. W. , Cheng T. S. , Wehrmeyer J. A. , Hess C. F.
Published: February 2023

Two-photon photodissociation of H2O by means of a narrowband tunable KrF laser at 248 nm is investigated for use as an H2O measurement technique in flames. To the authors' knowledge, these are the first reported spectral measurements of two-photo-dissociation fluorescence of H2O in flames.
Hysteresis of the electro-optic effect and its frequency dependence in ferroelectric vinylidene fluoride/trifluoroethylene copolymer films
Authors: Zhao Ji , Zhang Dapeng , Gamo Hideya
Published: February 2023

Vinylidene fluoride/trifluoroethylene (VDF/TrFE) has a stable ferroelectric property and can produce single-harmonic generation.1 We investigated the electro-optic coefficients in VDF/TrFE thin films and found that the electro-optic effect is relevant to the dipole alignment of the thin film and to the frequency of the applied field.
Photorefractive square-law converter
Authors: Vachss F. , Hong J. , Campbell S.
Published: February 2023

As a method of optically addressed spatial light modulation (SLM), real-time holography in photorefractive materials offers a highly desirable combination of dynamic range, resolution, and sensitivity. Unfortunately, true holographic recording requires a reference beam that is coherent with the information-bearing beam. This requirement was eliminated with the development of the photorefractive inchoherent-to-coherent converter (PICOC) device,1 which used an incoherent information-bearing input beam to locally erase a photorefractive grating. In attaining compatibility with incoherent sources, however, this device sacrificed the wide dynamic range of real-time holography. In particular, uniformity of response is obtained only with weak signal beams—a situation inherently resulting in a low-contrast output. To circumvent this difficulty we have devised a new photorefractive SLM technique in which high-contrast and incoherent-source compatibility are simultaneously achieved. Specifically, by superimposing our input image on an external fixed-amplitude grating and incoherently imaging the composite result into a photorefractive crystal, we may write a corresponding index grating into the material. We show that this grating may be read out by means of standard holographic techniques to yield a coherent replica of the image without the contrast loss associated with the PICOC device. In particular, we show that although this new device offers resolution equivalent to that of the PICOC, substantial improvements in both the dynamic range and the available range of response speed are achieved.
Video microscope with submicrometer resolution
Authors: Young Matt , Mechels Steven
Published: February 2023

We have constructed and evaluated a video microscope for performing dimensional measurements on optical fibers—what the industry calls the gray-scale method. We think that our results may be of interest to video microscopy, machine vision, and critical dimensional measurements.1 Our microscope consists of a common 40×, 0.65 N.A. microscope objective, a 550 nm interference filter, an illumination system, and a chargecoupled-device video camera, all mounted on a miniature optical bench. The field of view is 150 × 150 µm, so the image can encompass the entire end face of an optical fiber. Using a calibrated reticle, we have calibrated the distance scale of the microscope with an accuracy of ~±50 nm over the 150µm field of view. Using a calibrated linewidth standard, however, we have found a systematic error of 0.1-0.2µm on measurements of lines whose width is many times the resolution limit of the system. Because this systematic error depends on the illumination of the sample, we think it will be necessary to develop a calibrated fiber before the systematic error can be reduced below ~0.1 µm.
Additive-pulse mode-locking with a birefringent fiber
Authors: Feldman S. F. , Moore R. A. , Weinberger D. A. , Winful H. G.
Published: February 2023

This paper models the dynamics resulting from the use of a birefringent optical fiber as the nonlinear element in the external cavity of an additive-pulse mode-locked laser. Most previous researchers of this problem have taken a scalar approach that assumes that the state of polarization is preserved in the fiber. We build on their work by using a simplified laser model;1 however, we use the full nonlinear theory to model the evolution of the state of polarization in a birefringent fiber. We investigate the effects of nonlinear pulse shaping obtained with a highly birefringent fiberand the effects of the fast-axis polarization instability that arises in a weakly bire-fringent fiber.2 In either case the birefringence has a significant effect on the dynamics of the laser system.
Ultrafast coherent transients in chromophors in glasses
Authors: Moshary F. , Arend M. , Hartmann S. R.
Published: February 2023

Time delayed four-wave-mixing with incoherent light was used to study optical superposition state relaxation in sharp cut color filters (Coming CS2-58 and CS2-59) at room temperature and in Nile Blue in ethanol at 4.5 K. Laser light a 6240 A with a 400 A bandwidth and nsec duration was split and directed to an interferometric setup which introduced a controlled delay between the two split components. The two pulses (components) with wave vectors K1 and K2 were recombined in the sample. The resulting induced emissions along the phase matched relative pulse delay. In the color filters, the traces became gradually asymmetric about zero delay as the optical density (thickness) of the samples was increased. This asymmetric character is due to inhomogeneous broadening. We understand this chagne to be due to a modification of the spectral range of the excitation. The traces indicate T2 is approximately 30 fs. For the Nile Blue dye in ethanol glass, we find that the S0-S1 transition is inhomogeneously broadened with a Tx of approximately 10 fs.
Erbium-doped fiber amplifiers and their applications to optical fiber communications
Authors: Desurvire E. , Giles C. R.
Published: February 2023

In this paper current issues in Er fiber amplifiers will be discussed, including the comparison of device performance with 980-and 1480-nm pumps, fiber design optimization, spectroscopic characterization of Er:glass hosts, and theoretical modeling.
Retinal densitometry in the diagnosis of visual pigments
Authors: van Norren Dirk
Published: February 2023

The technique of retinal densitometry makes profitable use of the fact that visual pigments bleach under the influence of light. By comparing the amount of light reflected by the fundus in fully bleached and fully dark-adapted states, the spectral density and kinetics of visual pigments can be assessed in vivo. Since the 1950s the retinal densitometer has evolved from a room-filling apparatus to a handsome box the size of a fundus camera. The latest version from our laboratory scans at 20 wavelengths and is controlled from a microcomputer.
Recovery of nonrigid motion and structure
Authors: Horowitz Bradley , Pentland Alex P.
Published: February 2023

Previous work on motion has almost uniformly relied on the rigid-body assumption, which is motivated by the need to estimate a limited number of parameters. Nonrigid motion, however, is both common and difficult to deal with by means of rigid-body schemes. We introduce a model of nonrigid motion that is based on the finite-element method and which produces accurate, overconstrained estimates of both rigid and nonrigid motion by means of a simple linear-regression technique. This method is based on the realization that the intrinsic elastic properties of real materials imply that nonrigid motion is not completely unstructured; rather, it has limited variability and may be accurately described by a small number of easily estimated parameters. Several examples using gray-scale and x-ray imagery are presented.
Large decline of giant aerosols from solar aureole observations
Authors: Volz Frederic E.
Published: February 2023

Because of their strong forward scattering, large airborne particles of as much as 100 μm in diameter, which originate from industry and transportation (especially from roads and highways), can nearly always be detected at the edge of the sun’s disk. During May and June, airborne pollen sometimes makes these aureoles significantly brighter (and colored). In the balance between their production, dispersal by turbulence, and fallout, their brightness generally increases significantly during the day. By measurably suppressing the glare, it was found that in observations since 1974, the average brightness (and, therefore, the column load) of giant aerosols, gradually decreased to approximately one-tenth of that in 1979. This is approximately eight times faster than reduction of pollution measures in the central and eastern United States (~4% per year), which is related to small-sized haze and smoke. A corresponding, yet small, effect can be deduced from high-volume aerosol collections from urban areas in two size ranges. Had those 24-hour and all-weather collections been made in daytime and had they been for fair weather only, the results might have confirmed our conclusions more quantitatively.
Contour reconstruction in computer-aided holography
Authors: Ladas Kostas T. , Tsihrintzis George A. , Maleki Mohammad
Published: February 2023

In this presentation we generate images of the directional derivative and the Laplacian of the complex index of refraction of a weakly inhomogeneous scattering object. The object is embedded in a known background medium and is probed with coherent plane optical wavefields. The intensity distribution of the wavefields diffracted by the object is then recorded over lines perpendicular to the direction of propagation of the incident waves and is used as input data to a diffraction-tomographic procedure that generates estimates (reconstructions) of the directional derivative and the Laplacian of the index of refraction of the scatterer. The reconstruction algorithm is of the usual filtered-back-propagation type of diffraction tomography,1 in which appropriate modification of the tomographic filter is required. The success of the procedure, even when only intensity data are available (as is the case in the usual optical scattering experiments),2 is discussed briefly and is verified through a number of examples that use both synthetic and experimental data.
Coupling of self-pumped phase-conjugate oscillations to reflections and transmissions of incoherent beams in barium titanate
Authors: Venkateswarlu Putcha , Dokhanian Mostafa , Sekhar P. Chandra , George M. C.
Published: February 2023

If an extraordinary polarized laser beam A1 is incident on an electrically poled BaTiO3 crystal, the specular reflection A1R1 and the second surface reflection A1R2 decrease in intensity, while the phase conjugate A1* and the associated transmissions A1T1, A1T2 de velop.1 These effects can be dramatically observed with self-pumped phase conjugate oscillations, which are cross-coupled to the transmissions and reflections of an intersecting incoherent beam. A He-Ne laser beam A1 (25mw) with an angle of incidence,, enters the crystal at 2mm from one edge. A1*, A1T1, and A1T2 show regular oscillations, while A1R1 and A1R2 show oscillations, our of phase with those of A*l. If an incoherent beam A2 with ,, is put on, A*2 and A2T1 decrase in intensity and exhibit oscillaitons through beam coupling.
Silicon/liquid-crystal optoelectronic implementation of early vision models
Authors: Drabik Timothy J. , Guest Clark C.
Published: February 2023

Promising models for early vision have been developed based on (1) diffusive behavior of a unipolar activation quantity within a layer of neuromorphs, and (2) center-surround interactions among these layers.1 These two types of in teraction can map neatly into an optoelectronic paradigm: A neural layer can be realized as a silicon/liquid-crystal device2 carrying a two-dimensional array of elements with optical inputs and outputs. Diffusive intralayer interactions are implemented by means of horizontal resistive networks, as demonstrated by Mead.3 Center-surround receptive interactions correspond in the paradigm to space-invariant holographic optical interconnections between successive layers. Optical implementation of all except near-neighbor interactions enables a high density of electronic neural elements to be achieved. We discuss the optoelectronic implementation of functions similar to the neural analog diffusion-enhancement layer of Siebert and Waxman,1 and we treat the issues of circuit design, light modulator implementation, optical system design, cascading of devices, and elementary applications.
Optical implementation of visual cortex operations by means of incoherent optical neurons
Authors: Wang C. H. , Jenkins B. K.
Published: February 2023

The incoherent-optical-neuron (ION) model, in conjunction with optical weighted interconnections, can be used to implement arbitrarily connected neural networks. The ION model uses two different device responses, an inhibitory response and a nonlinear output response, to realize a complete neuron. We describe the use of this model to implement simple cells of the visual cortex. Such simple cells perform the operations of edge detection, orientation selection, and, in the case of moving objects, direction and speed selection. Hubei and Wiesel1 model the visual cortex as a hierarchical structure with simple cells as the building blocks. Higher-level operations are modeled as aggregates of simple cells.
Optical two-layer neural network for handwritten-character recognition alphabet
Authors: Qiao Yong , Psaltis Demetri
Published: February 2023

We describe a two-layer optical neural network. The system uses photorefraction holograms to implement synaptic interconnections and liquid-crystal light valves (LCLV's) to perform nonlinear thresholding operations. Kanerva's sparse, distributed memory was implemented using this system, and its ability to recognize a handwritten-character alphabet (A-Z) has been demonstrated experimentally in Kanerva's model; the first layer has fixed, random weights of interconnections, and the second layer is trained according to an outer-product scheme. A total of 104 handwritten-character patterns, with four patterns from each class, were used to train the network. When a character pattern is presented at the input of the trained network, one of the 26 output neurons is switched on, indicating which character the input pattern is. For this network to solve more complicated problems, it is desirable to make both layers adaptive. We propose a new local learning rule that trains both layers based only on local information and a global, scalar error signal. This learning rule is much easier to implement than the back-error-propagation algorithm, which requires the backpropagation of error signals through the network, but it still guarantees that the network is trained by energy descent.
Circularly symmetric, surface-emitting distributed-feedback semiconductor laser
Authors: Erdogan T. , King Oliver , Hall D. G.
Published: February 2023

Recently a novel circularly symmetric, surface-emitting distributed-feedback semiconductor laser has been proposed 1,2 Potential advantages of this laser include the emission of a narrow, circularly symmetric beam with a single, stable wavelength and its direct application to high-power two-dimensional laser arrays. To date, this laser has only been examined theoretically. Previously, we described the operation of this laser in terms of the familiar coupled-mode formalism.2 We predict its operation to be similar to that of the one-dimensional distributed-feedback laser, with the exception of more densely packed modes owing to the azimuthal dimension. Although we expect the major results of this analysis to be correct, it cannot accurately predict the intensity distribution in the circular laser because it ignores the dependence of gain and refractive index on intensity; this is especially true at the center, for which a very high intensity is predicted. In the present paper we consider the effects of this intensity dependence on the spatial variation of intensity in the laser. We also look at some details of fabrication of a working laser, including successfully fabricating second-order circular gratings and designing a laser with efficient current injection and surface light emission.
Miniature external cavities for coherent-beam combining
Authors: Leger James R.
Published: February 2023

External laser cavities have been used to improve the performance of single-stripe semiconductor lasers, broad-area lasers, and semiconductor-laser arrays. The external laser cavity is no longer restricted to a planar topology, and this provides a degree of flexibility and control over a two-dimensional array that does not exist for a monolithic structure. Micro-optics can be introduced to shape and modify each laser beam; other optical devices, such as spatial filters and gratings, can affect the entire ensemble of beams, assisting in establishing mutual coherence and wave-front control. Several different external cavity structures are compared and contrasted in this presentation. Cavities based on spatial filtering are shown to exhibit good discrimination between lateral modes and can be miniaturized by using graded-index lenses. Talbot cavities can also provide laser coupling and lateral-mode discrimination with overall lengths of a few millimeters. Fractional-Talbot cavities have recently been demonstrated with cavity lengths as small as 200 µm. Finally, interlaced arrays of microlenses and micromirrors have been shown to provide aperture filling and laser coupling with overall cavity lengths of 100 µm.
Effect of CO2 laser radiation on ion-exchanged glass waveguides
Authors: Albert Jacques
Published: February 2023

For some classes of ion-exchanged waveguides in glass, the index increase that results from the exchange process is strongly influenced by the associated buildup of surface stresses. In particular, for potassium-sodium exchange in soda-lime glass, the index increase is almost entirely from stress, and the waveguide is birefringent. This suggests modifying the waveguiding properties by altering the state of stress, either permanently or temporarily. One of the most convenient ways of doing this is to supply controllable amounts of heat to local areas of the surface by means of CO2 laser pulses, which are strongly absorbed in glass. Various experimental results are reported in which permanent and temporary modifications to optical waveguides are obtained with power densities as low as 0.38 W/cm2. In particular, efficient modulation at kilohertz rates, annealing of surface scattering losses, and damage thresholds are reported.
Hybrid mode-index/diffractive achromatic waveguide lenses
Authors: Spaulding Kevin E. , Morris G. Michael
Published: February 2023

Conventional waveguide lenses suffer from a large amount of chromatic aberration, which limits their usable wavelength range to a relatively small interval around the design wavelength. If unstable or broadband light sources are used in the system, this chromatic dispersion can significantly affect the performance of an integrated optical device. It is shown that an achromatic waveguide lens can be made with a hybrid mode-index/diffractive doublet. With this approach it is possible to cancel the longitudinal chromatic aberration of the refractive (mode-index) lens by an appropriately chosen diffractive lens element. With typical waveguide materials, a 10 mm focal length, f/5 hybrid lens can be made to have a usable wavelength range of approximately 80 nm; this is a more than order-of-magnitude improvement over conventional mode-index and diffractive lens elements. The magnitude of the wavelength range for a hybrid achromatic lens is approximately proportional to the f/number of the lens, so it is possible to get even larger wavelength ranges with correspondingly larger f/numbers. Under certain circumstances, the power of the diffractive element in the hybrid achromatic doublet can be shown to go to zero, producing an achromatic mode-index singlet.
In situ scattering measurements in the vacuum UV
Authors: Herren Kenneth A.
Published: February 2023

The National Aeronautics and Space Administration's In Situ Contamination-Effects Facility at Marshall Space Flight Center has been used to measure scattering from molecularly contaminated optical surfaces in the VUV. Light scattering has been measured on good optical surfaces in real time during molecular-contaminant deposition. The measurements used noncoherent VUV sources with a predominant wavelength of 1236 Å. The scattered light was detected by a set of three solar-blind VUV photomultipliers. These photomultipliers were arranged at fixed angles, coplaner with the source and target mirror, to measure the forward scatter, specular reflection, and backscatter from the material surfaces. These measurements have been complemented by BRDF measurements made with the Center for Applied Optics' BRDF facility. Optical samples were measured at 6328 Å before and after contamination to correlate with the in situ VUV measurements. This permits verification of possible scaling rules for BRDF measurements between the two regions. The experiment is described, and results of the ongoing program to characterize optical materials exposed to the environment of space are reported.
Learning in neural nets using projection methods
Authors: Yeh Shu-jen , Stark Henry
Published: February 2023

The method of generalized projections is applied to the multilayer feed-forward neural-net problem to derive a new learning algorithm.1 Unfortunately, strict adherence to the learning rule requires the solution of coupled, nonlinear algebraic equations. By permitting a simplification, a new learning rule results, which produces an implementable algorithm. We call this learning rule the projection-method learning rule (PMLR). We apply the PMLR to two well-known pattern-recognition problems, neither of which is solvable by a linear-discriminant scheme. The PMLR is compared with the error-back-propagation learning rule (BPLR) and is shown to converge faster than the BPLR for the problems being considered.
Fully planar glass waveguides with grating
Authors: Li M. J. , Najafi S. I. , Hill K. O.
Published: February 2023

In another paper we have reported ion-exchanged glass waveguides with submicrometer etched gratings. In this paper, we report on the fabrication and characterization of ion-exchanged waveguides with ion-exchanged gratings. A two-step process is used to make these components. First, potassium-ion exchange is employed to produce the channel waveguides. Second, silver-ion exchange is used to fabricate the submicrometer (0.4-0.5 μm) gratings. Two-layer photoresist processing and a holographic setup with a He:Cd laser are used to make the gratings. To determine the efficiency of the grating, light from a He:Ne laser is coupled into the waveguides, and the ratio of the power in the first-order diffracted waves to the guided power at the output of the waveguides is measured. This calculation yields a value of approximately 1% when the above pair of ion-exchange processes is used. The efficiency can be modulated by using other ion-exchange processes or by applying an electric field during ion exchange.
Fiber laser incorporating internal Bragg reflectors
Authors: Ball G. A. , Morrey W. W. , Waters J. P.
Published: February 2023

Holographic Bragg grating formation, in the core of a single mode Nd3+ doped fiber, has been used to fabricate a fiber laser. Bragg gratings were produced by exposing the core to a two-beam interference pattern, with the wavelength selected to lie in the oxygen-vacancy defect band of germania near 244 nm.
Laser-scan fluorescence microscope with annular excitation optics
Authors: Kawata Satoshi , Arimoto Rieko
Published: February 2023

We discuss the three-dimensional imaging characteristics of a laser-scan fluorescence microscope with an annular pupil in the excitation optics. As is well known, the use of an annular pupil in a conventional incoherent imaging system increases the depth of focus and the lateral resolution as compared with the use of a circular pupil.1,2 However, an annular pupil has not been practically used for fluorescence microscopy because it stops a large amount of the fluorescent light arriving at the objective lens. In the case of a laser-scan fluorescence microscope, we may place an annular pupil in the excitation optics rather than in the imaging optics. In this case, we do not waste fluorescent light because the objective-lens pupil is fully open.
Generation and detection of ultrashort pulsed electromagnetic radiation by large-aperture photoconducting antennas
Authors: Darrow J. T. , Zhang X.-C. , Mack M. , Auston D. H.
Published: February 2023

Large-aperture photoconducting antennas have been used to both generate and detect ultrafast submillimeter electromagnetic radiation. When photoconductively driven by optical pulses with energies in the range of 10-4 J/cm2, emitting antennas have produced far-infrared peak electric fields that are comparable to the bias field. Large-aperture emitters of semi-insulating InP with various electrode spacings (1,4, and 10 mm) have been measured. In contrast with previously used dipole antennas, which have a 5 μm radiation-damaged silicon-on-sapphire photoconducting gap, our new detectors have electrodes spaced 0.1-1 mm apart and also have a larger dynamic range.
Optical measurements of lateral depletion in high-speed GaAs quantum-well coplanar striplines
Authors: Ramaswamy M. , Knox W. H.
Published: February 2023

We investigate the spatial distribution of the light modulation that is produced by quantum-well excitons in an applied electric field. We study the field configuration in which the field is applied parallel to the quantum-well layers because this geometry is well-suited to high-speed traveling-wave modulation schemes.1 In our experiments high-resistivity (semi-insulating) quantum wells are required to reduce ohmic dissipation and dielectric losses for terahertz electrical signals. We have measured the spatial distribution of the electroabsorption of quantum-well excitons by constructing a high-magnification infrared microscope in which the sample is uniformly illuminated with narrowband laser-diode light at the exciton wavelength.2 We image the transmitted light onto an infrared charge-coupled-device camera and digitally subtract the image as we vary the bias voltage. We obtain a compete color-mapped difference image, which allows us to obtain the lateral depletion profile versus bias, polarity, etc.
Qualitative shape recognition
Authors: Pizlo Zygmunt , Rosenfeld Azriel
Published: February 2023

Machine-vision research shows that accurate three-dimensional (3-D) reconstruction of objects is difficult. Human-vision research shows that human judgments about 3-D features (depth and volume) are not accurate. This converging evidence, along with the known human ability to recognize shapes, suggests that accurate recognition of 3-D shapes is possible without accurate reconstruction. We consider only a part of this general problem, namely, recognition of 2-D shape in 3-D space. The proposed method is based on a decomposition of the differences between a presented and a standard shape into two components. One component is the amount of perspective difference (i.e., the difference produced by perspective transformation), and the other is the amount of nonperspective difference. An estimate of tilt is necessary for this decomposition, although the sensitivity of recognition to errors in tilt is small. The magnitude of the nonperspective difference is a measure of dissimilarity between shapes (if the dissimilarity is small, the shapes are considered to be identical), and the magnitude of perspective difference is an estimate of slant. Comparison of simulation results with psychophysical results suggests that the method is a plausible model of human shape recognition.
Measurements of multiscattered lidar returns
Authors: Hutt D. L. , Bissonnette L. R.
Published: February 2023

A multiple-field-of-view lidar has been developed, which makes simultaneous measurements of the direct backscattered lidar signal and multiscattered signals. The source is a Q-switched Nd:glass laser operating at 1.054 μm. The pulse energy is 1 J, the pulse duration is 25 ns, the pulse repetition rate is 0.1 Hz, and the divergence is 3 mrad. The novel feature of the instrument is the multielement detector, which consists of four concentric silicon photodiode detectors with active area diameters of 1.5,5,10, and 15 mm. Placed at the focus of a 150 mm f/1.33 lens assembly, the detector defines half-angle fields of view of 3.75,12.5, 25, and 37.5 mrad. The central field of view receives the direct backscattered power from the outgoing laser pulse, and the outer fields of view receive multiscattered contributions. The signal from the central detector constitutes a conventional lidar signal.
Nonuniform subband coding by retinal ganglion cells
Authors: Buchsbaum G. , Miller D. J.
Published: February 2023

The lateral geniculate nucleus (LGN) in primates comprises cells with greatly varying numbers and contrast sensitivities. Cells in the magnocellular layers are approximately eight times more sensitive to luminance contrast than are the color-sensitive cells in the parvicellular layers. Parvicellular cells, on the other hand, outnumber magnocellular cells by approximately a factor of 10.1,2 Kaplan and Shapley1 report that these differences in contrast sensitivity result from differences in the sensitivity of the retinal ganglion cells that provide excitatory synaptic impulses to the LGN neurons. These cells, with varying sensitivities and densities, constitute an imagecoding scheme in which dense high-rate samples of an image with relatively low contrast sensitivity are supplemented with lower-density samples with better contrast sensitivity. If the few higher-contrast samples are sufficient to compensate for the low contrast of the densely populated cells when coding the image, then considerable dynamic contrast range can be saved.
Picoliter viscosity measurements using nondegenerate two-wave mixing
Authors: Adler C. L. , Lawandy N. M.
Published: February 2023

Nondegenerate two-wave mixing (NDTWM) has been previously used to measure the relaxation time of solid-state systems. In an artificial Kerr medium, the relaxation time is the time for a particle to diffuse a portion of the index grating induced by radiation-pressure forces. This time is proportional to the viscosity of the medium. Our work demonstrates the use of NDTWM as a viscometer for picoliter volumes. Experiments were performed to measure the viscosity of suspensions of 0.090 µm diameter polystyrene microspheres titrated with known quantities of water and glycerol. Good quantitative agreement with tabulated values was found for the viscosity of water-glycerol solutions. Unlike viscometer techniques, which typically require large volumes of fluid, this technique can be implemented with fewer than 50 pL of sample volume, making it ideal for biological work.
Multifield, circularly polarized seeded stimulated Raman scattering
Authors: Flusberg A. , Fulghum S. , Tekula M. , Lotem H. , Rokni M.
Published: February 2023

Multifield Raman seeding may be applied to stimulated rotational Raman scattering and is promising for beam-quality control when the Raman gain-length product is large (50). The laser is seeded by two copropagating seed beams, e.g., 1st and 2nd Stokes. The rotational Raman polarization selection rules dictate that each beam must be circularly polarized with alternate heliocities. Because gain suppression is then absent, the Raman gain for seed amplification is high and discriminates well against noise (spontaneous Raman scattering). A parametric process coupling any four distinct waves recursively generates arbitrarily high-order Stokes waves. Because the beam quality of parametrically generated waves should be determined by that of the incident beams, good-quality conversion can occur at very high gain–length products. Code predictions support this conclusion under the following conditions: (1) seeds intense enough to discriminate against initiation by noise, (2) reasonably good beam quality and smooth edges of the incident beams, and (3) good spatial and temporal overlap between the incident beams. In a two-seed experiment on the S(1) transition of H2, it was found that a goodbeam-quality beam core was converted and amplified, but a poor-quality background was also amplified. The Raman code indicates that the observed mixture between good and bad beam quality arises from competition between noise and seeds, which is exacerbated by imperfect temporal overlap between the laser and seed pulses.
Frequency-doubled operation of a ground-state-depleted laser using the Nd3+4F3/2-4/9/2 transition in Y2SiO5
Authors: Beach R. , Albrecht G. , Mitchell S. , Comaskey B. , Solarz R. , Krupke W. , Brandle D. , Berkstresser G.
Published: February 2023

A ground-state-depleted laser has been demonstrated at 912 nm in the form of a Q-switched oscillator operating on the Nd3+4F3/2-4/9/2 transition in Y2SiO5. Samarium scandium gallium garnet has been demonstrated to be effective at selectively suppressing the competing and much stronger 4F3/2-4/11/2 lasing transition. Efficient harmonic generation has been demonstrated by using noncritically phase-matched KNbO3.
Two-dimensional imaging through diffusing media using 150 fs gated, electronic holography
Authors: Valdmanis J.A. , Leith E.N. , Chen H. , Chen Y. , Rudd J.V. , Dilworth D. , Lopez J.
Published: February 2023

We have developed a new ultrafast optical technique for two-dimensional imaging of absorbing objects embedded in diffusing media that appears to be well suited to imaging through human tissue. Preliminary experiments have demonstrated the capability of imaging, with sub-millimeter resolution, objects buried in diffusing material several centimeters thick that are not observable when normally illuminated. ⋙ When a femtosecond pulse propagates through a diffusing medium, it is greatly stretched in time due to multiple scattering.
Close-coupled diode-pumped NYAB green laser
Authors: Lin J. T. , Hwang M. Y. , Rowe S.
Published: February 2023

A new self-frequency-doubling crystal of NdxY(1−x)Al3(BO3)4 [NYAB] was investigated for the generation of green light. In a close-coupled scheme using a 3 mm flat/convex crystal, we have achieved more than 80 mW green with a pumping diode laser of 1 W.
Structure determination from intensity measurements using diffraction tomography
Authors: Maleki M. , Devaney A.J. , Schatzberg A.
Published: February 2023

A novel tomographic imaging procedure has recently been proposed [1] for reconstructing the complex index of refraction profile of a semi-transparent, three-dimensional inhomogeneous object from measurements of the intensity of the fields generated by the object in a sequence of scattering experiments. This procedure is briefly described and compared with earlier work of a similar nature by Wolf [2] and Carter [3], and is illustrated by a number of computer simulated examples. We also report on experimental results obtained using the procedure to reconstruct the index of refraction profile of an optical fiber suspended in an index matching bath. The experiment consisted of illuminating the suspended fiber with a collimated He-Ne laser beam, and measuring the resulting far-field intensity pattern with a scanned photomultiplier tube. The measured intensity pattern, which is effectively a Gabor hologram of the scattered field, is then employed in the reconstruction algorithm implemented on a PC. The reconstruction of the index profile of the fiber is found to be in very good agreement with the actual index profile as estimated from manufacturer specifications and optical microscopy. The talk includes a brief discussion of the use of the imaging procedure in coherent optical microscopy of optically thick samples.
Coupled-Cavity Resonant Passive Modelocked (RPM) Nd:YLF Laser
Authors: Keller U. , Woodward T. K. , Sivco D. L. , Cho A. Y.
Published: February 2023

Coupled cavity modelocking with a resonant nonlinearity in the coupled cavity is a new technique to passively modelock laser media with small gain saturation cross section and was introduced with a Ti:Sapphire laser [1,2]. As an extention of this work, we passively modelocked a cw pumped Nd:YLF laser with an InGaAs/GaAs strained layer MQW [3] resonant nonlinearity in the coupled cavity and generated 4 ps pulses at a wavelength of 1.047 µm with 390 mW average output power at a 250 MHz repetition rate, corresponding to a 1.6 nJ pulse energy. The RPM Nd:YLF laser consists of a linear folded, astigmatically compensated cw Nd:YLF cavity with a 3% coupler to an external low-Q cavity with a nonlinear InGaAs/GaAs quantum well end reflector. The overall tunable output coupler is in the coupled cavity and is typically ≈ 50%. The Nd:YLF rod is cw end- pumped with only 1.5 W at 798 nm from a Ti:Sapphire laser. RPM does not require active cavity length control for stable modelocked pulse trains [4], Diode pumping and higher repetition rates should deliver a practical high power high repetition source for photonic switching applications.
Investigation of conjugated polymers for nonlinear optical applications
Authors: Townsend Paul D. , Fann W. S. , Baker G. L. , Etemad S. , Jackel J. L. , Zoos Z. G.
Published: February 2023

Conjugated polymers such as polydiacetylenes possess large third-order optical susceptibilities in their transparent-gap regions, making them potentially attractive materials for applications in all-optical signal processing. To investigate this potential, we have developed techniques for fabricating a variety of integrated optical structures, such as channel-waveguide directional couplers, from thin films of soluble polydiacetylenes.1 At 1.06 µm these devices exhibit thermal switching at average power levels of 10 mW, and the ultrafast electronic part of the device reponse is dominated by two-photon absorption (TPA), which gives rise to intensity-dependent transmission.2 Spectrally resolved measurements of this TPA have been analyzed in terms of a correlated-electron model, and it has been found, contrary to expectation, that the nonlinear properties of the polymer are dominated by states lying well above the one-photon energy gap.
Chaotic behavior in the switching dynamics of periodic nonlinear media of finite length
Authors: de Sterke C. Martijn , Sipe J. E.
Published: February 2023

The properties of periodic nonlinear systems of infinite length are well understood. If the periodicity is somehow "weak," as would he the case in optical fibers and waveguides, the properties are similar to those of the Thirring model from field theory, which is well known to be integrable and to have soliton solutions. In the low-intensity limit, the system satisfies the nonlinear Schrodinger equation, which is integrable as well, and also has soliton solutions. With the introduction of boundaries, one finds, depending on the parameters, strikingly varied behavior. On one hand, one can find regimes in which the behavior is very soliton-like, which indicates integrability. On the other hand, one also observes "chaotic" regimes, which are characteristic of nonintegrable systems.
Nonlinear dynamics of four-wave mixing in photorefractive materials
Authors: Sterz S. , So D. , Saleh B. E. A. , Tataronis J. A.
Published: February 2023

The dynamics of optical phase conjugation by four-wave mixing in a single-grating, anisotropic photorefractive material are described by a system of five nonlinear partial differential equations, which govern the complex amplitudes of the waves and the grating. Exact solutions of these equations are available only in the steady state. Numerical solutions have been found by Bledowski et al.1 under the assumption that the phase difference between the probe and the forward pump is constant and that the transit time is negligible. We developed a finite-difference scheme to study the dynamics of the phase-conjugation process for various waveforms of the probe and pumps. In particular, we examined situations for which the grating and the conjugate wave have sustained values, even after the probe has been turned off. We was found that this occurs for certain values of the coupling coefficient and certain pump magnitudes. Sustained gratings initiated by pulsed probes may have applications to dynamic holography.
High-performance optical wavelength multiplexer/demultiplexer
Authors: Dobrowolski J. A. , Hara E. H. , Sullivan Brian T. , Waldorf A. J.
Published: February 2023

The principle of an optical-wavelength multiplexer/demultiplexer, in which the signals undergo repeated reflections from special filter elements that can be designed for any desired cross-talk ratio, is described. In theory, a cross-talk ratio of 60 dB should be easy to achieve. The insertion losses of these units can be quite small, allowing a number of filter elements designed for different wavelengths to be placed in series. The resulting device can provide simultaneous, multichannel two-way transmission. In a preliminary investigation of an experimental prototype, an insertion loss of 0.5 dB, a cross-talk ratio of 35 dB, and a wavelength separation of 0.05 μm were demonstrated. The multiplexer/demultiplexer is expected to have a long life and a high reliability. It is compact and should lend itself to low cost mass production.
Femtosecond degenerate four-wave mixing in multiple quantum wells at 300 K
Authors: Norwood D. P. , Swoboda H.-E. , Dawson M. D. , Boggess T. F. , Smirl A. L. , Hasenberg T. C.
Published: February 2023

We have investigated the decay of optically-induced, short-period (≅120 nm) gratings in GaAs/AlGaAs multiple-quantum-well structures with a barrier thickness LB of 1.4-15 nm. The crystals were excited with 100 fs pulses at wavelengths either resonant with the n = 1 exciton or into the bands. The decay of the diffraction efficiency of the gratings shows two components: a fast initial contribution that is independent of lb and has a decay time of 200-300 fs and a slower exponential contribution that depends on lb and has decay times between 1.5-90 ps. The latter component is determined by perpendicular transport by means of over-barrier hopping and phonon-assisted tunneling among adjacent wells and is consistent with a simple-rate-equation model based on these transport mechanisms and the recombination of the excited carriers. The fast initial component is observed only for resonant excitation and may be connected with the exciton ionization and/or reorientation. To investigate this feature, we performed two-beam self-diffraction experiments with cross-polarized pulses. Here, we observed signals in the background-free direction consistent with self-diffraction from an orientational grating, whereas we saw a fast increase of the transmission in the direction of the transmitted beam
Ionization of atomic hydrogen in superintense, high-frequency fields of circular polarization
Authors: Pont M. , Gavrila M.
Published: February 2023

We present the first quantitative results for the ionization of hydrogen at relatively high-frequencies, from low intensities to superintensities in excess of the atomic unit I0 = 3.51 ö 1016 W/cm2. We find that at superintensities the atom has the tendency to stabilize against multiphoton ionization, even at low frequencies. We signal, however, the existence of very short lifetimes for the ground state, at intensities just below those needed for stabilization. This fact will prevent the neutral atom from feeling the superintense peak values of the field even for the shortest existing laser pulses.