bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast optical matrix-vector multiplier
Authors: Anemogiannis Emmanuel , Kenan Richard P.
Published: February 2023

In recent publications1,2 a single-mode fiber signal processor was proposed capable of performing Toeplitz matrix-vector multiplication at gigahertz. We propose an integrated-optic Toeplitz matrix-vector processor that should be capable of operation at several tens of gigahertz. The multiplier is implemented in a Ti:LiNbO3 waveguide. It comprises an optical delay line, tapped by N electrooptic gratings of efficiency ηi = i/N, i= 1,2,…, N, located a distance L = Tc/n apart along the line. Here T is the period of the input data stream, and n is the effective index of the waveguide, typically around 2.2 for TE waves in Ti:LiNbO3 at 633 nm. The light from the taps is directed to a second set of gratings which implement the matrix entries. The resulting refracted beams are passed on to an integrating lens. Clearly, the smaller the distance L, the higher the data rate (1/T) that can be implemented. For L = 2 mm, data rates of 68 GHz are allowed. Design details for a 2 × 2 multiplier, planned for device demonstration, are presented.
Photorefractive nonlinear optics
Authors: Valley George C.
Published: February 2023

This tutorial starts with the single-defect single-charge-carrier rate equation model for the photorefractive effect first presented by Vinetskii and Kukhtarev in 1974. Results for important physical parameters such as beam-coupling gain, response time, and photorefractive sensitivity are given for this model, and extensions of the basic model to include two charge carriers, two or more defect levels, shallow donors, or acceptors are discussed. Correspondence between the rate equation model and the hopping model and conventional semiconductor results is given.
Photorefractive grating formation in Bi12SiO20 using high-intensity short-pulse write beams
Authors: Jones D. C. , Solymar L.
Published: February 2023

Photorefractive gratings recorded in a crystal of BSO and illuminated by a frequency-doubled Nd:YAG laser (wavelength, 533 nm; pulse length, 7 ns) at grating spacings of 1–8 m are studied using applied fields up to 7 kV/cm. The gratings are probed by a Bragg matched beam from a He–Ne laser. It is found that the diffraction efficiency initially increases linearly with pulse energy, then it saturates at a value which is proportional to the square of the applied field and which increases with increasing grating spacing. The energy density required to saturate the efficiency decreases with increasing grating spacing. The theory is based on the equations of Kukhtarev et al.,1 which are solved numerically using a Runge-Kutta-Merson integrating routine. The experimental results are in most cases in good agreement with the theoretical predictions.
Picosecond polarization rotation by photorefractive and free-carrier index gratings
Authors: Dubard J. , Smirl Arthur L. , Cui A. C. , Valley George C. , Boggess Thomas F.
Published: February 2023

Recently, we reported extensive measurements of the energy transfer in picosecond two-beam-coupling experiments in semi-insulating GaAs in which the photorefractive gain competed with (and was obscured by) nonlinear loss and transient energy transfer from the Drude-Lorentz free-carrier index grating.1 Here, by using a crossed polarizer-analyzer pair in a similar geometry, we measure the pump-induced rotation of the probe polarization as a function of pump fluence, time delay between the two pulses, pump polarization, analyzer angle, and sample orientation. The photorefractive and free-carrier index gratings are expected to have dramatically different dependences on these experimental parameters. We illustrate these dependences and use them to identify, separate, and contrast the contributions of the two gratings. One advantage of this technique, which we demonstrate, is that the contribution of the photorefractive grating to the cross-polarized two-beam coupling can be separated from the contribution of the free-carrier grating by controlling the pump polarization. Thus we separate and measure the photorefractive gain using this technique, and we compare it to the gain deduced from our conventional scalar two-beam coupling measurements and to a modified version of our original theory.
Feasibility of cleaning and recoating optical surfaces in space environments
Authors: Heaney J. B. , Herzig H. , Osantowski J. F. , Toft A. R.
Published: February 2023

The idea of coating mirrors in space was considered seriously as long ago as 1965–66. The inspiration for this arose from the realization that pure aluminum is the only mirror coating material that has a high reflectance in the 300-2000-Å spectral region, but this high reflectance is immediately lost due to oxidation in a terrestrial environment. Dielectric overcoats that are used to prevent oxidation inevitably reduce the high reflectance of pristine aluminum, especially at wavelengths below 1100 Å. In recent years, our knowledge of the space environment has increased greatly, and, in this era of the Space Shuttle and a future space station, our ability to operate and do manufacturing in space is vastly improved. In addition, the large scale optical surfaces that are now proposed for use in space should be considered candidates for in situ refurbishment.
Laser damage tests on reactive ion plating multilayer coatings
Authors: Guenther Karl H. , Mansour Kamjou
Published: February 2023

Dielectric multilayer coatings for laser mirror use at 532-nm wavelengths have been produced by low-voltage RIPD and tested for their laser-induced damage threshold.
Comparison of carbon films between dc glow discharge and dc magnetron sputtering processes
Authors: Oh Inki
Published: February 2023

Diamondlike hard carbon coatings were deposited on germanium in a dc plasma discharge vacuum chamber using a hydrogenated hexane fluid. Vapors from the hexane fluid were mixed with argon gases, keeping a 0.1-molar ratio of hexane to argon ions. The dc cathode voltage of 1.0 kV and the gas pressure of 50–100 mTorr were maintained for 30 min for the hard carbon coatings. The deposition process was reproducible and accurately controlled by the two independent parameters: dc cathode voltage V and gas pressure P. On the other hand, amorphous hard carbon films were prepared by dc plasma magnetron sputtering of a graphite target in ultrapure argon gas at a pressure of 5-10 mTorr at a substrate temperature of 100°C. The dc power range was 300-500 W. Both techniques of deposition of carbon films show extreme hardness, chemical inertness, high electrical resistivity, and optical transparency in the IR 8-12-μm region. The carbon films generated with the dc plasma of hydrocarbon gases result in high transmittance values compared with those of films generated by dc magnetron sputtering of graphite targets.
Chromophores in tissue for laser medicine and surgery
Authors: Goldman Leon
Published: February 2023

Early in the development of laser medicine and laser surgery, endogenous chromophores in mammalian tissue established the great value of the color selective action of lasers in the visible light range. From these models, there developed laser treatments of incurable birthmarks, cancer surgery, surgery of the heart and blood vessels, and even the treatment of tattoos. With the subsequent development of photodynamic therapy, a successful cancer treatment program was established. This was based on the localization and induced fluorescence of cancer tissue by derivatives of the chromophore and hematoporphyrin. The concept of laser thermophotolysis of Anderson and Parriash and the development of the technology of exogenous chromophores expanded the applications of lasers not only to cancer tissue but to potential uses for toxins, antigens, antibodies, etc. through conjugation and subsequent destruction of the combination by lasers with selective wavelengths. The uses then of selective localization and types of exogenous chromophore are limitless for laser applications for mammalian tissues.
Diffusion of light in turbid material
Authors: Ishimaru Akira
Published: February 2023

The diffusion characteristics of light in a turbid medium depend on whether the medium is dense, sparse, tenuous, or nontenuous. It also depends on whether the medium may be continuum or discrete scatterers. Plane-wave incidence, point-source excitation, and beam waves need to be considered as well as rough surfaces and interface effects. The range of validity of first-order scattering, multiple scattering, and diffusion approximation is discussed. Both volume scattering and surface scattering are considered as well as pulse diffusion, angular and temporal spectra and correlations, and polarization effects.
Propagation of femtosecond and picosecond laser pulses in turbid tissues
Authors: Jacques Steven L.
Published: February 2023

The steady-state distribution of light in a turbid tissue can be treated by diffusion theory. However, when laser pulse durations shorten into the picosecond (ps) and femtosecond (fs) range, the light does not have time to either become diffuse or achieve a steady-state distribution before absorption takes place. The propagation of ps and fs laser pulses is illustrated using Monte Carlo simulations. Two issues are addressed: (1) the time course of noninvasive reflectance measurements can specify the tissue optical properties in vivo, and (2) transient light distributions influence the distribution of two-photon chemistry in turbid tissues. (1) The total reflectance from a homogeneous turbid tissue is uniquely related to the dimensionless ratio of scattering to absorption. The time course of backscattered reflectance escaping the tissue allows a specification of the mean free path in the tissue in units of cm−1. Together, these measurements specify the tissue optical properties. (2) Two types of two-photon chemistry are considered: (i) simultaneous absorption of two photons and (ii) an initial absorption event which yields an excited state that absorbs a second photon. The duration of the laser pulse affects the spatial distribution of two-photon chemistry.
Measurement of vibrational frequencies of H3 using two-step photoionization
Authors: Lembo L. J. , Huestis D. L. , Helm H.
Published: February 2023

H3 is the simplest of all neutral triatomics and, therefore, of significant fundamental interest. We exploit the metastability of the rotationless levels of the

2
p

A
2
"

state to perform laser spectroscopy on a fast neutral beam of H3.1 In particular, a two-step photoionization scheme has been used to measure the positions of vibrationally excited levels of the n= 3 Rydberg manifold of H3. We find symmetric-stretch vibrational frequencies of 3315 and 3168 cm−1 for the

3

s
2

A
1
'

,and 3d
2
E″states, respectively; these lie close to the calculated frequency of 3189 cm−1 for the ion.2 Overtone frequencies of the 3p
2
E′state are determined as well.
Cyclopean experiment
Authors: Richards Whitman A.
Published: February 2023

An optical device, similar to that used in a microscope, presented the same view to each eye, as if both eyes were superimposed at the midline of the forehead. Surprisingly, the world seemed to possess the same rich depth sensations seen with normal binocular viewing. The depth obtained with the cyclopean view is enhanced compared with monocular viewing as if the disparity cue is available (although it is not). Flat postcard scenes inspected through the cyclopean device elicit a similar enhanced depth sensation. Normally, each eye receives identical images only for distant scenes, when the fixation directions are parallel. For such distant scenes the flat stereoscopic plane should not force a flat world percept. (Indeed it is very appropriate for strong monocular cues and high-level knowledge to modulate the depth-disparity relationship, even at all fixation distances.) For random fields, however, where the correct 3-D configuration can be recovered only with binocular parallax (such as a tree which has rich but complex occlusion clues), properly scaled stereoscopic disparity should dominate monocular information. Hence, for random textures, the cyclopean view appears flat (unless an element distinguishes itself by motion etc.).
Generalization by adaptive networks
Authors: Pavel Michael
Published: February 2023

Automatic feature extraction by adaptive networks is an attractive approach to pattern recognition for many visual and image processing tasks where the underlying features are unknown. These networks, composed of units that perform nonlinear memoryless transformation on the sum of their inputs, can be viewed as a particular implementation of adaptive pattern recognition schemes. For a given problem, the network is usually trained by repeated presentation of examples from a training set and modified until a solution is reached as defined by a criterion performance. As in any trainable pattern recognition scheme, the fundamental problem concerns the networks’ capabilities to respond appropriately to novel patterns. This ability is referred to as desirable generalization or induction. First, it is demonstrated that the specific generalizations depend on particular solutions obtained to the training set and that the number of possible solutions is likely to be very large. Consequently, even the simplest networks that can solve a given problem are usually too unconstrained to produce desirable generalizations. Thus the network pattern recognition problems are frequently ill-posed due to the fact that the power or ability to solve an arbitrary problem conflicts with constraints required for good generalization. For the networks to generalize in a desirable manner it is necessary to add additional constraints in the same manner as regularization is used for other ill-posed problems.
Neural network models that learn to extract visual cues
Authors: Sejnowski Terrence J.
Published: February 2023

Algorithms for analyzing visual scenes often depend on isolating the relevant visual cues and extracting the cues from images. For example, disparity is a cue for binocular depth estimation, and the depth is extracted by solving the correspondence problem. Problems arise, however, when more than one cue is relevant and the complexity of their interactions makes it difficult to analyze all possible contingencies. A new technique has recently been developed for neural network architectures in which the relevant cues and their contingencies are learned from examples. Images and their correct interpretations are supplied to the learning algorithm, which constructs feature detectors and parameter filters to solve the problem. The application of neural network learning algorithms to two problems in vision is presented: The extraction of shape parameters from shaded images of surfaces and the estimation of depth from binocular images. The properties of processing units in the trained networks are compared with those of neurons in mammalian visual cortex.
Enhanced 3-D reconstruction from confocal microscope images
Authors: Conchello Jose A. , Zelten J. Peter , Miele Frank C. , Davis Bruce H. , Hansen Eric W.
Published: February 2023

Confocal scanning microscopes are known to possess superior optical sectioning capabilities compared to conventional microscopes. Out-of-focus contributions in a through-focus series of images are significantly reduced by the confocal geometry but not completely removed. This paper reports our initial investigations Into a posterioriimage processing (i.e., deconvolution) for further improvement of depth resolution in confocal microscopy. This project is part of a larger effort In laser scanning fluorescence microscopy for biological and biophysical analyses in living cells. The instrument is built around a standard inverted microscope stand, enabling the use of standard optics, micromanipulation apparatus, and conventional (including video) microscopy in conjunction with laser scanning. The beam is scanned across the specimen by a pair of galvanometer-mounted mirrors driven by a programmable controller which can operate In three modes: full raster scan; region of interest; and random-access (point-hopping). After taking a scout image with laser scanning or video, the user will select isolated points or regions of interest for further analysis via a graphic user interface implemented on the system’s host computer. Experimental parameters such as detector integration times are set up with a window-style menu.
Optical homodyne phase metrology
Authors: Mertz Lawrence
Published: February 2023

The phase of interference fringes identifies with their position, and since we can measure the position of individual photons we can measure their individual phases. Estimating average fringe parameter information requires at least three detector channels to sort out the three parameters of brightness, contrast, and phase. A convenient Young’s fringe interferometer with a lenticular receiver and simple digital circuitry is demonstrated for displaying efficient ratiometric phasor diagrams in real time from discrete photons. A series of high-speed circuits that require no analog switching are also presented for resolving phase from somewhat brighter signals to four, six, or even 256 sectors per fringe at multimegahertz sampling rates. In all cases the digital format is suitable for real-time recursive phase unwrapping and averaging by using a read-only memory and multiplier-accumulator to implement the polar innovations filter.
Testing and evaluating high-power laser diodes
Authors: Landry M. J. , Rupert J. W. , Mittas A.
Published: February 2023

This paper describes the performance of high-power GaAIAs laser diodes (LDs) constructed by (1) Spectra Diode Laboratories (SDL) with multiple stripes containing single and multiple quantum wells and (2) McDonnell Douglas Astronautics Co., Optoelectronics Center (MDAC/OEC) with a single broad stripe containing a single quantum well. The LDs are fabricated using metalorganlc chemical vapor deposition (MOCVD) technology and are mounted in TO-3 cans, TO-18 cans, C-packages, or LCB-packages. All SDL and mix MDAC/OEC LDs had coated cavity mirrors. The characteristics described include (1) optical power vs current for LDs operating at rated power, four times rated power, and catastrophic power levels (SDL 2 and 3.4 W, MDAC/OEC 1–5 W), (2) efficiency vs optical power (SDL 13–40%, MDAC/OEC 25–50%), (3) cavity filamentatlon, beam steering, and divergence changes after LD turn-on and with increasing optical power as observed using near- or far- fleld LD beam intensity distribution, (4) changes in beam divergence as observed by monitoring changes in fiber-optic transmission, (5) changes in SDL-LD output power and efficiency due to aging and to thermal degradation, where aging is caused by operating at elevated optical power, and thermal degradation is caused by operating at different shot time intervals when LDs are fin- or thermoelectric-cooled heat sink.
Using high-power laser diodes as the optical source in hybrid optical firing set systems
Authors: Landry M. J. , Rupert J. W. , Mittas A.
Published: February 2023

This paper describes the performance of several hybrid optical firing set (HOFS) systems which Use Spectra Diode Laboratories (SDL) High-power laser diodes (LDs) as the sources. HOFS 3 and 4 of the first generation systems required 1.7 and 0.62 s to store 100 mJ in a 0.05-μF capacitor at an efficiency E of 2.1 and 2.0%, respectively. HOFS 3 used two 100-mW LDs operating at optical power (OP) = 400 mW (E = 21 and 32%), two 0.23-pitch Selfoc GRIN lenses optical couplers (OC) (transmission T= 97%), and two Applied Solar Energy Corp. PVA 1-V GaAs monolithic cells (E = 49%) connected in series to a Gamma High Voltage MC-10N-MOD to dc converter. HOFS 4 used a 500-mW LD operating at OP = 2 W (E = 23 %), 600-μm Enslgn-Bickford fiber optics (FO) OC (T= 80%), 2-V cell (E = 31%), and MC-10N-MOD dc to dc converter. HOFS 12 and 15 of the second generation systems required 0.35 and 0.80 s to store 100 and 200 mJ of energy in a 0.05- and 0.11-μF capacitor at E = 3.1 and 5.5%, respectively. HOFS 12 and 15 used a 4-W LD operating at OP = 3.4 and 1.8 W (E = 40 and 37%), 200-μm FO (T = 87 and 89%), 2-V hybrid cells (E = 32 and 45%), and model MC25/M143 dc to dc converter. If each component in HOFS 12 and 15 is optimized, the system efficiencies would increase to 19.1% and the capacitor charge times would decrease to 0.056 s.
Pattern discrimination using phase-only information
Authors: Chalasinska-Macukow Katarzyna
Published: February 2023

Correlation methods play an important role in optical information processing and pattern recognition. In some applications, however, the light efficiency of optical correlators is not sufficient. Recent works have shown that the problem may be avoided by using one of several types of pure phase filter. One type of phase-only filter was proposed by Homer and Gianino,1 another one, the tandem component filter by Bartelt.2 These results initiated a series of studies analyzing the possibilities of more complicated, both continuous and binary, phase-only filters. There are several advantages of restricting the filter function to binary values only. This approach is particularly useful for real-time applications with spatial light modulators.
Phase-only filters
Authors: Bartelt Hartmut O.
Published: February 2023

Spatial filters are important elements for optical signal processing systems, e.g., for correlation operations. In this context, phase-only filters which modulate only the phase of an optical wavefront are of special practical importance. The main advantage of phase-only filters is demonstrated by its high light efficiency, which is very attractive for passive optical systems without light amplification. In addition, such low-loss filters would be relevant to high-power applications. Phase-only filters also differ from conventional amplitude filters in their performance.
Family of optical signal processing algorithms which generalize correlation
Authors: Kallman Robert R.
Published: February 2023

We recently introduced a variant of the standard optical correlator architecture and algorithm which is quite useful for extracting objects of interest from complicated backgrounds. This new architecture differs from the standard correlator architecture only in the use of a detector element in the center of the Fourier plane. The spatial filtering step in this new architecture is done with a phase-only spatial light modulator, but good phase-only filter design is a bit challenging. We now show that this variant is just one of an infinite family of possible new algorithms and compares the performance of two of them vis-a-vis recognition and discrimination of realistic targets.
Hybrid optical/electronic implementation of adaptive neural networks
Authors: de Groot Peter J. , Noll Robert J.
Published: February 2023

A reconfigurable optical crossbar switch has been integrated into a microcomputer environment for modeling neural networks that learn. A differential technique is used for obtaining bipolar inputs, outputs, and connection weights in a mask-type crossbar using an inexpensive low-contrast spatial light modulator. Neural networks based on matched-filter techniques and competitive interconnects have been implemented for adaptive associative recall as well as unsupervised learning in an autonomous controller.
Selective focusing into rib waveguide arrays via large-aperture integrated lenses on LiNbO3
Authors: Rastani Kasra , Karim Zaheed , Tanguay Armand R.
Published: February 2023

The integrated optical synthetic aperture radar (IOSAR) processor is one of a number of potentially powerful optical signal processing components that combine many of the throughput characteristics of bulk optical processors with the cost, size, and power advantages of guided wave optical devices.1 As essential elements of such processors, we have developed large-aperture (2.5-mm) integrated lenses with short focal lengths (1.2–2.5 cm). These lenses were fabricated by masking desired concave-shaped lens regions of Ti:LiNbO3 with thin films of SiO2 followed by a proton exchange process.2 Titanium-indiffused proton exchanged (TIPE) slab waveguides were thus formed around the Ti:LiNbO3 concave lens, generating positive focal lengths. The measured throughput efficiencies for our lenses were of the order of 74%. Guided wave optical chips were developed on TIPE substrates with the following integrated elements: Tl lenses, large-area rib waveguide arrays (660 waveguides, 8 μm wide, 1 μm high, 1 cm long, with 2-μm separations), and uniform surface outcoupllng gratings on the rib waveguide arrays (4-μm grating spacing, 0.05 μm high, 1 mm long).1 Individual waveguides of the arrays were selectively coupled Into by the integrated lenses, and the guided modes were then outcoupled by the superimposed surface gratings to access the third processing dimension.
Three-level neural model and its analysis
Authors: Jin Yong , Zhang Jing , Yu Francis T. S.
Published: February 2023

By focusing the major functional operations of association cortex, we propose a three-level neural model to represent the mental activity. The model consists of the basic neuron level, the functional module level, and the module organization level. Notice that the basic neuron performs the physiological functions; the functional module is constructed with certain types of neuron as stimulus transformation; and the module organization operates the intelligent functions by interactions between the modules. Since the sensible information is encoded as the density of action potentials by the sensory, the mental information processes can be represented by potential-density distributions over the association cortex. Hence the mental operations can be analyzed through the potential-density transformation. A new concept, called sensation, is also introduced to indicate the distributed density of action potentials. With this concept, the mental activity, such as long-term memory, short-term memory, abstraction, comparison, and language, can be described as the sensation transformation and the sensation group transformation in the module organization level. The mathematical algorithms to analyze the mental activity within this model are given. The discrete algorithm can be used for analyzing the operations of individual modules, while the continuous algorithm describes the sensation transformation and sensation group transformation.
Training networks to compensate for irregular sampling
Authors: Ahumada Albert J. , Yellott John I.
Published: February 2023

A system with regular sampling at the photoreceptor level can reconstruct an accurate estimate of the original image by low-pass filtering the sampled image with a space-invariant filter. Quality reconstruction of images which are irregularly sampled demands space-variant filters. If the reconstruction is to be an interpolation (correct at the sample positions), the impulse response at each sample point must have zero response at all the other sample points. Two learning models are described that use spontaneous activity and lateral spread of activity to make a distance-preserving copy of the receptor arrangement at a higher level in the nervous system and construct a transformation network which converts lateral spread functions into interpolation functions. The learning models do not use external stimuli or reinforcement and can, therefore, be regarded as self-organizing processes, although the adjustment algorithms are error-correctihg rather than competitive in nature. The distance learning process is easily modified to allow variable magnification with eccentricity. The interpolation process provides a mechanism for adding effects of irregular discrete sampling to models of visual processes which take images as input.
Minimizing errors in analog neural networks
Authors: Caulfield H. John
Published: February 2023

We prove that the relative error in the result of a single-layer neural network is bounded by the product of the relative error in the analog representation of the interconnect matrix multiplied by the condition number of that matrix. We then show that the condition number is readily minimized in a way that does not affect system design substantially. Of the infinite number of equivalent transformations (so far as the condition number is concerned), some reduce the required dynamic range as well.
Fast and convergent Hopfield-Tank algorithms
Authors: Szu Harold H.
Published: February 2023

Difficulties of the Hopfield-Tank (H-T) model reported by Wilson-Pawley [Bio-Cybern. 58, 63 (1988)] in a scale up Travelling Salesman Problem (TSP) have been clarified and repudiated. H-T [Bio-Cybern. 52, 141 (1985)] have successfully integrated the decaying Hamiltonian, H(Vi; a, b, c) equation dU
i
/dt =(U
i
/t) – [∂H(V
j
; a, b, c)/∂V
i
] for the neuron input U
i
given the neuron output V
i
by a sigmoid mapping V
i
= g(U
i
).An improvement is the conservative dynamics eliminating time-consuming computation for every iteration of each analog neuron (good for a buzz-booming real world) and the self-denying interconnect T
ii
< 0 (necessary for VLSI resistive coupling). H-T dynamics is weakened by degenerate ground states due to those deliberately imprecise and inhibitory constraints (modeling the Mother Nature). In artificial problems, a hybrid neuron model consisting of a binary neuron with analog interconnects inputting intercity distances may be efficient. A decisive yes-visit-or-no-visit binary neuron may be better than a wishy-washy sigmoid neuron, for it can revive the ball-balancing game rolling over the energy landscape. The zero diagonal interconnect matrix means no self-talk (traceless T
ii
= 0), which is best for the absolute convergence theorem (self-promoting T
ii
> 0 may be OK, but self-denying T
ii
< 0 is bad) for asynchronous and democratic voting updates by peer neurons excluding itself.
Laser-based spectroscopic measurements in high-temperature gases
Authors: Hanson Ronald K.
Published: February 2023

Laser-based techniques are now widely accepted for nonintrusive probing of high-temperature gases. This paper describes progress in the development of two particular methods: laser-wavelength modulation spectroscopy and planar laser-induced fluorescence imaging. In the modulation spectroscopy, a single-frequency cw ring dye laser is rapidly scanned in frequency across a few cm−1 to record fully resolved absorption line profiles. Both absorption and fluorescence recording are utilized, with the latter providing improved spatial resolution at the expense of quantitativeness for absolute species measurements. The line shape data provide an important source of collision-broadening coefficients, and experiments of this type have been carried out in flames, rf-excited atmospheric pressure plasmas, and shock-heated flows. When multiple lines are scanned, these data can be used to infer temperature at kilohertz repetition rates. The method of planar laser-induced fluorescence offers potential for instantaneous 2-D imaging of several flow-field parameters including species concentrations, temperature, velocity, and pressure. Recent work has been directed toward extending the method to new species, to supersonic flows, and to simultaneous determination of multiple flow-field parameters. Progress in the development of new measurement strategies and improved intensified camera systems is described.
Effects of turbulent flows in a resonator cavity on ring laser outputs
Authors: Tarng J. H. , Yang C. C.
Published: February 2023

It has been found that turbulent flows may exist in the gain media of liquid and gas lasers. With the existence of turbulences in the gain medium, not only the propagation constant but also the gain becomes a spatially and temporally random function because of the random distributions of the number density of active molecules. In this research, the statistical properties, including the average intensity and contrast, of the output intensity of a ring laser are investigated. The gain medium is assumed to be partially homogeneously broadened and slightly saturated. For simplicity, a plane wave is considered. Combined with the perturbation approach, the method of phase screen is used. In this method, the turbulences are assumed to be concentrated within a thin screen which causes random phase modulation. Therefore, the stimulated wave is randomly modulated in phase and randomly amplified in intensity by the gain cell in each passage in a ring cavity. The number of passages is decided by the photon lifetime in this cavity. A Gaussian correlation function is used for describing the random structure. Effects of the turbulence characteristic length and fluctuation strength on the laser output are discussed. Several numerical examples for dye lasers are presented.
Effects of anisotropic behavior on the acousto-electrooptic interaction
Authors: DeCusatis C. , Das P. , Litynski D. M.
Published: February 2023

Both acoustooptic (AO) and electrooptic (EO) interactions can be generated simultaneously in piezoelectric crystals to produce an acousto–electrooptic (AEO) effect. Because these crystals are anisotropic, the AEO interaction is affected by factors such as the crystal cut and polarization of both the optical and acoustic signals. We have developed a general theory to describe such interactions while treating pure AO and EO modulation as special cases. The contributions of both longitudinal and transverse components of surface acoustic waves (SAWs) and EO transducers are developed separately; this theory also describes AEO interactions in isotropic media such as gallium arsenide, which is of interest for integrated optics applications. In particular, because of the anisotropic nature of the crystals, the diffracted light intensity is strongly polarization dependent. The position of the Bragg diffracted orders and the angular bandwidth of the interaction are also affected. We quantitatively studied these effects in lithium niobate and performed computer simulations of the diffracted light intensity in various conditions. In addition, our theory indicates that anisotropic media may exhibit optical properties similar to isotropic crystals in certain conditions.
Thermal self-focusing and stimulated scattering in photorefractive crystals
Authors: Lasher M. E. , Gookin Debra M.
Published: February 2023

Thermal self-focusing in photorefractive crystals differs from thermal effects in other crystals or glasses. Thermal lensing occurs when a refractive-index gradient is produced thermally by local heating with an intense laser beam. In photorefractive crystals, this index gradient creates a new phase matching condition, and light is preferentially scattered off photorefractive gratings. A significant difference between this and other stimulated scattering effects is that the beam divergence angle is intensity dependent. The diffraction angle of an incident probe beam can be significantly increased by the stimulated scattering phenomenon. We present the intensity dependence of the divergence angle for SBN, BaTiO3, and BSO. Divergence half-angles of up to 6° have been observed. We also describe the effects of polarization and orientation on beam divergence.
Construction and calibration of isotropic light dosimetry probes
Authors: Marijnissen Johannes P. A. , Star Willem M.
Published: February 2023

Measurements of the light energy fluence rate in turbid media (e.g., biological tissues) require an isotropic probe, which is a small bulb of highly light scattering material mounted on an optical fiber. For an isotropic response the bulb must be nearly spherical and the photon mean free path in the bulb must be small compared with its diameter. Using an epoxy mixed with a highly light scattering powder we occasionally obtained probes of good quality. Recently, bulbs with a diameter of not more than 0.8 mm have been reproducibly turned on a lathe from a hard white plastic. The probe can be calibrated in a parallel light beam in air. In a medium with refractive index n larger than one, the sensitivity of the probe decreases because more scattered light can escape from the probe. A calibration factor as a function of nis shown. The calibration for measurements in diffuse light has been checked in both a plane and spherical geometry. The optical constants of Intralipid were measured, and with these the energy fluence rate in optical phantoms of Intralipid with and without the absorber was calculated and measured, and good agreement was found.
Measurement of the index of refraction of mammalian tissue
Authors: Bolin F. P. , Preuss Luther E. , Taylor R. C. , Ference R. J.
Published: February 2023

The rapidly expanding field of research into tissue optics has introduced the need to make measurements of the refractive index of tissue. For example, theoretical calculations done on the transport of light in turbid material frequently must take account of boundaries and interfaces where the index changes. The use of traditional methods of measurement is difficult due to the highly absorbing and scattering nature of the tissue medium. A simple and rapid method for finding the refractive index of mammalian tissue is presented. Application to nonbiological materials is also possible. This method utilizes the light piping properties of standard fiber optics. The tissue to be measured replaces the cladding of the fiber, and the output of the fiber optic produces an angularly dependent intensity pattern, which is a function of the refractive index. Resultant values for several mammalian tissues (bovine, porcine, and human) have been determined at 632.8 nm. With few exceptions, for tissues at this wavelength, nwas in the 1.38–1.41 range. Measurements were also made on the effects of tissue hydration and lipid content.
Light propagation in a strongly scattering, weakly absorbing phantom at 633 nm
Authors: Moes Christian J. M. , van Gemert Martin J. C. , Star Willem M. , Marijnissen Johannes P. A.
Published: February 2023

The study of light propagation in tissue is complicated by the anisotropic, inhomogeneous, and variable character of tissue. We, therefore, used a reproducible tissue phantom to study the influence of beam diameter, absorption μ
a
, and scattering μ
s
coefficients and refractive index on the propagation of light in an optical phantom at 633 nm. We used mixtures of Intralipid 10% (scattering medium) and Evans Blue (absorbing medium); the optical parameters of these substances were determined by transmission measurements and by the added absorber method using the solution for a point source in an infinite medium in the diffusion approximation. These parameters are: μ
s
= 38.6 ± 0.4 mm−1 mliter−1 liter and asymmetry parameter g= 0.71 ± 0.03 for Intralipid 10% and μ
a
= (7.60 ± 0.05) × 10−3 mm−1 mliter−1 liter for Evans Blue (515 mg/liter). Measurements of the energy fluence rate using an isotropic probe were performed in media with scattering coefficient of 1 mm−1, anisotropy factor of 0.71, absorption coefficient of 1.3 × 10−3, 0.01 and 0.05 mm−1, and refractive index of 1.33. The experimental results were compared to an analytical solution of the diffusion approximation to the radiative transport equation. The influence of the beam diameter and the absorption coefficient of the medium on the light penetration appear to be large, and the experimental results show reasonable to good agreement with the theory.
Average amplitude matched filter
Authors: Horner Joseph L. , Flavin Mary A.
Published: February 2023

A matched filter in the Fourier plane is a complex function consisting of phase and amplitude. This cannot be written on a spatial light modulator (SLM) in a real-time system. The phase-only filter obviates this problem in that only a phase function needs to be written. We show that a good autocorrelation comparable to a matched filter can be obtained using the exact phase of the reference function and an amplitude mask which is an ensemble average of a class of similar object. Only the phase portion is updated in a library search and can be implemented in real time on a binary phase-only SLM. The position of the amplitude mask is not critical and could be changed by mechanical means to search for different classes of object. Computer simulation results are presented for the average amplitude matched filter with both continuous and binary phase and compared to the standard matched filter.
Contrast affects the perceived direction of a moving plaid
Authors: Stone Leland S. , Mulligan Jeffrey B. , Watson Andrew B.
Published: February 2023

Plaids, the sum of sinusoidal gratings of different orientations, have been useful in determining how the human visual system determines the motion of an object as a whole from information about the motion of oriented components within that object. The intersection of the constraints imposed by the motion of the two oriented components within a plaid (the constraints rule) provides a reasonable qualitative estimate of psychophysical results1 but has yet to be tested quantitatively. We made quantitative measurements of the perceived direction of motion of plaids whose component gratings were of different contrast. Human subjects viewed moving plaids composed of two achromatic luminance gratings of equal spatial frequency and fixed orientation. The perceived direction varied substantially and systematically with the contrast ratio of the components. Up to 20° biases were commonly observed. One possible explanation for our results is a scheme which uses orientation-tuned sensors that subject the estimated speed of the oriented components to a contrast-dependent distortion before applying the constraints rule. The observed significant deviation from the constraints rule represents an important restriction on future models of motion processing.
Factors affecting the perception of transparent motion
Authors: Mulligan Jeffrey B.
Published: February 2023

It is possible to create a perception of transparency by combining patterns having different motions. Two particular combination rules have specific interpretations in terms of physical phenomena: additive (specular reflection) and multiplicative (shadow illumination). Arbitrary combination rules applied to random patterns generate percepts in which the motions of the two patterns are visible but with superimposed noise. It is also possible to combine the patterns (using an exclusive-OR rule) so that only noise is visible. Within a 1-D family of combination rules, which include addition and multiplication, there is a range where smooth motions are seen with no superimposed noise; this range is centered about the additive combination. This result strengthens the finding of Anstis and Mather1 that the motion system deals with a linear representation of luminance and is consistent with the analysis of motion by linear sensors.
Acoustooptic channelized IFM receiver
Authors: Chang I. C.
Published: February 2023

This paper reports the theoretical and experimental work of an instantaneous frequency measurement (IFM) receiver using an acoustooptic (AO) implementation.1 A homodyne interferometric dual Bragg cells configuration was used wherein the reference is derived from the input signal by passing through a wideband delay line, a frequency shifter, and a limiting amplifier. Both the I and Q components of the input signal were measured within each channel by an array of IF phase discriminators. From these results the frequency data were determined. A demonstration breadboard of the AOIFM receiver was designed, built, and tested. A compact in-line interferometric bench was chosen which included a laser diode, a pair of efficient gigahertz bandwidth GaP cells, and an array of fiber-fanout detectors. A local oscillator of the IF of 65 MHz was used to provide the frequency offset between the signal and reference. An IF phase discriminator in turn measured the signal phase for frequency determination. Excellent results were obtained from the demonstration breadboard. These performances include 1-GHz bandwidth, 1-MHz resolution, and 55-dB dynamic range. More details on the theoretical and experimental findings are reported.
Comparison of two types of ellipsometer with rotating polarizers
Authors: Schrader K. N. , Tuenge S. R. , Carniglia C. K.
Published: February 2023

Ellipsometers having rotating polarizers are commonly used in cases where data are to be collected automatically. Two different configurations are frequently utilized. One type involves one fixed and one rotating polarizer. With this arrangement, a dc signal and a signal at twice the rotation frequency of the polarizer are generated. The data reduction involves these signals and thus is adversely affected by fluctuations in the dc illumination level. The second configuration involves two fixed polarizers and a rotating polarizer. The second fixed polarizer introduces an additional signal at four times the frequency of the rotating polarizer. In this case, the ellipsometric data can be reduced using only the phases of the ac components, without being influenced by the fluctuations in signal amplitude. However, the mathematical form of the data reduction makes this method extremely sensitive to errors in the phases in certain circumstances. A single ellipsometer was used to evaluate both of these configurations. The only variable in the system was the presence or absence of the second fixed polarizer; the same optics, detector, and electronics were used for both systems. The results of the comparison for measurements of uncoated silicon and single-layer films on fused silica are presented.
Characterization of thin films with linear inhomogeneity by spectroscopic ellipsometry
Authors: Carniglia C. K.
Published: February 2023

The measurement of the refractive index n and the physical thickness t of a homogeneous thin film by ellipsometry is a well-established technique. Spectroscopic ellipsometry provides n and t measurements over a broad spectral range, thus allowing determination of the dispersion in n. However, spectroscopic ellipsometry opens up the possibility of characterizing inhomogeneous thin films. This paper deals with the theoretical analysis of the sensitivity of ellipsometry to a linear refractive- index gradient within a single-layer thin film. The film is treated analytically by assuming that the reflectance depends only on the refractive indices at the surfaces of the film, thus ignoring the details of the index profile. The reflectance is also calculated by modeling the linear index gradient by a series of thin homogeneous layers and performing standard thin-film calculations. A major result is that to a first approximation, the ellipsometry of a film at a wavelength which is an odd multiple of four times the optical thickness (i.e., at the odd quarterwave points) is unaffected by the inhomogeneity of the film but depends only on the average refractive index of the film. The ellipsometric results for wavelengths which are multiples of twice the optical thickness (the halfwave points) are strongly affected by the inhomogeneity of the films. Results are presented relative to the sensitivity of ellipsometric measurements for the determination of the average index and the degree of inhomogeneity of a film.
Large-area laser diode amplifiers having high coherence
Authors: Andrews John R.
Published: February 2023

When a spatially and spectrally coherent signal is injected into a 100-µm wide traveling wave amplifier, the signal is amplified by up to 18 dB. Optical power in excess of 100 mW can be obtained while maintaining nearly diffraction-limited performance. A detailed examination of the far field of the amplifier allows extraction of quantitative information regarding the gain and thermal distributions in the amplifier. An examination of the coherent optical power-delivering capabilities of these devices is presented.
Monolithic 2-D surface emitting arrays of GaAs/AIGaAs diode lasers
Authors: Donnelly J. P. , Goodhue W. D. , Rauschenbach K. , Seielstad D. A. , Wang C. A. , Bailey R. J.
Published: February 2023

Monolithic two-dimensional arrays of GaAs/ AIGaAs diode lasers with light emission normal to the surface and as many as 100 lasers have been obtained by fabricating edge-emitting quantumwell lasers with external mirrors that deflect the radiation from the laser facets by 90°. All of the laser facets and deflecting mirrors were formed by ion-beam-assisted etching.