bluuebunny/crossref_metadata_2025 huggingface dataset:
Design of optical, wideband antireflective coatings with the Fourier-transform method: effect of constraints on the refractive indices
Authors: Verly P. C. , Dobrowolski J. A. , Willey R. R.
Published: February 2023

Graded index coatings and step down multilayers in which the index is progressively decreased from the substrate index to the medium index are known to be excellent broadband antireflectors. The modified Fourier transform techniques developed and implemented at the NRCC are well suited to the synthesis of graded index films. They can also yield homogeneous multilayers. However they normally assume that the media on either side of the coating are the same. This obviously rules out the design of AR coatings. We have recently described preliminary results obtained by applying our computer program to a problem in which the external media were different, namely to the synthesis of graded index broadband AR coatings on germanium.1 No constraints were imposed on the refractive indices. In this talk, we discuss the effects of restraining the refractive indices within a specified range of values. We show that good solutions continue to be possible. We give examples with refractive index profiles and optical performances that are quite comparable to published results obtained by other methods.
Development of Free-Electron Laser Sources for XUV Projection Lithography
Authors: Newman Brian E.
Published: February 2023

Future rf-linac-driven free-electron lasers (FELs), operating in the extreme-ultraviolet (XUV) range from 4 nm to 100 nm, could be excellent exposure tools for extending the resolution limit of projection optical lithography to ≤0.1 µm and with adequate total depth of focus (1 to 2 µm). The capability of such FELs to produce relatively high-average power at a tunable wavelength with selectable spectral bandwidth will enable these lasers to offset the inherently low throughput of the XUV projection optics which will have about five reflections at ≤50% each. When operated at a moderate duty rate of ≥1%, one XUV FEL should be able to supply sufficient average power to support high-volume chip production for multiple lithographic steppers operating simultaneously. In this multiplexed arrangement, the FEL photon-source cost per wafer level Is projected to be lower than proposed alternatives. Recent progress in attaining a very bright electron beam, short-period magnetic undulator, and high-reflectance resonator mirrors are described which make feasible a near-term demonstration of FEL operation below 100 nm.
Optical speckle correlator using the phase modulation property of liquid crystal television
Authors: Ogiwara Akifumi , Ohtsubo Junji
Published: February 2023

Recently, LCTV has been used in the field of optical information processing because of the low cost of the device and the ease of commercial availability. Although we can expect high diffraction efficiency of light by using a phase-only spatial light modulator, it is difficult to obtain perfect phase-only modulation in a conventional twisted nematic LCTV device. Both the intensity and phase modulation ratios through the LCTV panel are affected by the polarization of input light and the applied video signal voltage. In this paper, we study the intensity and phase modulation properties of a twisted nematic-type LCTV. A nonlinear optical correlator is proposed based on the phase modulation property of the LCTV. To obtain high diffraction efficiency, the video signal having 0 and π phase values is written onto the LCTV panel by the binarization of the input pattern. The dc component in the correlation pattern is suppressed by the phase-only modulation of the LCTV and a clear correlation signal is obtained.
First eleven years of the phase-shifting mask
Authors: Levenson Marc D. , Schellenberg Frank M. , Brock Phillip J.
Published: February 2023

These days, all mass-produced integrated circuits are fabricated using techniques where the image of a mask is projected with light onto a photosensitive polymer. The size of the smallest features formed by such optical lithography techniques is limited by the laws of optics, to ~0.4 µm, but the next generation of integrated circuits will require smaller features. While shorter wavelength light or larger aperture lenses might allow smaller minimum features, those proposed developments would also reduce the depth of focus to unfeasibly small values. The solution to this impasse was actually discovered 11 yr ago. If the mask used to form the image has information about both the phase and the amplitude of the light (rather than just the amplitude), the minimum feature size can be reduced by as much as a factor of 2. In such a phase-shifting mask, destructive interference cancels some of the effects of diffraction and ensures that individual bright features are separated by the required dark lines. The phase-shifting mask idea languished in obscurity before being developed beginning about three years ago by various Japanese semiconductor manufacturers. In the last year or so, remarkable accomplishments have been made public. Working electronic circuits with minimum feature sizes down to 0.17 µm have been fabricated using light of wavelength 0.365 mm. Unhappily the design of phase-shifting masks is an engineering exercise which requires knowledge of optics.
Recovering shape from the profiles of a rotating surface
Authors: Pollick Frank E. , Giblin Peter J. , Rycroft Jeanette
Published: February 2023

Given the profiles (self-occluding contours) of a surface undergoing fixed-axis rotation, it is possible to recover the shape and motion of the surface. The global symmetry of the envelope of profiles allows one to compute the projection of the axis of rotation. Once the projected axis is known, local measurements of the profiles are sufficient to determine the local surface, angular velocity and slant of the axis of rotation. It is also possible to find points on the profile where the image motion corresponds to the projected 3-D motion of points on the surface. This correspondence is instantaneous in the sense that the points that correspond are continually changing. The condition for a point on the profile to be in instantaneous correspondence is that its normal is parallel to the projected axis of rotation.
Extending the range of stereopsis
Authors: Kumar Tribhawan , Glaser D. A.
Published: February 2023

It is known that the disparity discrimination thresholds of a feature increase rapidly as the feature is offset from the fixation plane. We confirmed this fact by measuring the thresholds for a single 10-arcmin vertical line as a function of the offset disparity from the fixation plane. Thresholds for this line were also measured as additional features were added in the field of view. The added features were never in the plane as the single test line but had nonzero disparity relative to it. Thresholds were measured as a function of the number of added features, and configurations with the same minimum relative disparity (the relative disparity of the feature closest in disparity to the test line) were compared. The results show that thresholds are significantly lower when the additional features have crossed and uncrossed disparities with respect to the test line, and that the thresholds were unaffected when the fixation plane disparity was changed by as much as 10 arcmin with respect to the test line.
Third-order nonlinear optical interaction in bioengineered polymers
Authors: Fischer G. , Akkara J. , Aranda F. , Roach J. , Kaplan D. , Rao D. V. G. L. N.
Published: February 2023

A series of polymers and copolymers were synthesized from benzidine and para substituted phenols by the horseradish peroxidase catalyzed reaction in dioxane. Third-order nonlinear optical susceptibility χ(3) was measured for all these bioengineered polymers in a solution of dimethyl sulfoxide and methyl alcohol (in the ratio 4:1), using the technique of degenerate four-wave mixing. The source used for the measurements was a frequency doubled Nd:YAG laser giving 17-ps pulses. The vertically polarized output at 532 nm was split into three beams which are spatially and temporally overlapped in the sample contained in a cuvette of 1- or 2-mm light path. The χ(3) values of polymers were obtained by comparison with a reference sample of CS2. The molecular hyperpolarizability (γ) was obtained from the polymer solution measurements, and the macroscopic value χ(3) for the pure polymer was calculated. χ(3) values of polymers were in the range of 10-9 to 10-8 esu, three to four orders larger compared to the monomers. Relative magnitude of the electronic and orientation contributions to the nonlinearity were determined by changing the probe polarization from parallel to perpendicular orientation with respect to the pump beams. The phase-conjugate signal was also measured as a function of probe delay with respect to pump beams.
Potential seeing improvements using adaptive optics with a guide star
Authors: Aspinwall Dennis M. , Billman Kenneth W.
Published: February 2023

The nature and extent of atmospheric effects that degrade astronomical seeing are briefly reviewed. Based on the temporal and spatial characteristics of the effects, adaptive optics system concepts are considered as a means of ameliorating the atmospheric degradation. Guide stars, artificial beacons formed by laser interaction with matter in the atmosphere, are necessary elements in all these concepts. Starting with an assumed overall astronomical facility performance goal, a system performance error budget is developed which includes subsystem and important component contributions. The technical status of the important system elements, such as guide star laser, mesosphere physics, wavefront sensor, deformable mirror, etc., is then reviewed to allow definition of subsystem performance algorithms. Finally, these are incorporated into a system performance code to enable quantitative projection of overall astronomical facility near-term performance.
Dynamics of a monovelocity atomic beam maser
Authors: Oraevsky A. N. , Sarkisyan T. V. , Jones D. J. , Bandy D. K.
Published: February 2023

We explore the dynamic behavior of a monovelocity atomic beam. The equations describing the monovelocity atomic beam are similar to the Maxwell-Bloch equations. The steady-state, linear stability analysis, and dynamics verify that hysteresis, bistability, and even multistability of the output intensity exist in this system. Comparison of the loss–gain bifurcation curve of this system to the original maser model shows that the instability asymptote for the monovelocity atomic beam is κ = 0 where κ represents the loss parameter while the maser model asymptotes at κ > 1.
Pushing photolithography to its limits
Authors: Brunner T. A.
Published: February 2023

Over the past 16 years, eight generations of dynamic random access memory (DRAM) have been manufactured with each new generation delivering 4× more bits than the last. The 1-kbit DRAM of 1975 has steadily evolved into the 16-Mbit DRAM of today, driving steady advances in lithographic technology. Optical lithography has been used to manufacture all these DRAM generations, and is likely to extend another two or three generations. Optical lithographic tools are marvels of precision engineering with optics capable of projecting 2 billion pixels (distorted by <1/10 pixel) and with stages capable of positioning a 200-mm wafer to within 20 nm of any desired location.
Some new methods for restoring images of faint objects
Authors: Snyder Donald L. , Schulz Timothy J.
Published: February 2023

New methods are reviewed for forming images of faint objects. Camera data are modeled as a doubly stochastic Poisson process to account for quantum limitations and apparent jitter from propagation of light through weak turbulence, camera vibration, and tracking errors. The new methods fall into two categories depending on the model that can be assumed for the jitter. An approach that generalizes the Richardson-Lucy iteration for image restoration from quantum-limited data is obtained via statistical estimation theory when a stochastic model for the jitter is available. Faint-object (i.e., quantum-limited) and bright-object alternatives to the phase retrieval methods of Fienup for second-order correlation data and of Weigelt for third-order correlation data are obtained when the jitter is assumed to be piecewise constant during snapshots of the object.
Spectral properties of photocurrent fluctuations in avalanche photodiodes
Authors: Kahraman Gokalp , Saleh Bahaa E. A.
Published: February 2023

The photoelectric pulse generated by an avalanche photodiode (APD) as a result of a single injected photoelectron is regarded as a nonstationary random function of time (the impulse response function). A discrete stochastic model for the electron-hole motion and multiplication is defined on a spatio-temporal lattice and used to derive recursive equations for the mean, the variance, and the autocorrelation of the impulse response as functions of time. The power spectral density of the photocurrent in response to a Poisson-distributed stream of photons of uniform rate is evaluated. Correlation properties of the impulse response is studied for a conventional and multilayer (superlattice) APD with the same gain and carrier-ionization rate ratio. The conventional APD has a much more correlated response and a higher excess noise factor. Its response is slower; however, the signal-to-noise ratio (SNR) of its current response is greater. The photocurrent generated in the conventional APD from a stream of photons has a narrower power spectral density but a higher SNR. For both devices, the rms current is low at the onset of multiplication, sharply peaks at about the same time but does not die off as rapidly as the mean current. For a high-data-rate optical communication system, this type of noise will enhance inter­symbol interference and limit the bit error rate.
Three-dimensional optical data storage by two-photon excitation
Authors: Strickler James H. , Webb Watt W.
Published: February 2023

We have demonstrated high density (>1011 bits/cc) optical recording of information in a 3-D multilayer format. Information is written in a homogeneous negative photoresist as volume elements of increased index of refraction. The index increase results from crosslinking and shrinkage of the polymer induced by two-photon excitation of the photosensitizer at the waist of a highly focused beam of subpicosecond colliding pulse mode-locked laser. Quadratic dependence of two-photon excitation on incident instantaneous intensity confines crosslinking to the focal volume; therefore bit array layers may be written with thicknesses of the order of the confocal parameter. We have succeeded in writing eight well resolved bit planes in a ~30-µm thick polymer film. Information may be read by differential interference contrast or confocal microscopy. The potential for employing this technique in multilayered write-once-read-many optical storage disks is compared with other schemes for implementing 3-D optical data storage including two-photon fluorescence methods.
Particle image velocimetry fringe processing using an optically addressed spatial light modulator
Authors: Sharpe J. P. , Johnson K. M.
Published: February 2023

The optical fluid flow measurement technique of particle image velocimetry (PIV) relies on photographing, under pulsed illumination, small seeding particles carried along in the flow. After development of the film the local velocity in any area of the flow is given by the separation of particles in the image. One method of determining this separation is to pointwise interrogate the film with a laser beam and measure the spacing of the resultant fringes. This is generally performed by capturing the fringes with a video or CCD camera, digitizing them, and performing a Fourier transform. Automatic measurement systems for this task have been set up in several laboratories, and some researchers have also investigated the possibility of using photorefractives for processing the fringes. In this paper, we demonstrate using a ferroelectric liquid crystal, optically addressed spatial light modulator (OASLM) to perform the Fourier transform much faster than is possible with conventional methods.
Motion mislocation
Authors: Welch Leslie , MacLeod Donald I. A.
Published: February 2023

Pairs of dots in apparent motion are strikingly mislocalized in the direction of the motion. If two pairs of dots in horizontal apparent motion, moving in opposite directions are positioned one over the other (forming a tall rectangle), they do not appear vertically aligned. The rightward moving dots appear mislocalized to the right and the leftward moving dots appear mislocalized to the left. We have investigated this illusion focusing on the influence of added perturbing dots presented to reinforce or oppose the apparent motion created by the test dots. The apparent motion of the test dots can be either nulled or enhanced by such perturbing dots.1 When the test-dot apparent motion is nulled, the mislocation is abolished along with the motion. Likewise, the enhancement of test-dot apparent motion by a third dot on a consistent trajectory increased the mislocation. These observations support the idea that the motion of moving objects is signalled by the motion system rather than by static local signs2 and they also indicate that the motion system integrates or recruits information from multiple views (rather than only two views) of the moving stimulus.
Tunable solid-state lasers for laser radar
Authors: Ewing J. J.
Published: February 2023

The advent of tunable solid-state laser sources will impact a number of laser remote sensing uses. Currently, good spectral coverage can be obtained in the visible and ultraviolet with Ti:sapphire lasers excited by frequency doubled Nd:YAG lasers. Pollutant sensing of species such as NO2 and SO2 can be performed with these lasers. A brief review of the sensitivity calculations and hardware capabilities of Ti:sapphire and Ti:sapphire harmonic sources is given.
How to win SBIR contracts: a panel discussion
Authors: Furomoto Horace , Mortensen Robert , Moulton Peter F.
Published: February 2023

Summary not available.
Properties of glass for integrated optics
Authors: Houde-Walter S. N.
Published: February 2023

The observation of second-harmonic generation and all-optical switching in telecommunications fibers, optical amplification in rare earth-doped fibers, and the demonstration of lasing in rare earth-doped glass channel waveguides have collectively changed the notion of glass as being just a passive material for integrated optics. The diverse range of specifications to which glass properties can be tailored results from, in contrast to their crystalline counterparts, the ability to incorporate very large quantities of dopants and glass modifiers throughout a continuous range of stoichiometries.
Spatial limitations of diffraction-free beams
Authors: Katz Joseph , Marom Emanuel , Barkan Eric
Published: February 2023

Recently, a number of papers treated the so-called nondiffracting beams that exhibit an extended region of confinement. Axicons, when illuminated by uniform or Gaussian beams, generate distributions approaching this definition. Analysis of longitudinal and lateral variations of such beams along their propagating direction is presented. It is shown that some approximate results can be obtained by using geometric optics formalism or simplified integration techniques. Comparison with exact Fresnel integration results identify the usefulness of such approximations.
Time resolved x-ray measurements for the Livermore x-ray laser program
Authors: Carter Michael R.
Published: February 2023

Recent experiments at the NOVA Two-Beam Facility at Lawrence Livermore National Laboratory have concentrated on improving the spatial coherence of the neon like soft x-ray laser using a variety of laser architectures. We are also measuring the linewidth (longitudinal coherence) of the lasing transitions and developing nickellike x-ray lasers for use in biological microscopy and holography. These experiments employ a wide variety of time resolved diagnostic techniques using x-ray streak cameras including: spatial coherence measurements, high resolution (λ/Δλ ~35,000) x-ray laser spectroscopy, a medium-resolution (λ/Δλ ~300) streaked flat-field spectrometer, and a soft x-ray crystal spectrometer (1 Å < λ < 12 Å)(λ/Δλ ~ 400). Several time-gated x-ray laser diagnostics have been developed including an angularly resolved, microchannel plate intensified, grazing-incidence spectrograph and a far-field 2-D laser footprint diagnostic. Time integrated measurements including spectrally resolved x-ray emission and x-ray pinhole cameras are also utilized.
Squeezed solitons: quantum and thermal noise effects
Authors: Shelby R. M. , Rosenbluh M. , Drummond P. D. , Carte S. J.
Published: February 2023

Optical solitons are pulses that propagate in optical fibers without temporal or spectral distortion, owing to balancing of the second-order dispersion of the refractive index by nonlinear self-phase modulation induced by the intensity-dependent refractive index. Classically, this stationary distortion-free propagation only requires the proper choice of pulse width and shape for a given pulse energy. However, quantum fluctuations do not undergo stationary propagation but rather evolve due to dispersion and self-phase modulation. As a soliton pulse propagates, zero-point or vacuum noise associated with the pulse evolves into squeezed noise. Vacuum fluctuations are responsible for the fundamental detector noise floor known as the shot noise limit. Reduction of the noise in the detection of squeezed soliton pulses has achieved a noise level 34% below this limit. In optical fibers, thermal fluctuation of the refractive index limit the noise reduction that can be achieved by adding phase fluctuations in excess of the vacuum noise. At low frequencies thermal noise originates from local structural dynamics of fused silica which produces a 1/f power spectrum and from guided acoustic wage Brillouin scattering which modulates the refractive index at frequencies characteristic of the fiber structure. The bandwidth of these fluctuations is ~2 GHz; and for soliton pulses with spectral bandwidth much >2 GHz, we have shown that this noise can be made insignificant. For very short pulses, however, noise from Raman scattering must be considered. Recent calculations incorporating this noise mechanism are compared with experiment.
Billiard ball soliton interaction gates
Authors: Islam M. N. , Soccolich C. E.
Published: February 2023

We demonstrate a temporal conservative-logic interaction gate that can perform AND, inversion, and routing functions. The billiard balllike logic is based on elastic collisions between temporal solitons in optical fibers, and our cascadable logic gate has two temporally separated, identical frequency, input pulses that are combined using beam splitters. One of the inputs is reflected from a dichroic beam splitter that has a frequency dependent transmission; therefore, one input appears to have a center frequency shifted to longer wavelengths. The two inputs are then coupled into a polarization maintaining, dispersion shifted fiber 1.1 km long, which corresponds to about ten walk-off lengths. At the fiber input we start with two 17-pJ 430-fsec pulses at 1.67 μm separated by 4.5-pulse widths, and the pulses collide about midway in the fiber, completely pass-through, and emerge temporally separated at the output.
Nonlinearities and ultrafast charge transport in an all-binary strained InAs/GaAs hetero n-i-p-i
Authors: McCallum D. S. , Huang X. R. , Dawson Martin D. , Boggess Thomas F. , Smirl Arthur L. , Hasenberg T. C. , Kost Alan
Published: February 2023

We use differential spectroscopy with tunable psec pulses to temporally and spectrally resolve the formation and decay of optical nonlinearities and space-charge fields in a hetero n-i-p-i that contains quantum wells in the intrinsic region composed of all-binary InAs/GaAs short-period strained layer superlattices. The evolution of the resulting nonlinearity is determined by a delicate competition between excitonic bleaching and the excitonic shift caused by a screening of the builtin field. The relative contributions of the two nonlinearities are complicated functions of fluence, time and wavelength, with the detailed dynamics determined by thermionic emission from the wells, psec charge transport over nm dimensions, screening, and carrier recombinations. The distinctive spectral signatures for excitonic bleaching and shifting allow us to distinguish the two contributions to the nonlinear dynamics and to follow the charge transport and space-charge formation. At the lower fluences, the initial excitonic bleaching gives way to a blue shift of the exciton as the carriers escape the wells in ~3 psec and drift to screen the built-in field in <10 psec. This blue shift persists until the spatially separated carriers recombine nonexponentially on microsecond time scales. At higher fluences, excitonic bleaching and the blue shift are observed simultaneously, since only a fraction of the carriers are required to screen the field and the wells remain partially occupied. On the time scale of ~10 nsec, the bleaching contribution disappears as the carriers within the wells recombine, leaving only the persistent blue shift.
Modeling of cross–well carrier transport in a multiple quantum well pin structure
Authors: Hutchings D. C. , Park C. B. , Miller A.
Published: February 2023

We have previously measured the photoconductive response in a GaAs/AlGaAs multiple quantum well (MQW) pin diode on picosecond timescales by the excite-probe technique.1 These results give information on the fundamental operating limits of the quantum-well self electrooptic effect device (SEED) and MQW photodetectors. The rise time of the transient transmission change generally decreased as a function of reverse bias voltage with a pronounced minimum (30 ps) at a voltage level corresponding to resonant electron tunneling. A numerical algorithm has been developed to model this transient response. The electric field dependent time constants for the various carrier dynamic processes are determined first, such as escape of carriers from the wells by tunneling or thermionic emission, drift of carriers in the field, and recapture of carriers into the wells. The temporal and spatial evolution of the photoexcited electron and hole populations are then calculated self-consistently by including the space-charge contributions to the local electric field throughout the structure. The decay of the transmission change is included by allowing transverse diffusion of the excitation in the n-doped contact region. We also include the effects of a nonuniform applied field due to impurities. We obtain good agreement with experiment and are thus able to understand the nature of the cross-well transport.
There is more than one way of making you tritanopia
Authors: Mollon J. D.
Published: February 2023

Over a large range of stimulus conditions we can rely on the signals of our long- and middle-wave cones to sustain our visual discriminations. By comparison, there is a much more constrained range of conditions over which we obtain discriminable signals from the short-wave cones; and several stimulus parameters (e.g., luminance, duration, size, retinal locus) can be manipulated to make the normal observer behave like a tritanope. Some (following Farnsworth) have sought a single factor to account for these functional tritanopias, and indeed a factor underlying several of them may be the sparseness of shortwave cones, but it may be useful to distinguish at least four forms of functional tritanopia: (i) The complete tritanopia that arises from the absence of short-wave cones in the 20-min arc center of the foveola, (ii) The high detection threshold that results from the sparseness of short-wave cones in the rest of the retina. The absolute sensitivity of individual short-wave cones may be comparable with that of other cones, but spatial integration cannot extend the psychophysically measured sensitivity to low energies as it does for other cones, (iii) The spatial imprecision of the short-wave signals when the discriminanda are small juxtaposed fields, (iv) The saturation that can occur at a post-receptoral opponent site through which the short-wave signals pass.
S-cones, flicker and luminance
Authors: Stockman Andrew
Published: February 2023

The poor temporal resolution of S-cone vision is often attributed to the confinement of S-cone signals to sluggish chromatic pathways, but it could also reflect a deficit in the S-cones themselves. This question can be resolved psychophysically if the S-cone signal can be measured through the faster achromatic pathway. This is possible on intense orange fields, on which the S-cone response can be followed to nearly 30 Hz. Moreover, if S-cone flicker is combined with suprathreshold L-cone flicker at a slightly mistuned frequency, visible beats at the difference frequency can be detected for frequencies as high as 40 Hz. On these intense fields, we can distinguish two S-cone pathways: one with a low-pass frequency response that detects little above 12 Hz, and a second with a more extended bandpass response. The bandpass signal is capable of participating in flicker photometric nulls with M- and L-cone signals and shows a high frequency sensitivity loss that is not greatly different from the other cones, suggesting that S-cones are brisk and can feed the achromatic luminance channel. The luminance input is negative, but this is complicated by phase lags that can reach 180° at 18 Hz.
Two-dimensional multiple wavelength tunable surface-emitting laser arrays
Authors: Chang-Hasnain C. J. , Florez L. T. , Harbison J. P. , Maeda M. W.
Published: February 2023

Wavelength multiplexing and tuning capabilities are highly important for applications in optical interconnects and optical communications using wavelength division multiplexing (WDM). We review novel methods and experimental results to achieve these functions in vertical cavity surface-emitting lasers (VCSEL), we believe for the first time. By implementing spatially chirped layers in the laser cavities and obliquely aligning the array axes to the thickness chirp direction, we obtained 7 × 20 VCSEL array emitting 140 unique, non-redundant, controllable, uniformly separated single-mode wavelengths. A large total wavelength span of 43 nm is obtained without compromising the performance of the lasers. Moreover, our techniques are generic and can be readily extended to other wavelength regimes. In addition, we describe our work on wavelength-tunable VCSELs with a 1.8-nm (~540 GHz) continuous tuning and no mode hopping. The laser output power remains nearly constant throughout the tuning range. The wavelength tuning speed is ~400 ns. The multiple wavelength tunable laser arrays are promising for achieving ultrahigh bandwidth (hundreds Gbit/sec to Tbit/sec), high redundancy and reliability, reconfigurable optical networks.
Cognitive neural networks: collective computing with diverse attractors
Authors: Farhat Nabil H. , Babri Haroon
Published: February 2023

Nearly all neural networks for pattern recognition being dealt with today are associative classifier or identifier networks that are not cognitive. To be cognitive a net must be able to distinguish, on its own, between familiar and unfamiliar or novel sensory signals present at its input, and this can not be done by associative classifiers. It will be argued and shown that, to be truly cognitive, a network must be nonlinear and dynamic and able to manifest bifurcation. This means it should be able to carry out phase space computations with more than one type of attractor and to switch between these depending on whether the sensory input is familiar or novel. Cognition implies, therefore, bifurcation and computing with diverse attractors. Our reasons for adopting this view, which stemmed from known biophysical observations and from our neuromorphic target identification work, are discussed. An example of a cognitive network that computes with both stationary (limit point) and dynamic (periodic) attractors is given to illustrate our thesis. The elements of a neuromorphic radar target identification system which employs these concepts and is capable of distortion invariant recognition of three targets with perfect score is presented. The work presented elucidates the role of periodic attractors in feature binding and cognition and the significance of cognition in autonomous systems.
Optoneural system for invariant pattern recognition
Authors: Lejeune Claude , Sheng Young , Arsenault Henri H.
Published: February 2023

An optoneural system, which consists of an optical correlator and a neural network, is developed for invariant pattern recognition. The correlator uses Fourier-Mellin spatial filters (FMF) for feature extraction. The impulse response of a FMF is equal to the kernel function of the circular-Fourier and radial-Mellin transform. The filter itself contains no object information and yields an unique output for each input object. The features used as input to the neural network are the geometrical parameters of the 2-D pattern of the output local peaks. The neural network used is a multilayer feed forward net with a back propagation learning rule. The advantages of this approach are that a FMF may be used for all input objects without the need for training or updating the filter, and that the number of the extracted features is small, making it possible to use a small neural network. This Fourier-Mellin optoneural system shows multiple object recognition, which is invariant not only to rotation and scale changes but also to translations of the input objects.
Incoherent phase-conjugate backscattering in photorefractive media
Authors: Segev Mordechai , Yariv Amnon
Published: February 2023

Phase-conjugate backscattering in photorefractive (PR) materials, which can be considered a special case of two-beam coupling, involves interaction between two counterpropagating beams via light-induced reflection gratings. Since this backscattering results from interference between the beams, it requires mutual temporal coherence. Moreover, when one beam is self-generated, the input beam is required to have a long coherence length.
Scattering due to random space-charge field in photorefractive crystals
Authors: Gu Claire , Yeh Pochi
Published: February 2023

The fluctuation of refractive index introduced by the random space-charge field via the Pockels effect is one of the noise sources in photorefractive media. In EO crystals, including photorefractive crystals, charge particles such as ionized dopants and defects are randomly distributed. The space-charge electric field generated by these charge particles induces a fluctuation in dielectric constant (or equivalently index of refraction) from the Pockels effect. Such fluctuation exists even in the presence of hologram recording and two-wave mixing. When the crystal is illuminated by light, scattering occurs in a manner similar to that of Rayleigh scattering. This kind of scattering exists in all photorefractive crystals, where ionized dopants and/or defects are responsible for the formation of index gratings. Such scattering leads to statistical noise in photorefractive devices. In this paper, we consider the scattering from randomly distributed charge particles in photorefractive crystals. The scattering cross section is analyzed in terms of the fluctuation of refractive index induced by the random space-charge field via Pockels effect. We also discuss the noise due to scattering from these noisy index gratings. The signal to noise ratio and the minimum modulation depth for an index grating are discussed. The analysis can be extended to cover all EO media.
Self-canceling M-cone flicker signals
Authors: Stockman Andrew , Montag Ethan , MacLeod Donald
Published: February 2023

On a deep-red background the spectral sensitivity for 16-Hz flicker changes from M-cone to L-cone as the luminance approaches 5.5 log10 photopic td. The phase differences required to null middle-and long-wavelength flickering lights reveal that at frequencies below 16 Hz, M-cone flicker signals are delayed relative to L, but following an abrupt phase change of 180°, the reverse is the case at higher frequencies. These results suggest that at high light levels the usual rapid M-cone signal is combined with a slower one that has a relative phase lag of 180° near 16 Hz. On shorter wavelength backgrounds, qualitatively similar but smaller phases changes are found— as would be expected if the generation of the slow M-cone signal is not dependent on background color but is partly obscured by L-cone intrusion in the response to the middle-wavelength test light. In other conditions (for example, with less intense red background), phase differences are also found, but often with the opposite sign to those found on intense backgrounds.
Deblurring for a multiplepoint impulse response
Authors: Stossel Bryan J. , George Nicholas
Published: February 2023

We present analysis for the recovery of intensity images degraded by an incoherent point spread function (PSF) consisting of a set of randomly distributed impulses. In initial experiments with as many as 100 irregularly spaced impulses, we obtain excellent recovery of the original object provided we have precise knowledge of the PSF. In practice, there is little or no difficulty presented by the zeros of the transfer function of the multiple-point blur. We describe some interesting methods for counting these zeros over an appropriate region of the transform plane. In the second portion of our presentation, particular emphasis is placed on the effects of errors made in the determination of the point spread function. We simulate measurement errors in the recording of the PSF by randomly perturbing the location of each impulse about the actual position used to create the blurred image. Analysis and reconstructions based on variations in the measured psf are presented. Examples include the statistical deviation of the measured impulses from their true locations and the number of impulses present in the degradation.
Real-time optoelectronic Gabor detection of transient signals
Authors: Katz A. , Lu X. J. , Kanterikis E. G. , Li Yao , Zhang Yan , Caviris N. P.
Published: February 2023

Detection of transient signals in a noisy environment is an important topic in radar, sonar, and communications. An optoelectronic system for detection of transient signals has been constructed. The detection scheme is based on the Gabor representation of a signal, which can be used to represent transient signals of unknown shape and arrival time. The transient signal and Gabor window function are written to the laser beam profile via either transparency or spatial light modulator. The Gabor coefficients are detected by a 2-D CCD array. The use of a liquid crystal television, allowing for real-time detection of signals is investigated. Experimental results for exponentially decaying signals, are presented which clearly indicate the signal frequency and arrival time.
Rare earth-doped glass integrated optics: lasers and amplifiers
Authors: Stanford N. A. , Malone K. J. , Larson D. R. , McCollum M. J.
Published: February 2023

The rare earth-doping of planar and channel glass waveguide structures is evolving into a new device technology that is distinct from rare earth-doping fibers. This is particularly true since the VLSI processing techniques used for waveguide fabrication permit selective doping and formation of complex optical circuits. Such geometry-specific doping is important, for example, in making passive feed waveguides that direct laser diode pump light to active areas of the device. Dopants such as Nd, Er, Pr, Yb, and Ho permit a wide range of operating wavelengths. Green up-conversion lasers should be feasible with Ho as a dopant. Nd will provide laser action near 1060 nm and 1300 nm. Er enables amplification and lasing near 1500 nm. Pr has several lasing transitions through the visible red to near infrared. Some of the many potential uses for this technology include optical data storage read/write, super-luminescent sources for optical gyros, amplifying fan-out structures for distribution networks, compact single frequency lasers, and eye-safe lasers.
Optimization of cladding material in silica ridge waveguides and use in evanescent coupling to d-fibers
Authors: Maxwell Graeme D. , Hubbard Steven D. , Powling R. J. , MacKenzie F. , Beaumont C. J. , Cannard P. J.
Published: February 2023

In the flame hydrolysis deposition technique, halides of Si, Ge, P, and B are introduced into an oxyhydrogen flame and hydolyzed to form oxide particles that are subsequently deposited on a silicon substrate. After deposition, the low density film produced is sintered to form a clear glass. Buffer, core, and cladding regions can all be produced by this process with suitable choice of dopant and doping level for each region. The paper discusses the behavior of the two dopants P205 and B203 used in the deposition process with particular emphasis on optical and thermal properties of the cladding material produced. The unexpected distribution of phosphorous, giving localized regions of relatively high concentration can lead to undesirable secondary guidance and hence adversely effect the performance of the waveguides. Suitable codoping with boron has provided a solution to the problem and is discussed.
Laser ablative deposition of optical thin films
Authors: Narayan J.
Published: February 2023

A summary was not available at time of publication.
Pulsed laser deposition and its application to optical devices
Authors: Cheung J. T.
Published: February 2023

A summary was not available at time of publication.
Argon fluoride excimer laser ablation of cornea
Authors: Pettit G. H. , Ediger M. N. , Weiblinger R. P.
Published: February 2023

While the ablation of corneal tissue by pulsed 193-nm excimer laser light has been extensively studied for various clinical uses, the underlying laser-tissue interaction remains poorly understood. We have examined the process in vitro by several methods: monitoring the photoacoustic deflection of a He-Ne laser probe beam passing tangentially above the corneal irradiation site, examining the temporal profile of excimer pulses reflected from or transmitted through corneal samples in ablation conditions, and surveying the ArF laser light scattered from the ablation target. The photoacoustic technique indicates that the corneal etch process is consistent over an initial train of pulses, implying that a cumulative incubation effect does not occur with ablation of this tissue. Reflectometry measurements reveal that the trailing portion of the reflected excimer pulse is strongly attenuated. This reflected pulse clipping becomes progressively more pronounced with increasing incident pulse fluence, indicating dynamic optical properties of the material during laser irradiation. Scattered laser light signals and transmitted pulse profiles have been acquired to assess the cause of this reflected pulse distortion.
Waveguide devices modeling with a bpm-cad software system
Authors: Jakubczyk Zdzislaw
Published: February 2023

At the National Optics Institute we have developed a user friendly software system, BPM-CAD, to model integrated and fiber optical waveguide devices. The BPM-CAD is based on several numerical methods, the core of which is a 2-D and 3-D beam propagation method. Included is a correlation function method that gives modal properties such as propagation constants, relative mode weights, and mode eigenfunctions. To find modal properties of the more complicated waveguiding structures we also use a path integral method. This BPM-CAD system includes algorithms for solving the characteristic equation of planar multilayered structures and various types of fiber. Finally, to obtain the electric-field distribution of general electrode patterns for electrooptic devices, a mode projecting method is implemented. Each application has its own, multi-window data entry user interface with data editing abilities and geometrical device structure previewing. Acceptance of the edited and previewed data file starts the numerical procedure in a multiwindow, menu-driven graphical environment. Our research activities currently applying the BPM-CAD software include modal noise in short fiber sections, tapered fiber connector and, in integrated optics, the analysis of multi-mode branching structures and a Ti:LiNbO3 integrated Doppler velocimeter.
Soliton switching in erbium-doped nonlinear directional couplers
Authors: Wilson James , Wright E. M. , Stegeman G. I.
Published: February 2023

The nonlinear directional coupler has well-known optical switching properties. Unfortunately, since the characteristic coupling length depends on the instantaneous value of the power, ulstrashort pulses will break up in the coupler. However, if pulses are used which have pulse widths and peak powers comparable with those of solitons for the uncoupled guides, the pulses are essentially unperturbed by the switching process. We have numerically studied such soliton switching in a nonlinear directional coupler doped with erbium ions. We find that the presence of a small amount of gain (provided by the erbium) enhances the efficiency of the switching process. In addition, the switched pulses display the compression normally observed with adiabatically amplified solitons. Apart from this compression, little other distortion of the pulse shape occurs during propagation through the coupler. As a result, we have shown that it is possible to provide all-optical switching, amplification, and compression of short pulses in a single device. We have also considered the effects of gain dispersion on the switching. The main effect of a finite gain bandwidth is to limit the switching efficiency of the coupler.
Femtosecond monolithic colliding-pulse mode-locked quantum well lasers
Authors: Wu M. C. , Chen Y. K. , Tanbun-Ek T. , Logan R. A.
Published: February 2023

Monolithic mode-locked semiconductor lasers are compact and stable sources of ultrashort optical pulses with GHz to multihundred GHz repetition rates. By building the whole mode-locked laser on a single piece of semiconductor, the tedious optical alignment process with bulk optical elements are totally eliminated. The mechanical and temperature stability are also greatly enhanced because of the integrated cavity and the small volume.
Ultraviolet-induced distributed feedback gratings in Ce3+-doped silica fibers
Authors: Broer M. M. , Cone R. L. , Simpson J. R.
Published: February 2023

We report the use of nondegenerate four-wave mixing to write permanent refractive-index gratings in a Ce3+-doped (80 ppmw) aluminosilicate glass fiber at 300 K with one-photon excitation at 292 nm. After exposure for 30 min at an average power level of 13 mW at 300 K, a hole in the fiber transmission was observed at 808 nm, caused by the formation of a distributed feedback grating at this wavelength. The observed bandwidth of the grating was 0.18 nm, and the modulation depth of the transmission was 17%, from which the change in refractive index was calculated to be Δn = 2.5 × 10-5 at 808 nm. The photoinduced grating reflectivity monotonically decreased as the temperature was raised; the decrease in Δn from 300 to ≈640 K was less than 10%. Improvements are expected with increased Ce3+concentration or exposure. Very recently a refractive index change of 14 × 10-5 at 1060 nm was reported1 in a heavily doped silica fiber (11,700 ppm Ce3+), using single beam exposure at 266 nm in an interferometer configuration.