bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-resolved x-ray scattering using synchrotron sources
Authors: Tischler J. Z. , Larson B. C.
Published: February 2023

The pulsed time structure, high intensity, and tight collimation of synchrotron x-ray sources provide new opportunities for the investigation of transient physical properties. The time structure of the x-rays from a synchrotron source is typically 150-ps long pulses separated by 300 ns. By triggering an experiment in sync with these x-ray bursts, measurements can be made with 150-ps resolution as long as the x-ray detector can resolve the 300-ns pulse spacing. Using streak cameras, time resolution down to picoseconds can be achieved by time resolving within a single bunch since the peak intensity within a bunch is ~3 × 1016 photons/s. Next generation sources such as an undulator at the Advanced Photon Source will be 100–1000 times brighter than presently available sources. Nanosecond studies of pulsed laser melting and regrowth of Si and Ge are described, and related experiments involving high-heat flow and supercooled liquid structure are discussed. Single shot crystallography and experiments using the pulsed time structure for neV energy resolution x-ray diffraction via Mossbauer transitions are also discussed.
Hard x-ray image detectors
Authors: Cavailler A. , Veaux J. , Gex F. , Munier J. , Gex J. P.
Published: February 2023

Considering experiments using both synchrotrons and lasers we have been required to develop new x-ray image intensifiers having sizes considerably greater than CCD detectors and high yield in a wider spectral range, from 10 keV to 1 MeV. First, we present the compared performances of thick gold photocathode vs xenon solid photocathode in a spectral range from 5 keV to 1.3 MeV. Second, we present the results we have obtained with two x-ray image intensifiers made of 120-mm (60-mm) thin (thick) gold photocathode and hard x-ray incoming photons.
Microdiffraction gratings
Authors: Gallagher Neal C.
Published: February 2023

Electromagnetic theory is used to study grating diffraction when the grating period optical wavelength are about equal. When electromagnetic theory is used some interesting nonintuitive phenomena have been predicted and subsequently observed through microwave experiments. Consider the experiment of scattering light of wavelength from a grating with period d. To begin d ≫ λ and the usual multiple grating orders are observed. As the wavelength is increased, the grating orders fan out, away from the zero order or specular reflection term. As the orders become parallel to the grating surface they become evanescent or nonpropagating. We eventually reach a point where only three orders propagate: the 0 order and the ±1 orders. Now something interesting happens for oblique incidence. As the wavelength λ approaches the grating period in dimension, only one grating order, the -1 order, appears to be a propagating order. There is virtually no light propagating in either the 0 or +1 diffraction order. As the dimension of the wavelength continues to increase an abrupt, almost discontinuous, change in the diffracted light occurs as the light switches from the -1 order to the 0 order. This effect has been observed in the microwave region and predicted by numerical models. Numerical methods have been developed to deal with 2-D gratings as well as multiple layers of dielectric gratings. Our purpose is to foster greater communication between the microdiffractive optics community and the photonic band gap community.
Carrier capture and quantum confinement in AlGaAs/GaAs quantum wire laser structures
Authors: Walther M. , Kapon E. , Christen J. , Hwang D. M. , Bhat R.
Published: February 2023

We have recently demonstrated room temperature operation of quantum wire (QWR) lasers with threshold currents as low as 0.6 Ma.1,2 A potential problem in using QWRs in laser structures is the very small cross-section of the wire, which can reduce the capture efficiency of injected carriers. We investigated carrier-capture mechanisms in multi-QWR lasers using photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. In our lasers, the crescent-shaped QWRs are formed in situ by OMCVD on V-grooved substrates and are laterally bounded by thinner quantum well (QWL) layers formed on the slopes of the groove. At low temperatures (T < 100 K) we observe two luminescence lines, emitted from the QWR and QWL regions, respectively, as identified by CL imaging. At higher temperatures, the QWR feature becomes dominant, indicating efficient carrier capture from the QWLs to the lower energy QWR potential well. This is confirmed by PL excitation spectra. Two-dimensional quantum confinement in the wires is evidenced by the appearance of the second QWR subband at T > 140 K, with an energy separation of 19 meV, consistent with a model of the QWR subbands. The efficient carrier capture from the QWL region to the QWRs observed in these lasers is important for their efficient operation.
Optical amplifiers in lightwave systems
Authors: Giles C. R.
Published: February 2023

A description was not available at time of publication.
Exciton quenching in strained InGaAs/InP MQWs for Fabry-Perot modulators
Authors: Bradley P. J. , Calvani R. , Campi D. , Caponi R. , Morello G.
Published: February 2023

Multiple quantum wells (MQWs) exhibit large optical nonlinearities associated with saturation of excitonic transitions at the band edge. Here we present a study of nonlinear absorption and refraction in an InGaAs/InP MQW structure, for the design of an all-optical reflection modulator. The sample, grown by SBE, consists of sixty periods of 77 Å InGaAs wells and 70 Å InP barriers, sandwiched between two 0.5-μm InP buffers. The wells are affected by a tensile strain due to a 0.5% lattice mismatch. In the experiment, a cw Nd:YAG pump and a white light probe (tungsten lamp) are incident collinearly on the sample. The output light is wavelength scanned with a monochromator that blocks (together with a Nd:YAG laser mirror and a silicon filter) the 1064-nm radiation. Transmittivity spectra in the 1480–1600-nm range were measured for incident pump intensities up to 4.3 kW/cm2. Absorption spectra in the absence of excitation exhibit an exciton peak of unusual height (13,000/cm-1 for the wells only). Measurements at various pump powers demonstrated a 60% maximum quenching of the absorption peak with a saturation intensity of 183 W/cm2. The progressive peak lowering with increasing pump power is well fitted by a simple two-level formula, and a more complex model based on a numerical solution of the Bethe-Salpeter equation provides calculated spectra in good agreement with experimental data. A maximum index change of -0.076 at 1556 nm with 3.2-kW/cm2 incident pump intensity was estimated from the absorption curves through the Kramers-Kroenig relations.
Femtosecond dynamics in semiconductor laser devices
Authors: Ippen Erich P.
Published: February 2023

Pump-probe studies of active semiconductor waveguides have revealed a variety of gain and index dynamics that are relevant to ultrashort pulse generation and propagation in diode lasers and amplifiers.
Induced ultrafast phenomena in InGaAIP lasers
Authors: Tatum Jim A. , MacFarlane Duncan L.
Published: February 2023

We study the ultrafast gain dynamics of visible wavelength InGaAIP semiconductor diode lasers. Our experimental configuration makes use of an ultrafast dye laser system that produces a train of pulses forming a suitable signal with which to probe these diode lasers. When the pulse train is coupled into the active region of the diode laser, we observe a phase locking of a self-induced oscillation in the diode laser to the dye laser pulse train. This locking extends over a large range of probe wavelengths. In addition to this entrainment, we observe the gain recovery dynamics of the diode laser, which can in some situations be quite complicated. We have studied the properties of the gain media as a function of several parameters, including probe pulse duration, power and wavelength, as well as temperature of the diode laser. We compare the results obtained for both gain guided and index guided lasers, as well as for both traveling and standing wave diode laser amplifiers.
Buildup of steady-state picosecond pulses from an actively mode-locked laser diode array
Authors: Pan Ci-Ling , Chang C.-S. , Kuo Jahn-Chung
Published: February 2023

We have studied experimentally and theoretically the temporal and spectral characteristics of an actively mode-locked laser diode array in an external cavity as it evolves to the steady state. The steady-state pulse width of the laser output was 45 ps, assuming Gaussian pulse shape. The average output power was 15 mW at 433 MHz. During the first 40 ns after the onset of diode laser action, the laser pulse width sharpened quickly and finally reached the steady state in ~100 ns. This corresponded to forty-five round trips. The number of clusters of the longitudinal-mode spectrum reduces during the evolution and finally approaches a steady-state spectral distribution with the pulse energy mainly distributed among a few clusters near the line center. The narrowing of the spectrum as time goes by is a manifestation of homogeneous broadening. Unlocked, broad spectrum translates into random fast fluctuations in time. As the pulses become mode-locked, the spectrum becomes narrower and at the same time the pulsewidth or envelope can sharpen.
Use of Z-scan measurement technique to characterize nonlinear optical properties of imidazole-2-thiones
Authors: Toda Hiroyuki , Verber Carl M. , Campbell Daniel P.
Published: February 2023

The Z-scan technique for measuring third-order optical nonlinearities has been proposed and demonstrated recently.1 Using this technique, we measured the optically induced index change Δn for derivatives of imidazole-2-thione at a wavelength of 532 nm. In these materials, which are optically clear, we detected a quadratic relationship between the index change and the intensity in a 30–100-MW/cm2 range, as opposed to the linear relationship we observed in absorptive materials. We also observed that the distance between peak and valley of the Z-scan data was less than predicted for third-order effect. This implies that the index change is a result of either the higher-order nonlinear effect or a combination of lower-order effects. Further investigation of the possible mechanisms and measurements in related compounds is discussed.
S-cone pathway vulnerability and retinal disease
Authors: Greenstein V. C.
Published: February 2023

A summary was not available at time of publication.
Changes in color appearance from noncontiguous short-wavelength light
Authors: Wesner Michael F. , Shevell Steven K.
Published: February 2023

A fairly large concentric region of short-wavelength light (3-5° inner-outer diameter band) surrounding a small concentric 1° test causes a shift toward redness in the color of the test (red/green equilibrium shifts by as much as 20 nm). The effect of the band is not due to stray light or color contrast at the edge of the test (Wesner & Shevell, 1991). To investigate further the adapting effect of remote noncontiguous short-wavelength fields, we measured red/green equilibria of a 1° test, composed of an admixture of 549- and 660-nm light, surrounded by a concentric band of 32-td, 440-nm or 491-nm light. The inner-outer diameters of the bands were varied: 1–2° (contiguous to the test), 2–3°, 3–4°, 4–5°. We also measured the test superimposed on a 5°, 440-nm or 491-nm adapting field. The test was varied from 6 to 1000 td. As expected, the appearance of the test shifted toward redness when superimposed on a uniform short-wavelength adapting field. Replacing the adapting field with a short-wavelength band that was contiguous to the test, however, produced little or no shift in color appearance. A substantial shift toward redness was found with 440-nm bands at least 1° away from the edge of the test. Much smaller effects were measured for the 491-nm bands, which may be attributed to significantly less S-cone stimulation (~4% of that from the 440-nm light). These results indicate that short-wavelength light has a larger effect on color appearance when it is located some distance away from the test.
Room temperature photopumped 1.5-μm quantum well surface-emitting lasers with InGaAs/P/InP distributed Bragg reflectors
Authors: Tai K. , Choa F. S. , Tsang W. T. , Chu S. N. G. , Wynn J. D. , Sergent A. M.
Published: February 2023

Vertical cavity surface-emitting lasers (VCSEL) are attractive for various uses. For high bit rate (≥10 Gbit/s) optical data transmission over several km silica fiber, 1.3–1.55-μm emission is required in view of fiber dispersion. At present, room temperature cw operation on VCSEL has been achieved only in GaAs-based structures with emission wavelengths in the range of 0.77–0.98 μm. The key element of these lasers is the epitaxially grown distributed Bragg reflectors (DBR), which allow high reflectivity and thus low threshold current operation. In this paper we report room temperature photopumped performance of 1.5-μm InGaAs quantum well VCSEL with InGaAsP/InP DBR. The wafer was grown by one-step chemical beam epitaxy and, from the substrate, consisted of a 40-pair n-type InP/InGa-AsP (λE
g
= 1.42 μm), 8 InGaAs(40 Å)/InGaAsP undoped quantum wells are a p-type 0.4-μm InP cap. Reflectivity spectra of similarly grown DBR structures (without quantum well actives) exhibited a flat-topped stop band 80 nm wide with >99% reflectively that agreed with numerical calculation. After the epitaxial growth the wafer was e-beam coated with four pairs of quarter wavelength Si/SiO2 to form the top mirror. Excitation pulses of 1.06 μm 250 ns at 1-kHz rate were focused from the front side onto a 25–30-μm FWHM spot. The 1.5-μm lasing through the transparent substrate was clearly manifested in (1) a sharp hockey-stick shaped output vs input characteristic, (2) almost linearly polarized output above threshold, (3) greatly reduced linewidth at threshold , and (4) high differential quantum efficiency. Electric injection of the VCSEL here can be achieved through proper regrowth steps to form a capped mesa buried heterostructure, similar to CMBH edge-emitting lasers. The p-metallic electrode can serve as the top mirror, whose reflectivity can be enhanced (>99.5%) with an addition of once dielectric layer.
Bistable diode laser amplifiers as narrowband nonlinear wavelength filters
Authors: Pan Zeqi , Dagenais Mario
Published: February 2023

An optical wavelength division multiplexing (WDM) transmission system is attractive because of its potential to exploit the wide bandwidth of optical fibers. A tunable narrowband wavelength filter is a key device in a high capacity WDM system. Bistable diode laser amplifiers (BDLAs) based on an intensity-induced refractive-index change can be used as tunable, regenerative, nonlinear narrowband wavelength filters.1 Here, the operation is based on the strong dependence of the switching power on the wavelength detuning of the signal wavelength with respect to one of the Fabry-Perot resonances of the BDLA.2 The advantage of the BDLA as a nonlinear filter is that it can not only provide a narrow filter bandwidth with gain but can also perform pulse reshaping and thresholding on the selected signal, all in one step. We have studied the filtering characteristics of a bistable diode laser amplifier in a two-channel experiment. Bit-error-rate vs channel separation measurements were performed at 140 Mbit/s with a return-to-zero data format. Good performance is obtained with 12-GHz channel separation.
Technique to improve image fidelity in photorefractive media
Authors: Bashaw M. C. , Aharoni A. , Hesselink Lambertus
Published: February 2023

Most photorefractive crystals display a measure of imperfection, such as striations, curved surfaces, or other phase-disturbing properties, which distort transmitted images and degrade the fidelity of holographic storage. We consider strontium barium niobate crystals with striations perpendicular to the c-axis. In such crystals, extraordinary polarized light experiences the strongest photorefractive effect but scatters off the striae, degrading the image fidelity. Conventional four-wave mixing, using counterpropagating pump beams, does not completely alleviate the problem, as the pump beams are also scattered by the striae. This difficulty can be overcome by using two true phase-conjugate pump beams. We describe hologram storage in striated strontium barium niobate, using a true conjugate of the reference beam generated by internal self-pumping in a second photorefractive crystal. The conjugate of the reference beam generates a conjugate of the signal beam, which then reproduces the stored image with high fidelity. This technique enables the use of imperfect crystals in photorefractive applications. We explore uses of this technique for holographic data storage.
Photorefractive crystals in optical neural networks
Authors: Hong John
Published: February 2023

The photorefractive effect is a nonlinear optical phenomenon exhibited in a variety of electrooptic crystal whereby index gratings can be written holographically in real time and also be stored for extended periods of time. Because of this unique capability, photorefractive crystals have been used to implement the adaptive interconnections required for neural network systems. In such optical realizations of neural networks, complex interconnection patterns are stored in the form of volume holographic gratings in a photorefractive medium which can be accessed in parallel to achieve large data processing rates. Moreover, the dynamic nature of photorefractive holograms allow the implementation of learning algorithms whereby the contents of the stored interconnections are incrementally modified to optimize certain performance criteria.
Optoelectronic neurons using MSM detectors in GaAs
Authors: Grot Annette , Lin Steven , Psaltis Demetri
Published: February 2023

We present a new GaAs integrated optoelectronic neuron for use in neural networks. We have previously demonstrated that the integration of photodetectors, thresholding transistors, and a light source on a single substrate allows one to have high neuron density with acceptable power dissipation. In this paper we report a circuit in which we used a double heterostructure LED (light emitting diode) as the light source. We use an LED rather than laser diodes because LEDs can be operated with small currents, due to the lack of a threshold current. To minimize the total mesa height and maintain high LED quantum efficiency, MSM (metal–semiconductor–metal) photodetectors were used. The thresholding transistor was a MESFET (metal–semiconductor field-effect transistor). The total mesa height is less than 3 μm. Since no two devices share an epitaxial layer, each device is individually optimized. Our circuit uses two photodetectors, one to set the threshold voltage and the other to detect the signal. With this circuit, we can also build both excitatory and inhibitory neurons. Results from our experimental studies are presented.
Subthreshold flicker additivity of M- and L-cone signals with fast (π0′) rod signals
Authors: Knight Roger , Buck Steven L. , Sanocki Elizabeth
Published: February 2023

Fast flicker is detected on mesopic backgrounds by π0′, a rod mechanism with fast temporal response and bandpass temporal contrast sensitivity. The similar temporal properties of cone and π0′ mechanisms suggest a special relationship between cone pathways and π0′-mediated signals that is not shared with the more sluggish, π0 rod signals. This is also supported by field adaptation studies where both rod and M-cone mechanisms adapt π0′ and a single rod mechanism adapts π0. How π0′ and cone flicker signals combine in cone pathways was determined by measuring test additivity. Chromatic adaptation was used simultaneously to isolate rods and either M- and L-cones. Test additivity experiments then assessed the degree of additivity between mechanisms. The degree of additivity was substantially less for π0′ with M-cones (k = 3.8) than with L-cones (k = 2.2). This is consistent with π0′ signals combining L-cone signals according to vector addition and with M-cone signals weakly or not at all for flicker detection. The differential specificity of π0′ and M-cone and L-cones for temporal processing contrasts with the nonspecificity previously reported for these mechanisms for increment detection.
Silica on silicon integratedoptics technology: its uses in sensors and telecommunications
Authors: Grand G.
Published: February 2023

This paper presents details of what we call glass-on-silicon integrated optics, some applications developed at LETI, and future evolutions. If we consider the aim of mass production, there are now two possible ways for optoelectronics circuits: (1) the monolithic approach (sources, guides and detectors) built on the same III–V semiconductor substrate); (2) the hybrid approach, in which sources, detectors (and fibers) are separately made and assembled around the integrated-optics component. The technology presented here is of great interest in (2), concurrently to ion exchange in glass substrate, with the following advantages: easy truncation of devices by cleaving, large thermal conductivity of silicon which make feasible a direct hybridization of the source, and the wide range of technology already demonstrated in industrial microelectronics, including micromachining of silicon.
Laser-induced chemical vapor deposition diamondlike film
Authors: Hu Xiaoyong , Liu Fengmei , Tuan Longbin , Yin Ying , Zhu Taixun , Zhao Jingshan
Published: February 2023

Laser-induced chemical vapor deposition of thin films has many features, such as a low temperature reaction to produce photodissociation to deposit films, etc. Also, diamond film has the characteristics of high hardness, good heat conduction, etc., so it is of considerable practical importance in mechanical and electrical industries, semiconductor industry, etc. In our experiment, a 200-mJ/pulse 308-nm XeCl excimer laser light (EMG201) was used for irradiation. C6H6 or C7H8 was selected as a source gas, H2 was used as diluted gas. The pressure of the reactor was 10–20 torr, when it contained only a source gas; otherwise, it was ~40 torr when it had a source gas and H2. The gases were inert. The substrate was silicon wafers of (111) plane. The substrate was heated by a heating element, and its highest temperature was below 150° C. During deposition, laser-induced fluorescence and laser ionization spectroscopy were used to analyze the different photodissociated products. Also, x-ray diffraction, Raman spectra, scanning electron spectroscopy, etc., were used to analyze the composition of the film. From the above analysis, we obtained our diamondlike film.
Optothermal response of normal arterial tissue to laser irradiation: a theoretical and experimental investigation
Authors: Torres Jorge H. , Motamedi Massoud , Cilesiz Inci , Welch Ashley J.
Published: February 2023

To develop and validate a reliable mathematical model for prediction of tissue response to high-power laser irradiation, we utilized a 2-D axially symmetric optothermal model that uses the delta-Eddington approximation to calculate light distribution and the heat diffusion equation to estimate temperature distribution in tissue. Scattering and absorption coefficients of selected aortic human specimens were obtained from spectrophotometric data of tissue reflectance and transmission by an inverse solution to the radiative transfer equation. These values and the reported values for the thermal properties of normal aorta were used to predict tissue response during argon laser irradiation. Experimentally, the thermal response of tissue to laser irradiation was quantified by monitoring surface temperature during argon laser irradiation of human aortic samples via a thermographic technique. In the range of 25–100°C, and for similar irradiation parameters, calculated values of tissue surface temperature during laser irradiation were up to three times higher than the experimental values that were measured thermographically. However, for purely absorbing materials, excellent correlation between measurements and predictions was observed. Careful analysis of the data raised concerns about the accuracy of the methods for optical property measurement.
Theorem of Mannheim, formerly of Damien
Authors: Chastang Jean-Claude A.
Published: February 2023

A curious monograph entitled "Théorème sur les Surfaces d’Onde en Optique Géométrique” appeared in Paris in 1955. The author R. Damien claimed a novel theorem. Given a point source, a refracting surface, and the associated anticaustic (i.e., the refracted wavefront of algebraic path difference zero) an inversion in a sphere with the source as pole interchanges the transforms of the refracting surface and the anticaustic. The inverse of the refracting surface becomes an anticaustic and vice versa. S. Cornbleet in "Microwave and Optical Ray Geometry" (1984) and elsewhere dwelt a good deal upon this remarkable theorem and named it in honor of Damien.
Sub-Poisson photo-electron statistics from a semiconductor laser with electronic feedback
Authors: Churnside James H. , Gordov Evgenii P.
Published: February 2023

We detected a portion of the output from a semi-conductor diode laser and used the detected signal as a source of negative feedback to the laser drive current. At frequencies below the characteristic frequency of the feedback loop, the power spectral density of the detected photocurrent was as much as 15 dB below the shot noise limit. This reduction can be described by a semiclassical theory of the effects of feedback on an injected shot noise current. Physically, this corresponds to a process whereby the laser output is changed in response to changes in the noise current.
High-resolution time-resolved observations of x-ray laser lines using a spectrograph with a varied line spacing grating
Authors: Underwood James , Batson Phillip , Koch Jeff , Matthews Dennis , McGowan Brian
Published: February 2023

Measurement of the widths of lines emitted by x-ray lasers in the 50-250-Å range requires a spectral resolving power > 10,000. If it is desired to use a streak camera to measure the time variation of this width or the intensity of the line, the spectrum must be formed on a flat field that matches the configuration of the streak camera cathode. Conventional extreme ultraviolet/soft x-ray spectrographs—for example, those using a concave grating and that form the spectrum on an arc of the Rowland circle—do not have this combination of properties. We have designed and built a new form of spectrograph that uses a plane grating having a line spacing that varies across its width. In laboratory tests this instrument has achieved a resolving power of 35,000 in the 100–200 Å region. We have installed this instrument in the two-beam chamber at NOVA in LLNL and have measured the time-varying widths of lines from a number of x-ray lasers. Results from these experiments are presented.
Prospects for x-ray nonlinear optics
Authors: Shkolnikov P. L. , Kaplan A. E.
Published: February 2023

Recent developments of soft x-ray lasers (XRLs) sets a stage for the research on x-ray nonlinear optics. We theoretically demonstrate the feasibility of several x-ray resonant nonlinear effects (XRNEs) in the interaction of the radiation of XRLs with plasma. Among nonlinear x-ray effects considered by us in the other papers presented at this Annual Meeting are: (i) non-linear refractive index and related effects such as, e.g., self-focusing which can be expected for XRL radiation propagating in the XRL itself or in some low-ionized discharge plasmas; (ii) a VUV lasing in certain ions resonantly pumped by an available XRL; and (iii) near-resonant x-ray third-harmonic generation in a number of discharge plasmas. Some other XRNEs in plasmas as well as in metallic vapors are conceivable, in particular 2π pulses and self-transparency, as well as stimulated Raman scattering. The new area of nonlinear optics has great potential given the increasing availability of XRLs and new discharge plasma techniques.
Simulation of planar intergrated photonics devices with the finite-difference time-domain method
Authors: McLeod Robert R. , Hawkins Raymond J.
Published: February 2023

The FDTD method solves the complete vector Maxwell’s equations for a photonics device, and can be used to simulate devices of one, two or three-dimensional complexity. We show the use of this method with a number of 2-D integrated photonics devices including gratings, diodes, and complex guiding structures. These simulations will include both TE and TM polarizations as well as an infinite number of guided and free-space modes. The time-domain nature of the algorithm is exploited to produce the complete frequency-response of the devices under simulation. These FDTD results are compared with experiment, theory, and other simulations methods (such as the beam propagation method) to demonstrate their validity.
Fourier analysis and synthesis of adiabatic tapers in integrated optics
Authors: Song G. Hugh , Tomlinson W. J.
Published: February 2023

Taper structures are of great importance in integrated optics as mode size controllers buffering between integrated-optical circuits and fiber ends with large spot sizes. The fact that the designed taper should be fabricated cost-effectively often imposes the condition that the refractive index should not be graded. This increases the required taper length, and necessitates a method of optimal design.
Image deblurring in the context of astronomical science: problems of validating algorithms and results
Authors: Nisenson Peter
Published: February 2023

The spherical aberration in the primary of the Hubble Space Telescope has drastically affected the astronomical science that can be performed with the instrument. Optical correctors are planned to be installed but, until they are implemented, image restoration techniques will have to be used to recover some of the lost science from the data. The nature of the image restoration problem is unique, since while the aberrations can be accurately characterized, photon noise, detector noise, and the different instruments’ characteristics set different requirements on the restoration algorithm for each observation. Demonstration and validation of the scientific return from restored images (or spectra) will be crucial in establishing the accuracy of the results.
Modification of the Richardson-Lucy iteration for restoration of Hubble Space Telescope Images
Authors: Snyder Donald L. , White Richard L.
Published: February 2023

In the early 1970s Richardson1 and Lucy2 published an iterative method for recovering images of objects that appear blurred. Their method has been rediscovered by Shepp and Vardi3 for imaging radionuclides. The connection between these two seemingly disparate fields of science—restoring optical imagery and radionuclide imaging—occurs because they are inverse-problems having a common mathematical description. The purpose of this paper is to note some implications for restoring optical imagery of recent developments in radionuclide imaging.3-5 These developments yield extensions of the Richardson-Lucy iteration that have proved to be powerful for restoring images from quantum-limited data. An application to images acquired with the Hubble Space Telescope is given.
Deblurring extended astronomical objects with a maximum-a posteriori/expectation-maximization algorithm
Authors: Granrath Douglas J. , West Karen F. , Fisher H. Donald , Lersch James
Published: February 2023

It has been shown that expectation-maximization (EM) can be applied to a maximum-a posteriori (MAP) formulation of the image restoration problem, resulting in a nonlinear iterative restoration algorithm. This MAP/EM algorithm has been shown to be effective in the restoration and super resolution of point objects. When applied to ex tended objects such as planets, however, the algorithm produces ringing artifacts near edges in the object. We show that such artifacts can be overcome by decomposing the image into two terms, a background and a foreground. The background term is used to remove large scale variations in the data such that the foreground term retains primarily edge information. The MAP/EM algorithm then operates on the foreground term to produce the deblurring. This decomposition effectively generalizes the positivity constraint beyond a zero-level surface. We describe our means of performing the decomposition and show both simulated and actual results.
Collapse of optical solitons by superimposition with weak control pulses
Authors: DeLong Kenneth W. , Friberg Stephen R.
Published: February 2023

Solitons can be collapsed into pairs of pulses which are broad and weak, if simultaneously launched with low-intensity solitons. We describe soliton collapse and associated phenomena via numerical solutions of the nonlinear Schrodinger equation, and present the results of experimental investigations. When two optical solitons are simultaneously launched into a fiber, they under-go phase-dependent interactions which can either lead to the formation of single pulses (fusion) or to the emergence of two different pulses with phase-dependent velocity differences (steering). This has been numerically and experimentally demonstrated for two equal-amplitude N = 1 solitons,1 and also occurs when the soliton amplitudes are considerably different, in which case steering leads to pairs of broad, very low intensity pulses (collapse). As a consequence, the phase of small control pulses can be used either to collapse much larger solitons or to frustrate that collapse.
Propagation characteristics of GaAs/AlGaAs QW ridge waveguides
Authors: Chinni V. R. , Menyuk C. R. , Chen Y. J.
Published: February 2023

The effect of a quantum well in waveguides with simple geometries can be calculated with a simple model,1 but waveguides with more complex geometries require a more detailed calculation. The mode field distribution across the waveguide will be determined by the shape of the waveguide, and the electrostatic field experienced by the QW layer from voltage applied across the waveguide will be nonuniform. Therefore, the guided wave experiences different perturbations at different positions of the QW, which will affect the total absorption and phase. In the present work, we use the finite element method and perturbation theory to determine excitonic contributions to the propagation characteristics of a QW ridge waveguide for different voltages across the waveguide. The results show that a simple slab model overestimates the attenuation and phase change for wavelengths smaller than the peak absorption wavelength and underestimates them for wavelengths larger than the peak absorption wavelength.
Photorefractive phase conjugation in the ultraviolet
Authors: Laeri F. , Angelow G. , Hilgenberg D. , Jungen R. , Albers J.
Published: February 2023

Photorefractive materials for phase conjugation in the ultraviolet are studied. In the canonical understanding the photorefractive effect is due to crystal impurities. With doping of the UV-transparent crystals beta-barium-borate (BBO), lithium-tetra-borate (LTB), and potassium-dihydrogen-phosphate (KDP) with transition metals, photorefractive properties can be achieved. In nominally pure LTB pumped with 150 mW of the 364-nm cw argon-ion laser line, we already observed a phase-conjugate reflectivity of 0.15%.
Nonlinear dynamics in optics and neural networks
Authors: Anderson Dana Z. , Benkert Claus , Montgomery Don , Saffman Mark
Published: February 2023

By tailoring the interaction among modes of a photorefractive ring oscillator we have been able to produce a variety of neural network dynamics including winner-take-all and "voters paradox" behavior. Our systems take advantage of the mathematical similarity between neural network models and mode competition in lasers. In the present case, mode competition is mediated by two-beam coupling in photorefractive materials. Here we consider how tailored mode interactions can also give rise to self-organized information processing. We demonstrate a system that learns to demultiplex two signals from a multimode fiber, and another system that extracts features from a set of images, then subsequently performs feature correlation on a test image. Both systems perform their task by recognizing that the input environment consists of spatially and temporally independent components. There is very little a priori information given to the systems; instead, they self-organize based on the differences among the signals or images. Although the demonstrated systems are quite specific, the self-organizing principles demonstrated are quite general and may be used for a variety of signal processing and optical sensor tasks.
Foveal resolution limit for chromatic interference fringes
Authors: Sekiguchi Nobutoshi , Williams David R. , Brainard David H.
Published: February 2023

We measured chromatic contrast sensitivity using red–green interference fringes. This allowed the production of chromatic gratings with high intensity, spatial frequency, and contrast, without blur ring by optical factors such as axial chromatic aberration and diffraction. One persistent problem in producing isoluminant gratings is the possibility of a luminance artifact due to a phase shift between the red and green components, which can be caused by transverse chromatic aberration or, in the Maxwellian view, movements of the head and eyes relative to the optical system. We intro duce a new psychophysical technique that avoids this problem. Isoluminant 632.8-nm and 514.5-nm fringes of equal spatial frequency and orientation were drifted slowly in opposite directions (0.25 Hz). This stimulus changes in time from a luminance to a chromatic grating in each temporal cycle, so that a purely chromatic stimulus will always occur at some point despite eye and head movements. The observer adjusted the contrast of the pattern until the chromatic component was just detectable. We find that foveal chromatic gratings can be seen at spatial frequencies up to 20-25 c/deg.
Nonlinear distortion in the detection of unresolvable interference fringes
Authors: He Sheng , MacLeod Donald I. A.
Published: February 2023

Interference fringes pulsed at constant mean luminance can be detected at spatial frequencies far above the resolution limit. This is due at least in part to aliasing by the receptor mosaic (Williams, Vision Res. 1985), but another possible cue is provided by nonlinear distortion, which can create spatially uniform temporal transients when a pulsed fringe is presented. We have investigated the influence of spatially unstructured distortion products by superimposing the pulsed fringes on a randomly flickering uniform field. As expected, this manipulation has almost no effect on contrast sensitivity well below the resolution limit. Just above the resolution limit, however, the random luminance mask profoundly reduces fringe sensitivity, suggesting that spatially unstructured distortion products provide the basis for detection in this range. At higher fringe frequencies that ap proximately match the cone mosaic, the random luminance flicker again becomes ineffective for some observers. This creates a pronounced secondary maximum in contrast sensitivity in the flicker mask condition, presumably due to spatially structured cues provided by aliasing with the cone mosaic. Apparently in the unmasked case, distortion products bridge the gap between the resolvable frequency range and the aliasing range.
Genes and encoded M-cone pigments from two types of protanope
Authors: Neitz Maureen , Neitz Jay , Jacobs Gerald H.
Published: February 2023

Small differences in spectral peak among cone pigments appear to underlie several different human color vision polymorphisms. We recently identified three amino acid substitutions that can account for spectral differences among X-encoded pigments. Each of these appears to produce spectral shifts of specific magnitudes. The one that produces the smallest spectral shift (5–7 nm) is a serine for alanine substitution at amino acid position 180. This substitution may be responsible for cone pigment variations among color normal observers (Science, 252, p. 971). We have tested the hypothesis that this same substitution causes spectral variation in the M pigments of protanopes. Spectral sensitivities of protanopes were measured using ERG flicker photometry. We found two types of protanope (in a sample of seven); one phenotype has an M pigment with a spectral peak of 530 nm and the other has a pigment with a 537-nm peak. The amino acid sequences of the M pigments, deduced from nucleotide sequences of genes from one of each of the two protanope phenotypes, differ at position 180 just as predicted. These results confirm that this substitution produces a 5–7-nm shift in human cone pigments. Several different color vision polymorphisms may be caused by this single substitution.
Color vision polymorphism and red-green learning
Authors: Ahumada Albert J.
Published: February 2023

Our version of the idea that the red-green distinction is associatively learned by experience was spawned by the notion that neural connections in the retina and LGN may be blind to their originating cone types [middle-wavelength-sensitive (M) or long-wavelength-sensitive (L)]. Red-green learning theory provides an alternative perspective for describing polymorphic models of color perception (Piantanida, Am. J. Optom. & Physiol. Opt., 1976). In these models, the M-cone spectral sensitivity maximum for a man is genetically selected from a range of wavelengths that begins with that of the normal M-cone but extends to that of the normal L-cone. A similar situation occurs for the L-cone, so a small number of men are predicted to have M-cones more sensitive to long wavelengths than their L-cones. Also a small number of heterozygotie women are expected to have two different M-cones with most sensitive wavelengths surrounding those of their L-cone. From the learning point of view there are no M- and L-cones: genetics supplies cones which the learning process groups. For opponency models that combine L- and M-inputs with opposite signs to obtain a red-green opponent process, different phenotypes emerge than those predicted by associative learning.
Directional sensitivity of the cone systems
Authors: Walraven Pieter L.
Published: February 2023

The specific character of the S-cones with respect to the Stiles-Crawford effect can be deduced from measurements by Stiles and Crawford (1939), and Alpern and Katahari (1983), who showed that the luminous efficiency of the S-system at an eccentricity of entry of the light of 3 mm, is ~30–40% lower than the efficiency of the M- and L-systems. Bouman and Walraven (1960) assumed a reduced luminous efficiency of the same order of magnitude of the S-system to explain the chromatic Stiles-Crawford effect in the 400-500-nm wavelength region. I suggest that this difference between the S-system on the one hand and the M- and L-systems on the other, be attributed to the scarcity of the S-cones. In the case of the S-cones the oblique incident light leaks to M- and L-cones but not to other S-cones because the separation between the cones is too large. It is suggested that all cones exhibit a Stiles-Crawford effect equal to that of the S-cones, but that the effect for the L- and M-cone-systems is reduced because of the leakage of light to neighboring cones.
Invariance of the white point with age
Authors: Werner John S. , Schefrin Brooke E.
Published: February 2023

Mixtures of monochromatic lights that appear achromatic were measured for fifty normal trichromats ranging in age from 11 to 78 years. Stimuli were presented to one eye as a 1-sec flash (10 sec interstimulus interval) in Maxwellian view. The achromatic locus was found by varying the intensity ratio of a mixture of unique blue and unique yellow while maintaining constant overall retinal illuminance. Measurements were obtained at three intensity levels (10, 100, 1,000 td) for each observer. Additional verification of the position of the achromatic locus in color space was obtained for twenty-three subjects using a mixture composed of 600 nm and its spectral complement. There were no significant changes in the achromatic loci as a function of age. The mean achromatic locus in CIE chromaticity space was x,y = 0.31,0.31, or u'v' = 0.21,0.46. These results imply that the three classes of cones undergo parallel losses in sensitivity (specified at the cornea) as a function of age.
Hue cancellation on binocularly fused adapting fields
Authors: Gille Jennifer , Larimer James , Piantanida Thomas , Lanham Jennifer
Published: February 2023

We studied hue cancellation on adapting fields whose color appearance was altered using binocular fusion. Test stimuli were mixtures of red and green light that were either increments of decrements superimposed on an adapting back ground in one eye. The observer’s task was to set a red-green equilibrium ratio for each magnitude of increment or decrement. Monocular adapting fields appeared red, orange, yellow, yellow–green or green, and fused binocular fields appeared either yellow, orange, or yellow–green because of green or red contralateral fields. Hue-cancellation functions were found to depend on the color appearance of the adapting field. We found differences in the slopes of the linear functions describing cancellations for increments and decrements, and differences in slopes between binocular conditions and their monocular control conditions. We also found small but consistent changes in the intercepts of these linear functions. Results relate to two-process models of color vision. The slope changes reflect gain changes that must be centrally rather than retinally controlled. The different slopes for increments and decrements implies differential gains mediated by the putative central mechanism and related to color appearance. Intercept changes are related to simultaneous color contrast effects and failures of color constancy.
Wet-chemical processed single-leakage-channel waveguide grating couplers for integrated-optic applications
Authors: Roncone Ronald L. , Bates Keith A. , Li Lifeng , Burke James J. , Weisenbach Lori , Zelinski Brian J. J.
Published: February 2023

In general, waveguide grating couplers have output coupling efficiencies of ~50% (i.e., a 50/50 split of modal energy into the waveguide’s cover and substrate regions) and input coupling efficiencies of ~40% (due to beam-profile mismatch). Agrawal et al.
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proposed placing a dielectric stack on the substrate to reflect the light diffracted toward the substrate back up and out of the device, thus creating a single-leakage-channel grating coupler. We demonstrate this concept by showing the design considerations, fabrication, and characterization of such a device. A wet-chemical process, readily adaptable to existing microelectronic fabrication lines, is used to fabricate the dielectric stack on a glass substrate. A solgel buffer layer of appropriate thickness is deposited on top of the reflector stack to isolate the guiding layer from the stack and to provide constructive addition of the reflected light with the light diffracted into the cover region. A variable-depth grating, etched into a solgel processed SiO2-TiO2 waveguide, serves to reduce beam profile mismatch, and thus enhance the coupling of Gaussian beams.