bluuebunny/crossref_metadata_2025 huggingface dataset:
Supermode competition in adjoint-coupled unstable resonators
Authors: Shakir Sami A. , Erkkila John H.
Published: February 2023

Supermode competition in coupled lasers has previously been analyzed for plane wave geometries in which the self-feedback and mutual coupling parameters are defined explicitly in terms of beam splitters that are used to couple the lasers. In adjoint-coupled unstable resonators, it is convenient to couple the resonators through holes in the scraper mirrors. Hole coupling complicates the analysis of these devices since diffraction and transverse field variations related to the hole geometry play a significant role in dictating the characteristics of the resonator output. In this work we use diffraction theory to estimate the self-feedback and mutual coupling parameters. Overlap integrals are used to reduce the diffraction equations to a form similar to plane wave dispersion equations. We also develop an efficient nonlinear plane wave method for assessing cross-saturation effects in the coupled laser. Dependence of the coupling parameters on hole size, hole position, resonator detuning, and resonator Fresnel numbers is explored.
Single- and two-mode theory for coupled lasers with Doppler-broadened gain media
Authors: Jelonek Mark P. , Roh Won B.
Published: February 2023

Existing coupled laser theories have been extended to analyze the case of two coupled lasers supporting two modes in each cavity. Sets of four single-mode and eight two-mode equations describing the amplitudes and phases of the modes oscillating in each cavity have been solved numerically to evaluate the operating characteristics of the coupled laser system. Locking ranges were computed for both single- and two-mode coupled laser systems. A number of oddities in behavior have been found including a mutually reinforced hole-burning minimum in intensity (whose existence was verified using a single, two-mode He-Ne laser) and a phase dependent shift of the laser frequency from the bare cavity frequency.
Two-mode switching in semiconductor lasers
Authors: Gray George , Roy Rajarshi
Published: February 2023

We present an experimental study of the statistics of the switching between two modes of an AlGaAs semiconductor laser. We collect statistics of the dwell times (the time that a mode remains on or off) of the two modes and form probability histograms. From these histograms we are able to calculate the mean, variance, and higherorder moments of the dwell times. In particular, we examine this switching as a function of the ratio of the mean intensities of the two modes, i.e., the mode suppression ratio (MSR). The depth and frequency of the mode hopping is greatly dependent on the laser operating temperature and the injection current. We also present results of computer simulations on the mode hopping phenomena, based on a model which we have previously applied to intensity noise and line shape calculations.1 We show the effect of the injection current on the mean dwell times for different values of the mode suppression ratio. We also consider the effect of different reference levels (the level at which a particular mode is considered on or off) on the dwell time distributions.
Modal discrimination in diode lasers with a variable reflectivity output facet
Authors: McCarthy Nathalie , Champagne Yves , Mailhot Sylvain
Published: February 2023

Conventional broad area (BA) diode lasers tend to oscillate on several lateral modes as the injection current is increased above threshold, causing a spatial coherence degradation and a far field pattern broadening. We present numerical simulations of BA diode lasers with smooth spatial filtering at the output facet. The spatial filtering takes place only in the direction parallel to the junction and is provided by an output facet having a laterally varying reflectivity profile. Beam characteristics and discrimination against high-order lateral modes of the laser devices are calculated with the beam propagation method and Prony algorithm. An output facet with a Gaussian reflectivity profile favors the fundamental lateral mode. Discrimination between the two lowest-order modes increases significantly with narrow reflectivity profiles. The Gaussian facet slightly broadens the far field distribution of the fundamental mode. However, multimode oscillation is inhibited and the beam coherence is preserved higher above threshold. The threshold current increase is compensated by the crossing of the L-I curves, yielding a better wall-plug efficiency at high drive levels for broad area lasers with a Gaussian output facet.
Ti:Al2O3 laser with optical phase conjugated feedback pump
Authors: Kong Hongzhi , Chen Bosu , Cronin-Golomb Mark
Published: February 2023

A modified photorefractive total internal reflection self-pumped phase conjugate (cat) mirror has been used to feedback the residual pump beam to a TiAl2O3 laser as an additional pump. By using a prism, the cat mirror reflected all lines from an argon laser with a total reflectivity of 15%. This feedback reduced the laser threshold from 2.4 to 2.23 W while the feedback of an ordinary flat mirror with 15% reflectivity could only slightly reduce the threshold to 2.36 W. The effectiveness of the feedback pump depends on the beam quality and its coincidence with the first pump. After passing through the laser rod, the residual pump is distorted due to effects such as thermal distortion and diffraction. This distortion is sufficient to seriously degrade the effectiveness of a flat mirror feedback. The phase conjugate reflection, by its nature, is capable of correcting this distortion and automatically traces back to where it came from so that both the pumps could overlap optimally.
Improved photorefractive response using a cylindrical lens
Authors: Salamo G. J. , Monson B. D. , Clark W. W. , Miller M. J. , Wood G. L. , Sharp E. J. , Neurgaonkar R. R.
Published: February 2023

It is well known that the photorefractive response time improves with incident laser intensity.1 This characteristic behavior can be exploited by focusing incident laser , light to a small waist in the photorefractive medium. Unfortunately, the use of a spherical lens to focus the incident beam to a tight waist may also result in a decrease in the magnitude of the photorefractive effect. We report on experimental observations which demonstrate that the photorefractive characteristic response time can be improved by more than an order of magnitude by using a cylindrical lens to focus the incident laser light into the photorefractive material. In particular, the photorefractive response time for two-beam coupling,1 beam fanning, and self-pumping2 are each observed to improve while maintaining approximately the original coupling strength.
Photorefractive properties of doped strontium barium niobate
Authors: Vazquez Rene A. , Ewbank M. D. , Neurgaonkar R. R. , Cory W. K.
Published: February 2023

Photorefractive strontium barium niobate (Sr
x
Ba1-
x
Nb2O6 or SBN) is an attractive material for photorefractive applications because of its large r33 electrooptic coefficient. One attractive feature of the tungsten bronze structure of SBN is the large number of vacant lattice sites which can be occupied by dopants to provide a high density of photorefractive charge carriers.
Resonance light scattering in coated microspheres
Authors: Lin H-B. , Eversole J. D. , Campillo A. J.
Published: February 2023

Small spherical particles (from 5 to 30 µm) were charged and suspended in an electric quadrupole trap which was mounted in a chamber connected to a vacuum pump. The particles were coated with a liquid having a relatively low vapor pressure at normal atmosphere (such as glycerine). When the chamber was evacuated, light from a He-Ne laser scattered at right angles from the coated particle was monitored as the liquid coating shrank due to evaporation. Sharp variations in the scattered light intensity as a function of changing particle size are termed Mie resonances or partial wave resonances occurring when the wavelength and particle radius match a spherical cavity-type mode. For the large layer thickness, this resonance spectrum matches that of the homogeneous liquid droplet (both experimentally and computationally). However, significant deviations occur when the layer thickness comes within a few optical wavelengths of the core surface. Observations were made for glass, mercury, and polystyrene cores. An exact computational model for concentric spheres with differing indices of refraction accounts for much of the observed spectra. Discrepancies are discussed in terms of the model restrictions of concentricity and step function layer-core boundary.
Optical instability in optoelectronic bistable circuits
Authors: Wang Ruibo , Liu Shutian , Li Chunfei , Zhang Lei
Published: February 2023

Relaxation oscillation can occur in a bistable system when the nonlinearity has two contributions of opposite sign and different time constants.1 Optoelectronic bistable laser diode or light emitting diode (BILD or BILED) circuit is a promising optical logic device.2 Here we present the observation and analysis of optical regenerative pulsation in a system constructed of a liquid crystal switch and BILED circuit. Sine wavelike pulses were obtained in our experiment, and the experimental results show that the oscillation frequencies were stable. The frequency can be adjusted by changing the voltage supply. In our experiment the output from an argon laser is controlled by a LC switch, which is connected to a BILED circuit. The voltage applied across the LC switch is provided by an amplifier, and it is proportional to the output of the BILED circuit. The BILED circuit has two output signals of feedback. One is the light emitted from the LED in the circuit. The other is a part of the laser beam transmitted from the LC switch. As oscillation occurs in the circuit, the laser beam is modulated into a train of pulses by the LC switch.
Nonlinear dynamics of semiconductor injection lasers under large-signal modulation
Authors: Farley-Leadbetter V. L. , Mcinerney J. G. , Mancha S. , Dente G. C. , Gallant D.
Published: February 2023

Semiconductor injection lasers are ideal vehicles for studying complex dynamics and chaos in nonlinear optical systems. Moreover, in view of the extensive applications of these lasers to optical communication and data storage, it is essential to study their dynamic properties in detail. Most previous studies of modulated semiconductor lasers have addressed small-signal responses,1 and more recently the effects of strong modulation on devices which exhibit spontaneous self-pulsations.2 Here we consider the dynamic behavior of semiconductor lasers, which are well behaved in isolation, under large-signal modulation at gigahertz rates. Nonlinear rate equations, including a strong sinusoidal external modulation term, are solved numerically to obtain the optical mode spectra, relative intensity noise spectra, and output power times series, for various laser biasing and modulation conditions. Various frequency locking and mixing phenomena have been observed. For fixed dc bias at a given modulation frequency, the laser undergoes complex transitions from periodic to chaotic responses, as the modulation depth is increased. However, these transitions occur at modulation depths considerably lower than those predicted by the authors of Ref. 2. Numerical and experimental results are discussed.
Instabilities of counterpropagating waves in a Kerrlike medium
Authors: Pare C. , Firth W. J.
Published: February 2023

Over the last few years, different types of instability associated with counterpropa- gating waves in a bulk Kerrlike medium have been discovered.1−3 Considering a finite response time for the nonlinearity, Silberberg and Bar-Joseph1 first predicted temporal oscillations and chaos at high intensity. Taking into account the extra degree of freedom due to the polarization state of the fields, Gaeta et al.2 have then shown that this polarization state is actually unstable above some intensity threshold. Finally, we have recently shown, using a linear stability analysis, that transverse instabilities also occur above another intensity threshold and an analytical expression has been given for this threshold.3 Here, we present a simple explicit analytical expression (in terms of the length of the medium and the tensor components of the susceptibility) for the polarization instability threshold in the case of equal beam intensities, considering only the first bifurcation. This instability can then be interpreted as a four-wave mixing self-oscillation. This result allows us to compare the different instability thresholds associated with contra- directional waves. It is shown that the transverse instability should dominate, not excluding however the possible onset of polarization instability at higher intensity.
Quantum fluctuations in a singlemode pulsed dye laser
Authors: Cameron Seth A. , Littman Michael G.
Published: February 2023

We have studied the intensity fluctuations from a pulsed single-mode grazing incidence dye laser. The laser operates using rhodamine 6G dye pumped by 308-nm radiation from a XeCl excimer laser. Pump laser pulse duration is 6-8 ns and dye laser pulse duration is 2-3 ns. The spectral purity of the dye laser radiation is within 20% of the Fourier transform limit. Using event-mode recording involving the monitoring of pump laser intensity and dye laser intensity, we have determined the amplitude distribution from the dye laser at pumping conditions below, slightly above, and significantly above threshold. The amplitude fluctuations from the dye laser are well characterized by a Fokker-Planck model and they appear to be quantum in origin.
Comparison of longitudinally diode laser pumped Nd:YAG and Nd:GGG lasers
Authors: Labranche Bruno , Lavigne P. , Belanger P. A.
Published: February 2023

Nd:GGG is a laser medium similar to Nd:YAG but it can support a greater concentration of neodymium. We have measured a concentration of ~2.5% of neodymium in a Nd:GGG rod compared with 1% for Nd:YAG. For this high concentration of neodymium in Nd:GGG, we have observed a broader and higher absorption peak at 810 nm. We have used the rod of Nd:GGG in a half-symmetric resonator using a cw low power laser diode as the pumping source. We have tried two different laser diodes and two output couplers having different transmissions at 1.06 µm. With the Nd:GGG rod at 1.062 µm, 3.5 mW was obtained. The Nd:YAG rod was pumped in similar conditions with an output power of 6.3 mW at 1.064 µm. The Nd:GGG laser has shown a lower slope efficiency and a higher threshold than Nd:YAG.
Receptor position learning from known motions
Authors: Ahumada Albert J. , Pavel J. Misha
Published: February 2023

L. T. Maloney has described a method of calibrating a visual system consisting of input units and output units that are initially imperfectly connected. The information available is the known translation and the output images. The desired condition forcing adjustment of the connecting weights is that the output images be related by the translation. We have simulated three algorithms. Two of them resemble the Widrow-Hoff algorithm except that one of the output images is translated and used as the reinforcer for the other. The third is a combination of these two and includes the gradient descent solution for a single motion presentation. The translation condition is not affected by a space-invariant linear filter, so we force a unique solution by fixing the receptive field of one unit. For appropriate image subspaces and positioning and motion paradigms, all three methods can correctly train the weights for connecting an irregular array to a regular array, independent of the initial weights.
Periodicity of interference structure of the Mie extinction cross section
Authors: Chylek Petr , Zhan Jiyu
Published: February 2023

Slow oscillation of the normalized extinction cross section of a dielectric spherical particle (extinction efficiency) is usually referred to as an interference structure. Sharp resonance peaks superimposed on this smoothly varying curve form the so-called ripple structure of the extinction curve. The van de Hulst anomalous diffraction approximation predicts correctly the interference structure and its periodicity Δx = π / (m − 1 ), where x is the size parameter and m is the refractive index. The fact that the above expression, derived under the assumption m − 1 ≪ 1 (anomalous diffraction approximation), provides results in agreement with numerical calculations for refractive indices up to about m = 2.5 suggests that it can be derived under less restrictive assumptions on the refractive index m. Using asymptotic expansions of the Riccati-Bessel functions valid for size parameter x ≫ n, we show that the whole expression Re(an + bn) becomes independent of the summation index n. For refractive indices 0.5 ⪯ m ⪯ 2.5 the Re(an + bn) contains only one term periodic in the size parameter x. Periodicity of this term is Δx = π /(m − 1). Consequently, periodicity of the interference structure of the extinction curve is Δx = π/(m − 1) for all refractive indices 0.5 ⪯ m ⪯ 2.5.
Finite aperture effects on intensity fluctuations in a random medium
Authors: Mazar Reuven , Bronshtein Alexander
Published: February 2023

Investigation of the intensity correlation properties of laser radiation propagating in the lower atmosphere is of great importance in constructing laser communication and surveillance systems. Significant effort in theoretical research is concentrated on obtaining approximate analytical and numerical solutions for the fourth-order statistical moment of the high frequency random field propagating in a weakly fluctuating medium. Most of the numerical results deal with infinite plane wave excitations, and there are very few results for finite aperture fields. One of the powerful analytical methods allowing one to obtain explicit analytical expressions for the fourth-order statistical moment is the two-scale expansion procedure. These solutions are expressed in a multiple integral form and their computations are in good agreement with the numerical solutions of the fourth-order moment equation for a plane wave initial excitation. However in the finite beam case, the number of the integrations increases and the numerical evaluation of the solutions requires considerable computational resources.
Small area buried GaAs mesa photodiode for high speed applications
Authors: Kovacic S. J. , Jessop P. E. , Garside B. K.
Published: February 2023

A major problem inherent in the design of any small area photodiode is making electrical contact without greatly increasing the extrinsic capacitance. The planar surface area of the device must be large enough to accommodate both an adequate light collecting area and a metallized contact. We have investigated a scheme where the metallized bonding area is removed from the light collecting surface by extending a 10-µm wide gold finger from the bonding pad to the top surface of the mesa photodiode (15-µm mesa diameter). This is made possible by burying the mesa beneath a layer of transparent passivating dielectric (polyimide) and then patterning the polyimide to produce a hole through to the mesa. The photodiode itself consisted of n-type GaAs (3E16/cm3) which was ion implanted with Zn (3.3E13 Zn/cm2 at 100 keV). The small area of the device and passivating qualities of the polyimide yielded devices with dark current typically 0.1 nA at 75% breakdown voltage (33 V). The impulse response of the photodiode at 20-V reverse bias was found to be 48-ps (±2) (FWHM). This was consistent with measured device capacitance (0.325 pF) and a 50-Ω load.
Effect of energy buildup on noise in avalanche photodiodes
Authors: Saleh Bahaa E. A. , Hayat A. Majeed , Teich Malvin C.
Published: February 2023

The random multiplication process in avalanche photodiodes (APDs) is complicated by the existence of an inherent dead space D within which a newly created carrier cannot initiate a new ionization. A threshold energy must be built up before the carrier can reionize. We provide an analysis of noise in single-carrier-initiation single-carrier-multiplication APDs, including the effect of dead space. The carrier counting process is cast as an age-dependent branching process, whose statistics are well known. Expressions for the mean gain and excess noise factor are determined. Dead space reduces the excess noise factor F; however, this is accompanied by a reduction in the mean gain G. For APDs with ionization coefficient α = 2 × 103 cm-1, D = 0.5 µm, and a 10-µm multiplication region, reductions in F and G are 14% and 33%, respectively. Expressions for the mean and variance of the impulse response function arising from a single photon absorption are determined as functions of time. Asymptotic expressions for the mean gain and the excess noise factor, describing devices in which the length of the multiplication region is much larger than the dead space, are derived in terms of the Malthusian parameter of the branching process. In this case F is time independent.
Silicon photodiode self-calibration using white light for photometric standards
Authors: Ohno Yoshihiro
Published: February 2023

Photometric standards can be attained using silicon photodiodes whose absolute spectral responsivity is self-calibrated. However, this method requires laborious measurements at many wavelengths over the visible region. In this paper a new method is proposed in which the responsivity [A / m] for the photometric quantity of a silicon photodiode, which is combined with a Vλ-correction filter of known spectral transmittance, can be self-calibrated by measuring the surface reflectance and the recombination loss of the silicon photodiode for white light. The derived equation for this method requires that the spectral responsivity of the monitor detector for the surface reflectance measurement be proportional to wavelength. This requirement is approximately fulfilled by high quality silicon photodiodes, but this approximation causes a systematic error. To clarify this error, computer simulation has been applied using the measured spectral data of commercially available silicon photodiodes both for the self-calibration sample and for the monitor detector, and of Vλ-correction filters, with data of CIE Standard Source A as white light. The simulation revealed that the systematic error was <0.2%.
Theory of phase memory in interference between two pairs of downconverted photons
Authors: Ou Z. Y. , Wang L. J. , Mandel L.
Published: February 2023

An analysis is presented of the interference between two photon pairs produced in the process of downconversion from two identical crystals, which are then mixed by beam splitters. Because the downconverted photons are entangled with the vacuum, they carry information about the phase of the pump field. As a result the two-photon detection probability is shown to depend on the phase of the pump, even when the resolving time of the detector electronics is far longer than the coherence time of the light.
Corrections to the theory of photocounting
Authors: Tatarskii V. I.
Published: February 2023

It is shown that causality is violated in the well-known Glauber theory of photon statistics. This is caused by use of the wave rotation approximation. Corrections to the theory of photocounting are proposed. For coherent states, the corrected probability distribution of photocounting has the same formula as previously but a different mean.
Cavity QED
Authors: Meystre Pierre
Published: February 2023

Cavity QED is the study of the interaction between one or a few atoms or electrons and the tailored electromagnetic environments that can be realized, e.g., in optical or microwave cavities. This tutorial gives a broad review of this field, illustrating how it impacts our understanding of the transition from truly microscopic to macroscopic dynamics in light-matter interactions, as well as challenges our conventional (ensemble average) interpretation of quantum mechanics.
Infrared look at the high Tc superconductors
Authors: Tanner D. B. , Cao Feng , Herr S. L. , Porter C. D. , Kamaras K. , Etemad S. , Venkatesan T. , Chase E. , Inam A. , Woo X. O. , Hedge M. S. , Datta B. , Bonn D. A. , Reedy M. , Garrett J. D. , Greedan J. E. , Stager C. V. , Timusk T.
Published: February 2023

The optical properties of high Tc films, crystals, and bulk samples have been studied between 15 and 300 K, over the 50-40,000-cm−1 (6-meV-5-eV) range. The normal state spectra have a low frequency Drude contribution and a strong midinfrared absorption; Drude parameters correspond to the clean limit (1/τ < 2Δ or l >ξ). Below Tc the reflectance of high quality (Tc~ 88 K and ΔTc, ≈ 0.5 K) YBa2Cu3O7 films prepared by laser deposition is 100% ± 1% below 150 cm−1. There is an onset of absorption at 150 cm-1 and structure in the 450-500-cm−1 region. These, however, are not the superconducting energy gap because they (1) are present above Tc (2), do not shift when Tc decreases due to oxygen deficiency, and (3) occur at a substantially higher energy in Tc ≈ 85 K Bi2Sr2CaCu2O8 crystals. Instead, they are temperature independent midinfrared excitations. The superconducting gap may (perhaps) be inferred (with rather wide bounds) from the temperature dependence of certain infrared vibrational features, which narrow below Tc.
Raman measurements of superconducting gap in Tl2Ca2Ba2Cu3O10 single crystals
Authors: Rosen H. J. , Krantz M. C. , Wei J. Y. T. , Morris D. E.
Published: February 2023

The superconducting gap represents a key parameter in the understanding of new high temperature superconductors. The magnitude of the gap relative to the superconducting transition temperature will determine the viability of a strong or a weak coupling theory. In spite of the importance of this parameter there is wide disagreement between infrared and tunneling results on its magnitude. We present Raman determinations of the superconducting gap in thallium 2223 single crystals which have the highest known transition temperature. Raman spectra taken from several well-characterized superconducting Tl2Ca2Ba2Cu3O10 (2223) single crystals (Tc = 105 K) display the characteristic phonon spectrum of the 2223 phase on top of a broad background continuum interpreted as electronic scattering. The electronic scattering continuum decreases substantially below Tc for frequency shifts smaller than 400 cm-1, indicating the formation of a superconducting gap. The gap does not show the expected abrupt onset but a linearly rising electronic scattering background possibly indicating a distribution of gaps or the presence of electronic excitations within the gap.
Theoretical basis for XUV free-electron lasers
Authors: Goldstein John C.
Published: February 2023

The free-electron laser (FEL) is a promising new source of high power, tunable, coherent radiation in the extreme ultraviolet (XUV, 10-100-nm) and soft x-ray (<10-nm) regions of the electromagnetic spectrum. The basic principles of an FEL have long been verified experimentally by the operation of about one dozen devices at wavelengths ranging from 240 nm to ~8.8 mm. To shorten the operating wavelength of a given FEL by increasing the energy of the incident electrons is possible according to the FEL resonance relation which shows that the laser wavelength is proportional to the undulator's wavelength and inversely proportional to the electrons' energy. However, the optical gain decreases with decreasing wavelength. Also, mirror reflectivities decrease sharply in the XUV, thus raising the threshold gain for an oscillator. Brighter electron beams and longer undulator magnets are required to exceed the threshold gain. Thus, operation of an XUV FEL requires specific improvements in each of the three major contributing technologies: relativistic electron beams, optics, and undulator magnets. We shall discuss in detail the required improvements for the operation of an XUV FEL oscillator and also requirements for mirrorless amplified spontaneous emission sources.
Optoelectronic quadratic associative memory
Authors: Von Lehmen A. , Paek E. G. , Carrion L. C. , Patel J. S. , Marrakchi A.
Published: February 2023

Optical implementations of neural networks have attracted substantial attention because of the promise of relative ease in achieving a high degree of parallelism. First demonstration of optical neural net systems include those based on the Hopfield associative memory model. However, the Hopfield associative memory has found limited application due to its relatively small storage capacity. This has given impetus to the investigation of alternative associative memory algorithms which yield increased storage capacity. Among these is the high-order associative memory, in which the increase in degrees of freedom provides large additional memory capacity. In this paper we describe the first optical implementation of a quadratic associative memory based on an inner product representation. Our experimental system is the first execution of an optical chip design, which eliminates the need for bulk optics, and results in an extremely compact network. The system utilizes liquid crystal modulators to represent input vectors and ambient room light as the light source. The experimental system also incorporates bipolar storage of memory vectors. The ten-input system stably stores up to four memory vectors and exhibits error correction capability.
Optical neural network using a mutually pumped phase conjugate mirror
Authors: Dunning Gilmore J. , Owechko Y. , Soffer B. H.
Published: February 2023

We implement a neural network which takes advantage of the parallelism of optics and the flexibility of electronics by using a mutually pumped, phase conjugate mirror (MP-PCM) and computer controlled feedback. This device has the potential of modeling networks with 105 neurons and 1010 interconnections. The states of all neurons are represented as a 2-D array of pixels on a spatial light modulator. Each pixel is interconnected optically in 3-D via a photorefractive hologram in the MP-PCM which uses two beams input into a photorefractive crystal. Each beam forms a continuum of gratings through two-wave mixing within the crystal. The two beams mutally interact and each pixel in the input plane writes gratings with all the other pixels to form the phase conjugate beams. The gratings produced by one beam are read by the second beam, forming a multiport device. The neuron nonlinear activation functions are implemented electronically using a frame grabber and host computer. Homoassociations are performed by addressing the crystal with the same spatial information on both input beams. The mutual interaction in the crystal allows a partial version of one image to reconstruct the entire version of the second image. For heteroassociations, the crystal is addressed with different images on each beam.
Interpattern association neural network for heteroassociative memory
Authors: Lu Taiwei , Xu Xin , Wu Shudong , Yu Francis T. S.
Published: February 2023

For a given set of reference patterns, many neural network algorithms construct the interconnection weight matrix (IWM) by emphasizing the association of elements within each reference pattern (i.e., intrapattern association), while paying little attention to the association among patterns (i.e., interpattern association). The special features of the reference patterns play an important role in pattern recognition.
Prototype reflecting coronagraph
Authors: Smartt Raymond N. , Koutchmy Serge , Cross Eugene W.
Published: February 2023

A miniature 1-m focal length coronagraph has been constructed at NSO/SP, based on a 5-cm diam superpolished silicon mirror from Zeiss, F.R.G. The optical configuration is simply off-axis reflection from the objective to an occulting disk, Lyot stop, and secondary optics. The coronagraph has been designed to be used either with photographic recording or a video CCD camera, both in combination with a VARO image tube. Details of the design and preliminary results are presented. It is shown that a mirror-objective-based coronagraph has many advantages and few disadvantages compared with the traditional singlet-objective-lens Lyot coronagraph. This reflecting coronagraph design is shown to be appropriate for larger systems, using other types of mirror substrate, with the prospect of very small net instrumental polarization, of crucial importance for certain coronal studies, with excellent aberration correction over modest fields.
Incorporating coronographs with segmented telescopic systems for extrasolar planetary imaging
Authors: Watson Steven M. , Mills James P.
Published: February 2023

Detection of nonsolar planets is a challenging problem. Due to the planets' faintness relative to the star, it is necessary to suppress the stellar diffracted energy to very low levels. In addition, the planets' proximity to the star requires large diameter optics. This research examines various multiaperture and segmented telescopic systems incorporating Lyot coronagraphs to suppress the stellar diffracted energy and provide the required resolution so that the extrasolar planets can be directly imaged. Because of the phasing tolerances and required contrast ratios, the IR band of 10.5-13.5 µm was examined over a field of view of 0.2-2 arcsec. Various telescopic objectives with the Lyot coronagraph were examined for their ability to suppress the stellar diffracted energy levels. The designs examined were comprised of segmented circular, hexagonal, apodized hexagonal, and rectangular systems. The Lyot coronagraph consists of a Gaussian apodizer at the focal plane of the objective and a Lyot stop at the image plane of the objective.
Simple method for measuring polarization retardation
Authors: Greivenkamp John E.
Published: February 2023

We present a simple method for characterizing polarization retarders based on the fact that a retarder of a given thickness will have a phase retardation Δϕ that varies with wavelength. In fact, Δϕ is directly proportional to the thickness and birefringence of the material and inversely proportional to the wavelength. If the retarder is placed between two polarizing filters (either parallel or crossed), the transmitted intensity will be colored as a result of this variation. A spectral transmission curve of this sandwich can be obtained from a spectrophotometer, and the resulting data are very easy to interpret. The curve will be sinusoidally varying function of the optical frequency, and the frequency of this variation depends on the retardation. The wavelengths where the transmission is a maximum, a minimum, or one-half, respectively, correspond to the wavelengths at which the retardation has a value of a full wave, a halfwave, or a quarterwave.
Robustness of the four-detector photopolarimeter and the general concept of polarimetric light traps
Authors: Azzam R. M. A.
Published: February 2023

In the four-detector photopolarimeter (FDP) a light beam, whose state of polarization is to be measured, experiences three successive partial specular reflections at the surfaces of three photodetectors, with noncoincident planes of incidence, before the remaining flux is substantially totally absorbed by the last detector. A 4 × 1 current vector I is thus developed which is linearly related, I = A S, to the input Stokes vector S, where A is called the instrument matrix. The FDP is robust in that its operation is unaffected by (1) the insertion between detectors of beam-forming elements such as apertures and lenses, (2) spatially uniform optical anisotropy of the detector surfaces, (3) differences of the photoelectric responses associated with the nonreflected p- and s-polarized radiation, as may be caused by the excitation of surface plasmons, and (4) optical feedback as may occur if the last detector is placed exactly normal to the beam. Whereas all these factors influence the A matrix, this is inconsequential inasmuch as A is determined by calibration.
Current-ratio uniform-sensitivity thickness monitor (CRUST-M)
Authors: Nick David C. , Azzam R. M. A.
Published: February 2023

A new method of monitoring layer thickness during thin film deposition is presented. A beam of linearly polarized light of fixed azimuth strikes the film at oblique incidence. The reflected light is intercepted by a fixed arrangement of two partially specularly reflecting photodetectors with nonparallel surfaces. As the layer is deposited, the ratio of the detector currents varies as a periodic, linear sawtooth function of film thickness. Precalibration of the two-detector arrangement provides a plot of the expected path of the ratio. The thickness of the growing film is monitored continuously by comparing the measured ratio to its predetermined value. An example of the application of the CRUST-M is given for the deposition of common dielectric films on Si. The resolution is ~1 Å for monitoring the oxidation of Si.
Propagation of high energy laser beams through metallic aerosols
Authors: Zardecki Andrew , Armstrong Robert L.
Published: February 2023

By combining the results of the hydrodynamic code CON1D and the beam propagation code LASER, we investigate the propagation of high energy laser beams through vaporizing metallic aerosols in the regime for which the plasma generation becomes important. The plasma is formed in the spherical vapor layer surrounding the irradiated particle. Both the plasma initiation and dynamics are studied. To describe the plasma absorption, we include the effects of inverse bremsstrahlung and photoionization. An effective plasma absorption coefficient allows us to set up a coupled system of equations describing the system consisting of the beam and vapor.
Optical techniques for plasma diagnostics
Authors: Selwyn Gary S.
Published: February 2023

Optical diagnostic techniques can provide unique insight into plasmas and the interaction of reactive plasmas with material surfaces. However, the application of these techniques to plasma processing is not straightforward and may present formidable difficulties even to experienced spectroscopists. One commonly used technique– optical emission spectroscopy (OES)–may provide misleading results due to strong spatial variations in a discharge and the dependence of OES signal intensity on the electron excitation rate. Other techniques such as laser-induced fluorescence (LIF) are often better suited for plasma diagnostics. Due to experimental requirements, however, this method is more difficult to implement and more limited in scope than OES. Variants of LIF, such as two-photon excitation and spinchange transitions, may be used to enhance the scope of LIF as a plasma diagnostic technique and thereby provide a greater understanding of material processing. Laser-initiated photoelectric emission may be used as an in situ optical probe of surfaces during plasma processing. The use of these techniques is reviewed and illustrated with recent applications.
Nonlinear optical techniques for investigation of surfaces and interfaces
Authors: Shen Yuen-Ron
Published: February 2023

Optical second-harmonic generation and the related technique of infrared-visible sum- frequency generation are extremely versatile tools for studies of surfaces and interfaces. Numerous examples will be discussed to illustrate the power of the techniques.
Ultrafast laser studies of vibrational relaxation on surfaces: CO (v = 1)/Pt(111)
Authors: Beckerle J. D. , Casassa M. P. , Cavanagh R. R. , Heilweil E. J. , Stephenson J. C.
Published: February 2023

Energy transfer at surfaces is important in physical processes such as sticking, desorption, diffusion, and the chemistry of oxidation, catalysis, and electronic materials processing. Early information on rates and mechanisms of vibrational energy dissipation came from theory or were inferred from linewidths of optical spectra. We have used tunable subpicosecond IR laser pulses in pump-probe experiments to obtain time-resolved information about the vibrational energy relaxation time (T1) and homogeneous dephasing time (T2) for the high-frequency CO (v = 1) stretch mode of an ordered monolayer of CO on the surface of a Pt(111) single crystal. The observed Ti ≈ 4 ps at low temperatures is slower than would be inferred from the IR bandwidth for top-site CO. Transient IR spectra of the excited adlayer suggest that the CO (v = 1) → (v = 2) band is anharmonic- ally shifted from the fundamental v = 0 → v = 1 transition by only 4-5 cm−1. These results are compared with the long values of T for OH, NH, or SiH stretch modes on nonmetallic surfaces, and to spectral data and time-resolved studies of very fast CO-CO coupling in metal carbonyl molecules and very short T1 for CO (v = 1) chemisorbed on amorphous metal particles. These comparisons, as well as theory, suggest the important energy relaxation mechanism for CO (v = 1 )/Pt(111) is coupling of the vibrating dipole to metal free electrons, i.e., damping by electron-hole pair formation.
Diffraction of Gaussian beams by superposed volume gratings
Authors: Tu Kun-Yii , Tamir T.
Published: February 2023

The behavior of spatially bounded beams diffracted by superposed volume gratings is investigated by using a recently reported multiple-scattering method.1 This method provides a numerically efficient and physically meaningful approach to deriving and evaluating the fields diffracted by multiple gratings inclined at arbitrary angles with respect to each other. In particular, we consider Gaussian-beam incidence and examine both the near and far scattered fields. We then find that each diffracted beam undergoes a displacement, a distortion in the intensity profile, and an efficiency loss. These effects are most pronounced if the width of the incident beam spectrum is wider than that of the grating response spectrum. Quantitative examples will be given, and the significance of the beam-modification effects for applications in holography and optical interconnects will be discussed.
Holography of fields of small objects by using limited coherence
Authors: Hinsch Klaus D. , Hinrichs H. , Kuhfahl G. , Meinlschmidt P.
Published: February 2023

The study of a large number of small objects distributed over an extended 3-D field in space often calls for holographic recording. Examples are: bubble dynamics in cavitation research or particle image velocimetry (PIV) in flow studies. When instantaneous recording of the complete scene is required, ordinary photography fails since there is no time for subsequent focusing on separate planes in depth. The holographic image, however, can be investigated in detail later on. Sometimes, the evaluation of the holographic image encounters difficulties, since information from a certain region in depth is disturbed by signals from other regions. In PIV of 3-D flow fields, for example, photographs of different planes in depth must be made for further Young's fringes analysis. Noise from the many out-of-focus particles obscures these fringes. A solution to this problem makes use of the limited coherence of the light. Illumination is made by folding a light sheet back and forth through object space. Light scattered by the objects is recorded on a holographic plate, each light sheet being matched in optical path length to a separate reference beam. Careful adjustment of path differences between the light sheets eliminates holographic cross-talk. Upon reconstruction, each reference beam reproduces the objects within one light sheet only, thus reducing disturbances by light from other sheets. Application of the technique to particle image velocimetry of windtunnel flows is demonstrated.
Generation of an optical Wigner distribution of complex signals by using an optical phase-conjugate device
Authors: Li Y. , Ha B. , Eichmann G. , Caviris N. P.
Published: February 2023

The Wigner distribution, which is a useful means of signal time-frequency or space-patial-frequency joint representation, has found various applications in communication, signal and image processing, and pattern recognition. For these applications, the design of various optical Wigner-distribution processors has been accomplished. By using linear space-invariant optical elements, the optical Wigner distribution of a real signal can easily be displayed. To process a complex signal, spatial filtering and holographic techniques were also proposed. These techniques usually have operating restrictions that limit their real-time optical-processing applications.
Modeling of surface-emitting lasers
Authors: LaViolette Kerry D. , Liu Pao-Lo
Published: February 2023

Short-cavity surface-emitting semiconductor lasers are potentially useful for highly parallel, optical signal processing and for communication systems. Because of their short cavity lengths, surface-emitting (SE) lasers are expected to be single-mode and to have a high modulation bandwidth. By means of noise-driven rate equations1,2 typical SE lasers are modeled so as to produce photon statistics, relative-intensity noise (RIN) as a function of frequency, and relaxation-oscillation frequency. An example is a 2.7/µm long bulk SE laser simulated at a computer work station. The one µs run at 0.1 ps steps takes 4 minutes to produce photon statistics when the SE laser is biased at 1.5 times the threshold current of 0.89 mA. The main mode is well defined, and the side mode acts as noise with an average side-mode suppression ratio of -29 dB. The relaxation oscillation frequency is 1.7 GHz and is lower than expected. However, the photon lifetime is 2 ps because of the high mirror reflectivity. The characteristics of other SE laser structures, such as quantum-well structures, will also be discussed.
Analysis of coupled grating-surface-emitting diodes
Authors: Salví Theodore , Shakir Sami
Published: February 2023

Efficiency and mode control are important in the proper design of coupled grating-surface-emitting diodes. In these devices, radiation-emission properties depend on the injection current of the different sections, phase effects from length variations, grating phase effects, and the boundary conditions of the particular design. We model an array of grating-surface-emitting diodes coupled either as a linear array or in a ring configuration. A realistic model incorporating power saturation, saturable losses, and saturable dispersion effects is established. Optimization of operation by injection-current control and phase modulation is studied. Modeling of the general (nonsymmetric) ring geometry is more complicated than the corresponding linear array because the number of unknowns is four instead of two. We show an approach that reduces the number of general ring unknowns to two.
Laser-induced fluorescence diagnostics of silicon and silicon monoxide
Authors: Hynes Anthony J.
Published: February 2023

Silicon monoxide (SiO) and atomic silicon (Si) are potentially important intermediates in a variety of combustion and plasma environments. Examples are silicon-doped flames used in vapor-phase axial deposition for the manufacture of optical waveguides and a variety of CFD and plasma-enhanced chemical- vapor deposition processes for the deposition of silicon and silica in microelectronics fabrication. We have observed laser-induced-fluorescence (LIF) from SiO in low-pressure glow discharges in Ar/O2/SiCl4 mixtures and in atmospheric-pressure CH4/O2 flames doped with SiCl4. Fluorescence was excited by means of the A1π-X1E+ transition at approximately 230 nm. Spectra were assigned by comparison with simulated spectra computed from previously reported line positions and rotational and vibrational temperatures were obtained. Measurements of resolved fluorescence with a gated optical multichannel analyzer were used to determine energy transfer rates in the A1π state. Si atoms were observed in low-pressure discharges and also as a product in the 193 and 251 nm multiphoton dissociation of silanes. Si atoms were monitored through LIF by scanning the laser across the 4s 3P0→3p23P transition at approximately 250 nm and monitoring the undispersed fluorescence. Production of Si 4s 3P0 in the 193 and 251 nm multiphoton dissociation of silanes was also observed, and its implications for Si atom diagnostics will be discussed.