bluuebunny/crossref_metadata_2025 huggingface dataset:
Modification of Monte Carlo codes for use with sharply peaked phase functions
Authors: Walker Philip L.
Published: February 2023

In comparing the Monte Carlo code (MSCAT) in EOSAEL1 to analytic multiple scattering,2 it is clear that MSCAT underpredicts the effects of multiple scattering for cases discussed in Ref. 2. The reason for the underprediction is that the angular scattering algorithm used by this and other Monte Carlo codes may not be adequate for sharply peaked phase functions. Angular scattering is computed from a lookup table of the angular integral of the phase function tabulated at fixed angles. If the phase function is too sharply peaked for the chosen angle spacing, the generated scattering distribution will not reproduce the phase function. Because the lookup table is an integral of the phase function, this leads to the idea of picking the tabulated angles automatically by means of an adaptive quadrature. Use of adaptive quadrature means that the angles at which the phase function is tabulated are closest together where the phase function is sharpest.
Use of geometrical and physical optics to model free-electron lasers
Authors: Tombrello Joseph F. , Shealy David L.
Published: February 2023

The design and analysis of free-electron laser (FEL) systems require accurately modeling both the propagation of the light beam through the resonator and the light beam’s interaction with the electron beam in the wiggler. In a previous work,1 a design algorithm was reported for determining the mirror parameters for a class of soft-x-ray grazing-incidence resonators for use in FEL systems. The unloaded resonator designs were analyzed by means of the diffraction-based optical design code GLAD and were shown to support stable Gaussian TEM00 modes. However, substantial distortion of an unloaded mode owing to the inclusion of the electron beam interaction could significantly affect the gain of the FEL.2 If the electron beam effects could be included in the FEL design algorithm, the resonator parameters could be adjusted to offset the distortions. We will present a preliminary report on an attempt to integrate a FEL electron-beam modeling code with the optical-design code GLAD in hopes of addressing some FEL design problems that each code alone is unable to handle.
Monolithic waveguide/photodiode integration on InP with integrated mirrors or vertical-impedance matching
Authors: Deri R. J. , Wada O. , Sanada T. , Yasuoka N. , Makiuchi M. , Kuramata A. , Hamaguchi H. , Yamakoshi S. , Hawkins R. J.
Published: February 2023

Monolithic waveguide/photodetector coupling is essential for optoelectronic integration using waveguide optical signal processing. Previous coupling approaches have restricted applicability because of poor detector bandwidth for thick waveguide layers,1 difficult fabrication (multiple material-growth cycles on nonplanar surfaces),2-4 or strongly wavelength-dependent coupling.3,4 Here we describe two alternatives suitable for thick waveguides; they exhibit low input coupling loss from optical fibers (< 1 dB) at 1.5 μm wavelength. Our approaches are variants of conventional butt-coupling2 or vertical-coupling1 schemes and achieve efficient absorption with minimal detector area for high-bandwidth detection. Our butt-coupling variant uses integrated mirrors to couple guides on one wafer surface to detectors on the opposite surface, as was first suggested by Trommer.5 In contrast to Trommer's system, however, our large waveguide size minimizes diffraction in the substrate. This reduces the detector area (50 × 80μm2) and capacitance (0.25 pF) and maintains low optical insertion loss (~1 dB). Our vertical-coupling variant uses impedance-matching for coupling enhancement, resulting in a low input coupling loss (0.8 dB), a guide/diode coupling of 70–99%, and bandwidths of 5.5-11 GHz.
Third-order nonlinearity of chínese tea—a good phase conjugator
Authors: Abdeldayem H. , Xikiang He , Sekhar P. Chandra , Mago V. K. , Venkateswarlu P.
Published: February 2023

The phase conjugate reflectivity of a solution of chínese tea1in ethanol, at different concentrations, was measured, using degenerate four wave mixing of a Nd:YAG laser beam at 532 nm. A reflectivity ≅ 7% was obtained at a concentration of 3.6 × 10-4g/cm3, The square root of the reflectivity versus the pump intensities

(

I
1

I
2

)

was plotted. The slope, of the straight line obtained, was used to estimate the value of the third order non-linear susceptibility, χ(3)≅ 1.1 × 10−7esu, assuming that absoprtion is the dominant mechanism for phase conjugation. At a higher concentration of 3.7 × 10−3g/cm3, χ(3)is 1.4 × 10−6esu. χ(3)was also estimated from the critical power Pcfor self trapping of a CW ar+ laser beam at 514.5 nm. χ(3)is linear at low concentrations, and saturates at the optical density (O.D.) = 0.5. At this O.D., χ(3)= 1.2 × 10-6esu, which is comparable to that obtained from DFWM. Other self-action effects, such as self-defocussing, spatial self-phase modulation and beam fanning, were also observed. Further investigations are in progress.
One-way imaging through a random phase aberrator using real-time holography
Authors: Chang Tallis Y. , Hong John H. , Yeh Pochi , Neurgaonkar R. R. , Avizonis Petras V.
Published: February 2023

Several methods of one-way image transmission have been proposed in the past. Early methods1 used conventional holography, and the recent ones have used nonlinear optics.2 We present experimental studies of a nonlinear optical (i.e., real-time holographic) implementation of one early proposal.1 This technique consists of first recording a hologram of an aberrator and then reading it out with an image beam that is distorted through the same aberrator. The hologram is formed by the interference between a plane-wave beam transmitted through the aberrator (objectbeam) and another plane-wave beam (reference beam). Any image beam traversing the same path as the object beam of the holographic recording, including the transmission through the aberrator, can thus be diffracted from the hologram and will yield the distortion-free original image. We have demonstrated this technique in real time by using a thin photorefractive strontium barium niobate crystal (0.7 mm thick) as the holographic medium. Preliminary results showed that the processed image resolution, as limited by the combination of our optics and the crystal, was ~70μm. The constraints of this technique on the crystal thickness, severity of aberration, and field of view will be discussed.
Direct image recovery from speckle patterns in the presence of additive noise
Authors: Lees David E. B.
Published: February 2023

It has been shown1 that imagery can be recovered directly from speckle patterns without the intermediate step of estimating image autocorrelation.2 The present work demonstrates the imagery recovered from simulated speckle patterns with additive Gaussian noise. Excellent images can be reconstructed, even when corrupted by significant noise, if support constraints are tight and outline the objects. Images were reconstructed from 100 speckle patterns with an ideal reconstruction signal-to-noise ratio of 10. Gaussian-distributed speckle noise with 10% of the speckle-pattern energy had a negligible effect on the signal-to-noise ratio in the reconstructed imagery. When the noise content was increased to 50% of the speckle pattern energy, the signal-to-noise ratio decreased by a factor of 5. The image quality also dropped rapidly with decreasing support tightness.
Optical neural network using pocket-sized liquid-crystal televisions
Authors: Yu Francis T. S. , Lu Taiwei , Yang Xiang Y.
Published: February 2023

The improvements in liquid-crystal-television (LCTV) technology have made it possible to build a miniature optical neural network using high-resolution LCTV's to replace the cathode-ray-tube video monitor.1 Two LCTV's built by thin-film-transistor techniques have been used in an optical neural network for displaying the associative-memory matrix and input pattern, respectively. A compact optical neural network of 8 × 8 = 64 neurons has been synthesized, and the number of neurons in the network can be further increased to 15 × 20 = 300. The size of the optical neural network is only ~65 × 12 × 12 cm3, and the system size can be further reduced to ~20 × 7 × 7 cm3 by employing smaller LCTV's. The processing speed of the neural network can be as high as 3 × 106 interconnections.
Longitudinal behavior in semiconductor laser diodes
Authors: Thompson William E. , Osinski Marek
Published: February 2023

Much of the analysis of high-power semiconductor-laser diodes that has been reported in recent literature is based on the assumption of uniform carrier density (and, consequently, on uniform gain) in the longitudinal direction (z-axis). It was noted long ago1 however, that this assumption is inconsistent with the actual behavior inside the optical cavity. The differential equations that describe the carrier density and the optical fields inside the cavity are coupled, so that a nonuniform total optical field must of necessity introduce a nonuniform carrier density as a function of z. The assumption of uniform carrier density is reasonable for equal facet reflectivities, such as that determined by the GaAs-air interface, but becomes increasingly suspect for situations in which the reflectivities are different (for example, the extraction all optical power from one end in high-power applications) or in which fabrication techniques purposefully introduce variations on longitudinal pumping. To address these situations, an analysis that does not depend on the assumption of uniform carrier distribution is needed. This paper reports on efforts to analyze the behavior of semiconductor laser diodes, including possible variation of parameters in the z-direction. An iterative method has been adopted to examine analytically (to the extent possible) the nonuniformity of carrier density distribution. The first-order correction in the iterative solution is obtained by using two separate approaches. Numerical examples that illustrate the importance of this correction for asymmetric-cavity lasers will be presented.
Rastered operation of row-column addressed vertical-cavity surface-emitting laser array
Authors: Von Lehmen A. , Orenstein M. , Chang-Hasnain C. , Banwell T. , Wullert J. , Stoffel N. , Florez L. , Harbison J.
Published: February 2023

The use of vertical-cavity surface-emitting (VCSE) laser arrays has been discussed for a number of applications; these include optical interconnects1 and beam steering in display and holographic-memory applications.2 Previously, we have described the fabrication of a matrix, or row-column-addressable InGaAs/GaAs VCSE laser array.3 This array is a 32 × 32 element array in which the electrical contact geometry is such that all lasers in each row and in each column are connected. Such a scheme is extremely attractive because it requires only 2N bond pads to address N devices. It is particularly appropriate for the two applications described above. In this paper, we will discuss rastered operation of matrix-addressed laser arrays. This involves sending streams of time-multiplexed data along the row/column address lines. Each laser device is enabled only when it simultaneously receives "on" pulses from the row and column data streams. The uniformity of various operating characteristics across the array under row–column addressing will also be discussed.
Birefringence in integrated optical devices made by double Ag+ ion exchange in BGG 31
Authors: Stierlin R. , Horst F. , Fabricius N. , Fuest R.
Published: February 2023

Passive integrated-optical (IO) devices may be important in future telecommunication, e.g., as cheap power splitters, combiners, and demultiplexers, as well as in optic and fiber-optic sensors.1-2 In numerous applications of single-mode sensors, the state of polarization is a crucial physical parameter (e.g., in Faraday sensors). Therefore, the polarization properties of the IO components are, in addition to the price, of utmost importance in sensor design. In our work we focus on IO linear waveguides and Y-couplers. The birefringence in buried IO waveguides is at least an order of magnitude smaller than in surface waveguides made by a K+ ion exchange in BK7 (n ≤ 10–4).2,3 However, the exact value of the birefringence and its variation with temperature is not known. We measured the temperature dependence of the birefringence at 1300 nm and compared it with the theoretical value. For the measurements, we set up an ellipsometer with a rotating analyzer. With this arrangement an absolute accuracy of ~ ±0.5° phase difference between the TE and TM modes was achieved over the complete temperature range of –35°C to +80°C provided that the phase differences were not too small(≥5°).
Examination of ion-exchange processes for single-mode waveguides
Authors: Tervonen A. , Pöyhönen P. , Honkanen S. , Tahkokorpi M. , Tammela S.
Published: February 2023

A computer model was used to study important two-step ion-exchange processes for the fabrication of single-mode optical waveguides with fiber-like mode-field distributions. The ion-concentration distributions were solved from the ion-exchange equations by a finite-difference method1 and were transformed into waveguide refractive-index profiles. The guided-mode solutions were then obtained by an iterative finite-difference method. As a first step, a field-assisted method was calculated to have better controllability than a diffusion method. As a second step, field-assisted burial was found to optimize the coupling loss to fiber to 0.05 dB, but losses lower than 0.2 dB were calculated for less complex diffusion-process steps. On thebasis of modeling, a solid-state fabrication process was proposed,1 and the near-field distributions of fabricated waveguides were measured to confirm the model. Additional results are presented for a special two-step process that is an inverse ion-exchange method. This method has advantages in the control of the lateral dimension of waveguides.
Structured targets for enhanced absorption and x-ray emission in femtosecond laser-produced plasmas
Authors: Mumane Margaret M. , Kapteyn Henry C. , Falcone Roger W.
Published: February 2023

Laser-produced plasmas created by intense femtosecond laser pulses have been demonstrated to be subpicosecond soft-x-ray sources.1 The use of terawatt femtosecond lasers will permit the generation of more intense x-ray sources; however, increased target reflectivity at high fluence will result in less efficient coupling of laser energy into the plasma. We demonstrate experimentally that structured targets can dramatically increase this coupling. Our targets were lithographically produced SiO diffraction gratings with a 150 nm depth and a 300 nm period. An incident laser pulse of 3 mJ in 150 fs at 620 nm was focused to 1016 W/cm2 on the target. For polarization perpendicular to the groove structure, an effective-index approximation shows that the laser light penetrates the grooves; a reflectivity of only 8% was observed experimentally. The x-ray yield for 30 eV to >1 keV increased by >11 times. This increased absorption is a result of the enhanced electric field in the voids between the grooves, resulting in an enhanced energy deposition, a hotter plasma, and a more intense x-ray emission.
Simulation of weight noise in back-propagation architectures
Authors: Levy James J. , Athale Ravindra A. , Haney Michael W.
Published: February 2023

The back-propagation algorithm1 has become increasingly popular in the neural-net research community. Various optical implementations have been proposed, with the hope of increased performance by means of the parallelism of optics. Realistic models of optoelectronic implementations must include the effects of noise. Although noise often anneals, increasing the convergence rate, excessive noise can effectively transform any updating algorithm into a random search among weight configurations. For the purpose of evaluating the effects of component noise on the performance of the back-propagation algorithm, we have developed a simulation program that permits insertion of noise terms wherever appropriate. The program introduces noise processes into the weight-updating process during the learning phase. The learning curve is defined as the mean-square error versus the iteration number. Our analysis emphasizes examination of learning curves rather than simply the probability of convergence.
Synchronous amplification of femtosecond pulses at 10 kHz with a solid-state pump laser system
Authors: Sweetser J. , Palese S. , Dunn T. J. , Walmsley I. A. , Miller R. J. D.
Published: February 2023

We have developed a Nd:YLF regenerative-amplifier-based (RA-based) system for synchronous amplification1 of femtosecond pulses from a passively mode-locked dye laser (CPM) at rates approaching 20 kHz. The higher repetition rates, which are desirable for improved signal-to-noise ratios in scientific experiments, are limited only by the pumping rate of the RA. The two significant features of the system are (1) the operation of the RA at greater than 10 kHz and (2) the synchronization of the output of the RA to the colliding-pulse mode-locked (CPM) laser. This is achieved by FM mode-locking a diode-pumped Nd:YLF seed laser with an RF signal that is phase-locked to the output of the CPM. The use of an FM mode-locking scheme relaxes the requirement that the CPM and seed laser cavities be exactly matched. The picosecond pulses from the seed laser are amplified to the millijoule level in the RA, frequency doubled, and then used to synchronously pump a dye amplifier. Synchronous amplification with short pump pulses (d ps) permits more efficient amplification and creates the potential for higher pulse energies than are currently available with copper-vapor laser-based asynchronous amplifiers.2
KTP: a new electro-optic device for high-power, high-repetition-rate applications
Authors: Wang X. D. , Basseras P. , Miller R. J. D. , Vanherzeele H.
Published: February 2023

In addition to having attractive nonlinear-optic characteristics, KTP has promising electro-optic and dielectric properties that make it useful for various electro-optical applications, such as modulators and Q switches.1 We report an experimental study of a 4 × 8 × 4 mm hydrothermally grown KTP Pockels cell that uses the effective electro-optic coefficient rcl = r33 − (ni.n3)3r13 = 28 pm/v. Compensation of the static birefringence of KTP, when it is used as an amplitude modulator has been realized. Our investigations revealed that, unlike other electro-optic materials, KTP Pockels cells do not suffer from piezoelectrically induced parasitics (acoustic ringing). Because of this, we have been able to demonstrate stable high-repetition rate pulse slicing. The dynamic half-wave voltage is close to the static one (2.9 kV) and it is significantly lower than that of the commonly used LiNbO3 Pockels cell. An additional advantage of KTP over other Pockels-cell materials, such as KD*P and LiNbO3, is its high damage threshold. Therefore, KTP is an ideal material for intracavity electro-optic applications, in particular for high-peak power regenerative amplifiers operating at high-repetition rate.2 We have successfully studied this mode of operation, and our results will be discussed.
Ultrashort electrical pulse generation and spectroscopy
Authors: Nuss Martin C. , Roskos H.
Published: February 2023

The optoelectronic generation and detection of extremely short electrical pulses with bandwidths exceeding 1 THz have made it possible to study materials and electronic structures in a previously barely accessible region of the electromagnetic spectrum.1 These electrical pulses can be propagated in free space and can be collimated and focused with such optical elements as lenses and mirrors.2 We use these submillimeter-wave electrical pulses to study the electronic structure of low- and high-temperature superconductors,3 intersubband absorption in GaAs/AlGaAs quantum wells, and resonance splitting in coupled quantum wells. We will also discuss the anisotropic dielectric constants of the two-dimensional electron gases in semiconductor quantum wells and the weakly coupled conducting layers of high-temperature superconductors.
Femtosecond carfier transport in nonuniform depletion regions
Authors: Knox W. H. , Ramaswamy M. , Goodnick S. M.
Published: February 2023

We use excitonic effects in thin GaAs quantum-well stripline samples to generate, propagate, and detect femtosecond electrical signals.1 The time dependence of the initial photocurrent transient depends sensitively on the bias and excitation conditions. One possible explanation for this is a velocity-overshoot effect, which is strong because the carriers that we excite are initially at the band gap, with no excess energy. In this presentation, we point out that in metal-semi-insulating semiconductor contact systems, the actual electric field experienced by the quantum-well excitons can be highly nonuniform,2 making quantitative comparison with theory difficult. In fact, Monte-Carlo studies of carrier transport should be developed to account for this effect.
Three-dimensional surface curvature from binocular stereo disparity II
Authors: Wildes Richard P.
Published: February 2023

How is it possible to recover the three-dimensional (3-D) curvature of a surface given its differential projections in a stereoscopic view? Previously, it has been shown that the differentially imaged curvature of markings (e.g., texture) on a 3-D surface theoretically allow for the recovery of the surface's 1-D curvature. However, numerical studies suggest that this solution is unstable in application.1
Fourth-order interference between two independent sources of stimulated Raman scattering
Authors: Smithey D. T. , Kuo S. J. , Raymer M. G.
Published: February 2023

Stimulated Raman scattering (SRS) produces Stokes light pulses that originate from quantum noise and exhibit macroscopic amplitude and phase fluctuations from one shot to another.1 By using transient stimulated Raman scattering, we generated light pulses with macroscopic energy and temporal coherence times that were long compared to their duration. Stokes pulses were generated independently in each arm of an interferometer and were interfered together at the output. The position, modulation depth, and brightness of the interference fringes fluctuated randomly from one shot to the next. We obtained both the ensemble-averaged intensity and the fourth-order intensity correlation function of the interfered Stokes light. The average has a smooth envelope and shows no modulation, whereas the intensity correlation function has a depth of modulation of 0.35 for Stokes generation in the unsaturated (thermal-like) regime and 0.46 in the saturated (laser-like) regime. This is in agreement with predicted values of 0.33 and 0.5.2
Transient lineshapes in nonlinear mixing of pump and probe fields
Authors: Kumar M. Sanjay , Eberly J.H.
Published: February 2023

We present results on transient lineshapes in signals generated coherently via the nonlinear mixing of pump and probe fields in a model two-level system. We confine ourselves to times short compared to atomic relaxation times and hence treat both fields nonperturbatively. However, the probe is assumed to be much weaker than the pump. Transient lineshapes have been studied earlier in the case of atomic relaxation times and hence treat both fields nonperturbatively. However, the probe is assumed to be much weaker than the pump. Transient lineshapes have been studies earlier in the case of atomic inversion. Here, we identify the lineshape with the intensity of the light scattered in a particular direction and around a particular central frequency. The lineshapes are found to exhibit, in the neighbourhood of subharmonic resonances, highly non-lorentzian profiles.
Observation of subharmonic resonances of the Rabi frequency in the absorption spectrum of a two-level atom in a bichromatic field
Authors: Papademetriou Stephanos , Chakmakjian Stephen , Stroud C. R.
Published: February 2023

The interaction of a strong bichromatic field with a two-level atom has been the subject of numerous experimental and theoretical investigations. We performed an experimental study of the Interaction of an intense 100% amplitude-modulated (AM) field with an ensemble of homogeneously broadened two-level atoms. An optically pumped sodium atomic beam is used as the atomic medium. The field is created by interfering two intense frequency-stabilized laser fields. The frequency of one field is held fixed while another field of equal amplitude is scanned in frequency. The beat frequency produced by interfering the two laser fields is equal to twice the modulation frequency of a 100% AM field. We monitor the time-averaged fluorescent intensity of the atoms as the frequency separation between the two fields is varied. A similar experiment was done in the past in the rf regime.1 For the first time, we report the observation of multiple, well-resolved resonances in the absorption spectrum of the field in the optical regime. We present data and theory for several values of detuning between the fixed field and atomic resonance. The experimental results are in good agreement with theory.
Sensitive measurements of anisotropy of third-order nonlinearities
Authors: DeSalvo J. R. , Hagan D. J. , Sheik-Bahae M. , Van Stryland E. W.
Published: February 2023

We have performed measurements of the anisotropy of the nonlinear refractive index n2 of crystalline solids using a variation of our recently developed Z-scan technique.1 This method gives a sensitive, absolutely calibrated measurement of n2 using a single beam. In the Z-scan technique the transmittance of a sample with an aperture placed in the far field is measured as a function of the position of the sample with an aperture placed in the far field is measured as a function of the position of the sample with respect to the Gaussian beam waist. This results in a peak signal followed by a valley for n2<0 and the opposite for n2 <0 as the sample is moved from before to after focus. If the aperture is removed the Z-scan is sensitive only to nonlinear absorption. For the measurement of the anisotropy of n2 we modify this technique by placing the sample at a position of peak (valley) signal and rotate a wave plate, either a half-wave plate to rotate the linearly polarized light or a quarter-wave plate to change from linear to circular polarization. By placing samples at the beam waist with the aperture fully open, this method yields the anisotropy of the nonlinear absorption coefficient. We demonstrate this technique on materials such as KTP where we haveseen n2 vary by a factor of 1.6 as the linear polarization is rotated . Thus, n2 of flux grown, z-cut KTP at 1.06µm varies from0.62 × 10−12esuto 1.0 × 10−12 esu. In x-cut KTP, we have observed the unusual effect of a reversal in sign of the nonlinear refraction from self-sign change is correlated with the generation of second harmonic light, with the maximum defocusing occurring at the polarization producing the maximum second harmonic.
Alternative Z-scan geometries for n2 measurements
Authors: Said A. A. , Sheik-Bahae M. , Hagan D. J. , van Stryland E. W.
Published: February 2023

We have recently reported a simple experimental technique,1 Z-scan, for determining nonlinear refraction and nonlinear absorption. By using a Gaussian beam in a tight focusing geometry, the transmittance of a nonlinear medium through an aperture in the far field is measured as a function of the sample position with respect to the focal plane. A typical Z-scan transmittance signal has a maximum before focus and a minimum after focus, or vice versa, depending on the sign of the nonlinearity. In this paper, we explore different geometries of the Z-scan experimental arrangement to further enhance the sensitivity of this technique. Adding an imaging lens in the far field and placing the aperture at the imaging plane of the new system enhances the sensitivity of the Z-scan technique threefold. Moreover, the positions of the peak and valley are reversed with respect to the focal plane.
Stimulated rotational Raman scattering in air with a high-power broadband laser
Authors: Skeldon M. D. , Bahr R.
Published: February 2023

For laser fusion applications, many highpower laser beams must propagate over long air paths to a target chamber. Because of the high intensity–length products required for this application, the generation of stimulated rotational Raman scattering (SRRS) in air is of utmost concern.1,2 addition, schemes for smoothing the intensity distribution on the target require large optical bandwidths. We report for the first time the effects of laser bandwidth on the SRRS process in air at a wavelength of 351 nm. A 600 ps pulse from a Kuizenga oscillator at a wavelength of 1054 nm is sent through a LiNbO3 electro-optic modulator to impose a sinusoidal phase modulation with a 2.4 GHz modulation frequency onto the beam. The beam is then amplified by one beam line of the OMEGA fusion laser, frequency-tripled in a pair of KD*P crystals, down-collimated to a 5 cm beam diameter, and propagated over a distance of 35 m in air. The energy of the beam is varied up to 25 J per pulse and is measured with calibrated detectors. At the end of the path, the spectra are recorded on calibrated film with a 1 m spectrometer, digitized with a microdensitometer, and analyzed.
Optoelectronic implementation of neocognitron for pattern recognition
Authors: Chao Tien Hsin , Yu Jeffrey , Cheng Li Jen
Published: February 2023

The neocognitron is a multilayered feedforward hierarchical neural network model for visual pattern recognition that was first proposed by Fukushima.1 As opposed to some of the neural-network models (e.g., associative memory, perceptron, and adline), the neocognitron is a featu re-extraction-based system that offers the unique advantage of deformation and shift invariance. The primary difference between a neocognitron and other neural networks is that the neocognitron recognizes and extracts simple features from an input pattern in its beginning layer. Extracted features are then grouped for recognition in a deeper layer. This feature-extraction mechanism provides the foundation for deformation invariance. We have devised an optoelectronic system for the implementation of the neocognitron. Each generic layer of the neocognitron consists of a multichannel correlator and a two-dimensional electronicschip array. The function of the correlator is to perform parallel feature extraction with shift invariance. The electronics chip performs realtime thresholded correlation detection. The basic layer is used iteratively to realize a multilayer functionality. In this paper, the system issues in implementing this neocognitron will be discussed. Preliminary experimental results will also be presented.
Wavelength-multiplexed volume holograms for fully interconnected neural networks
Authors: Weverka Robert T. , Wagner Kelvin , Saffman Mark
Published: February 2023

The degrees of freedom available in a three-dimensional volume hologram only permit interconnection between sparse fractal planes1 of optical neurons that do not adequately sample dense two-dimensional images. We are investigating the capabilities of spectral-hole-burning volume holography2 for implementing N
2
to N interconnections, and will present several approaches for using the wavelength domain as the necessary extra dimension. Global interconnection architectures based on N wavelength-multiplexed N-to-N
2
mappings do not produce any unwanted Bragg degenerate diffractions, but those based on N
2
-to-N mappings produce unwanted diffractions that can be eliminated with a spatially dependent wavelength filter, such as a wedged Fabry-Perot étalon. Other architectures based on N
3/2
-to-N
3/2
mappings with N multiplexed wavelengths to achieve N
2
-to-N
2
global interconnections as well as local mappings requiring fewer wavelengths are also possible. These schemes access all of the degrees of freedom of a spectral-holeburning hologram simultaneously, allowing as many as 1012 multiplications per readout, and learning architectures allow these interconnections to be adaptively modified based on a sequence of training patterns.
Custom electro-optic devices for optically implemented neuromorphic computing systems
Authors: Robinson Michael G. , Zhang Lin , Johnson Kristina M. , Jared David A.
Published: February 2023

Previously we presented optical implementations of neural networks that use parallel optical interconnections between neurons.1 In these systems, the nonlinear neural response is carried out electronically after detecting the optical signal. However, this would be better implemented by means of custom, parallel-optical in–optical/out–electro-optic spatial light modulators (SLM's). Here we present two types of such SLM's fabricated from amorphous silicon (a- Si:H)/FLC and VLSI/FLC technologies. In both cases, the input beam carrying the information from the previous layer of neurons is detected and is transformed into an electric current. This current is then electronically processed, and the resultant voltage is applied across an FLC modulator, which spatially modulates the output optical beam. For the a-Si:H/FLC device, simple linear subtractions between spatially separated optical channels can be achieved, with a nonlinear response obtained through the switching characteristic of the FLC.2 This is particularly applicable in optical implementations that use bipolar interconnection weights.1
Spatial-mode cleanup of a pulsed laser beam through mutually pumped phase conjugation with a cw reference
Authors: Chang Tallis Y.
Published: February 2023

A novel method of cleaning up the spatial mode of a pulsed laser beam without using a pinhole is proposed and demonstrated. The method consists of performing mutually pumped phase conjugation (MPPC)1 between the pulsed laser beam and a cw reference, thereby transferring the desirable spatial mode of the cw beam onto the pulsed beam. In my experiments, I have observed MPPC in a photorefractive BaTiO3 crystal between the beams from a Q-switched flashlamp-pumped Nd:YAG laser and a diode-pumped cw Nd:YAG laser, both operating at 532 nm. The experimental results showed convincing spatial-mode cleanup of the pulsed beam. This method can also modify the incident pulsed beam to any desirable spatial pattern by using a spatial light modulator in the cw reference. To my knowledge, this is the first reported MPPC between pulsed and cw laser beams.
Optical novelty filtering using a phase-grating carrier
Authors: Yu Francis T. S. , Wu Shudong , Rajan Sumati , Mayers Andy
Published: February 2023

Techniques using photorefractive effects for novelty filtering have recently been suggested by several investigators. Examples include the four-wave-mixing interferometric method,1 the two-wave-mixing beam-depleting method, and the beam fanning method.2
Variable-frequency cross-grating interferometer
Authors: Cheng Yih-Shyang
Published: February 2023

A holographic plate doubly exposed to the two-beam interference pattern with a rotation between two exposures can be processed to direct considerable light in four desired directions. This plate serves as the beam splitter in the interferometer. With four line gratings in the other plane to redirect the light and produce an interference pattern, an elliptical (or circular) dotted array with unity contrast can be produced with white-light extended-source illumination.1 However, this interferometer cannot produce a dotted structure with continuously variable fringe periods in orthogonal directions. When the first grating is replaced by two line gratings, with their line structures inclined at an angle (almost in contact), one of these line gratings combined with two line gratings from the other plane, would constitute a line-grating interferometer. The conditions under which a dotted interference pattern with unity contrast can be produced are discussed, together with some experimental results.
Optically Encoded Phase Matched Second Harmonic Generation in Semiconductor Microcrystallite Doped Glasses
Authors: Lawandy N.M. , MacDonald R.
Published: February 2023

Since the discovery of efficient SHG in germanosilicate fibers no other materials have been found which exhibit optically organized χ(2) gratings(1,2). We have discovered that CdSexS1−x semiconductor doped glasses can be prepared for efficient SHG when the glass is simultaneously exposed to 532 nm and 1.06 µm radiation from a Q-switched modelocked laser(3). The effect has resulted in the generation of SHG visible in room lights corresponding to a χ(2)~10−16m /V. Sidelight studies show that a phase-matched process is responsible for the SHG.
Electro-Optic Sampling Measurement of the Built-In Field in Fibers Conditioned for Second Harmonic Generation
Authors: Kamal A. , Stock M. L. , Szpak A. , Thomas C. H. , Weinberger D. A. , Frankel M. , Nees John , Ozaki K. , Valdmanis J. A.
Published: February 2023

We have confirmed, for what we believe to be the first time, the presence of a built-in field in fibers which have been optically conditioned to produce second harmonic (SH) radiation. It is now well accepted that the optical conditioning occurs through the mixing of the fundamental and SH seeding fields such that a periodic χ(2) is established in the fiber.(1) The details of the mixing mechanism are still open to considerable debate, although most of the proposed models necessitate the existence of an internal dc electric field.(1),(2),(3) a measurement of the magnitude of the internal field is thus of fundamental importance.
All-Fiber Modelocked Figure Eight Laser
Authors: Duling Irl N.
Published: February 2023

The introduction of Er doped fibers for amplification in optical communication systems has led to the possibility of long-haul, high-speed soliton based communication. There is a need for a simple inexpensive efficient source of solitons at a wavelength compatible with the Er amplifiers and a repetition rate suitable for the new high speed communication systems. In the past these pulses have been provided by either color center lasers (which are bulky, expensive and inefficient) or modelocked laser diodes (which have little output power and have a limited lifetime). In this paper we present an all-fiber laser that produces solitons by passive modelocking a fiber ring with a nonlinear amplifying loop mirror (NALM) containing an Er-doped section of fiber. This laser produces transform limited pulses as short as 2 ps, at an average power of 1.3 mW and a wavelength of 1.535µm, by passive modelocking. The idea of modelocking a laser using a nonlinear fiber mirror has been proposed[1] and demonstrated[2] for a linear laser using either a bulk optic or a fiber reflector at the other end of an amplifying segment. In this paper we would like to present a ring laser which is modelocked by a NALM, so that no active modelocking is necessary in the laser.
He 2 +: A potential for a New Laser
Authors: Hill Peter C.
Published: February 2023

The discovery of

He

2
+

(

C
2

∏
u

)

resulted from a study1 of the feasibility of neutral helium dimer lasers. These lasers (as with the majority of excimer lasers) operate in a pulse afterglow. Such operation limit the power and efficiency of any potential laser and make the lifetime of the lasing molecule critical to the feasibility. Thus the explanation of wepreviously unassigned intense continua seen during arc discharges was sought as it may lead to an excimer laser that did not suffer the disadvantages of afterglow operation.
Squeezing spectrum in second-harmonic generation in the self-pulsing domain
Authors: Fabre C. , Pettiaux N. , Mandel P.
Published: February 2023

We consider the process of second-harmonic generation (SHG) in a cavity that is resonant for both the fundamental- and second-harmonic modes. It has been previously shown that such a system displays a phase instability leading to self-pulsing and that one gets important intensity squeezing on both the fundamental- and second-harmonic mode for pump intensities close to but below, this instability threshold. Such a squeezing has been recently observed. We have analytically calculated the intensity-noise spectrum of the frequency-doubled field above such an instability threshold, using a semiclassical input-output technique for the treatment of quantum fluctuations.1 Such a method, introduced to deal with fluctuations around a stable solution, has been generalized to the self-pulsing domain. As expected, the instability brings excess noise to the output beams, especially around the zero-frequency, the self-oscillation frequency, and its harmonics. Remarkably, some regions exist in which where the noise increase is small; squeezing still occurs in this frequency range far in the instability domain.
Photorefractive resonator with gain and loss: weak- versus strong-field theory
Authors: Lininger D. M. , Crouch D. D. , Anderson D. Z.
Published: February 2023

We compare weak-field theory to strong field numerical results for the bistable ring resonator with saturable photorefractive gain and loss.1,2 Our weak-field model, which only considers the time dependence of the gratings and neglects the spatial dependence,3 captures an approximate picture of the two-crystal resonator. The basic model consists of two coupled complex equations for the gain and loss gratings. An analytic equation is derived for the self-pulsing period and amplitude for the case of zero detuning and near threshold gain. In this case, the system is analogous to a Van der Pol oscillator. We solve for the stationary points of the system and discuss their stability conditions. We also present computer simulation of the nonzero detuning case.
Characterization of thin Al films by grating coupling to surface plasma waves
Authors: Zaidi S. H. , Reicher D. , Draper B. , McNeil J. R. , Brueck S. R. J.
Published: February 2023

We present the first detailed investigation of the optical properties of thin Al films using grating coupling to surface plasma waves (SPW). The films ranged in thickness from 5 to 50 nm and were deposited on a holographically etched grating in Si by means of evaporation, sputtering, and ion-assisted sputtering (IAS) techniques. For comparison, the films were also deposited on gratings etched in photoresist and SiO2. The dielectric constants determined from SPW data were in good agreement with the ellipsometric measurements on films deposited on Si wafers at the same time. The deposition rate for all films was maintained at 2-3 Ǻ/s, and background vacuum was low at 10-7 Torr. The evaporated films had the highest quality, followed by sputtered and IAS films. Resistance measurements (taken with a standard four-point probe) also agreed with the optical data; this demonstrates the potential of this measurement as a non-contact resistivity monitor for semiconductor manufacturing. In addition, SEM measurements of the film grain sizes were carried out to relate the higher resistance of sputtered and IAS films to the additional scattering from the grain boundaries.1 Previously, prism coupling to surface plasma waves has been used to characterize dielectric properties of metal films.2
Optimization of the fidelity of color reproduction by reflection holography
Authors: Hubei P. M. , Neil M. A.
Published: February 2023

A theoretical model of color reproduction by holograms has been shown to agree with experimental results.1 The theory models the holographic process: recording at three laser wavelengths and replay in white light by three wavelength-selective display holograms. The hologram replays the three components with a finite bandwidth and a certain level of background-scatter noise. Considering the set of eight Munsell-chip surfaces we can predict corresponding color points on a CIE diagram when it is reproduced by a holographic image. We compare these points with points corresponding to the same eight surfaces illuminated by a standard reference source and matched to white (the same white used to illuminate the hologram). Using typical levels for replay bandwidth and noise2 (determined experimentally from holograms), we can vary the three wavelengths and compare the resultant image colors with the reference colors. The wavelength combination 444, 526, and 609 nm gave the least average squared shift in the positions of the eight colors when they were plotted on the 1976 CIE USC diagram (0.00018 CIE diagram units). The wavelength combination of 442, 531, and 623 nm gave the largest gamut area (57.7% of the area of the octagon formed by the eight reference points).
Buildup of modulational instability from wave breaking in birefringent fiber propagation
Authors: Rothenberg Joshua E.
Published: February 2023

Optical modulational instability (MI), the spontaneous generation of intensity modulation on a continuous wave, has received much recent attention as a novel nonlinear process and as a source of terahertz repetition-rate subpicosecond pulses. The MI phenomenon was first observed for anomalous group-velocity dispersion,1 and it has recently been observed with coupled nonlinearly visible waves in a birefringent fiber.2 For use of generated terahertz pulse trains it is desirable to have oscillations with stable relative delay. However, MI, being a parametric process, typically builds up from noise. As a result, the oscillations are not coherently generated, and their timing relative to the input is not controlled. In this paper it is observed that when coupled, visible picosecond pulses propagate in a birefringent fiber, the pulses first form "wave-breaking" oscillations, which then, owing to MI gain, grow exponentially. When cross-correlation with a 100 fs probe pulse was employed, terahertz oscillations were observed to form on orthogonally polarized pulses copropagating in the fiber. The correlations were measured by averaging over many pulses, and they demonstrate the stability and relative coherence of the oscillations. Numerical simulations confirm this description, showing the formation and amplification of wave-breaking oscillations, and they also indicate the formation of dark soliton trains.
Further evidence of suppression of ionization and atomic electron localization by an intense laser field
Authors: Su Q. , Eberly J. H.
Published: February 2023

Ionization supression and electron localization have been reported for very intense and very short-wavelength laser fields.1 In this paper we present results for a one photon ionization process. We find that the responses of an atomic electron seems to fall into one of three categories as the field changes from a low value to tens of an atomic unit. In the first category ionization can be described by perturbation theory. The second category of response is characterized by a very rapid approach to nearly 100% ionization. The third category features ionization suppression and electron localization, and both of these seem to be strongly correlated with the deformation of the atomic binding potential associated with the KramersHenneberger frame transformation.2