bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of a microcavity laser with optical injection
Authors: Jin R. , Boggavarapu D. , Grantham J. , Hu Y. , Gibbs H. M. , Khitrova G. , Koch S. W. , Sargent M. , de Colstoun F. Brown , Lowry C. , Chow W.
Published: February 2023

The dynamics of a surface-emitting laser (SEL) with cw optical injection are studied. The molecular-beam-epitaxy grown SEL has an active region consisting of 0.72-μm bulk GaAs and two high-reflectivity mirrors with 22 and 27 periods of 62.5-nm AlGaAs (x = 0.143)/73.3-nm AlAs, respectively. Lasing occurs in a single longitudinal and transverse mode at 886 nm when pumped by an 830-nm edge-emitting diode laser. A tunable dye laser generates the injection signal. At a detuning of 3 GHz, the lasing frequency is pulled toward the injection until complete locking is achieved at an injection level of 5 μW. Asymmetric relaxation-oscillation side-bands spaced about 6 GHz from the injection frequency are observed at higher injection powers, the low-frequency band being stronger. When the injected power is increased further to 0.2 mW, a new lower-frequency sideband appears and is pushed away by as much as 30 GHz as the injection power is increased to 5 mW. Preliminary calculations show that the observed complex phenomena can only be understood by including both the gain saturation and nonlinear index change in the theoretical modelling.
Imaging three-dimensional diffuser-obscured surface objects by enhanced backscatter processes
Authors: Welch Gisele , Rhodes William T.
Published: February 2023

A simple model, based on the decomposition of an object into a superposition of overlap ping lens-like components, aids significantly in the understanding of recently proposed enhanced-backscatter-based methods for imaging diffuser-obscured objects. This model shows clearly why enhanced back scatter imaging of planar objects allows retrieval of only the modulus of the Fresnel transform of the object reflectance distribution. The model can be extended to include the case of non-planar surface objects. Interpretation of the model allows one to determine whether information in some sense comparable to the object Fresnel transform modulus can be inferred from the enhanced backscatter radiation field.
One-transistor VLSI ferroelectric-liquid-crystal spatial light modulator element
Authors: Handschy M. A. , Drabik T. J. , Cotter L. K. , Cunningham J. D. , Pattee A. M.
Published: February 2023

A top layer of light modulating ferroelectric liquid crystal (FLC) turns a suitably designed VLSI memory into an electrically addressed spatial light modulator (SLM). The one-transistor dynamic register developed for electronic DRAM application also produces the densest SLM. We present such an SLM with elements on 20 μm centers and having a 72% fill factor. Its layout utilizes 2 μm CMOS design rules. We show 32 × 32 and 256 × 256 arrays; 512 × 512 arrays are feasible within standard silicon foundry photolithography capabilities. Address decoding electronics integrated with the modulator elements permit the SLM to be refreshed at frame rates up to 10 kHz; FLC response time limits optical cycling to 4 kHz. The refresh effectively overcomes photoinduced erasure of the dynamic element at incident intensities up to 10 mW/cm2. We will discuss other SLM performance characteristics and project improvements that could be achieved with finer design rules and circuit planarization.
Optically addressed VLSI/FLC bipolar device for optical learning machines
Authors: Mao C. C. , Robinson M. , Johnson K. M.
Published: February 2023

Recently, the combination of VLSI and liquid crystal technologies has resulted in a number of compact, high density spatial light modulators. In this paper, we present results on the design, fabrication, and demonstration of a novel, optically addressed VLSI/FLC de vice for insertion into a bipolar optoelectronic learning machine. The device consists of eight pixels, representing neurons. Each nueron consists of two photodetectors, a comparator, circuitry for target values, switching circuitry, and two output modulation pads. A thin layer of smectic C* FLC (SCE-13) is sandwiched between the VLSI chip and a piece of glass coated with a transparent conductive oxide (TCO) acting as an electrode. To operate the device, a 2.5 V de bias is applied on the TCO electrode, and either 5 V or 0 V is provided on the modulation pads. The voltage on each pad is determined by the optical intensity and the target value. The device demonstrated an on / off contrast ratio of 5:1, a 10 90% rise time of 3 ms (limited by the low voltage placed across the FLC), and an input optical sensitivity of 2 nW/cm2.
Polarization dispersion in randomly varying birefringence fiber
Authors: Wai Ping-kong A. , Menyuk Curtis R. , Chen Hsing-hen
Published: February 2023

Polarization dispersion of single-mode fibers with random birefringence is studied statistically. The extent of coupling between local polarization eigenmodes depends on the relative strength of the twist length and the beat length of the fiber. When the beat length is much shorter than the twist length, weak coupling theory applies.1 However, in regions where the fiber birefringence is nearly zero, the beat length is much longer than the twist length, so that the polarization eigenmodes are strangely coupled. In previous work, the region of zero birefringence has been modelled by a sudden random rotation of the fiber at regular interval. In this work, arbitrary random birefringence is considered. The correlation length of the electric fields is calculated in terms of that of the random variation of the fiber.
Wide range beam diameter measurements using periodic exponential and rectified sinusoidal gratings
Authors: Awwal A. A. S. , Cherri A. K.
Published: February 2023

A knowledge of the effective beam diameter of a laser beam is essential to many laser applications, such as material processing, read-out of optical disks, and metrology using laser sensors. Among many beam diameter measurement techniques, a relatively fast and inexpensive technique is provided by the use of ruling1, 2 where a beam scans the ruling and a detector placed directly on the other side detects the maximum and the minimum transmitted power through the ruling. The ratio P
min
/P
max
is found to be a function of the ratio r
O
/L (beam radius to ruling period).
A GRIN endomicroscope
Authors: Prescott Rochelle
Published: February 2023

The

y
,

y
¯

diagram clearly illustrates the remarkable properties of a GRIN endoscope with an objective less than one-quarter period in length. The entrance tunnel may be clearly visualized, as well as the remarkable transformation of the entrance pupil and the field of view as the instrument is focussed at progressively shorter distances. I will show how such a system may be designed to allow the transformation of properties from that of a low numerical aperture, wide field endoscope to a large numerical aperture, high magnification microscope by simply changing the ocular system focus.
Focusing of an atomic beam by a J1 laser beam
Authors: Choi Kurnyul , Lee HeeS. , Fenichel Henry
Published: February 2023

Atomic beam focusing by a gradient (dipole) force of the near resonant first order Bessel beam is considered in the paraxial approximation. The first order Bessel beam (J1) is a nondivergent laser beam whose intensity profile is the square of the first-order Bessel function of the first kind. A co-propagating J1 beam, tuned above resonance, provides the minimum rate of spontaneous emission on axis where the intensity is minimum. It has a steeper intensity gradient and a longer interaction range than does TEM01*. Numerical results for the focal length and the atomic beam density distribution in the focal plane are presented. Focusing limitations due to aberrations are discussed. The results are compared with those of the TEM01* mode beam.
Quantum-well modulators at 1.55 μm grown by single-step selective CBE
Authors: Chen Y. , Zucker J. E. , Chiu T. H.
Published: February 2023

In an effort to reduce the number of growth steps and increase device yield, we developed novel InGaAsP/InP quantum-well electro-optic modulators based on single-step growth on masked substrates by chemical beam epitaxy (CBE). The operating wavelengths of these devices are accurately tuned by controlling the CBE mask patterns. For the initial measurement we have fabricated patterned stripes on 3000 Å SiO2 with stripe width varying from 2 μm to 50 μm. A p-i-n structure with 20 InGaAsP/InP quantum wells lattice-matched to InP substrate was grown on n-type InP substrate. Rib waveguides were formed without any further processing. Excellent electroabsorption modulation was observed for these structures. We have measured an on/off ratio of 14.5 dB for a voltage swing of 10 V at 1.55 μm. In addition to the ease of fabrication and improved device yield, this technique also offers the unique tunability of the band edge of the waveguide by controlling the width of the SiO2 mask stripes. The photoluminescence spectrum of the rib waveguide shifts from 1464 nm to 1429 nm when the rib width is increased from 2 μm to 50 μm. The tunability is promising for laser-modulator integration without a complicated regrowth process.
Electronic spectroscopy and predissociation of Ne–CN
Authors: Heaven Michael C. , Fei Suli
Published: February 2023

In previous work we have recorded and analyzed three bands of Ne-CN that were associated with the CN B-X 0-0 transition. These features were assigned to the electronic origin and two internal rotor levels.1 More recently we have re-examined the Ne–CN B–X spectrum in the vicinity of the CN 0–0 transition, and we have recorded spectra for the bands associated with the CN 1–0 transition. Ten previously unobserved bands have been characterized. Features corresponding to multiple excitations of the internal rotor mode, and excitation of the –Ne stretch, are present. We are in the process of fitting a two-dimensional potential energy surface to this data.
Singly and doubly doped Pr fluoride fiber amplifier operation at 1.3 μm
Authors: Ohishi Yasutake , Kanamori Terutoshi , Sudo Shoichi
Published: February 2023

Pr3+-doped fiber amplifiers show promise for use as optical amplifiers operating at 1.3 mm. They have generated immense interest in the field of telecommunications because most current optical fiber systems operate at wavelengths around 1.3 mm, which is the second telecommunication window.
Raman gain in a Λ three-level system with closely spaced ground states
Authors: Shahriar M. , Hemmer P. , Donoghue J. , Cronin-Golomb M. , Kumar P.
Published: February 2023

Recently, there has been much interest in the Raman gain observed in a Λ system with closely spaced ground states such that both ground states are initially equally populated and are both coupled to the excited state by a single frequency pump. This system is of interest for applications such as frequency shifting and self phase conjugation and as an example of a system that exhibits large optical gain for very small excited state populations. We have studied the mechanism of this gain theoretically and have found qualitative agreement with previously observed experimental results. Briefly, a single frequency pump interacts with both legs of the system. The gain spectrum is interpreted in terms of the energy intervals of the dressed states of the pumped system. The gain mechanism is analogous to the process of sideband amplification in a strongly driven two level system. However, the width of the gain is not limited by the linewidth of the excited state and, for low pump power, is determined by the transit time only. This is consistent with the extremely narrow gains (less than 1 kHz) observed in sodium vapor.
Self-similarity in stimulated Raman scattering
Authors: Menyuk Curtis R.
Published: February 2023

The equations that govern stimulated Raman scattering in the transient limit have long been known to have soliton solutions, a Lax pair, and other properties that are normally associated with integrable systems. Yet the behavior of this system is known to be quite different from the usual behavior associated with integrable systems. Solitons are transient, and the behavior of the system at long distances is dominated by self-similarity. This behavior was first discovered numerically.1 In this presentation, we show that the self-similar solution can be derived by using symmetry reduction. We then show for fairly general initial conditions precisely which self-similar solution the system tends toward at long distances. These results are verified numerically. We argue that this behavior in which self-similar solutions dominate the longdistance evolution should often appear in nonlinear systems with memory. Finally, we outline an experiment that could observe the self-similar solutions.
Self focusing, filament dragging logic in anisotropic nonlinear saturating Kerr media
Authors: Wagner K. , McLeod B.
Published: February 2023

A new approach to a cascadable three terminal optical logic gate with gain and input–output isolation is proposed, simulated, and analyzed. The interaction between two orthogonally polarized self focusing filaments in an anisotropic nonlinear Kerr medium results in the spatial dragging of a strong pump beam by a weak signal wave. As a result, the strong pump is transmitted as a stable filament when the weak signal is not present, and it is dragged to the side by more than a beam width in the presence of the weak signal wave. This dragging interaction is stronger than spatial soliton collisions,1, 2 and as a result absolute gain is achievable. High speed, phase insensitive, cascadable logic operations, such as NOR gates, can be realized with this technique by dragging a pump filament by any of several inputs, just as in the Islam temporal soliton dragging gate.3
Amplification of 100 fs laser pulses to terawatt peak power levels
Authors: White W. E. , Donnelly T. , Hunter J. R. , Price D. F. , Shepherd R. L. , Van Woerkom L.
Published: February 2023

Many groups around the world are involved in development of high peak power 100 fs laser systems. One technique common to most of these systems is chirped pulse amplification in broad gain–bandwidth solid state laser materials. Our system begins with a commercially available, actively started, self-mode-locked Ti:sapphire oscillator that is pumped by 9.2 W of all lines argon ion laser light. The 100 fs, 800 nm, 12 nJ pulses from this oscillator are then stretched in a double pass, antiparallel grating pair stretcher. The resulting 450 ps, 3 nJ pulses are then amplified to 10 mJ in a Ti:sapphire regenerative amplifier pumped by 60 mJ of doubled Nd:YAG light. The pulses are then further amplified in a double pass Ti:sapphire rod that is pumped by a 550 mJ pulse from the same Nd:YAG laser. Further amplification has been performed in dye converted flashlamp pumped Ti:sapphire, laser pumped Ti:sapphire, and flashlamp pumped Cr:LiSAF. In all three cases, pulses have been recompressed to peak powers in excess of 1 TW. Performance of these amplifiers as well as issues pertaining to recompression, propagation, and focusability will be discussed.
Contrast sensitivity functions measured on flashed backgrounds in the dark-adapted eye
Authors: Kortum P. T. , Geisler W. S.
Published: February 2023

Flash-on-flash thresholds for sinusoidal targets were measured in the dark-adapted eye. The stimuli were increment-Gabor patches (the sum of a Gaussian-damped sine wave and a simple Gaussian of equal amplitude) presented for 50 ms at the onset of a 500-ms flashed background. The Gabor stimuli had a bandwidth of 0.5 octaves, with center frequencies of 1, 2, 4, 8, and 12 c/deg. For all spatial frequencies, the slopes of the increment-threshold functions increased continuously with flashed-background intensity, reaching values greater than 2.0 at 10,000 td. In the dark, the thresholds for the different spatial frequencies were spread out over a range of 1.0 log unit, with the higher spatial frequencies yielding the higher thresholds (i.e., the CSF fell off steeply with spatial frequency). However, the spread of the thresholds decreased as the flashed-background intensity increased; above 100 trolands the CSFs were nearly flat over the range of spatial frequencies tested. These results suggest that different mechanisms mediate detection at low and high flash intensities.
Real time wideband correlator based on optical implementation of the overlap- and-add algorithm
Authors: Kanterakis E. G. , Katz A. , Lomp G. , Lu X. J. , Zhang Y. , Li Yao
Published: February 2023

A real-time optical cross correlator of very long digital or analog data streams and capable of operating in excess of 40 million samples per second is described. The overall system architecture relies on the implementation of the overlap-and-add algorithm. The algorithm has been appropriately modified to implement the cross correlation of two long signals by first computing the cross correlation of many shorter ones. The optical system utilizes a cw laser source, an acousto-optic cell, a liquid crystal display and a multichannel parallel photodiode array to implement the short correlations. A digital system is used to form the final correlation output. Preliminary results are described.
Spectral texture optical classifier
Authors: Guest Clark C.
Published: February 2023

Several requirements distinguish remote sensing applications of pattern classification. The shape of the pattern may not be of primary importance. Often, observed regions (cities, crops, forests, oceans, etc.) display characteristic textures but no particular shape. Even when objects to be recognized have particular shapes, they are often embedded in such large fields of view that too few pixels are available to distinguish shape. An optical processor has been designed that recognizes objects based on their spectral and texture characteristics. A pixel-wide strip of the scene is processed at a time. The spectrum of each pixel of the scene is spread in the direction perpendicular to the strip by a grating. The resulting spatial spectral image that results can drive the input side of an optically addressed spatial light modulator. Conversion of the multispectral information into coherent monochromatic light allows further processing by using optical correlation techniques.
Binary phase-only filters with improved discrimination: maximized amplitude difference method
Authors: Yang Hedong , Guest Clark C.
Published: February 2023

A new design method, the maximized amplitude difference (MAD) method, is developed to improve the discrimination of binary phase-only filters (BPOFs). In this approach, we construct an energy function that represents the weighted difference between the amplitude of the autocorrelation peak caused by the target signal and that of the cross-correlation peak caused by the nontarget signal. The weight determines the suppression level of the crosscorrelation peak. The best phases for each of the two peaks are determined through a fast deterministic search algorithm. A BPOF that minimizes the energy function is encoded by assigning each filter pixel according to the projections of the spectral components of the target and the nontarget signals onto the phasors representing the two peaks. For the examples used in the numerical studies, a 7.0 dB increase of the ratio of the autocorrelation peak intensity to the cross-correlation peak intensity has been achieved. For a given suppression level of the cross-correlation peak, the MAD method also maximizes the intensity difference between the two peaks. The MAD method is also compared with the simulated annealing method through numerical studies. The two methods generate almost identical filters, but the MAD method is much faster.
Fast design algorithm for ternary phase-amplitude filters with maximized signal to noise ratio
Authors: Yang Hedong , Guest Clark C.
Published: February 2023

Ternary phase–amplitude filters (TPAFs) are essentially binary phase-only filters (BPOFs) with a carefully selected passband or support function. Results have been reported on TPAFs used to maximize the signal to noise ratio (SNR)1 of an optical correlator. In this paper we present a simple algorithm to design a TPAF yielding maximum SNR. The algorithm, similar to the one of Ref. 1 in that it optimizes both the decision line and the passband of a BPOF, is more general (applicable to complex input) and increases computation speed further by reducing the 2-dimensional search to a 1-dimensional one. A decision line for the value assignment (π or - π) of a filter pixel is defined by its normal. A threshold along the normal determines the passband of the filter. For each tested decision line, the spectral components of the reference signal are projected onto its normal. The threshold is iteratively determined such that only those components with the largest projection are included in the passband. It is shown that the threshold thus selected results in a maximized SNR. The algorithm also makes it easier to achieve a trade-off between the SNR and correlation peak intensity.
A multimode laser model with population dynamics and coherent-wave mixing
Authors: Hodges S.E. , Adkison D. , Munroe M. , Gadomski W. , Raymer M. G.
Published: February 2023

A stochastic, semiclassical model is developed for a multimode, homogeneously broadened laser with rapid-dipole dephasing (e.g., semiconductor or dye lasers). The theory is valid for systems with a free spectral range that is large compared to the decay rates of the population inversion and the field. Population dynamics including temporal beating effects and relaxation oscillations, spatial-hole burning, coherent-wave mixing, and quantum noise are all included self consistently. A principal restriction of the model, when applied to coupled-cavity systems, is that the modes satisfying the boundary conditions of the gain medium must form a subset of the modes defined by the cavity and that only modes in this subset are above threshold. For typical systems, the model is valid for pump rates up to several times threshold. The effects on the emission linewidth of coherent-wave mixing and the number of modes included in the simulation at various pump rates are discussed.
Phase dynamics of spatially coupled lasers
Authors: Fabiny Larry , Colet Pere , Roy Rajarshi
Published: February 2023

We have experimentally and theoretically investigated the coherence properties and phase dynamics of spatially coupled Nd:YAG lasers as a function of overlap of their fields. The dependence of phase locking on the overlap and detuning of the lasers, as well as noise sources, are studied. The coherence of the laser sources is directly determined from the visibility of interference fringes as recorded by a video camera system. Heterodyne measurements are used to determine the detuning between individual lasers. It is found that the lasers remain phase locked even when the overlap of their fields is extremely small, but then they display an abrupt transition to incoherence. When phase locked the laser fields are 180° out of phase with each other. A phase mask fabricated by binary optics procedures may be used to alter the phase relationship between the laser sources. Numerical solutions of a laser model that show very good agreement with experimental measurements will be presented, and the coherence-incoherence transition will be interpreted theoretically.
POINTS: micro-arcsecond optical astrometry in space
Authors: Noecker M.C. , Babcock R.W. , Chandler J.F. , Phillips J.D. , Reasenberg R.D.
Published: February 2023

We present an introduction to POINTS (Precision Optical INTerferometer in Space), a next-generation space-borne optical instrument and a leading candidate for the Astrometric Interferometry Mission recommended by the Bahcall committee. POINTS uses two starlight interferometers to measure the angle between two 10th magnitude stars (90° ± 3° apart) with 5 μarcsec precision (25 prad) in ~1 min. Analysis of many star pair measurements using measurement redundancy and 360° closure gives best-fit values for star positions and a full set of instrument biases. Each interferometer measures the angle between the starlight wave fronts and its baseline; internal laser metrology monitors the starlight path lengths and the angle between the two baselines. With instrument dimensions known to a few picometers, the star angle measurement can be nearly shot-noise limited; with stability over a few hours, the instrument can recalibrate its biases anew. Occasional measurements during a 10-year mission allow measurements of parallax to 0.4% at 10 kpc, a search for other planetary systems, calibration of the cosmic distance ladder, determination of 3-space positions and velocities, establishment of a stable reference grid, a possible second-order test of GR light deflection, and many other investigations.
Concentric-circle grating, surface-emitting, semiconductor lasers
Authors: Erdogan T. , King O. , Wicks G. W. , Hall D. G.
Published: February 2023

Recently a semiconductor laser with a resonator defined by a concentric-circle waveguide diffraction grating (CCG) was shown to oscillate in a circularly symmetric mode.1 In this paper we discuss the nature of the optically pumped laser emission from such a laser. We have observed in the best case a relatively narrow-divergence beam (<1°) that is circular in cross-section. Superpositions of higher-order azimuthal modes are obtained when asymmetries in the pump or grating structure are introduced. Despite the broad-area geometry of the laser, it does not show evidence of filamentary behavior. The polarization and spectral properties of the laser emission are also examined. The CCG laser resonator shows great promise for producing a high-power, narrow-divergence, coherent beam that is two-dimensionally symmetric.
Degradation of mode-suppression ratio due to longitudinal-mode beating in semiconductor lasers
Authors: Gray George R. , Agrawal Govind P.
Published: February 2023

Gain nonlinearities play an important role in determining the static and dynamic characteristics of semiconductor lasers. Most of the theoretical work to date considers primarily the nonlinear gain arising from intraband effects, for which the total carrier density remains essentially constant. Recently it has been shown that beating of the total carrier density at the longitudinal-mode-spacing frequency also gives rise to important effects, even when the longitudinal-mode spacing is much greater than the reciprocal carrier lifetime. In this paper we show that the nonlinear gain arising from such interband effects can lead to degradation of the side-mode suppression ratio in single-frequency lasers. We utilize a general multi-longitudinal-mode model that includes self- and cross-saturation, as well as four-wave mixing, arising from both intraband and interband sources. Particular attention is paid to the influence of the linewidth enhancement factor and the laser length on the mode suppression ratio. Our results suggest that relatively long distributed feedback lasers (length >500 μm) are likely to operate in a single longitudinal mode only over a limited current range.
Extracting transformable parts from object categories
Authors: Schyns Philippe G. , Bulthoff Heinrich H.
Published: February 2023

When people have to extract parts from a new category of objects, where they do not know the shape of the parts, they must extract shape from the raw data. If the parts composing the objects transform from one instance to another, part extraction is difficult because any part could potentially correspond to any other part in the object category. However, our perceptual apparatus rules out many correspondences. We investigated the role of sign of curvature in part extraction in categories of 2D B-spline stimuli. Subjects were exposed to one category of objects composed of a random and a target part whose shape was unknown a priori. In the concave and convex conditions, the target part was transformed, but its signs of curvature remained constant throughout the category, while in the mixed condition, the transformation included a change of sign. Results showed that subjects in the concave and convex groups extracted a single part from the category while subjects in the mixed group extracted two parts: one with a positive and one with a negative sign of curvature.
Higher harmonic emission and the helical Cerenkov effect in a dielectric medium with the uniform magnetic field
Authors: Soln Josip
Published: February 2023

A description is being developed of the spontaneous emission in a dielectric medium with the uniform magnetic field. A new, helical Cerenkov effect is deduced, whose power spectrum depends on the radius of curvature of the electron trajectory (measured in the plane perpendicular to the direction of the electron guiding center). In general, for beam energies of up to 10 MeV, indices of refraction (n) of up to 2, and magnetic fields of up to 5 T, the spontaneous emission may go into a large number of harmonics, covering the 10 to 106 GHz spectral region, where, for simplicity, one assumes that n varies slowly with the radiation frequency.
Initial performance of Los Alamos Advanced FEL
Authors: Nguyen D. C. , Sheffield R. L. , Chan K. C. D. , Meier K. L.
Published: February 2023

We report recent results on the high-brightness electron linac and initial performance of the Advanced FEL at Los Alamos. The design and construction of the Advanced FEL beamline are based upon integration of advanced technologies such as high-brightness photoinjector, high-gradient compact linac, and permanent-magnet beamline components. The beamline transports an extremely bright electron beam at energy up to 20 MeV and peak currents up to 300 A in 10-ps pulses. With use of an electromagnet microwiggler and short optical resonator, the Advanced FEL will be the first of its kind small enough to be mounted on an optical table and yet capable of providing high-power optical output spanning the near-IR and visible regions. To obtain good photocathode lifetimes and high peak surface fields, the linac has been conditioned with an rf glow discharge, high-temperature bakes, and high-power rf conditioning. Initial results of the Advanced FEL performance will be presented.
Influence of the pulse repetition rate in single-photon decay spectroscopy by means of Fourier transform methods
Authors: López R. J. , Gónzalez F. , Moreno F.
Published: February 2023

Recently we have shown that the application of Fourier transform methods (and more specifically, the sine transform) to experiments of single-photon decay spectroscopy (SPDS) can be advantageous when the fluorescence intensity is low and the lifetimes to be measured are long (lifetimes longer than 1 μs are usually found in experiments of solid state physics). For monoexponential decays, the sine transform of the probability density of the time of arrival of the first fluorescence photoelectron presents a peak whose maximum corresponds to a value of the frequency equal to the inverse of the lifetime. The presence of this peak allows us to obtain the value of the lifetime with better accuracy than that obtained by means of the conventional multichannel technique. Because of the finiteness of the period of the excitation pulse chain, a periodic ripple appears superimposed to the main peak. Its influence increases as the period of the excitation signal decreases. For multi-exponential decays, this effect may mask some of the components if their fluorescence intensities are very low. In this paper we have studied the influence of the pulse repetition rate in SPDS experiments when sine transform methods are applied. As a first step, this study has been performed for monoexponential decays, and the optimum pulse repetition rate has been obtained. An experimental verification has been carried out by measuring the lifetime of the excited level 4T1, of Mn2+ in a TMMC crystal.
Phase locking of semiconductor diode lasers by phase conjugate injection
Authors: Wu Cunkai , McInerney John G. , Liby Bruce W.
Published: February 2023

We describe a phase-locking technique for combining two and three separate commercial laser diodes (Hitachi 1400) by using phase conjugate injections. The injections come from a double phase conjugation mirror (DPCM). The photorefractive crystal is a single crystal of BaTiO3. Two phase-locking schemes were experimentally demonstrated: unidirectional injection and bidirectional mutual injection. Because the phase conjugate wave is backward going, this injection is more effective. Only a fraction of a microwatt injection from the DPCM is sufficient for phase locking. The visibility of the interference fringe between the two locked lasers is about 0.95. For the unidirectional phase conjugate injection, the curve of the dependence of locking bandwidth on the injection power was measured. The intermodal injection locking was studied. In this case the mode-coupling effect was included in the coupled wave equations. Our theoretical curve is in agreement with the experimental results. For the bidirectional phase conjugate mutual injection, the phase locking between two lasers is stable only when the injection level is less than −40 dB. When the injection level is larger than −30 dB, the output spectra of the lasers are broadened and the case is driven into multimode operation.
Investigating state filling effect on high speed modulation dynamics in quantum well lasers
Authors: Zhao B. , Chen T. R. , Yamalda Y. , Zhuang Y. H. , Kuze N. , Yariv A.
Published: February 2023

In quantum well lasers, a separate confinement heterostructure is usually used to obtain a quantum confinement of the injected carriers in the active region and a confinement of the optical field in the waveguide. It is shown that the appreciable carrier population in the electronics states of the optical confinement layers (state filling effect) has a large influence on the differential gain in the quantum well lasers. By using different separate confinement structures for quantum well lasers, we find experimentally that the differential gain difference can be as large as a factor of three, which significantly affects the high speed modulation bandwidth in such lasers. These experimental results agree well with the theoretical predictions.
Holographic recording in photopolymer films
Authors: Curtis Kevin , Psaltis Demetri
Published: February 2023

Angularly multiplexed holograms have potential applications as optical memories, information processors, and interconnections. DuPont's photopolymer HRF-150 has already been shown1 to have excellent sensitivity and resolution, making it a good candidate for an optical storage medium. Ten image plane holograms were recorded in DuPont's HRF-150 photopolymer. Agreement with Kogelnik's two wave diffraction theory2 for phase gratings, time considerations and their effect on dynamic range, the necessity of a pre-illumination pulse, and good image quality at read out are also reported.
Early visual mechanisms in texture processing
Authors: Geisler W. S.
Published: February 2023

The inputs to high-level mechanisms (such as those responsible for boundary extraction, grouping, surface interpolation, and object identification) ultimately arise from the parallel and automatic processes at the front end of the visual system. Hence, any rigorous psychophysical analysis of high-level mechanisms requires either controlling for, or computing the effects of, the initial low-level mechanisms. In recent years, understanding of the front end has increased substantially, raising hopes for a more rigorous study of high-level mechanisms. To illustrate how models of the front end might be applied to the study of higher-level mechanisms, two examples will be considered: texture segmentation and visual search for texture regions. It will be shown that the front end can account for a rather large percentage of the variance in the performance of these tasks, suggesting that the high-level mechanisms are able to extract, with nearly uniform efficiency, the relevant information provided by the parallel front end. Thus, many aspects of texture processing might be adequately accounted for by combining current models of the front end with relatively simple models of the high-level mechanisms.
Progress in optimization of the magnitude and stability of nonlinearity in poled polymers
Authors: Twieg Robert J. , Burland D. , Carter K. , Moylan C. , Hedrick J. , Miller R. D. , Staehelin M. , Walsh P. , Volksen W.
Published: February 2023

General issues related to the design and optimization of second order nonlinear optical chromophores and polymers are presented and exemplified by recent work. Nonlinear chromophores have been prepared that contain only thermally stable functionality, their optical nonlinearities β and βo have been evaluated by the EFISH technique, and their thermal stability has been evaluated by a combination of thermogravitimetric, calorimetric, and spectroscopic techniques. These special chromophores have been incorporated into high temperature thermoplastics in both guest/host and covalent configurations, and the anisotropic polar order in thin films of these materials is created by electric field poling (corona or electrode). The magnitude of the optical nonlinearity and the stability at elevated temperatures (>200°C) is evaluated, and the materials design and processing features required for minimization of the relaxation of polar order are described.
Color codes in visual search
Authors: Palmer John , Teller Davida
Published: February 2023

The present experiment explores the influence of color on set-size effects in a visual search task. In the initial measurements, discrimination thresholds for luminance-defined and chromatically defined (red) targets were measured for set sizes of 2 and 8. Displays were constructed with white surrounds to control adaptation, restricted eccentricities and spacing to control these effects, randomized stimulus positions to avoid configural cues, and lowpass stimuli in both time and space to maximize chromatic sensitivity. Preliminary results show approximately equal set-size effects for both luminance and chromatic targets. Results will be compared with those of earlier studies of how different tasks affect the shapes of discrimination ellipsoids in 3-dimensional color space.
Calculation of sparse interconnection matrices for a Hopfield network
Authors: van den Bulck Pierre
Published: February 2023

Finding a binary pattern that matches a two-dimensional signal in some sense is a problem that arises, for example, in the area of digital holography. It is known1 that this problem can be solved by means of a Hopfield network. To calculate the interconnection weights and the thresholds of the network, the problem is formulated in terms of finding the minimum of the network's energy function.
Accuracy enhancement of optical vector–matrix processors
Authors: Ellett Scott A. , Krile Thomas F. , Walkup John F.
Published: February 2023

The accuracy of an optical vector–matrix multiplier (OVMM) is directly related to the number of levels that can be accurately detected at the output. In an OVMM, the linear range of the input devices and detectors is divided into a number of levels that provides an acceptable error rate at the output. By using error-correcting codes it is possible to increase the number of levels at the output and still provide the same error rate. The output levels are spaced closer together, and an error-correcting code is used to reduce the error rate to an acceptable value. A higher dimension vector-matrix product is computed along with the encoding and decoding operations. Computer simulation results will be presented.
Optical implementation of a cyclic error control code decoding system
Authors: Reeve C. D.
Published: February 2023

Optical correlation techniques have been used to decode a binary cyclic error control code in real time. The code is generated by using a primitive polynomial as the characteristic function of a linear feedback shift register. Each code word generated is a cyclic shift of every other code word, except for the zero code word. It is shown that a set of 15-bit code words is able to correct three errors per word and detect up to seven errors. The cyclic nature of the code lends itself to a correlation approach for decoding. I present results of decoding systems using both twin and single Bragg cell correlators showing that all code words, including the zero code word, can be correctly identified with up to three bit errors per word. With currently available acousto-optic cells chip rates of several hundred megahertz are achievable.
Defects in VLSI/FLC spatial light modulators
Authors: Serati S. A. , Turner R. , Sharp G. D.
Published: February 2023

A new device that has been developed for optical signal processing applications is the VLSI/ferroelectric liquid crystal spatial light modulator (SLM). These SLM's are attractive for optical processing applications because of their size, speed, and resolution. They can also be used for either binary phase or intensity modulation. However, these SLM's operate in reflection mode where surface variations and other device defects are enhanced. The types of defects that affect phase and intensity modulation are pixel-to-pixel height variations, surface curvature, nonuniform liquid crystal alignment, and zonal effects caused by ion migration. These defects are examined specifically for optical-correlator applications since the correlation process is senstive to both phase and amplitude. Optical correlators can also use either binary phase or intensity modulation at the image plane. This allows direct comparison for device defects on optical correlator performance for intensity vs. phase modulation. Theoretical and experimental analyses are presented.
Time-dependent birefringence in end-pumped cubic crystals
Authors: Farrukh U. O. , Hardesty C. E. , Barnes N. P.
Published: February 2023

An expression for transient thermally induced birefringence for uniformly pumped cubic crystals was previously derived by Koechner and Rice.1 The formulation has been applied to the case when the crystal is being end-pumped by a Gaussian beam. Time-dependent temperature data obtained from a numerical code2 is used to estimate the transient birefringence in Nd:YAG crystals that are repetitively end-pumped by a dual-pulsed sequence. Data on time-dependent birefringence and polarization loss in output power will be presented.
Photon transport measurements in dense random media
Authors: Watson George H. , Saulnier Paul M. , Tarhan I. Inane , Zinkin Martin P.
Published: February 2023

Recent results of photon transport measurements in highly multiple-scattering random media will be presented. Time-resolved transmission of picosecond laser pulses, coherent backscattering, and laser speckle statistics are several of the experimental probes being used to examine factors that influence photon transport and localization. The effects of spatial correlations among scatterers on the transport mean-free-path lengths obtained from time-resolved transmission and extinction compare favorably with corrections introduced into the Mie formalism via a static structure factor. Transport parameters determined from coherent back-scattering have been significantly improved by current theoretical models for samples with high surface reflectivity. Finally, new applications of laser speckle to the study of dense random media will be discussed. The recent advances in this field, including improved understanding of photon transport probes, should facilitate the search for localization of light in disordered optical superlattices.
Comparison between propagation of focussed Gaussian beams through nonlinear Kerr-type and thermal media
Authors: Misra Raj M. , Banerjee Partha P.
Published: February 2023

We calculate the exact diffraction pattern in the far-field resulting from the incidence of focussed Gaussian beams on thin samples of Kerr-type and thermally nonlinear media. For the latter case, we model the temperature profile by using the heat equation with a source term proportional to the beam intensity. Our results show the essential differences between the effects of these two types of nonlinearities. Notably, the thermal non­linearity is responsible for a distinctive far-field on-axis minimum when the nonlinear sample is placed beyond the focus of the incident Gaussian beam. We have also verified this result experimentally by using a solution of chlorophyll in alcohol.
Raman spectroscopy of suspended microparticles
Authors: Tang I. N. , Fung K. H.
Published: February 2023

Raman spectroscopy is applied to the physical and chemical characterization of an electrostatically charged single microparticle, which is trapped and suspended in an electrodynamic cell. The microparticle, 10 to 15 pm in diameter and composed of deliquescent salts, such as sulfates and nitrates, is present either as a solid particle or as a solution droplet under controlled humidity conditions. While the chemical composition of the microparticle is characterized by the distinct Raman shifts observed, the physical state is revealed by the Raman bandwidths. Since a solid microparticle under laser illumination is largely free from both the sample inhomogeneity and optical transmission degradation that are normally encountered in a single crystal or crystalline powders, it follows that single particle suspension is a unique experimental method for measuring relative Raman scattering cross-sections for solid compounds. Furthermore, a suspended solution droplet, which evaporates to attain high supersaturations inaccessible with bulk samples, provides an ideal condition for a spectroscopic investigation of solute-solute interactions in concentrated electrolyte solutions. Experimental results are presented, which illustrate the uniqueness and versatility of single particle Raman spectroscopy.
Multichannel absorption spectroscopy applied to solid-propellant flames
Authors: Vanderhoff John A.
Published: February 2023

Absorption spectroscopy has been used to characterize the steady-state flame structure of solid propellants burning over a pressure range of 0.3 to 2.0 MPa. A high-intensity-xenon-arc-lamp light source and two spectrometer-intensified photodiode-array detectors formed the experimental setup from which NO and OH absorption spectra were acquired. Vibrationally resolved transitions in the A2Σ - X2Π (0,1) electronic system of NO from 230 to 250 nm comprise the absorption spectra from which temperatures and absolute NO concentrations are determined in the dark-zone region of the solid-propellant flames, and rotationally resolved transitions in the A2 Σ - X2 Π (0,0) vibrational band system of OH from306to311nm comprise the absorption spectra from which luminous-flame temperatures are determined. These spectra can be least-squares fitted with respect to a variety of parameters, which include an instrument response function, an absorption baseline, as well as the temperature and concentration. Several different propellants that exhibit a dark zone have been studied as a function of pressure. The dark-zone temperatures ranged from 1300 K to 1500 K, and NO concentrations varied from 13 to 30 mole percent, depending on the propellant type and pressure. Temperatures in the luminous flame region reached adiabatic values.
Vibrational energy transfer from v' = 3 of OH(A) in flames
Authors: Steffens Kristen L. , Jeffries Jay B. , Crosley David R.
Published: February 2023

Excitation of v' = 3 in A2Σ+ OH is popular for flame LIF measurements, in part because of the reduced influence of collisional effects on this rapidly predissociating level. However, at atmospheric pressure a large fraction (over 50%) of the total light emitted can originate from levels populated by vibrational transfer from v' =3. Fluorescence scans following laser excitation of v′ = 3 made in the burnt gases of flames between 14 and 760 Torr show an increasing population in v' = 0, 1, 2 with increasing pressure. H2/O2 flames where H2O is the primary collider show the least transfer, CH4/O2 (H2O and CO2) more, and CH4/air (with N2 collider, efficient for VET but poor at quenching) the most. There is a small decrease in VET with increasing ro­tational level.