bluuebunny/crossref_metadata_2025 huggingface dataset:
Yellow and green upconversion in Nd:YLiF4
Authors: Novo-Gradac A. D. , Jacobsen S. M. , Yen W. M.
Published: February 2023

Yellow and green upconversion fluorescence is observed at room temperature in Nd:YLiF4 by pumping with a high power diode laser around 795 nm (12.6 × 103 cm-1). The upconversion fluorescence was found to be strongest with the pump light polarized parallel to the crystallographic a-axis and was found to be weaker than the 4F3/2 → 4I11/2 near infrared (NIR) fluorescence by about five orders of magnitude. At room temperature, Nd:YLiF4 absorbs strongly between 780 nm and 815 nm because of the 4I9/2 → 4F5/2 and 4I9/2 → 4F3/2 transitions. The 4F5/2 level relaxes non- radiatively to the metastable 4F3/2 level, which is in turn responsible for the NIR fluorescence transitions occurring between 9.2 x 103 and 11.6 x 103 cm–1. The green upconversion emission lies between 18.5 × 103 and 19.3 × 103 cm–1, and the yellow upconversion emission lies between 16.7 × 103 and 17.4 × 103 cm-1. Considerable structure is visible in the spectrum. A possible route to the observed upconversion is absorption of a second laser photon from the 4F3/2 metastable level into the 2D(1)5/2 and/ or 2P1/2 levels. We will present power dependent and time resolved studies of these fluorescence bands and discuss the dynamics of the upconversion process.
Quantum mechanics and physics of 2.0-μm lasers
Authors: Barnes Norman P.
Published: February 2023

Unlike Nd lasers, 2.0 μm lasers are complicated by the presence of a sensitizer as well as an active atom, by the plethora of energy levels associated with Ho and Tm, and by the quasi three level laser operation. With two different atoms in the laser material, each having a multitude of manifolds and levels, several energy transfer processes are possible. Some of these energy transfer processes are necessary for the efficient operation, while others are clearly deleterious. Understanding all three of these effects is aided by a quantum mechanical model of the various atoms. In addition, quantum mechanics is used to unravel the spectroscopy of the laser material. Consequently, a quantum model is highly useful in understanding the physics of existing lasers and to help predict new laser materials with high potential. A quantum mechanical model can be used to unravel the spectroscopy of the Ho and Tm atoms either with ab initio calculations or by using an iterative approach with experimental data. With either method, energy levels, degeneracies, and transition probabilities can be determined. These directly affect pertinent ground state absorption and every transfer process, including upconversion. Details of these calculations will be given.
Dependence of alexandrite laser pumped 2.1 μm laser performance on pump pulse-width
Authors: Lee Chang J. , Han Gooywan , Bair Clayton H. , Barnes Norman P. , Hess Robert V.
Published: February 2023

A normal mode alexandrite laser has been used as a laser diode simulator for pumping a Tm:Ho:YAG laser to investigate performance as a function of pump laser pulsewidth. An end pumped geometry utilizing several 4 mm long Tm:Ho:YAG crystals in a standing wave resonator and various output couplers was used to determine threshold and slope efficiencies. Output coupler reflectivities varied from 0.7 to 0.99, and the pump pulse-width, varied from 200 μs to 650 μs. A maximum slope efficiency in terms of the absorbed energy of 0.4 was observed for all pulse-widths. However, the output coupler reflectivity for which this maximum was achieved was pulse-width dependent. A significant reduction in slope efficiency was observed for the lower reflectivity output couplers. This reduction was more severe for the longer pump pulse-widths and has been attributed to increased upconversion losses. Experimental details and a theoretical discussion will be presented.
Temporally variable, tunable solid-state laser source for scientific and medical applications
Authors: Heller D. F. , Fisanick G. J. , Walling J. C.
Published: February 2023

Solid-state vibronic lasers can now be made whose pulses tune in time as well as in frequency. "Pulse stretching," or control of temporal pulse duration and shape, is effected by newly developed techniques involving intra-resonator electro-optic feedback. The physics of electro-optic feedback controlled lasers is discussed, and an alexandrite laser employing a very fast electro-optic feedback system is described. Experimental characterization of this system has demonstrated broadly tunable (720-800 nm), variable duration (100 ns–5 μs) pulses with pulse energies of several hundred millijoules. Although a number of factors affect pulse temporal and amplitude stability as well as pulse shape, temporal fluctuations of only a few percent have been achieved. These pulses are sufficiently stable and reproducible to be reliably upconverted into the UV by second harmonic generation (SHG). The characteristics of microsecond duration "stretched" pulses are reported and a number of interesting new applications in science and medicine are described. These include applications in sodium lidar, lithotripsy, and dentistry.
High-resolution vacuum ultraviolet spectroscopy: toward the measurement of the 1s-2s interval in atomic helium
Authors: Melikechi N. , Gangopadhyay S. , Eyler E. E.
Published: February 2023

As part of our ongoing effort to measure the ionization energy of helium, we have developed a scheme to generate VUV radiation in the vicinity of 120 nm by resonant four-wave mixing in molecular hydrogen. We use two single-mode pulsed dye lasers, one at a fixed 606 nm wavelength and the other tunable in the visible red region. The first laser is tuned to two photon resonance with the ν=0, N=0 level of the EF state. The required 202 nm wavelength is generated by frequency tripling of 606 nm light, by using frequency doubling in KDP and sum- frequency mixing in β-BaB2O4. The second laser is tuned to produce tunable coherent difference-frequency radiation in the vicinity of 120 nm. This scheme is similar to that developed by Strauss and Funk1 but has the advantage of using two red-dye lasers, greatly simplifying the metrology needed for high resolution VUV spectroscopy. We will report progress on a preliminary measurement of the 1s-2s interval in helium using two-photon excitation at 120.28 nm.
Nonlinear photorefractive mixing theory
Authors: Khoury J. , Cronin-Golomb M. , Woods C.
Published: February 2023

Linear photorefractive mixing is limited to constant intensity. However, the operation theory becomes nonlinear when one of the inputs is amplitude modulated. As in electronics, it is often essential to have amplitude modulation. In this paper, we introduce the solution for the nonlinear photo-refractive mixing theory for weakly absorbing materials with constant intensity response-time product in the low coupling limit. The correction terms to the linear solution appear as a low frequency term (possibly including a dc component) and a high frequency multiplicative absorptive ripple that modulates the sum of the linear and low frequency terms. For many photorefractive mixing devices, any amplitude modulations used produce corrections that are extremely small, typically less than 1%, and these vary inversely proportional to the amplitude modulation frequency.
Effect of relative intensities of coherent beams in beam couplings in self-pumping BaTiO3 crystal
Authors: Moghbel Mehdi , Venkateswarlu Putcha
Published: February 2023

Two coherent beams, A1 and A2 were horizontally polarized, crossed in the center of an electrically poled BaTiO3 crystal at an angle of 46°, with angles of incidence 20.5° and 25.5° respectively. The powers were varied from 2 mW to 10 mW for A1, and 5 mW to 13 mW for A2. A1 was found to self pump individually, but not A2. They both self pumped under simultaneous pumping. With A1 = 5 mW and A2 = 9 mW, A1, under individual pumping, showed minor pulsations in its self pump A1. A2 does not self pump. However, under simultaneous pumping, strong coherent oscillations were observed in both. When A1 = 0.16 mW and A2 = 13 mW, A2 barely showed self pumped oscillations at the rate of 3 per 15 min while A1 did not individually self pump. Under simultaneous pumping, A1 and A2 both showed self pumped oscillations at the rate of 5 per 15 min. As 2 mW and 13 mW represent approximately the threshold values of the laser power required for A1 and A2 in their respective quadrants to cross the threshold of γL = 2.34 to self pump, the coupling parameter γ per unit length can be estimated to be 6.5 times larger than that for A2 in their respective quadrants.
The effect of carrier sweepout on performance of semi-insulating multiple quantum well photorefractive devices
Authors: Partovi A. , Glass A. M. , Chiu T. H. , Feldman R. D. , Knox W. H. , Olson D. H. , Zydzik G. J. , O’Bryan H. M. , Austin R. F.
Published: February 2023

We have studied the effect of barrier design on the grating period dependence of the diffraction efficiency in semi-insulating GaAs/AlGaAs and CdZnTe/ZnTe multiple quantum well photorefractive devices in the geometry with the applied field perpendicular to the quantum wells. In GaAs/ AlGaAs devices, lowering of the barrier height by reduction of Al concentration from 40% to 30% and reduction of barrier thickness from 100 Å to 35 Å increases the diffraction efficiency by 5 orders of magnitude. This is due to the reduction in carrier sweepout time from the quantum wells reducing the lateral diffusion of carriers in the wells. In an optimized structure, a diffraction efficiency of 3% at 852 nm from a 2 pm thick sample is obtained by application of 20 V. We will discuss the relevant parameters in the design of a photorefractive multiple quantum well device. As an application, the performance of a joint transform correlator based on a multiple quantum well photorefractive device and diode lasers will be discussed.
New photorefractive crystal KNbO3:Co
Authors: Shen Dezhong , Ma Xiaoyan
Published: February 2023

KNbO3 crystals doped with Co have been grown by the Kyropoulos method. The density of Co ions in the crystal is about 100 parts in 106. The inhomogeneity of refractive index Δn < 10−5/cm. The ordinary dimensions of single-domain KNbO3:Co crystals are about 10 × 10 × 10 mm3. Photorefractlve properties of KNbO3:Co have been compared with those of KNbO3:Fe. It has been found that the photorefractive diffraction efficiency of KNbO3:Co crystal without being reduced is one order of magnitude larger than that of reduced KNbO3:Fe crystal. The two wave coupling intensity distribution with the change of entering direction of the laser beam has been measured, and the distribution curve has been given. Other photorefractive experiments are being carryied out, and the results will be reported.
The gating of x-ray images in the 30–40 psec region for laser plasma experiments
Authors: Kilkenny J. D. , Bell P. E. , Landen O. , Hanks R. , Wiedwald J. D. , Bradley D. K.
Published: February 2023

High-gain laser-driven intertial confinement fusion requires that the target be driven to high densities while maintaining a high degree of symmetry. The need to measure, with minimal velocity blurring, both the symmetry of the imploding shell and the size and shape of the compressed core has driven the continuing development of gated x-ray detectors with framing times in the tens of picoseconds range. X-ray framing cameras based on the electrical gating of proximity-focused microchannel plate (MCP) devices are now routinely in use at the University of Rochester's Laboratory for Laser Energetics and at Lawrence Livermore National Laboratory and have been in use for several years to produce up to 14 channels of 100 ps gated x-ray images for laser plasma research. This gate width of x-rays is readily produced from 200 ps electrical pulsing of the microchannel plates, because the non-linearity of gain vs voltage reduces the x-ray gate width. To achieve faster gating thinner microchannel plates with a reduction in the electron transit time have been used.
Diffractive analysis of low-Fresnel-number unstable optical resonators with super-Gaussian mirrors
Authors: Bowers Mark S.
Published: February 2023

Unstable optical resonators that use a super-Gaussian reflectivity output coupler have been demonstrated to be successful in generating coherent radiation from high gain and large transverse-mode volume lasers. In the past, the design procedure for these resonators has been based on the geometrical-optics solution for the transverse eigenmodes, in which the fundamental mode is also a super-Gaussian of the same order as the mirror. In this work, the limits of validity of the geometrical-optics solution are quantified by numerically solving for the diffractive (Fox-Li) transverse eigenmodes as a function of the Gaussian-aperture-equivalent Fresnel number Neq. It is shown that for low cavity magnifications (M ≤ 2), the diffractive eigenvalues deviate from the geometrical-optics value for Neq ≤ 2. At these low equivalent Fresnel numbers, the transverse mode structure is determined by propagation effects (diffractive spreading), in addition to the mirror reflectivity profile. Furthermore, resonators with super-Gaussian mirrors of order n ≥ 6 exhibit transverse mode crossing points when Neq ≤ 2. Near a mode crossing point, the fundamental mode resembles a higher-order ring mode with a large central depression. The implications for the design of these resonators will be discussed.
Efficient coherent beam combining of multiple high-power AlGaAs amplifiers
Authors: Schuster Gregory L. , Andrews John R.
Published: February 2023

We are exploring power scale-up of semiconductor lasers through the use of arrays of amplifiers and coherent beam combining. In this paper we report 93%-efficient interferometric beam combining of the output of two saturated amplifiers yielding 372 mW in a single diffraction-limited lobe. A single mode semiconductor laser was amplified in a single stage and divided into two beams of 53 and 71 mW. Each beam was injected into a 10-stripe gain guided amplifier array having dimensions of approximately 100 μm wide by 250 μm long. Each amplifier had antireflective coatings (R~2%) on the front facet and high reflectivity mirrors (R~99%) on the rear facet. The output from the double-pass amplifiers produced 206 mW and 195 mW in the respective central lobes. Relative phase control was implemented by a mirror mounted on a piezoelectric stack before the beams were recombined at a common beam splitter. Proper adjustment of the relative phase resulted in a single beam having an output power of 396 mW with 94% or 372 mW, in a single diffraction-limited lobe. The corresponding single-lobe combining efficiency is 93%. The out-of-phase far-field profile was also recorded and indicated some mode mismatch as well as less than ideal coherence. Based on this data, we have shown that higher saturated gain may be achieved with multi-element systems.
Tolerances on the flood beam experiment beam train for maintaining linear polarized laser output
Authors: Carniglia C. K.
Published: February 2023

The Flood Beam Experiment being performed at the Starfire Optical Range at Kirtland Air Force Base involves illuminating satellites with a linearly polarized laser beam. This laser beam reflects from nine components before emerging from the telescope beam director. Each of these components has the possibility of changing the state of polarization from linear to elliptical. The initial state of the linear polarization is maintained if the polarization vector is in or perpendicular to the plane of incidence on each component (p or s polarization, respectively) or if the phase shift on reflection is 0° or 180°. For the optics on the optical bench, it is possible to maintain the polarization state close to s-polarization. For the mirrors that direct the beam, a variety of incidence angles are possible, and reflected phase shifts of close to 180° must be maintained. Tolerances on the position and alignment of the laser table are determined. Based on the measured phase shifts of the mirrors, the actual depolarization of the system is estimated to be less than 0.1%.
Determining the optimal numerical aperture for micro-lithographic systems
Authors: Sklar E. , Viswanathan V. K.
Published: February 2023

Micro-lithographic optical systems require a high modulation at a spatial frequency representing the smallest circuitry detail to be imaged, and also require that this modulation be maintained over a fixed depth of focus representing the longitudinal position error of the photoresist material. This somewhat unusual requirement for micro-lithography optics leads to a rethinking of the basic assumption that higher numerical apertures always lead to better system performance. We observe that for a fixed longitudinal focus error the wave-front error is proportional to the square of the numerical aperture, and for a fixed spatial frequency the 1-D modulation is equal to 1 - (spatial frequency) X (wavelength)/2(numerical aperture). Based on low frequency approximation for the 1-D MTF for defocus, the authors calculate the optimum numerical aperture as a function of both allowable depth of focus and minimum modulation at a fixed spatial frequency. The results are shown to agree with several micro-lithography optical designs. This seemingly trivial application of MTF relationships is not widely appreciated in the micro-lithography community.
Bipolar optical vector-matrix product for neural networks applications
Authors: Abramson S. , Saad D. , Marom E. , Konforti N.
Published: February 2023

Optical processors for neural networks are based on vector-matrix multiplication machines that can, potentially, compete with serial computers, because of their parallelism and ability to facilitate densely connected networks. However, in most systems proposed, the multiplication supports only two quadrants, disallowing the possibility of using both positive and negative neuron outputs to increase network capacity and learning rate. We propose and demonstrate a fast four quadrant optoelectronic multiplying machine, for a bipolar vector and a continuous matrix containing both positive and negative values, which can be used during both the recall and learning sessions of feedforward neural networks. The machine is based on two adjacent liquid crystal television displays (LCTVs), which enable it to perform an exclusive-OR (XOR) operation between the patterns presented on the displays, thus implementing a complete signed multiplication. The absolute values of the matrix elements are presented on a monitor while the sign is applied to a LCTV to achieve the said four quadrant multiplication. The summation of the vector-matrix multiplication is done either by cylindrical optics or by lenslet arrays. Experimental results, obtained by using common commercial components, present a novel useful and reliable approach for a four quadant electro-oprical matrix-vector multiplication in general and for feedforward neural network training and recall stages in particular.
Characteristics of high-spatial-frequency dielectric and lossy surface-relief gratings for arbitrary incidence and polarization
Authors: Brundrett D. L. , Gaylord T. K. , Glytsis E. N.
Published: February 2023

The transmittance and reflectance of dielectric and lossy surface-relief gratings in the limit of periods small compared to the incident wavelength are determined for an incident wave of arbitrary angle of incidence and polarization. The orientation of the plane of incidence is also arbitrary; it may contain the grating vector, be normal to the grating vector, or have some other orientation. In the small period limit, all diffracted orders except the zeroth order are cut off, and the grating behaves like a slab of uniaxial material with the optic axis parallel to the grating vector. The refractive indices of the slab are determined by the grating filling factor and the indices of the substrate and surrounding medium. We compare the diffraction efficiencies predicted by this model with those obtained by the rigorous coupled-wave analysis. The wavelength-period ratio necessary for the validity of this model is determined, and we show that antireflection behavior can be obtained by appropriate combinations of groove depth and filling factor.
Why is chromatic sensitivity greater than luminance sensitivity?
Authors: Eskew R. T. , Stromeyer C. F. , Chaparro A. , Kronauer R. E.
Published: February 2023

At ARVO 1992, we reported that when detection thresholds for luminance and red–green chromatic spots are expressed in cone contrast energy (the space–time integral of the squared contrast produced in either the L or M cones), sensitivity for the optimal chromatic spot was 5–8× higher than for the optimal luminance spot. For an ideal observer of chromatic signals, the noise level in P ganglion cells would not be limiting, suggesting that limits are imposed at subsequent stages. The large chromatic sensitivity advantage could result from (a) lower detection uncertainty in the red–green mechanism; (b) common mode rejection (removal of correlated noise) in the opponent pathway; (c) reductions in luminance gain, relative to chromatic gain, at cortical levels; or (d) greater spatial and temporal integration in the chromatic pathway.
Localized 2D-exciton energy relaxation due to acoustic deformation potential interaction
Authors: Takiguchi Yoshihiro , Shum Kai , Alfano R. R. , Koteles Emil S. , Bertolet D. C. , Lau Kei May
Published: February 2023

Localized exciton energy relaxation in three undoped lattice mismatched Al0.35Ga0.65As/GaAs1–
y
P
y
strained quantum wells (QWs) has been investigated by using picosecond time-resolved photoluminescence (PL) spectroscopy. The well widths (LZ) for two QWs with y = 0.05 are 121 Å (QW-A) and 79 Å (QW-B). Another QW (QW-C) is LZ = 121 Å with y = 0.08. QW-C is more strained. For these structures, the lowest energy excitons are degenerate exciton, heavy exciton, and light exciton in QW-A, B, and C, respectively. We have evaluated the time varying average exciton energies  for the QWs from the time-resolved PL spectra. The  decreased exponentially with time. Using the exciton masses and exciton deformation potentials (DP) determined by linearly interpolating the values for GaAs and GaP, we have obtained excellent agreement in QW-A and B between  data and the theoretical curves. The relaxation rate is confirmed to be proportional to the square of the product of exciton mass and DP and to be inversely proportional to LZ. For QW-C, the energy relaxation rate is between the light and the heavy exciton relaxation rate. This indicates that the exciton in QW-C relaxes because of not only acoustic DP interaction but also because of other scattering mechanisms.
Ultrafast gain dynamics in semiconductor laser amplifiers
Authors: Mørk J. , Mark J.
Published: February 2023

We report pump–probe measurements of sub-picosecond gain dynamics in InGaAsP bulk and quantum well laser amplifiers. Pulses with a width of 120 fs and tunable from 1485 to 1540 nm are generated by using a color center laser with additive pulse mode locking. The probe signal, measured as a function of pump–probe delay for different wavelengths and different bias currents, shows that the gain undergoes very complicated dynamics with several time constants involved. By comparison with theoretical calculations based on the density matrix approach and including pulse propagation effects we conclude that the dynamics reflect the combined effect of two-photon absorption (TPA), spectral hole-burning (SHB), and carrier heating (CH). TPA leads to a gain decrease for all wavelengths and follows the pump pulse instantaneously, whereas SHB (associated with a ~50 fs intraband relaxation time) changes sign at the transparency point. Carrier heating, with a temperature relaxation time of 700 fs, leads to a gain decrease and is seen in almost the entire wavelength region. Carrier cooling is observed deep into the absorption region. These results have implications for fast modulation of laser diodes and amplification of ultrashort pulses and could find application in laser diode mode locking.
Spectroscopic measurements on single molecules in solution
Authors: Davis L. M. , Soper S. A. , Shera E. B.
Published: February 2023

We have developed a technique that allows individual dye molecules in solution to be detected as they pass through a tightly focused laser beam.1 A mode-locked laser is used for excitation, and time-gated single photon counting is used to discriminate the weak bursts of fluorescence from the promptly scattered Raman noise arising from the many solvent molecules. By developing a dual-channel instrument that incorporates a dichroic beam splitter in the fluorescence collection optics and two synchronized mode locked lasers for excitation, we have demonstrated the ability to detect and spectrally identify single molecules in an ultradilute solution containing both rhodamine 6G and sulforhodamine 101.2 Also, by digitizing and storing the arrival times of the fluorescence photons with respect to the excitation pulses we are able to measure the fluorescence lifetimes of individual molecules to a reasonable precision by using maximum entropy techniques. We also discuss recent progress towards making other spectroscopic measurements on single molecules, such as the quantum yield, absorption cross-section, and photodegradation rate, and future applications.
Optical spectroscopy using a solid as a single-molecule trap
Authors: Moerner W. F. , Basché Th. , Orrit M.
Published: February 2023

As was recently demonstrated1 by using highly efficient fluorescence excitation spcctroscopy with N.A. = 0.98 collection optics, single impurity molecules in both crystals and polymers may now be detected with single-to-noiseratios (SNRs) as high as 30 in a 1 Hz bandwidth at low temperatures. In essence, the solid may be regarded as a trap for the single molecule, which effectively quenches rotation. Our measurements include precise determination of the lifetime limited linewidth and the temperature dependence of the optical dephasing, as well as spectral diffusion (wandering in frequency space due to perturbations by lattice degrees of freedom). For the polymeric host, light-induced changes in resonant frequency of a single molecule can be produced, a process that is normally termed spectral hole-burning when large ensembles of molecules are probed. In recent studies, nonclassical light emission from a single molecule (photon antibunching) has been observed in a fashion analogous to that reported earlier for a single atom in a trap.
Violations of classical probability in optics
Authors: Mandel Leonard
Published: February 2023

A number of optical experiments in which quantum mechanics is tested and the laws of classical probability are violated will be described. These all make use of the phenomenon in non-linear optics known as parametric down-conversion, in which an incident pump photon effectively splits into two correlated, lower frequency signal and idler photons in an entangled quantum state. We show that two-photon coincidence measurements of signal and idler photons violate classical probability by at least 600 standard deviations. Other measurements involving two down-converters lead to the realization of a ″quantum eraser″ and suggest a reinterpretation of the usual quantum wave function.
Development of an imaging system for soft x-ray projection lithography
Authors: Jewell Tanya E.
Published: February 2023

An imaging system to be used in photolithography must provide high resolution images over large fields that must be flat and free of distortion. An additional requirement is the image- space telecentricity, in order to prevent magnification changes due to wafer defocus. In the soft x-ray region only mirrors can be used for imaging. This imposes several specific requirements for reflective system design: to insure high system throughput, the number of mirrors used in the system should be a minimum; the aperture stop should be accessible (e.g., it may be placed at one of the mirrors) to prevent any possibility of creating differences in image across the field; and finally, elimination of the obscuration in the pupil is desired since it may reduce the image contrast. Various system configurations that use two, three, and four mirrors have been investigated. It was found that in order to satisfy all the above requirements, the system design naturally evolves into the scanned ring-field configuration. Four mirrors were found to be the minimum number to satisfy these requirements simultaneously. In reduction ring-field scanned systems, image distortion must be corrected not only across the ring width in the radial direction, but also in the cross-scan direction.
High-resolution soft-x-ray projection imaging using Schwarzschild optics and a laser plasma source
Authors: Tichenor D. A. , Kubiak G. D. , Malinowski M. E. , Stulen R. H. , Haney S. J. , Berger K. W. , Brown L. A. , Sweatt W. C. , Bjorkholm J. E. , Bokor J. , Freeman R. R. , Himel M. D. , Jewell T. E. , MacDowell A. A. , Mansfield W. M. , Tennant D. M. , Waskiewicz W. K. , White D. L. , Windt D. L. , Wood O. R.
Published: February 2023

High resolution soft-x-ray projection images are recorded by using a laser plasma source (LPS) and multilayer coated optics. Soft x-rays are generated by focusing a KrF excimer laser beam on a moving gold target. The resulting 15-eV plasma emits an intense burst of soft x-rays from a spot 150 μm in diameter. The soft x-rays are collected by an off-axis ellipsoidal condenser to illuminate a transmission mask composed of a germanium absorber supported by a silicon membrane. A magnified image of the LPS is projected by the condenser into the entrance pupil of the imaging objective to provide Kohler illumination of the mask. The imaging objective is a 20× reduction Schwarzschild having a numerical aperture of 0.08 and operating at a wavelength of 14 nm. Features as small as 0.1 μm have been recorded in polymethylmethacrylate (PMMA). A second-generation system is being fabricated based on a 10× Schwarzschild objective and a near-on-axis ellipsoidal condenser. This system is designed to image a reflecting mask onto a resist-coated wafer.
Numerical modeling of the linear propagation characteristics of periodically segmented waveguides for blue light generation by quasi-phase-matched SHG
Authors: Li Lifeng , Burke James J.
Published: February 2023

Quasi-phase-matched second harmonic generation of blue light in optical waveguides is currently of great interest since it holds the potential for miniaturized blue light sources for various applications. Many fabrication methods for periodic domain inversion and waveguide formation have been investigated in several material systems.1 The method of the periodically segmented KTP waveguide studied by the du Pont group2 has certain advantages over the others: it forms the waveguides and the inverted domains in one processing step, and it offers greater degrees of freedom in waveguide fabrication. The segmented structure does not form a conventional waveguide since it consists of alternating guided and unguided sections; however, very low radiative loss was observed experimentally.3 The linear properties of this type of waveguide have not been studied so far. In this paper, we present results of numerical modeling of the linear propagation characteristics of the periodically segmented waveguides, which include propagation constants, radiative loss rates, and guided wave profiles of the fundamental and the second harmonic frequencies.
Counterpropagating waves in periodic nonlinear structures
Authors: Aceves A. B. , De Anglis C. , Moloney J. V. , Wabnitz S.
Published: February 2023

In this paper the authors will present both numerical and analytical results on the coupled mode equations that describe the evolution of counterpropagating beams in periodic waveguides. These coupled mode equations are a generalization, by an addition of one transverse diffraction term, of the equations that describe wave trains in a fiber filter whose grating is of the order of the wavelength of light. We will first show how the well known gap soliton solutions of the fiber filter extend to the planar waveguide structure. These solutions represent beams that are cw in the transverse direction and have the soliton profile in the direction of propagation. We then numerically show whether transverse modulational instabilities arise. Finally, we numerically study the dynamics of pulses propagating along the waveguide. Here, we compare the dynamics of the coupled mode theory with that of the two dimensional nonlinear Schrodinger equation (2D NLS), which has been suggested as a good limit when the pulses are spatially broad. An interesting feature of this comparison is that for the 2D NLS in the anomalous regime, a collapse in finite time is predicted. We then want to determine if this also happens in the coupled mode analysis.
A low-noise modulation technique for Fabry–Perot cavity gravitational-wave detectors
Authors: Drever R.W.P.
Published: February 2023

A modulation technique using an external modulator and cavities slightly unequal in length, or in distance from beamsplitter, is described. Advantages are outlined.
Low-voltage, 1.3 um Fabry-Perot reflectance modulator from lattice-mismatched (InAlGa)As materials
Authors: Fritz I. J. , Hammons B. E. , Howard A J. , Brennan T. M.
Published: February 2023

We have demonstrated the first Fabry-Perot-cavity reflectance modualtor operating near 1.3 um. A peak contrast ratio of 2.7 was achieved at 4V bias.
Nonclassical correlation of total photon using macroscopic twin pulses of light
Authors: Smithey Daniel Tod , Beck Mark Kevin , Belsley M. , Raymer Michael Gordon
Published: February 2023

We measured probability distributions of the photoelectrin difference number of twin light pulses. These are the only nonclassical photoelectron distributions measured to date in the macroscopic regime.
Cerenkov emission in photonic handgap structures
Authors: Lawandy Nabil M. , Kweon Gyeong-il
Published: February 2023

We use a simple dispersion relation to study the process of spontaneous Cerenkov emission in a photonic bandgap material. The calculation uses the Klein-Gordon equation to treat the electrons and the truncated density of states of the 1-D dispersion relation to approximate the photon density of states in the photonic bandgap material. The Jesuits show that these structures which can have a void fraction greater than 80% can produce equivalent or higher Cerenkov radiation rates as solid high density matter. Since these structures are porous and in some cases are comprised of a series of intersecting open channels which go through entire structure, the relativistic electrons can traverse the material without suffering inelastic scattering losses. The conclusion is that these light weight porous 3-D structures could be used to slow relativistic particles by purely radiative means without the generation of heat. Potential applications of our findings are in the areas of reactor engineering, particle detection, and Cerenkov lasers.
Laser properties of Mn:Ba3(VO4)2
Authors: Merkle Larry D. , Pinto Albert , Verdun Horatio R. , McIntosh Bruce
Published: February 2023

Laser oscillation has been obtained using a new solid-state laser ion, Mn5+. The material is Mn:barium orthovanadate, in which Mn occupies an approximately tetrahedral site.
Phase-sensitive switching in a rocking rotator fiber filter
Authors: Krautschik Christof G. , Stegeman George I. , Stolen Roger H.
Published: February 2023

To date all-optical switching experiments in fibers and channel waveguides have relied on having a high intensity beam switch itself between two waveguides or two polarization states or on switching a low intensity beam by a strong control beam. We demonstrate for the first time, to our knowledge, phase-sensitive switching in a periodically twisted birefringent fiber where the switching of a strong signal beam is controlled by the phase of a weak control beam. The switching was implemented by using 30 ps pulses from a dye laser tuned to the resonant filter wavelength of 590 nm. The fiber was cut to a coupling length of 185 cm, and a strong signal beam, which contained 90% of the input power, was launched along one of the two principal axes. The remaining 10% was coupled orthogonal to the signal polarization direction. The phase difference between the two polarization states was controlled by a Soleil–Babinet compensator. When the phase was varied between 0 and 2π, phase-sensitive switching was observed at input powers of 1.3 kW. The contrast of the bar state could be varied between 22% and 75% of the input power by using the configuration described above. The phase-sensitive process vanished at input powers much smaller than the critical power.
Spatially incoherent joint transform correlators
Authors: Rosen Joseph , Segev Mordechai , Shamir Joseph , Yariv Amnon
Published: February 2023

A rotational shearing interferometer was used to demonstrate a spatially incoherent joint transform correlator. This system has the ability to perform a correlation between two complex functions (images) borne on spatially incoherent illumination, without the use of holographic filters. The output intensity distribution of the interferometer includes a term that is related to a Fourier spectrum of the input image. We digitally processed this output intensity, removed its dc level, and squared the remaining function. The resultant joint spectrum was displayed on a spatial light modulator, located at the input plane of a conventional coherent 2-f reconstruction system. We demonstrate various cases of transversely and axially separated auto and cross correlations using this system.
Extended linearized gray scale with a CRT monitor
Authors: Swift Dan J. , Smith Robert A.
Published: February 2023

Pelli and Zhang1 describe techniques for combining the outputs of the three color DACs with a CRT monitor to achieve extra gray-scale resolution, particularly at low contrast levels. While there should theoretically be 24 bits of gray scale available, this is unachievable in practice because of monitor nonlinearities and DAC inaccuracies. We measure and account for the inaccuracies in the DAC (which are extremely stable) and build a single look-up table containing over 32,000 (15 bits) RGB values that are converted by a passive resistive circuit into equal-interval gray-scale values. Dynamic inaccuracies resulting from DAC bit changes have been examined and found to be negligible. A novel smoothing technique that does not depend on assumptions of the form of the monitor non-linearity has been used. The final correlation of measured vs. specified luminance has been as high as 0.999996 (and is typically higher than 0.99995).
The effect of eye movement tracking strategies on the velocity distribution of the retinal image
Authors: Eckert Michael P. , Buchsbaum Gershon
Published: February 2023

Tracking eye movements significantly alter the image motion statistics reaching the retina. To describe these motion statistics and the effect of tracking, we define a space dependent velocity distribution for the image, computed by sampling velocity through time at every spatial location in the image. The velocity distribution provides a measure of the average spatiotemporal variability of images rather than instantaneous velocity variations. The mean and variance of the velocity distribution are related to the bandwidth and variability of the spatiotemporal spectrum of images. We implemented several visual tracking strategies on digitized image sequences to investigate their effect on the velocity distribution. We find, as expected, that retinal motion statistics are markedly different from those of the external image. Smooth pursuit provides consistently small mean velocities in the center of the visual field (the point of tracking), but at the expense of increasing the mean velocity in the periphery. On the other hand, visual full field tracking (vestibulo-ocular and optokinetic eye movements) reduces velocity equally across the entire visual field.
Contrast sensitivity functions measured during early dark adaptation
Authors: Hahn L. W. , Geisler W. S.
Published: February 2023

Last year we reported measurements of contrast sensitivity functions (CSFs) during long-term dark adaptation and on steady backgrounds. The targets were "increment-Gabor" patches—the sum of a 2D Gaussian-damped sinewave and a simple 2D Gaussian of the same amplitude. The major finding was that the CSFs changed shape as a function of steady background luminance (as expected from previous studies) but did not change shape during long-term dark adaptation. This latter result supports the hypothesis that long-term adaptation is both local and multiplicative. The present study measured CSFs (1 to 15 c/deg) during short-term dark adaptation and on steady backgrounds. For the short-term adaptation conditions, the adapting background was presented with a duty cycle of 3% (9.7 s on and 0.3 s off), and the target was presented 175 ms after background offset. We found, in the short-term adaptation (and steady-background) conditions, that the CSFs became flatter as adapting illumination increased from 0 to 100,000 trolands. This implies that short-term dark adaptation cannot be simultaneously local and multiplicative.
Optical gradient filter using a liquid crystal modulator
Authors: Guo Zhuanyun , Zhang Yanxin , Mu Guoguang
Published: February 2023

Various methods of optical edge enhancement have been developed. Among them the real-time absolute grad operator is the most attractive. Although hologram based local nonlinear filter can perform the |grad| operation, there are many difficulties in making and operating this holographic filter. In addition, relatively low optical efficiency is a problem. We present here a novel optical filter with the function of AJ1(2πr0ρ)ejϕ, where ρ and ϕ are the variables in polar coordinates at the Fourier plane. This filter is a local nonlinear device that can perform the |grad| operation in real time and can be realized by using a liquid crystal modulator. The transmission-type device has the advantages of high optical efficiency, ease of fabrication, and insensitivity to mechanical vibrations. Experimental results have shown the proposed device has promising applications.
Real-time preprocessing of large field of view images using optical spatial filtering
Authors: Powell Kirk J. , Clark Natalie , Giles Michael K.
Published: February 2023

Cueing or locating objects of interest in a large field of view (FOV) image is a preprocessing requirement for recognition systems. Cueing operations generally utilize computationally intensive digital methods that can limit system performance, especially for large images. This digital bottleneck can be overcome by taking advantage of the speed and massive parallelism of optical cueing techniques that utilize spatial filtering to locate objects of interest. A well established design for optical filtering is the coherent imaging architecture with an accessible filter plane. This architecture (used for Vander Lugt correlation) is ideal for any type of spatial frequency filtering, and it can be implemented by using liquid crystal televisions as gray level filters and input image transducers.1, 2 This paper examines real-time optical gray level spatial filtering techniques for object location. Bandpass gray level windows can be designed by using Hamming or other apodizing methods to reduce the image ringing caused by binary bandpass filters. Filter optimization results in fewer object location false alarms and more accurate cueing. Gray level filters based on object scale have been used successfully to locate objects of interest in large FOV gray scale images.
Phase-only photorefractive joint transform correlator
Authors: Khoury J. , Golomb M. Cronin- , Kane J. , Woods C.
Published: February 2023

The joint transform correlator (JTC) is well known to be one of the most convenient devices for correlating two images in the sense that there is no need to fabricate a holographic filter. It has been shown previously that by applying additional non-linearities in the joint spectrum plane, such as binarization or saturation, to the joint spectra it is possible to achieve correlation with phase-only information (i.e., correlation with an inverse filter). This process increases the discrimination ability of the correlator. Previously, the nonlinearities for conventional and enhanced JTC required a camera, a computer, and a spatial light modulator for their implementation. We propose an arrangement of degenerate four wave mixing using two barium titanate crystals. This enables us to replace the previously used electronic processing while simultaneously achieving the two non-linearities, binarization and saturation, for a phase-only JTC.
An invariant joint transform correlator
Authors: Lin Jian , Lin T. W. , Lu Jianyi
Published: February 2023

Classical optical correlators are known to be very sensitive to distortions, such as scaling and rotation. Various invariant systems have been studied and reported based on the conventional spatial filtering technique. In this paper, we have investigated the feasibility of implementing an invariant system based on optical joint transform architecture. A polar-log coordinate transformation is used as the technique to retain scaling and rotation invariances. This transformation is achieved through the Fourier transform of a universal mask from a computer generated hologram and the power spectra of input objects. An energy compensating process is employed to accommodate size variations from input objects and the nonlinear weight factor caused by the coordinate transform mask. The dynamic range of the proposed system is evaluated and compared with those obtained from conventional approaches. A method of preserving shift in variance and the phase information of input objects in the system has also been studied and suggested. Results verifying the proposed system are presented.
Star-field recognition
Authors: Scholl Marija S. , De Antonio Luisa , Dennison Edwin W. , Alexander James W. , Bartman Randy K.
Published: February 2023

A star-field pattern recognition algorithm has been developed. It is based on matching the stars on a full celestial sphere observed in a CCD-based star tracker and the reference stars, by using the Yale bright star catalog. The correspondence between the observed and the reference star pattern is found by constructing three-star triangles in spherical geometry. Star angular coordinates and stellar magnitudes are matched to build up a celestial region of correspondence. Gaussian errors in the cataloged star coordinates and brightnesses are modeled to test the algorithm against faulty reference data. Random values are selected for the origin and the orientation of the observed star field. The possibility of hardware-introduced stars is modeled by introducing spurious data. The computer simulations and the preliminary experimental results at the Table Mountain Observatory are described.
Nonlinear dynamics of semiconductor lasers with phase-conjugate feedback
Authors: Agrawal Govind P. , Gray George R.
Published: February 2023

Semiconductor lasers are known to be extremely sensitive to external optical feedback. This paper considers the case in which the feedback is due to reflections from a phase-conjugate mirror, rather than from an ordinary mirror. Such phase-conjugate feedback (PCF) introduces novel features into the laser dynamics. A steady state in which the laser phase is locked to a fixed value can be reached when the amount of PCF is below a critical value. With increasing PCF, the steady state is destabilized and the laser output becomes chaotic following a period-doubling or a quasi-periodic route, depending on the amount of feedback. The effects of spontaneous-emission noise on the laser instabilities are studied by solving numerically a set of stochastic rate equations. Numerical simulations are used to study how the noise characteristics such as relative intensity noise, frequency noise, and the line shape are affected by the PCF.