bluuebunny/crossref_metadata_2025 huggingface dataset:
Dispersion-free femtosecond optical autocorrelator
Authors: Takiguchi Yoshihiro , Alfano R. R.
Published: February 2023

A novel dispersion-free autocorrelator (DFA) has been developed to achieve subfemtosecond temporal resolution. Autocorrelation methods are widely used to measure ultrashort pulses because of their high temporal resolution. However, when the pulse duration is reduced to femtosecond region, pulse broadening by optical components occurs, owing to group velocity dispersion (GVD), and this broadening causes error in pulse-duration measurements. Because GVD depends on spectrum width, shorter pulses increase the broadening effect. To eliminate such pulse broadening, each optical component in the autocorrelators must be ultra thin or dispersion-free. Our novel autocorrelator, which is a noncollinear configuration, consists of a focusing concave mirror, an ultrathin pellicle 5 m beam splitter, corner reflectors, and a 100 m KDP crystal to generate the second harmonic of the laser pulses. Each component causes subfemtosecond time broadening for 10 fs pulses at 620 nm. For example, broadening of a 10 fs pulse due to the pellicle is only 10.0004 fs (negligible) while the KDP crystal broadens the 10 fs pulse to 10.21 fs. Using a colliding-pulse mode-locked (CPM) ring dye laser generating 60 fs pulses at 620 nm, the characteristics of DFA have been investigated by comparing them with an ordinary, noncollinear autocorrelator. Various DFA designs, along with their characteristics and GVD effects, will be discussed in detail.
Spontaneous emission from a moving atom in a standing-wave cavity
Authors: Ren Wensheng , Carmichael H. J.
Published: February 2023

The spectrum of spontaneously emitted photons from an atom interacting with a single cavity mode depends on the relative sizes of the mode coupling constant and the cavity loss rate. If the coupling constant is not too large, perturbation theory can he used to show that the atomic linewidth is either increased (enhanced spontaneous emission) or decreased (inhibited spontaneous emission) by the presence of the cavity. When the coupling constant is so large that perturbation theory cannot be used, the spontaneous- emission spectrum forms a doublet, with peaks located at the normal-mode frequencies for the coupled-dipole and cavity-mode oscillators (the so-called "vacuum" Rabi splitting).
Observation of enhanced photosensitivity in hydrogen-treated fibers
Authors: Ouellette François , Gagnon Daniel , Poirier Michel , Harvey Steve
Published: February 2023

We report the observation of enhanced photosensitivity in fibers exposed to a 12-atmosphere hydrogen pressure and 400C heat for 4 hours. Photosensitivity was measured with two different techniques. First, short Bragg gratings (1 cm long) were written with counterpropagating pulses from a mode- locked continuous-wave frequency-doubled Nd:YAG laser. The 80 ps pulses meet at the middle of a short strand of single-mode fiber, and the standing wave induces the formation of the grating. This portion of the fiber is maintained at a constant temperature. The grating reflectivity is measured by means of time-domain reflectometry with a fast photodiode and a sampling oscilloscope (resolution of 75 ps). Reflectivities as low as 0.1% are easily detectable. The rate of grating formation was found to be much faster in the treated fiber than in the untreated fiber. Our second technique uses the fact that the index change follows the polarization of the writing light. Thus, polarized light from the doubled Nd:YAG laser was sent through a short fiber strand, and we measured the transmission through an analyzer of a HeNe laser probe that was polarized at 45 with respect to the green beam. This technique minimizes the influence of thermal effects. Again, an increase by as much as a factor of five in the polarization rotation rate was observed in the treated fiber.
Threshold characteristics of Hill grating formation
Authors: Guo Baiming , Leitch James , Anderson Dana Z.
Published: February 2023

We have observed an intensity threshold for Hill grating formation in single-mode polarization-preserving fibers from Andrews Corporation. A single-frequency argon-ion laser operating at either 488 nm or 514 nm is isolated from fiber reflection an acousto-optic modulator. The fiber rests in a loose coil on a copper plate that is temperature controlled to a few millidegrees Kelvin, cooled somewhat below room temperature. The fibers we report on here are "D" type and have core sizes 12 m and 1.52.5 m, although we have observed thresholds in other Andrews fibers as well. Grating formation commences at an intensity in the region of 130 mW, depending on the length of the fiber and on the initial intensity of the light reflected from the output end. The threshold behavior resembles that observed for self-seeded growth of second-harmonic light: Below threshold, no grating is written, above threshold, grating formation begins. However, reducing the light intensity below threshold stops further growth. The threshold may occur because of a balance between gain and loss for the growth process; it could also indicate the underlying physics and chemistry. Much, but not all, of our evidence seems to point to the former.
Retinal locus for fixation and identification
Authors: Schuchard Ronald A.
Published: February 2023

Retinal locus for fixation/identification are defined as the retinal locations used to fixate/identify temporally or spatially different objects. I determined the characteristics of the retinal locus for fixation/identification by using a scanning laser ophthalmoscope (Rodenstock) to display objects on the retina with a 5 × 5 grid (16 × 15). To determine retinal locus for fixation, each object (0.25 circle, 1 circle, 1 crosshair with 6 ft. bars) was first presented in the center grid position and then randomly presented (without replacement) in an eccentric grid position (giving 24 central and 24 eccentric fixations for each object). To determine retinal locus for identification, a randomly rotated C (0.25) was randomly presented (without replacement) at eccentric grid locations and observers determined the orientation (up, down, right, or left). Data were obtained from three experienced psychophysical observers and three naive observers. The results show that the retinal locus for fixation is nearly constant for different shapes and sizes of fixation objects. However, the retinal locus for fixation is larger for eccentric grid locations compared to central grid locations. The retinal locus for identification is larger than either the central or eccentric retinal locus for fixation. The implications for eye tracker calibration and perimetry will be discussed.
The design of an optical scope for the visually impaired
Authors: Wolken Jerome J.
Published: February 2023

In search of innovative optical systems, I have turned to the eye structure of rare, deep sea animals that live at depths of 200500 m, where the light is limited and comes primarily from bioluminescence. Eyes have considerable inventiveness in their optical design among these marine animals. In the crustacean copepod copilia the eye has a biconvex lens (L1) and, at some distance away, an aspherical lens (L2), a telescopic eye. The shape of the L2 lens is optimized for light collection and acts like a fiber-optic bundle. The focal point of the optics is a small area of the retina. The retina rapidly moves back and forth, and the organism's eye scans much like the electron beam scans a television screen. The animal's eye structure was modeled, and lenses L1 and L2 (glass and polymers) were shaped into the calculated curvatures and geometry. The focal lengths of the lenses were determined and fitted into an optical scope. The scope was tested for imaging of near, far, and moving objects. Sharp images were obtained from 25 cm to infinity at relatively low light levels. Tests of prototype scopes indicate that they have application for the visually impaired, and the lens L2 can be adapted for optical scanning, photography, and solar energy collection.
Ultrashort pulse generation using a pulsed alexandrite laser
Authors: Miller E. J. , Boyd R. W. , Steteyick K. A. , Potember R. S. , Harte D. J.
Published: February 2023

There has been widespread interest in generating ultrashort pulses from broadband solid-state materials. However, until recently, pulses generated from solid-state materials have been limited to the order of picoseconds. Pulse widths obtained by most active/passive mode-locking schemes are limited by the relatively slow relaxation time of the saturable absorber. Here we report results of an experimental demonstration of a new technique for producing high-energy, subpicosecond pulses from a solid-state laser system, namely, active/passive mode-locking using a reverse saturable absorber. This technique has been predicted to produce pulses that are shorter than the relaxation times of either absorber. Our laser configuration is similar to a conventional active/passive mode-locked alexandrite oscillator but has an added reverse-saturable absorber, i.e., a material that exhibits absorption that increases with intensity. The saturable and reverse-saturable absorbers shorten the leading and lagging edges of the pulse, respectively. In addition to the pulse-shortening mechanisms of the two absorbers, our preliminary results show that an additional mechanism, similar to that in coupled-cavity lasers, is present in our system. We have determined that an intracavity reflection effectively couples energy from a 150 ps pulse into the ultrashort pulse. Similar results were obtained with an extracavity reflection. To date, our system has produced streak camera limited pulses (ps) containing several microjoules of energy at a repetition rate of 10 Hz.
Simultaneous self-starting additive-pulse mode-locking and second-harmonic generation of Nd:YAG lasers using self-organized germanosilicate optical fibers
Authors: Lawandy N. M. , Bernardin James P.
Published: February 2023

Several reports of self-starting additive pulse mode-locking (APM) of solid-state lasers have appeared recently. These include diode pumped Nd:YAG and Ti:Al2O3 lasers. The nonlinear cavities used in these experiments were formed by using single-mode optical fibers as the nonlinear medium. The fibers are typically on the order of 1 m in length and, along with the reflector, provide the intensity-dependent reflectivity required to trigger and sustain mode-locked operation. The use of fibers for the nonlinear index medium for APM presents a second exciting possibility of simultaneously frequency doubling and tripling the laser output. This combined process is made possible by the well-known but not well understood phenomenon of second-harmonic generation in germanosilicate optical fibers. Fibers that have been self-prepared or conditioned with seeded preparation have resulted in conversion efficiencies as high as 13% when used outside a laser cavity. We will discuss experiments on a cw Nd:YAG laser, which is passively mode-locked by means of a conditioned germanosilicate fiber external cavity for APM and SHG. The external-cavity reflector is 100% reflecting at 1.06 mm and 90% transmitting at 532 nm. The coupled-cavity laser emits pulses at 532 nm that are shorter than 5 ps.
Atmospheric fluctuations in an 11-μm stellar interferometer with movable telescopes
Authors: Bester M. , Degiacomi C. G. , Danchi W. C. , Townes C. H.
Published: February 2023

A stellar interferometer involving two movable telescopes is in use on Mount Wilson in the 10 m atmospheric window. Baselines between 4 m and 35 m have been laid out but observations so far have been with baselines of 4 m and 13 m. Heterodyne detection and lobe rotation are used as they are in a radio interferometer, with frequency of the resulting interference fringe reduced to a fixed 10 Hz. Fluctuations of the fringe frequency about the central 10 Hz frequency are measured, along with fluctuations of the 11 m optical paths within each telescope. The spectrum of each of these fluctuations is calculated, as is the spectrum for transmission in the atmosphere excluding the internal optical path. For a 4 m baseline and reasonably good seeing 2", the relative optical paths are stable enough to provide no change in average frequency for observation times of as long as 1 h, which produces a fringe signal width of ~0.3 mHz with small modulation sidebands. Atmospheric path-length fluctuations at frequencies between ~1 and ~10-3 Hz are obtained from the sidebands or from measured phase fluctuations. Wide variations in characteristics of the power spectrum occur for various atmospheric conditions.
A constant-time optical parallel sorting engine
Authors: Huang Kung-Shiuh , Liu Lishing , Peir Jih-Kwon
Published: February 2023

Sorting a sequence of N items requires at least O (NlogN) time steps on a conventional uniprocessor computer and O(log2 N) for most electronic parallel sorting networks, such as old-even merging network, bitonic merging network and perfect shuffle network. In this paper we provide an optical facility for sorting a sequence of N items in only O(1) time steps.
Changes in coherence and spectral properties of light reflected from a dielectric slab
Authors: Wang Weijian , Simon Rajiah , Wolf Emil
Published: February 2023

We have studied the changes in the spectrum and in the degree of coherence of light reflected from a stratified medium. The incident field was assumed to be produced by a planar, secondary, partially-coherent source. We analyzed reflection from a homogeneous dielectric slab in detail the case in which the incident field is produced by a Gaussian, Schell-model source. We will present results of numerical calculations for several representative values of the parameters. We found drastic changes in the spectrum and in the degree of coherence in some cases.
Coherence in two-photon down-conversion induced by a laser
Authors: Wang L. J. , Zou X. Y. , Mandel L.
Published: February 2023

Normally, the signal and idler photons produced in the process of parametric down- conversion exhibit no second-order interference. However, in the presence of an external laser beam that follows the path of the signal photon and induces stimulated down-conversions, one would expect coherence to develop between the down-converted signal and idler beams. We have demonstrated the existence of this phenomenon in an experiment with two down-converters pumped by similar pump beams, in which two mutually coherent laser beams followed the paths of the two down-converted signal photons and induced coherence between the two down- converted idler beams. The induced coherence gives rise to observable interference effects, which are presented.
Polarization properties of waveguide focusing grating couplers
Authors: Hwang Shiow-Hwei , Gong Qian , Li Lifeng , Lawrence George
Published: February 2023

Waveguide focusing grating couplers are being developed for application to optical data storage and for other applications where the advantages of light weight, monolithic construction, and ease of use with integrated optics are important. The polarization theory of these devices differs from the theory of conventional gratings because of the conversion of guided waves to radiated waves. We describe a simple theory based on induced dipoles that agrees well with experiment and that gives the designer insight into the process. The use of TM guided waves presents special difficulties. We discuss the design of waveguide focusing grating read-write heads for optical data storage applications, and show that it is possible, in principle, to achieve ideally mode-matched input and output, including intensity, phase, and polarization mode forms. Results of calculations of end-to-end performance will be presented. The experimental justification for the induced dipole theory will be described in a companion paper by Li et al.
Power-spectrum measurements using radial shearing interferometry
Authors: Sudol Ronald J. , Jungquist Robert K.
Published: February 2023

A common-path radial-shearing interferometer is examined for its ability to measure the power spectrum of an object's intensity transmittance. For an object illuminated with spatially incoherent light, it is shown that the contrast of the intensity distribution observed at the interferometer's output contains a cosine transform of the object's intensity transmittance. From a coherence theory point of view, this manifestation is shown to be the result of a direct mapping between the object's frequency spectrum and the coherence properties of the interfering beams. Alternatively, when one considers the system impulse response, each point in the object is found to produce a set of cosinusoidal fringes whose frequency is directly related to that object point. The subsequent incoherent summation over the object results in an interference pattern that contains a bias term plus the transform of the object's intensity transmittance. Experimental results obtained for standard objects such as circular apertures and Ronchi rulings compare favorably with theoretical predictions.
Parameters that influence the distribution of activity in visual codes
Authors: Brady Nuala , Field David J.
Published: February 2023

This study explores the relation between the activity of 'sensors' in different visual codes and the statistics of natural scenes. The codes that will be described have properties similar to those of the mammalian visual cortex. They represent images with a Gabor-like transform in which sensors have parameters of position, orientation, frequency, and phase. It has been shown that the amplitude spectra of natural scenes falls as a function of frequency by a factor of roughly 1/f. Using 25 images, results will be described which show that codes with constant spatial frequency bandwidths (in octaves) and constant orientation bandwidths (in degrees) can produce equal activity across the entire array of sensors. The variance of activity across the above parameters was determined, and will be discussed in relation to efficient coding of the environment. Codes of this type often make use of quadrature pairs with even and odd symmetry although the physiological evidence for these particular symmetries is weak. We will demonstrate that although quadrature pairs may be useful, no particular symmetry (e.g., phase relations) adds to the efficiency of the code.
Contrast linearity and superposition of visual potentials evoked by pattern-reversing weighted Hermite polynomials
Authors: Reeves Adam , Yang Jian
Published: February 2023

We discuss exponentially weighted Hermite polynomials (WHPs) as basis functions for spatial visual information processing. WHPs are spatially localized, complete, and orthogonal (Gabors and sine-waves lack some of these desirable properties). We stimulated the visual evoked potential (VEP) with different WHPs (of orders 0 through 31), using almost the full range of possible contrasts. The field was 15 across and 104 cd/m2. The subject's eye fixated above the field to avoid cancellation of VEPs from upper and lower visual hemifields. Steady-state VEPs were recorded, from 4 cm above the inion, by reversing pattern contrast at 6.35 Hz. The power of the VEP was very close to being linear with contrast, although there was a tendency to saturate with the most effective WHPs at very high contrasts. Responses to pairs of stimuli, in which one WHP at a fixed contrast was added to a second WHP of variable contrast, were obtained with WHPs of orders 10 and 3, 1 and 16, and 6 and 23. Superposition held well in two cases for the second author (J. Y.) and in all three cases for a naive subject.
Light-controlled gate
Authors: Strobl Karlheinz , Golub Ilya
Published: February 2023

It is believed that optical computing will be able to vastly outperform electronic computing in the future. Logic gates performing data manipulations are essential building blocks of any computer. All-optical logic gates can eventually replace the electronic and electro-optic ones. We demonstrate how specific material properties can be used to construct such an all-optical logic gate based on light interaction with a nonlinear interface (NI). The NI comprises a linear dielectric material in contact with an intensity-dependent nonlinear material, which exhibits a controllable intensity-dependent reflectivity and transmissivity. We chose a material with a thermal nonlinearity that is among the largest, has a response time well suited for real-time visual demonstration, and requires a minimum of high-technology equipment. A thin film of nonlinear material is sandwiched between two identical linear materials. Both types of material have the same index of refraction for low light intensity. Absorption of light in the nonlinear medium results in heating, leading to density decrease and corresponding refractive index reduction. The angle of incidence of the air-cooled Ar+ laser beam is chosen so that a low-intensity light beam is transmitted, and a high-intensity light beam is totally reflected. Thus, by changing the light intensity, one controls light switching with this optical gate.
See 7-μm capillaries in your own eye
Authors: Applegate Raymond A. , Bradley Arthur , Zilio Colleen
Published: February 2023

We are in the process of constructing a vascular entoptoscope suitable for hands-on museum use. The entoptoscope will allow the user to view the smallest details of the retinal vasculature in the macular area and to measure the size of the foveal avascular zone. The device is user friendly and requires only that the user have a pupil size of at least 3 mm and be able to hold his or her head relatively still. Furthermore, because the vascular entoptoscope produces an entoptic view of the vessels, it is not sensitive to variations in refractive errors. Therefore, appropriate refractive corrections (i.e., spectacles or contact lenses) are not necessary for sharp visualization of the capillaries. The device uses the optical principles of Maxwellian view to image a uniformly illuminated blue test field on the retina. The exit pupil of the system is 0.75 mm in diameter and rotates at a rate of 3 4 Hz around a point approximately centered in the entrance pupil of the eye. Depending on the embellishments added, a museum-durable vascular entoptoscope can be built for $12,000 $16,000, or less if several units are constructed at one time.
Acousto-optical spectrometers
Authors: Pustovoit I.
Published: February 2023

We demonstrate an acousto-optical spectrometer useful for remote spectral analysis. The spectrometer is built around a quartz single crystal. The spectral region of 420–780 nm is scanned in 2 s with a resulution of 0.1–0.25 nm.
Photon tunneling microscope applications
Authors: Guerra John M.
Published: February 2023

Photons incident on a total-internal-reflection surface that is also the object plane of a reflected light microscope will tunnel through a submicron gap in the presence of a dielectric sample. Tunneling increases exponentially with sample height for a homogenous refractive index and is quantified by empirical calibration to a known geometry. Video photometry of the gray-scale tunneling image is converted by a three-axis oscilloscope to real-time three-dimensional topography featuring variable perspective. Vertical resolution is detector-limited to less than 1 nm over a field depth of about 0.75; lateral resolution is enhanced to about 0.3 on extended objects. Photon tunneling images of diamond turned surfaces, optical data structures, polished optical surfaces, and lithographic structures are among those presented. Comparison and correlation with other methods for measuring surface topography in this regime are discussed.
Four decades of optical information processing
Authors: Goodman Joseph W.
Published: February 2023

While the origins of optical information processing can be traced to the work of Ernst Abbe, active research in this area began in the 1950s. During this first decade, the idea of coherent Fourier plane filtering flowered, and applications to image processing were the initial focus of attention. Filtering power was limited by the difficulty of controlling both amplitude and phase transmission through the focal plane. The 1960s saw the introduction of the laser and the discovery of the interferometrically generated filter, which allowed implementation of much more sophisticated filters than had previously been possible. Emphasis increased on the processing of radar signals, and matched filter pattern recognition. The 1970s saw a major shift of attention to discrete data, and development of a multitude of techniques for processing vectors and matrices. The shortcomings of analog (as opposed to digital) processing became especially clear in this decade. In the 1980s attention turned to three subjects, often all included in the term optical computing: all optical digital processing, optical interconnects, and optical neural computing. These three areas remain active at the beginning of the 1990s. Some speculation will be provided regarding what emphasis we will see throughout the 1990s.
High-resolution spectroscopy of chemical intermediates
Authors: Sears Trevor J.
Published: February 2023

Infrared and UV-vis laser spectroscopy has been used to study a number of small free radicals, with an emphasis on those exhibiting large amplitude vibrational motion or a strong breakdown of the Born-Oppenheimer approximation. Recently, we have detected gas phase infrared absorption spectra of the ethyl (C2H5) and carboxyl (HOCO) radicals for the first time. The C2H5 spectrum shows clear evidence for nearly free rotation of the CH2 radical center relative to the CH3 end of the molecule, while the HOCO spectrum appears to be somewhat simpler although analysis is far from complete. Following spectroscopic characterization of these species, infrared absorption will offer a precise and sensitive method for monitoring the radical concentrations on a state-by-state basis. Other species including NCS, C3, and NCO have been studied by laser induced fluorescence in a supersonic free jet expansion. Highly excited vibrational states have been accessed by optical double resonance (stimulated emission pumping) and the experimental data demand extensions of the standard effective Hamiltonian treatments describing the energy levels of these and similar species as increasing amounts of energy are placed in the internal modes.
Future of neutral atom clocks
Authors: Chu Steven , Kasevich Mark , Moler Kathryn , Riis Erling , Weiss David
Published: February 2023

The measurement on energy levels suitable for time standards can be made more precise with the use of neutral atom manipulation techniques. We have created a fountain of atoms where the measurement time of atoms in ballistic free-fall can be on the order of a fraction of a second. With these long measurement times, the linewidth of the transition between the ground state hyperfine transition of sodium was measured to be 2 Hz. Transitions in the optical frequency range could then have Q's of 1014 to 1015. In addition to creating high Q resonances, it is desirable to have a continuous, high density fountain of atoms. Towards this end, we have constructed an "atomic funnel" capable of creating a cold, high flux stream of atoms suitable for injection into a continuous atomic fountain. In the first embodiment of the funnel, the brightness of the atomic beam (defined as the number of atoms per second/( ΔvxΔvyΔvz) was increased by a factor of 3 × 103. With more recent improvements for both funneling more atoms from the hot atomic beam and better use of polarization gradient cooling to launch the atoms, we will describe how to improve the brightness enhancement by another four orders of magnitude.
Laser interferometric system for displacement measurement with high precision
Authors: Hosoe Shigeru
Published: February 2023

The practical method for measuring displacement with laser interferometric system is proposed. The newly developed interferometric system has a simplified optical layout, less number of optical elements and a shortened optical pass. As a result, the thermal drift and nonlinearity in the displacement measurement has been decreased in both the bifrequency heterodyne system and the fringe-counting system. In addition, simplified electrical fringe counting logic has made it faster to get the measured value without losing any accuracy in fringe counting, and to calculate the corrected light wavelength by the subtraction of the two fringes' value. This two-fringe value system can make the displacement measurement more precise than single value system, and would be more useful for the position feedback system. This method achieved (1) subnanometer resolution [less than (λ)/1000], (2) 5 MHz throughput of the differential light wavelength compensated value and (3) nonlinearity within (λ)/1000.
Nonlinear-optical properties of InAs/GaAs superlattice quantum wells
Authors: Kost Alan , Jupina Mike , Garmire Elsa , Haseriberg Tom
Published: February 2023

We have examined the nonlinear optical properties of InAs/GaAs superlattice quantum wells imbedded in a n-i-p-i structure. Measurements were made over a wide range of optical intensities from ≈100 mW/cm2 to 1 kW/cm2. At the smallest intensities, optical nonlinearities are observed which arise from the screening of the built-in electric fields and the corresponding electroabsorption. These nonlinearities are very energy efficient; however, they saturate at a relatively small value (ΔT/T = 10%). At higher intensities, changes ΔT/T ≈ 50% or larger are observed due to state filling and saturation of the excitonic absorption resonance. These effects are similar to those observed in conventional quantum wells; however, the presence of the doped material causes a separation of electron and hole populations which lengthens photocarrier lifetimes and reduces the required intensities. All material in the structures, grown by molecular beam epitaxy, is GaAs (transparent at the wavelengths of interest) except for the quantum wells which are short period InAs/GaAs superlattices. The use of alternating InAs and GaAs layers to produce a region of smaller band-gap avoids the problem of alloy disordering arising from the use of an InGaAs ternary.
Sublaser linewidth absorption spectroscopy
Authors: Whittaker Edward A. , Pattnaik R. K. , Supplee James M. , Sun H. C.
Published: February 2023

When using a laser source for frequency domain spectroscopy, source linewidth invariably limits spectral resolution. Consequently, high-resolution spectroscopy generally requires frequency stabilized single frequency lasers. In extending the theory of frequency modulation absorption spectroscopy to the case of multimode dye lasers1 we noticed that when both laser mode spacing and modulation frequency are small compared to absorber linewidth, the absorption spectrum measured by such a source could show features which were much sharper than the effective laser linewidth. We have now been able to demonstrate this effect using a coherent 599-01 dye laser tuned only with a biréfringent plate. We will present some experimental results on atomic sodium and molecular iodine absorption. We will also discuss the high-resolution tuning and frequency stability properties of the dye laser itself.
Large-aperture phase plates for beam smoothing on Nova
Authors: Dixit Sham , Woods Bruce , Thomas Ian , Henesian Mark , Powell Howard
Published: February 2023

We have designed, fabricated and tested 80-cm aperture phase plates for smoothing the Nova laser beams in the target plane. The phase plate consists of a spatially random two-dimensional pattern of bilevel hexagonal elements which introduce a relative phase delay of 0 or π. Fabrication of phase plates involves deposition of a SiO2 pattern on a SiO2 substrate through a sequence of chemical and optical steps. We have measured and analyzed the near field diffraction pattern produced by such phase plates and chosen to place them behind the target focus lens to minimize the threat of optical damage. The phase plate properties are characterized by measuring the far-field and the quasi far-field diffraction pattern in terms of the spatial Fourier distribution of intensities. We will discuss theoretical modeling of the performance of these phase plates including a description of the phase aberrations of the Nova beam. We will compare the predicted performance of the phase plates with the measured characteristics.
GaAs-based real-time optical correlator
Authors: Liu Duncan T. H. , Cheng Li-Jen , Gregory Don A.
Published: February 2023

Compared with the conventional oxide photorefractive crystals, the main advantages of GaAs are the short response time (submilliseconds at about 1 W/cm2 write beam intensity), the availability of large-size crystals, and the compatibility with other semiconductor devices such as near-infrared semiconductor lasers. In this paper, a realtime VanderLugt optical correlator is demonstrated using a GaAs crystal as the dynamic volume holographic medium and liquid crystal spatial light modulators (SLMs) as the input devices. The advantage of using the VanderLugt configuration here is that the reference image (located in the read beam) can be changed at a rate not limited by the response time of the dynamic holographic medium; thus the size and orientation invariance can be, in principle, achieved in real time by displaying many different orientations and sizes of a reference image in a very short period. The potential system frame rate, limited by the response time of the photorefractive GaAs crystal, can be a few kHz or higher, depending on the total write beam intensity. The limitations of this optical correlator will be discussed.
High-efficiency joint-transform correlator for symbolic substitution
Authors: Yu Francis T. S. , Tanone Aris , Tam Eddy C. , Gregory Don A.
Published: February 2023

Symbolic substitution is essentially a combination of recognition and substitution process. In addition, although symbolic substitution is not restricted to space invariant operations, it is indeed based on the space invariant connectivity of optics. Therefore, holographic associative memory techniques may be alternatively applied to implement a symbolic substitution logic system. In this paper, we propose to use a high efficiency joint transform correlator to implement a symbolic subsitiution operation. Consider that the search pattern fs(x,y) and an input image which consists of M × N input patterns fm,n(x−xm,y−yn), are to be displayed on an input plane in a joint transform correlator architecture. The power spectrum recorded by a CCD camera is then replicated, using dedicated video hardware, to produce a L × L spectra array on a second spatial light modulator. Upon illuminated by a quasi-monochromatic, partially coherent parallel light source, the object irradiance at the correlation plane will be increased by L2 times that of a conventional joint transform correlator.1 Therefore, if fmn matches with the search pattern fs, a bright correlation peak would be located at a specific location determined by (xm,yn). To eliminate all of the unwanted light, a set of sampling pinholes is placed at the correlation plane. As a result, a spatial distribution of point light sources is generated. The substitution process can be accomplished by inserting a multiplexed Fourier transform hologram of the substituting pattern behind the collimating system after the sampling plane. Each of the point light source produces a plane wave front to read out the hologram. Consequently, each correlated input pattern produces a substituted output pattern at the corresponding position on the output plane, and symbolic substitution operation is completed.
Photorefractive time correlation filter
Authors: Ryan V. , Khoury J. , Woods C. L. , Cronin-Golomb M.
Published: February 2023

Previous photorefractive holographic filters are limited by the response time of the material. The time correlation filter recognizes a sequence of individual temporal features which are limited only by the speed of the phase modulators in the reference and input signals. This filter operates using photorefractive time-integrated degenerate four- wave mixing with a reference beam and signal beam incident on one side of the bismuth silicon oxide crystal and an incoherent counter-propagating beam to read out the time correlation hologram. When no modulation is applied to the signal, the phase variations in the reference beam erase the hologram. Experimentally measured signals include square wave phase and amplitude modulation for the detection of similar signals, constant longitudinal velocity signals, and periodic transverse signals. The time correlation filter performs a special form of correlation of temporal signals which is not linearly proportional to the modulation. For example, the velocity detection output is strongest at the reference encoded velocity but also gives smaller peak responses at odd integral multiples of the velocity. The device operation agrees with theory developed from time varying photorefractive theory for general signals.
A New Kind of Laser Schlieren Instrument
Authors: Tianxin Shan
Published: February 2023

This paper describes the principle and structure of a new kind of laser schlieren instrument. In this instrument, the Ronchi grating bearing 180°-shifting phase is used as a Hilbert transform filter.
Greater statistical efficiency for viewpoint-invariant differences in the categorization of curves
Authors: Mansfield J. S. , Biederman I. , Legge G. E. , Knill D. C.
Published: February 2023

Some models of 3-D object recognition rely on the detection and classification of viewpoint-invariant geometrical features of 2-D images. In an investigation of the extent to which the human visual system is differentially tuned to viewpoint-invariant features vs viewpoint-dependent features, we measured the statistical efficiency with which observers can classify curved contours (circular arcs of fixed arc length) into pretrained categories. Optimal tuning to viewpoint-invariant features predicts that higher efficiencies would be obtained when one experimental category included contours with zero or near-zero curvature (i.e., near-straight lines). Prior to experimental trials, observers were shown sample curves from two curvature categories, defined as sets of curves with Gaussian distributions of different mean curvature. In a forced choice procedure, they then indicated the category of origin for curves randomly selected from the two categories. Efficiency was taken as the ratio of the observer’s d-prime and the d-prime of an ideal detector. Typically, efficiencies were greatest (as large as 80%) when one of the category means was close to zero curvature. When the categories were moved from zero curvature, efficiencies decreased by as much as 30%. These results are consistent with the exploitation of viewpoint-invariance differences in object recognition.
Near-field microwave imaging of buried objects: a sampling array and Fourier transform processing
Authors: Davis W. , Roussos J. , Tricoles G.
Published: February 2023

Microwave images of buried dielectric objects were formed from near-field complex-valued reflectance measured with a sampling array. The array contained seven printed circuit antennas in a staggered pattern of two parallel lines. Each antenna was individually and sequentially connected to a transmitter and receiver. The array was translated laterally to scan an area and the data arranged into lines of seven samples. Because the measurements were in the near-field, the spatial distribution of reflectance resembles the object’s shape. The measured data were processed to improve the resemblance. Individual rows of reflectance samples were first Fourier transformed. The transform was propagated to the air–soil interface and then inverted. The final step was forming the difference between inversion results for separate rows and there normalizing to the values for a preceding row. The processed reflectance more nearly resembled the object shape than did the measured data. In addition, subsidiary maxima, or clutter, were somewhat reduced.
Analysis of a white light joint transform correlator and its use in color object detection
Authors: Cheng Yih-Shyang
Published: February 2023

A joint transform correlator in which the size of the joint transform spectrum (JTS) can be varied by using diverging or converging white light illumination is analyzed. A JTS is formed where the illuminating source is focused. Certain coherence requirements should be met for the joint transform hologram (JTH) to be formed successfully. The separation between the input object to be identified and the reference object should be smaller than the lateral coherence length of the source in the corresponding direction. The number of overlapped spatial frequency spectra (SFS) is proportional to the SFS bandwidth of the object in the direction perpendicular to the grating line over the SF of the grating. Hence, both input and reference objects should lie on a line parallel to the grating line for successful recording of the JTH. In the readout process, white light is dispersed by a pair of gratings to shine on the recorded JTH. Light of different wavelength transmitted by the JTH is Fourier transformed by a lens and is superimposed to give the correlation output. In the experiment with a black-and-white reference object, from the color of the correlation spot we can identify that the object with a specific color is present. For detection of an object with a primary color, the reference object with a primary color can be used. If the reference object is replaced by one harmonic component of the object, the proposed system can readily be extended to rotation-invariant and scale-invariant pattern recognition.
Concept of a fundamental tracking frequency for atmospheric turbulence tilt correction
Authors: Tyler Glenn A.
Published: February 2023

In this work the bandwidth requirements for compensation of the turbulence-induced tilt associated with a ground based telescope used for astronomical observation are studied in detail, Kilmogorov statistics and the Taylor hypothesis are used to develop expressions for the G-tilt and Z-tilt power spectra in the presence of wind. A closed loop servo with a transfer function of the form, H(f) = (1 + if/f3dB)-1 is applied to these disturbances and the one axis one sigma jitter σθ is computed. The analysis illustrates that σθ = (λ/D)(f
T
/f3dB), where λ is the wavelength, D is the telescope diameter, f3dB is the servo bandwidth, and fT is the fundamental tracking frequency for atmospheric turbulence-induced tilt compensation. As a consequence, a servo bandwidth equal to fT results in a one-axis one-sigma jitter equal to the diffraction angle, λ/D. For G-tilt the fundamental tracking frequency is given by

f
T

=
0.331
 

D

−
1
/
6

λ

−
1

(
sec
ψ
)

1
/
2

[

∫
d

h

C
n
2

(
h
)

V
2

(
h
)

]

1
/
2

where ψ is the zenith angle,

C
n
2

(
⋅
)

is the refractive-index structure constant, and V(·) is the wind velocity profile.
Projection lithography using soft x-rays
Authors: Bjorkholm J. E. , D'Souza R. , Eichner L. , Freeman R. R. , Jewell T. E. , MacDowell A. A. , Mansfield W. M. , Pastalan J. , Szeto L. H. , Taylor D. , Tennant D. M. , Waskiewicz W. K. , White D. L. , Windt D. L. , Wood O. R.
Published: February 2023

Our group has undertaken a program to determine the feasibility of using soft x-rays (wavelengths of ~13 nm) to carry out advanced projection lithography. Such a technology is attractive since it would offer the ability to use reduction imaging to achieve a resolution of >0.1 µm while maintaining a depth of focus >1 µm. We have already demonstrated the printing over a small area of features as small as 50 nm using a 20×-reduction optical system composed of mirrors and synchrotron radiation from an undulator at the National Synchrotron Light Source. A review of our most recent results is presented.
Optical computing module using SLR and LCLV
Authors: McAulay Alastair D. , Wang Junquing , Xu Xin
Published: February 2023

Optical computers may be constructed from memory, logic, and interconnection modules.1 A module is proposed in which the output of a spatial light rebroadcaster (SLR) is fed into an image intensifier, followed by an optically addressable liquid crystal light valve (LCLV). Experimental characterization of the devices is provided. The cascadability of the module is demonstrated experimentally. First, an 8 × 8 array is stored onto the SLR by writing the argon light. Later, infrared is used to read out the stored pattern from the SLR. The orange output is amplified by the image intensifier and the output of the image intensifier illuminates the photoconductive side of the LCLV. Argon light reads the information off the other side of the LCLV. The output pattern is seen to be identical to the original input. Storage is provided by the SLR and gain by the image intensifier and the LCLV. The LCLV also converts the frequency back to the original frequency for cascadability. The module is acting as a memory module or buffer that permits synchronization for a computing system. Further, two inputs may be merged using a polarizing beam splitter to provide a parallel logic module for performing OR or NOR. The module may be combined with an interconnection module to permit the construction of a digital optical computer.
Optical matrix-vector multiplication using pinhole holograms
Authors: Song Q. W. , Lee M. C. , Talbot P. J.
Published: February 2023

Matrix-vector multiplication is a fundamental operation in linear algebra. A large variety of optical processors for performing matrix multiplication have been proposed and some experimentally demonstrated. We propose the use of a pinhole hologram to perform matrix-vector multiplication. The method is capable of storing and addressing a large number of holograms inside a photorefractive crystal, and achieving better resolution while performing the multiplication. On recording, each matrix (M × N) pattern passes through a pinhole and then stores its hologram in a photorefractive crystal. A phase-conjugate beam is used to read the holograms. Among them the desired one, say matrix A, is selected by opening a corresponding pinhole. An NX1 vector B is illuminated by the selected matrix pattern. The overlap of matrix A and B produces a matrix C. Using a cylindrical lens we can convert the M × N matrix C into a MX1 vector c which is the product of A and B. A 2-D detector array is used to detect the intensity distribution of vector c and to decide its value according to some preset threshold value. This method can be programmed to implement a parallel real-time matrix-vector processor.
Induced coherence without induced emission resulting in nonlocal interference
Authors: Zou X. Y. , Wang L. J. , Mandel L.
Published: February 2023

Although induced coherence is generally associated with induced or stimulated emission, in the process of spontaneous parametric downconversion of light induced coherence can be generated on its own. This is demonstrated in an experiment in which the signal beams from two coherently pumped downconverters come together and interfere whenever the idler beam from the first downconverter (DC1) passes through and is aligned with the second downconverter (DC2). Misaligning the idlers, or blocking the first idler, makes the interference disappear. More generally, the degree of coherence can be controlled by varying the amplitude of the idler from DC1 that reaches DC2. The interpretation of this nonclassical interference is discussed in terms of the interpretation of the wave function or state vector of the emitted photon.