bluuebunny/crossref_metadata_2025 huggingface dataset:
Scattered internal and external near fields for spherical and cylinderlike particles
Authors: Alexander Dennis R. , Barton John P. , Schaub Scott A. , Emanuel Mark E.
Published: February 2023

Calculations of the electric-field strengths internal and external to the particle but in the near field have been performed for both spheres and infinite cylinders. Results are presented that illustrate the magnitude of the internal and external electric-field strengths in terms of a source function defined as S = |E/Einc|2. The location of the maximum source function external to the particle depends on the optical size parameter. For example, a spherical aerosol particle of water with an optical size of 12–13 and an index of refraction of 1.18 + 0.07 i has a maximum source function of ~3.25 occurring at a normalized radial position of ~1.8 radii on the shadow side.
Wave interactions with bandlimited fractal structures
Authors: Jaggard Dwight L.
Published: February 2023

The fractal concept, introduced by Mandelbrot in the 1970s, complements Euclidean geometry when used to describe irregular or variegated structures such as those that occur in nature. This concept embodies ideas of both dilation symmetry and nondifferentiability and introduces a fractional dimension. The fractional dimension, denoted the fractual dimension D, exceeds the topological dimension d of the structure of interest. For example, the fractal dimension can vary from one for smooth curves to two for irregular area-filling curves while the topological dimension remains unity. These concepts can be generalized to higher dimensional structures where the topological dimension is two or three. Here we introduce the fractal concept and review its application to several wave problems in scattering, diffraction, and reflection. Of particular interest are the following topics that have been under investigation at the University of Pennsylvania: scattering from fractal fibers; diffraction by fractal screens and surfaces; applications to turbulence; reflection from fractal structures. The models for these applications are based on the concept of bandlimited fractal structures which provide the appropriate tool for analysis.
Broadband atomic response to incommensurate modulation frequencies
Authors: Koch Karl , Chakmakjian Stephen H. , Stroud C. R. , Hillman Lloyd W.
Published: February 2023

Pomeau et al.1 have shown that a two-level atom strongly driven by a single 100% amplitude modulated (AM) field can show broadband spectral behavior when the two-frequency components of the field are incommensurate. The equations describing the atomic response to a monochromatic field without making the rotating wave approximation (RWA) are identical to form to the equations describing a 100% amplitude modulated field within the RWA. This fact makes the observation of the predictions of Pomeau et al. experimentally realizable in an optical resonance experiment. We present theoretical and numerical results of a pair of 100% AM fields interacting with a single atomic resonance. The effects on the atomic response of commensurate and incommensurate modulation frequencies are discussed as are the experimental implications.
Fiber-optic refractometer and its applications to erosive fluids
Authors: Miyazaki Takao , Yamada Yoshiro , Komine Isamu
Published: February 2023

We have made a quantitative sensitivity analysis of a fiber-optic refractometer based on a geometrical model. The analysis shows that, by bending the sensor probe in a figure eight and varying its curvature, the sensitivity range can be adjusted arbitrarily and that high sensitivity in the lower refractive-index ranges is possible. The curvature giving optimum sensitivity at refractive index n is determined as a function of n/n0, where n0 is the probe refractive index. These results have been experimentally confirmed, and resolution of the order of 10−5 near the refractive index of water has been achieved.
Fiber-optic illuminators for microscopy
Authors: Goodman Douglas S.
Published: February 2023

Many illumination problems in microscopy can be solved easily and economically with fiber optics. Modes of illumination not provided by the designers of a microscope can be added. A modular illumination system involving a number of microscopes and sources can be assembled. Various types of illumination can be used simultaneously, e.g., bright field and dark field, transmitted and reflected. Realignment on changing lamps is simplified, since it is merely necessary to align the lamp relative to the fiber input end. Older microscopes with small lamps can be upgraded by using a fiber bundle terminated at the lamp filament location.
Polarization sensitivity of noise gratings recorded in bleached silver halide emulsions
Authors: Kostuk Raymond K. , Sincerbox Glenn T.
Published: February 2023

The formation of holographic gratings in silver halide emulsions is limited by scattering from photosensitive grains embedded in these materials. The most serious consequence of this effect is a sharp reduction in the diffraction efficiency of a volume hologram reconstructed at the formation angle. This has been attributed to a noise grating resulting from the interference between a beam illuminating the emulsion and scattered light. Although the presence of these gratings has previously been reported, their dependence on the relative orientation of the polarization of the construction and reconstruction beams has not been discussed. In this presentation it is shown that the correct relative polarization orientation reduces the detrimental effects of these gratings and improves the efficiency of holograms formed in silver halide emulsions.
Effects of a sinusoidal modulation on a period doubling and chaotic acoustooptic device
Authors: Marriott C. , Delisle Claude
Published: February 2023

Our system has been thoroughly studied and demonstrated to show the Feigenbaum period doubling route to chaos. We have extended the study of such a system by applying an external signal and observing both experimentally and numerically its effects. One such effect is modal amplification, amplification of the external signal at frequencies which coincide with the natural frequencies or modes of the system. This is observed before both Hopf and period doubling bifurcations. Normally the first mode determines the system’s behavior, but when a signal at the same frequency as a higher-order mode is applied the system can be made to oscillate in this mode. Once the system has bifurcated to a periodic solution it is possible, if the amplitude and frequencies are properly chosen, to fix the frequency of oscillation of the system at that of the external signal. Even when the system shows chaotic behavior, certain modulation frequencies force the system to become periodic. The frequency spectrum both before and after a bifurcation exhibits an effect, called candelabra, or the presence of many additional lines in the spectrum.
Infrared laser amplification and switching using nematic liquid crystal films
Authors: Khoo I.C. , Finn G.M. , Michael R.F. , Liu T.H. , Yan P.P.
Published: February 2023

Nematic liquid crystals possess sizable birefringence, good transparencies, and a large thermal refractive-index gradient in the visible as well as the IR regime. Using the orientational (Kerrlike) and thermal (diffusive-type) nonlinearities we show that large probe beam amplifications can be achieved with moderate power pump beam (with an intensity of the order of just a few W/cm2). The dependence of the experimentally observed beam amplification process on beam ratio, grating constant, intensity, film thickness, temperature, and frequency shift between the pump and probe has been measured and shown to be in good agreement with our detailed theories.
Two-step pseudocolor encoding
Authors: Yu Francis T.S. , Mayers A.W. , Chen X.X.
Published: February 2023

This paper describes a method of achieving the positive, negative, and cross-product encodings in a two-step process. The cross-product information is carried on the moire pattern produced by crossed gratings in the encoding process. The removal of the final encoding step simplifies the original procedure and increases the available dynamic range of the film for the two encodings. Experimental results are presented.
Eye size and neuron counts in mouse and rat
Authors: Hallett Peter E.
Published: February 2023

Photoreceptors and neurons at various levels to cortex have been counted in mouse and rat. The ratios of neuron numbers (rat/mouse) are similar to the ratio of retinal areas or the squared ratio of eye sizes; so to a first approximation the two species have linearly scaled eyes, equal photoreceptor spacings (in microns), and visual pathways scaled numerically by the number of photoreceptors. The following points can be made: Photoreceptors outnumber neurons at all subsequent levels. Cone sampling exceeds the behavioral acuity limit, the latter being largely determined at or before the ganglion cell level. There are few neurons in the LGNd, and so the corresponding eye parameter p is very large, as is appropriate for nocturnal animals. The general organization is economical, if not sparse, so undersampling occurs even for central vision when the pupils are small. However, aliasing should be slight if the retinal and geniculate sampling lattices are irregular with only a local spatial order. Cortical encoding appears less explicit than in the primate. This seems compatible with either the photon limitations of nocturnal vision or the advantages of a fast though limited range of behavior.
Two-layer model of visual undersampling
Authors: Smith Robert A.
Published: February 2023

In treating the common case of 2-D lattice sampling of a nominally 1-D grating, the Nyquist limit is commonly defined in terms of the average nearest neighbor spacing. In fact, the redundancy in this stimulus permits the grating to be recovered at frequencies many times the nominal Nyquist limit. To show this, a two-layer visual system is simulated with the following properties: (1) The first layer is a physiologically plausible array of receptors, which undersamples the stimulus. (2) The second layer consists of line detectors, which oversample the output of the first layer. Such a model shows a number of interesting behaviors including (1) the resolution of super-Nyquist gratings and (2) a reversal in apparent orientation at twice the nominal Nyquist frequency.
Phase-stable laser sources for sub-Hz-linewidth optical spectroscopy
Authors: Hall John L. , Hills Dieter
Published: February 2023

We are investigating the problems of transferring the intrinsic stability of a thoughtfully designed and isolated stable interferometer into an equivalent frequency stability of the laser locked onto its fringe.1 Tests with two lasers independently locked to adjacent orders show locking precision below 50 mHz (1 × 10−16) for times ~1–100 s, degrading toward shorter times as t−1/2 due to finite SNR. Toward longer times performance degrades as t+1 due to changes in the systematic offsets. Locking to various orders gives an estimate that the locking accuracy is <2 Hz (4 × 10−15). Reductions in lock inaccuracy seem possible using harmonic detection.2 Experiments to measure drift of the Zerodur etalon use a 129l2-stabilized laser as a reference. Unfortunately its SNR is too low to give useful information quickly: the cavity’s 2-Hz/s drift just matches the reference laser’s random noise at ~40 s. Comparison over longer times (~days), taken with precise thermal controls operating, is consistent with the idea that the observed 75-Hz noise amplitudes near 24 h are due to remaining thermal problems, and the 1 % uncertainty in the observed drift rate is due to limitations of the iodine-stabilized reference laser. We are considering use of the powerful method of modulation transfer spectroscopy with an external l2 cell,3 in combination with five massive copper shielding walls with independent stabilization and hence thermal gradient control.
Absolute cross sections for two-photon absorption to and ionization from the EF(v = 6) level of H2
Authors: Buck Jesse D. , Bamford Douglas J. , Bischel William K.
Published: February 2023

We have measured the two-photon cross section for the Q1 branch of the EF(v = 6) ← X(v = 0) transition in H2, using techniques developed in this laboratory. The population of the excited EF level was determined using a calibrated fluorescence detection system to monitor B(v = 1) ← EF(v = 6) fluorescence at ~750 nm. Ions resulting from absorption of a third photon were collected on parallel wire electrodes and amplified with a calibrated charge-sensitive preamplifier. The temporal and spatial profiles of the laser pulses were characterized with a fast photodiode and a linear pyroelectric array, respectively. The 100-μm resolution of the pyroelectric array and the necessity to avoid saturation both dictate the use of a gently focused beam geometry. Our results (4.3×10−47 cm4/s−1 and 1.6 × 10−18 cm2) are in reasonable agreement with previous experiments and calculations, and we are currently working to upgrade our accuracy.
Dynamics of UV-induced vibrational fluorescence of the CN− Ion in alkali-halide crystals
Authors: McWhirter J. T. , Ambrose W. P. , Sievers A. J.
Published: February 2023

We have used UV laser excitation at 308 nm to excite nonresonantly the vibrational stretch mode of the CN− ion doped in alkali-halide crystals. The energy transfer stems from the creation of color centers near the molecular defect. IR fluorescence from the deexcitation of the CN− vibrational manifold in the ground electronic state is observed to yield a series of anharmonically shifted (~25-cm−1) transitions near 2000 cm−1. The strength of the IR emission grows with increasing host lattice constant and a shift in population to higher vibrational states v with decreasing sample temperature is observed. For Rbl + 0.5% KCN, this shift results in population inversion between the v = 2 and v = 1 levels at liquid He temperature. Inversion is quickly obtained (<1 μs) after the 40-ns UV laser pulse. At longer times a redistribution of the vibrational population is driven by V-V transfer between the molecular defects. (12 min)
Coherent optical interactions in semiconductor lasers
Authors: Hillman Lloyd W.
Published: February 2023

Fluctuations in the amplitude, frequency, and polarization of the light emitted by semiconductor laser diodes limit the performance and reliability of many optical systems, such as communication links, optical radar, and optical disk read/write memories. Although it seems logical to explain this noise as random or stochastic, such erratic behavior can arise from completely deterministic models of lasers that retain the dynamics of the coherently induced polarization. This has been shown to be true for the simple model of a laser based on a two-level resonance, which results in the coupled Maxwell-Bloch equations. These equations are termed coherent because the Bloch equations describe, not only the inversion, but also the induced polarization of the gain media. The Maxwell-Bloch equations predict an array of instabilities and novel operation conditions for a laser: the output light may oscillate regularly or pulsate chaotically. Such behavior is not found in the incoherent population rate equation that adiabatically eliminated the dynamics of the polarization. However, the Maxwell-Bloch equations apply only to dilute materials in which the total gain is described by an ensemble average over many noninteracting molecules, such as gas lasers.
New method of optical short-pulse generation using diode lasers
Authors: Lee Chang-Hee , Cho Kun-Ho , Shin Sang-Yung , Lee Soo-Young
Published: February 2023

Various methods of optical short-pulse generation using diode lasers have been reported. Recently, optical short pulses were generated by applying the electrooptic feedback to self-pulsed solitary1 or external cavity diode lasers.2 We propose a new method of optical short-pulse generation based on negative electrooptic feedback.
Use of the matrix beam propagation method in Y-junction and star-coupled laser arrays
Authors: Yevick David , Hermansson Bjorn
Published: February 2023

In the first part of our presentation, we summarize our previous BPM results in integrated and fiber optics, semiconductor laser, coherence and coherent backscattering theory, and band-structure analysis. We also review our reformulation of the BPM in terms of matrix operators.1 The second half considers the application of the matrix algorithm to strongly coupled laser arrays. In particular, we calculate the band-structure associated with the higher-order supermodes of periodic Y-junction laser arrays of infinite transverse extent. Our results here demonstrate that while many salient properties of Y-junction array modes can be reproduced by simple models incorporating adiabatic lossless Y-junctions, the supermode gain displays several previously unreported features associated with the finite Y-junction lengths.
Oscillation mode control of gain-guided phased-array lasers
Authors: Gacusan L. Z. , Wang S. C.
Published: February 2023

The oscillation mode properties of multiple-stripe gain-guided phased-array GaAIAs laser diodes are investigated in cw operating conditions. Both longitudinal mode spectra and spatial mode patterns of the laser arrays are measured. With a simple collecting optics arrangement and in certain operating temperature and driving current conditions, a single longitudinal mode (with intensity ratios as high as 70 to 1) and a single spatial lobe are observed from an otherwise multimode multi-lobe laser output. The effects of operating temperature and current on the single-mode oscillation are investigated, and the results show that the single mode can be controlled over certain current and temperature ranges. The use of various collecting optics such as microscope objectives and GRIN lenses is explored, and in each case a different mode intensity ratio is obtained. It is also found that insertion of a quarterwave plate between the laser array and collecting lens spoils the single-mode oscillation, which suggests that optical feedback due to the collecting lens may be responsible for the single-mode occurrence. The details of these mode control investigations are described.
Grating coupling of radiation from thin-film DFB lasers
Authors: Emmons R. M. , Hall Dennis G.
Published: February 2023

We have theoretically studied the threshold-longitudinal-mode structure and the grating coupled radiation of thin-film or semiconductor-type distributed feedback (DFB) lasers. Grating form, including tooth shape and chirp, influences mode confinement and coupling. Using coupled-mode theory and Green’s function methods, we have estimated far-field radiation patterns, mode spectrum, and longitudinal field distributions of DFB lasers with several grating forms. Results indicate that the choice of grating form can strongly influence radiation coupling and mode discrimination. We discuss the far-field radiation patterns of these structures and the effects of this coupling on laser mode spectrum.
New rainbow holograms with extended viewing angles
Authors: Hansen Matthew E.
Published: February 2023

Two new rainbow holograms avoid the viewing limitations of the slit-master rainbow hologram. These two holograms are the rainbow-master rainbow hologram and the ring-master rainbow hologram. Both holograms are displayed horizontally while white light is reilluminated from directly above or below. The rainbow viewing window of the ring-master rainbow hologram is a truncated cone shape which hovers above and around the horizontal holographic image. This allows for 360° viewing. The rainbow-master rainbow hologram exhibits the same properties with a 180° viewing window. As with the slit-master rainbow hologram, the ring and rainbow shaped H1 master holograms are simple laser transmission holograms. Holographic table geometries for two-step ring and rainbow-master rainbow holograms have been developed for translucent and opaque objects. Multiple-exposure pseudocolor encoding may be used to enhance display work or to color tag concentric zones of information in optical processing systems.
Holographic stereograms generated with a liquid crystal spatial light modulator
Authors: Andrews John R. , Damm Reinhart
Published: February 2023

Holographic stereograms are an important way of generating 3-D images from photographs of real objects or of computer-generated 3-D objects. We have made holographic stereograms using several commercially available liquid crystal spatial light modulators, which have been modified in some cases, to produce transmission masks for a video image. The spatial light modulator is used to form the image in producing holographic stereograms. The image source has been either a video camera (or VCR) or the video signal from a computer graphics terminal. The use of the liquid crystal spatial light modulator eliminates the photographic intermediary in the process of forming the holographic stereograms. This technique speeds the process of forming holographic stereograms, increases automation of the process, and reduces the time required in creating the hologram. The major limitations of this technique are the contrast and resolution of the spatial light modulators which are commercially available and of low cost. Since the spatial light modulator technology is rapidly advancing, better performance in both of these areas is expected. Therefore, we believe that this technique offers an attractive means of producing holographic stereograms, especially from computer graphics, in situations where a short cycle time is important.
Interconnect density limitations for free space optical interconnects of electronic circuits
Authors: Feldman Michael R. , Guest Clark C.
Published: February 2023

The number of interconnects that can be provided by any optical interconnect system is fundamentally limited by Gabor’s theorem1 to AΩ/λ2, where A is the hologram area, Ω is the solid angle of the beam spread, and λ is the optical wavelength. Even with thin computer-generated holograms (CGHs), this fundamental limit can be achieved for space-invariant interconnects or for a single-source broadcasting information to many detectors (e.g., clock distribution). Several CGHs have been fabricated by electron-beam lithography for these types of application. For example, one experimental CGH is designed to connect 64 sources to 256 detectors in a space-invariant pattern.
Using a double-wavelength shift scheme in the construction of low f/No. HOE objectives
Authors: Chen Hsuan , Shan Qi-Zhe
Published: February 2023

The method of using wavelength shift in the construction of IR holographic lenses1 has brought forth a new dimension of flexibility in approaching the difficult problem of making low f/No. diffraction-limited HOEs. This paper considers a particular aspect of the technique, namely, the double-wavelength shift scheme in the construction of low f/No. HOE objectives or collimators. A phase corrector plate H
c
is first fabricated holographically at the shorter wavelength (for example, argon blue line). The required HOE objective lens is then constructed by interference of a plane wave and a spherical wave at the intermediate wavelength (for example, argon green line). The corrector plate is placed in the plane wave beam during the recording, so that the beam emerging from H
c
has the necessary phase correction function for the HOE H. The HOE is then used in the designated longer wavelength (for example, He-Ne red line). By carefully adjusting the scale factor v between the recording configuration of H
c
and that of H, we succeed in reducing the bulk of the residual phase errors in the single-wavelength shift scheme where the third-order Seidel aberrations have been eliminated.
Origin of anisotropic second harmonic generation in elemental metals
Authors: Janz S. , Van Driel H. M. , Sipe John E.
Published: February 2023

The optical second harmonic (SH) response of single-crystal cubic metals, Al and Cu, have been investigated theoretically and experimentally. Experimentally, we have observed SH generation from (111) and (100) surfaces in air using linearly polarized nanosecond pulses with wavelengths of 1.06, 0.68, and 0.63 µm. The SH generation efficiency was found to exhibit strong anisotropy as a crystal is rotated about its surface normal. These results together with the experimental results of Tom and Aumiller1 on (111) Cu indicate that, whereas the free electron contribution dominates the linear optical properties, interband effects must dominate the nonlinear optical properties. We investigated this conjecture theoretically in the case of Al by calculating the SH susceptibility tensor from a simple pseudopotential model2 for the band structure. The calculations indicate that the anisotropic part of the tensor is due to interband effects only and that the interband contribution to the tensor is as large as or larger than the conduction electron response at optical frequencies.
Effect of the polarizing property in a frequency-doubled YAG laser
Authors: Yao Jianquan , Li Yu , Zhang Dapeng
Published: February 2023

For type II phase-matched frequency doubling in a biaxial crystal, we proved that the amplitude ratio of two vertical fundamental wavefields in the crystal is an important factor affecting the frequency doubling efficiency (FDE). The plot of FDE as a function of the ratio has been drawn by numerically solving the coupling wave equations. The optimum orientation of the frequency-doubling crystal to the laser rod has been determined in which the FDE is the highest. When KT/OPO4(KTP) crystal is used as the frequency doubler in a YAG laser, the optimum orientation is the position in which the ratio is 0.976.
Achromatic phase shifter or modulator
Authors: George Nicholas , Stone Thomas
Published: February 2023

Achromatic optical systems forming phase shifters and phase modulators are presented and shown to impart phase angle shifts in incoming waves which are independent of wavelength over extremely broad spectral ranges. Three major classes of achromatic phase shifter are described including the triangle, close cascade, and imaged grating configurations. When constructed utilizing volume holographic optical elements, very high diffraction efficiencies may be achieved over the large bandwidths used. These systems may similarly be utilized to impart achromatic frequency shifts and frequency modulations which are wavelength independent over extreme spectral ranges and usually do not disturb the spatial form of the input waves. These devices may be used in ac interferometers and allow such interferometers to be operated in white light. Achromatic phase shifts of 0, +ϕ, −ϕ, +2ϕ, −2ϕ,…, may be simultaneously obtained.
Orientation anisotropy for the detection of aliased patterns by peripheral vision is optically induced
Authors: Cheney F. E. , Thibos Larry N.
Published: February 2023

Detection of aliasing varies throughout the peripheral visual field in a systematic way we call radial tuning; the cutoff spatial frequency is always maximized when the grating is oriented radially.1 Because this orientational anisotropy occurs for grating stimuli produced as interference fringes directly on the retina with an achromatic interferometer, we suggested previously that radial tuning is of neural, not optical, origin. New theoretical analysis has caused us to reconsider the basis of radial tuning.
Equivalent blur in human vision
Authors: Levi Dennis M. , Klein Stanley A.
Published: February 2023

We measured thresholds for two-line resolution and for two-line separation discrimination in central and peripheral vision using Gaussian blurred lines. For both tasks and at all eccentricities the dependence of thresholds on stimulus blur can be characterized as follows: Th = [U* (1 + B
s
/B
E
)2])1/2, where Th is the threshold, U is the unblurred threshold, B
S
is the stimulus blur, and BE is the equivalent intrinsic blur of the visual system. When the stimulus blur B
S
is small, it has no influence on the thresholds; when it is larger than the equivalent intrinsic blur B
E
the threshold is proportional to the stimulus blur. B
E
is contrast invariant.
Predictions of separation discrimination for multipoles
Authors: Klein Stanley A. , Levi Dennis M.
Published: February 2023

The challenge in modeling spatial vision is to predict resolution acuity and separation acuity (hyperacuity) in a variety of different conditions. Our viewprint model1 successfully predicts thresholds for a high-contrast three-line bisection task. We now apply the viewprint model to a variety of two-multipole separation tasks. Several parameters were varied: contrast; stimulus blur; multipole type (ramp, edge, line, dipole, and quadrupole); polarity (opposite polarity edges, for example, correspond to a bar); and viewprint blur (to simulate sparse sampling). Three regimes of separation are found, each with different dependence on the above parameters. The resolution regime, for the smallest separations, is quite sensitive to most of the parameters varied. The null point regime between 2 and 10 min is relatively independent of parameter manipulations. The large separation regime requires modification of the model to take into account the cortical sampling grain. Our predictions will be compared to psychophysical separation thresholds for the same and opposite polarity lines in several blur and contrast conditions. To a first approximation there is good agreement between theory and experiment including some counterintuitive results.
Light scattering for surface inspection
Authors: Nyman B. M. , Mendenko B. J. , De La Rosa J. G.
Published: February 2023

The laser scanning inspection system (LSIS) is a large-area high-speed surface inspection system based on laser scanning and light scattering. It currently inspects patterned and unpatterned photolithography masks for particulates, down to 2 μm in size. Masks up to 6 × 6 in. can be inspected in <60 s. The LSIS is controlled by an AT&T PC6300. Light scattering for inspection of smooth surfaces is a well-established technique. For patterned surfaces light is scattered from the pattern and the defects. There may also be diffraction from the pattern.
New-type moire fringe alignment aid
Authors: Yu Xin
Published: February 2023

The moire method has been successfully used as a null position sensor for a long time, but the sensor can indicate the null position only; neither the shift value nor direction can be determined.
Imaging sources in the near field of a radio telescope
Authors: Carter William H.
Published: February 2023

The methods presently used to image sources with a radio telescope or similar optical interferometer all require that the sources are far enough away to be in the far field of the antenna array. However, new radio telescopes are being considered with spaceborne antenna arrays that are so large that some radio sources of interest are within their near field. A theoretical model is developed which appears to be useful for refocusing a radio telescope to image sources which are within the near field of the antenna array.
Probability distribution for the photocount associated with a K distribution for laser intensity
Authors: Lure Y. M. , Gao M. , Yang C. C.
Published: February 2023

It is well known that the photocount possesses approximately a gamma probability density function when the detected optical intensity follows a negative-exponential distribution. In some problems of laser propagation through turbulent media, however, it has been shown that the detected laser intensity possesses a K distribution. Here the notation K represents a modified Bessel function with an imaginary argument. The probability density function for the photocount associated with such a K distribution for laser intensity is evaluated. The theory presented can be applied to two situations: (1) probability distribution for photon number in a certain time interval collected by a small detecting aperture; (2) probability distribution for photon number per unit time interval collected by a large detecting aperture. To this end, the probability distribution for the detected energy or detected power, depending on different situations, is computed first. For this purpose, a standard procedure in probability theory can be followed through using the characteristic function. Then a Poisson distribution is introduced to compute the probability distribution of the photocount. Different orders of K distribution are to be considered.
Time-reversal principle and fiber-optics coupling
Authors: Jannson Tomasz , Nq Joel
Published: February 2023

Using very general phase-space formalism applied to a Hamiltonian model of monochromatic light rays with an axial z coordinate as a parameter,1-3 it is shown that there is a fundamental relation between intrinsic optical losses of expanded-beam fiber-to-fiber coupler/connector and the boundary shape of its 4-D phase-space volume located in the symmetry plane of the system. This relation, based on the so-called time-reversal principle, holds for all axially symmetrical expanded- beam connectors with two twin parts representing the collimator and concentrator. Thus this principle can be applied to any practical fiber-fiber connector including both imaging and nonimaging2-3 optical systems. The proof of the time-reversal principle is based on comparison of geometrical rays reflected either from phase conjugate or conventional mirrors situated in the symmetry plane of the coupling system (including Liouville theorem). It is shown that for ideal couplers, the angular spectrum of light at any point in the plane of symmetry should have inversion symmetry.
Space-variant joint transform image correlation using a binary spatial light modulator at the Fourier plane
Authors: Javidi Bahram
Published: February 2023

A joint Fourier transform image correlator that uses a binary spatial light modulator (SLM) at the Fourier plane is described. It is shown that binarizing the Fourier transform’s interference intensity does not seriously degrade the performance of the system. The implementation of joint transform correlation using an electrically addressed SLM at the Fourier plane requires magnification of the Fourier transform's interference intensity to pre serve spatial invariance. Due to the low resolution of available SLMs, magnification is necessary to resolve the high spatial frequencies of the interference intensity which are associated with the input signals’ positions. However, such magnifications can result in the loss of the space-band width product (SBWP), which can degrade the performance of the system. Here we also show that by compromising the spatial invariance, the correlation signals can be formed without such magnifications and without significant losses in the SBWP. Undersampling of the interference intensity results in aliasing effects and loss of the high spatial frequencies associated with the input signals’ locations which also correspond to the correlation positions at the output plane. We show that the correlation functions can be recovered if their positions are known; therefore, the proposed technique is called space-variant image correlation. It is shown that the correlation performance of the proposed system is superior to those systems that employ magnification at the Fourier plane to preserve spatial invariance.
Measurements of intensity distributions in a laser with a saturable absorber
Authors: Mortazavi Mansour , Singh Surendra
Published: February 2023

Steady-state intensity probability densities for the light coming out of a laser with an intracavity saturable absorber have been measured in a variety of operating conditions. The experiments were carried out on a He–Ne laser operating at λ = 633 nm with an intracavity Ne absorber cell. The operating point of the laser was varied with the help of a variable loss element. Measurements were made for several operating points of the laser spanning both bistable and monostable operations.
Phase stabilization and digital phase locking of laser subharmonic signals
Authors: Chung K. P. , Marcano A. O.
Published: February 2023

Laser subharmonic signals generated through degenerate parametric oscillation exhibit interesting properties that may be used for optical signal phase stabilization and digital phase locking. The laser subharmonic generation may be described by a set of differential equations coupling the fundamental and subharmonic fields in a nonlinear medium with second-order susceptibilities. It is shown analytically that phase stabilization and bistability are possible for type 1 phase matching but not for type 2 phase matching or harmonic generation. Results of numerical integrations confirming analytical results are also presented. In normal pulsed optical parametric oscillation, the noise phase determines the mode of each pulse with equal probability. However, the ambiguity in-phase shift can be removed by an application of a seeding wave whose frequency is the same as that of the subharmonic. The seeding signal should be comfortably above the noise level but much weaker than the pumping wave. In addition, the seeding signal should be controlled to overlap the particular moment of creation of the subharmonic pulse. This concept leads to digital phase locking, and a design of an optical switch based on this principle is given in the literature.1
Advanced deformable mirror system development
Authors: Price Thomas R.
Published: February 2023

Lincoln Laboratory is developing deformable mirror systems to be used in adaptive-optics systems intended to compensate for the deleterious effects of the atmosphere on a propagating laser beam. We developed a digital mirror-control system for the automatic measurement of important mirror parameters. Calibrated actuator transfer functions, measured with this system, are stored in the mirror driver as look-up tables. Using these tables, the mirror can be commanded to a specific figure in a single step with an error of <λ/30 rms. A 241-actuator deformable mirror has been flattened routinely to λ/50 rms. A facility has been assembled to evaluate advanced adaptive-optics components including liquid-cooled deformable mirrors. Computer-controlled equipment allows the evaluation of actuator response, mirror jitter, and heat exchanger performance.
Characterization and performance of a new membrane mirror
Authors: Schmutz Lawrence E. , Wormell D. C.
Published: February 2023

A new deformable membrane mirror (DM2) was simulated, designed, and constructed using contemporary membrane materials. The resulting prototype was characterized and the results compared favorably with simulation. A 2-D model of mirror dynamic response was defined and evaluated both by analysis and numerical simulation. The model incorporated mirror tension, mass, geometry, and the effects of damping due to the surrounding medium. Finite boundary conditions were considered. The analytical results were compared to the corresponding numerical cases and found to agree within the grid resolution of the simulation. Detailed design of the DM2 was performed, and a prototype fabricated using a 4-in. diam chrome coated Mylar pellicle suspended in a precision-lapped Macor frame between a 16 × 16 array of electrodes and a window with a transparent electrode. The membrane cavity was at atmospheric pressure.
Optimal control of an adaptive segmented mirror
Authors: Downie John D. , Goodman Joseph W.
Published: February 2023

The optimum feedback matrix for a general segmented mirror adaptive optics system is derived assuming the use of two different wavefront sensors. The system is assumed to be used as an imaging telescope encountering wavefront degradations due to both propagation through a turbulent atmosphere and mirror figure errors such as segment misalignment. The first study is for a sensor that measures the average wavefront gradient over each segment, as a Hartmann sensor might do. The second case assumes that the instantaneous phase derivatives in the x and y directions are measured at an array of discrete locations over the mirror aperture. A shearing interferometer wavefront sensor could provide this type of data. In both cases a feedback matrix is derived that converts the measured data into linear displacements for the actuators positioned on the back of the segments. The feedback matrix derived is optimum in the sense that the mean residual wavefront variance is minimized. This optimization assumes knowledge of the statistics of the atmospherically induced aberrations, the mirror figure errors, and the wavefront sensor errors.
Visual texture segmentation based on oriented energy pyramids
Authors: Bergen James R.
Published: February 2023

An oriented pyramid is an image representation consisting of the outputs of a set of linear filters that are tuned for a range of spatial frequencies and orientations. An oriented energy pyramid is derived from an oriented pyramid by squaring the outputs of the filters and spatially averaging them over a region that is proportional to the size of the underlying filter. This is somewhat analogous to computing the amplitude of the local response while discarding phase information. The distribution of intensities in an individual energy image gives an indication of the amount of a particular kind of structure within the integration region but not the spatial configuration of activity within that region. In this sense, oriented energy responses are similar to what is computed by complex cells in visual cortex. One interesting property of this image representation is that it converts many conspicuous texture differences into intensity differences. This suggests that the phenomenon of perceptual texture segregation may not require the postulation of novel physiological mechanisms.
Class of stimuli for studying correspondence in apparent motion
Authors: Papathomas Thomas V. , Gorea Andrei
Published: February 2023

Correspondence matching is the process by which the visual system matches the attributes of objects (luminance, color, shape, contrast, etc.) at different places and at different times to determine the path of the objects in apparent (stroboscopic) motion. We present a class of stimulus which allows the existence of simultaneous motion paths carried by a multiplicity of attributes. Each attribute can be matched independently of the others to elicit unambiguous (unidirectional) or ambiguous (multidirectional) motion perception. When two or more attributes are matched, we can either match them along the same direction (concurrent matching) or along different ones (competitive matching). This flexibility in manipulating the stimuli makes it possible to (a) isolate specific motion mechanisms in the absence of stimulation of any other mechanisms, (b) assess the summation index for motion strength when two or more attributes are matched concurrently, and (c) directly assess the relative strength of pairs of attributes by using competitive matchings to determine which of them prevails in determining the perceived direction of motion. These stimuli have been employed in psychophysical experiments to assess the relative strength of several attributes (color, orientation, luminance, spatial frequency, binocular disparity, and polarity) in eliciting motion.
Inverse perspective mapping and parallel optical flow computation
Authors: Bulthoff Heinrich H. , Mallot Hanspeter A. , Little James J.
Published: February 2023

Autonomous navigation based on optical flow can be significantly simplified by coordinate transformation or retinotopic mapping prior to motion detection. For detection and avoidance of obstacles in the plane, the appropriate map is the inverse perspective. This mapping transforms the egomotion field into a parallel vector field where the velocity vector of points in the ground plane has equal length. Obstacles will stand out by the length of their flow vectors, the deviation scales with their height above the plane.1 We tested this mapping paradigm with moving synthetic images. Motion detection was performed in the transformed images using a parallel motion algorithm which is not subject to the aperture problem.2 This area-based voting is computationally expensive when performed in a 2-D neighborhood (4 s for 2562 images on the Connection Machine) and less suited for natural and artificial real-time vision systems. The inverse perspective mapping speeds up the voting step considerably by restricting the 2-D search to a 1-D search along the egomotion vector. The map thus implements the ecological constraint that movement occurs in a plane while all derivations from that assumption are considered obstacles.
Discrete contour descriptors
Authors: Walters Deborah K. W.
Published: February 2023

A parametric representation of discrete image contours is presented which has the following properties: the parameters capture the perceptually relevant qualities of the contour, and the representation is compact. The descriptors are based on a segmentation of arbitrary image contours using constraints from both human visual perception and human generation of contour images. It can be proved that any such segment can be uniquely represented by a single 2-D real-valued vector. This compactness can be attributed to the use of a discrete representation and a perceptually significant segmentation. But for the parametric descriptors to be perceptually valid, a redundant representation which includes a few additional parameters is required. In addition to descriptors for individual segments, the relations between segments must be included in a complete contour description. The complete contour descriptors provide a representation which is translation, scaling, and rotation invariant for objects yet encodes relative scaling, translation, and rotation for object parts. The usefulness of the complete contour descriptors is demonstrated by their use in a neural network which learns to recognize individual handwritten digits.
Compact cavities for coherent array lasers
Authors: Roychoudhuri C. , Siebert E. , D'Amato F. , Macomber S. , Noll R.
Published: February 2023

The key elements in fabricating stable coherent array of lasers are (1) a suitable phase reference, (2) selection (discrimination) of one single array mode out of a closely spaced multitude of eigensolutions. We describe several approaches including Talbot cavity that lend themselves to practical and compact system design with currently available technologies.
Imaging process in the oblique viewing microscope
Authors: Chastang Jean-Claude
Published: February 2023

The oblique viewing microscope is an optical instrument comprised of three parts. The first part is a relay lens which is a unity magnification system, preferably telecentric. The second part is a planar diffraction grating. The third part is a small to moderate numerical aperture microscope. With the appropriate geometric configuration, monochromatic illumination and suitable grating choice, microscopic observation at enormous oblique angles (e.g., 50°) becomes possible without defocusing or distortion problems whatsoever. The imaging process in this instrument is described, particularly the rendition of 3-D objects as well as the function of the grating, which is used here as a genuine imaging component. With the proper choice of pupil location the resolution is completely isotropic. Defocusing in this instrument is a little more complicated than in the conventional case as an astigmatic term appears. The defocusing tolerances are given.