bluuebunny/crossref_metadata_2025 huggingface dataset:
Second harmonic/subharmonic generation in a medium with quadratic and cubic nonlinearities
Authors: Banerjee Partha P. , Choe Wonha
Published: February 2023

Second harmonic generation is traditionally analyzed by assuming a quadratically nonlinear medium which may or may not be dispersive. In the former case, the second harmonic asymptotically builds up during propagation, while the fundamental monotonically decreases. In the latter case, however, there is a periodic exchange of energy between the fundamental and second harmonic. In our analysis, we first assume a medium with quadratic and cubic nonlinearities without dispersion (i.e., assuming perfect phase matching). Results from the initial value problem for harmonic generation indicate asymptotic behavior for low values of the cubic nonlinearity and periodic behavior for higher values. In other words, the role of dispersion (phase mismatch) in the traditional treatment is served by an adequate amount of the cubic nonlinearity. The same problem is also solved for the more general case when dispersion may be present. Finally, subharmonic generation in a quadratically and cubically nonlinear medium is also studied for the nondispersive and dispersive cases.
Observation of stochastic resonance in a ring laser
Authors: Roy Rajarshi , Mcnamara B. , Wiesenfeld K.
Published: February 2023

The phenomenon of stochastic resonance was introduced by Benzi et al.1 to explain the periodicity of the earth’s ice ages. The primary signature of the phenomenon is that the addition of random noise can improve the SNR of a periodically modulated system, relative to that observed with no externally injected noise. We report the first observation of stochastic resonance in an optical device, the bistable ring laser. The experiment exploits a new technique to modulate periodically the asymmetry between the two counterrotating lasing modes. Large enhancements of the SNR of up to 11 dB have been obtained. A theoretical model of a particle in a double well potential is used to obtain a theoretical fit to the experimental data.
Scattering of light of finite bandwidth from fluctuating media
Authors: Wolf Emil , Foley John T.
Published: February 2023

With the increasing use of laser light in scattering experiments, a good deal of information has been obtained in recent years about thermal fluctuations in gases and liquids, lattice vibrations in solids, phase transitions, etc. The underlying theory has, however, been largely restricted to scattering of monochromatic plane waves by density fluctuations in equilibrium conditions. We present new results, valid within the accuracy of the first Born approximation, relating to scattering of partially coherent light by a linear medium whose macroscopic response is a random function of space and time, characterized by a statistical ensemble that is homogeneous and stationary. The incident light is assumed to be linearly polarized, statistically stationary, and homogeneous and to have an arbitrary spectral profile. The analysis is based on statistical continuum theory, and the results apply both off-resonance and in the resonance region of the medium. The central quantity of this theory is a generalization of the van Hove correlation function,1,2 well known in the theory of neutron scattering.
Spatial coherence discrimination by Bragg holography
Authors: Jannson J. , Jannson T. , Wolf E.
Published: February 2023

In recent years considerable efforts have been made to produce devices which discriminate between laser light and diffuse (ambient) light.1 All these investigations were based on differences between the spectral characteristics, i.e., the temporal coherence properties of the light. We first summarize the results of a recent analysis2 which indicates that discrimination on the basis of spatial coherence might also be possible. More specifically, we consider light of any state of coherence incident on a scattering medium, and we show how the angular distributions of the scattered intensity depend both on the properties of the scattering medium and on the degree of spatial coherence of the incident light. We then illustrate the results by application to Bragg holographic filtering. Some experimental results illustrating the theoretical analysis are also presented.
New developments in the minimum-negativity iterative algorithm for restoration
Authors: Howard Samuel J.
Published: February 2023

A new means of bringing about more rapid and complete convergence of the nonlinear equations that implement the minimum-negativity constraint by summing over a modified spatial function (instead of only the negative values) was discovered. This improved spatial function was found by seeking a trend in succeeding major iterations of the equations that may be extrapolated into a future iteration. This new function replaced only those values between the negative regions that were previously set to zero. It was noted that this modified spatial function closely approximated the one given by Fourier transforming the true high-frequency values, which is evidently the reason for its success. A constant ratio between the sum of the new function that is substituted for the zeros and the sum of the negative values had to be maintained, however, to prevent divergence in the solution of the equations. The considerable advantage entailed by this new procedure is that convergence may be obtained for even the recalcitrant data with approximate procedures that use only the fast Fourier transform to perform the major calculations. Results are shown for field-widened interferometer data of 32K points and both simulated and experimental IRAS data of 64 × 64 pixels.
Applications of multilayers to UV and XUV grating spectrometers
Authors: Rife J. C.
Published: February 2023

Potential multilayer grating applications for the wavelength region from 6 to 350 Å are discussed. Multilayer bandwidth and efficiency as affected by stack construction as well as the effects of roughness and corrections to Bragg’s law are considered. Grating performance is reviewed with attention to the limitations to high resolving powers and spectral purity in fabrication and instrument design. Combined multilayer grating performance is discussed for broadband and narrowband coatings. Designs for 10-20 Å, 105 resolving power spectrograph, 6-60-Å scanning monochromator, and a 170- or 45-Å region, medium resolution, broadband spectrograph/monochromator are explored.
Weber’s law for separation discrimination is not due to eccentricity
Authors: Palmer John , Murakami Nobuko
Published: February 2023

Weber’s law is usually found for relative localization tasks, such as discriminating the separation of two squares or discriminating the vernier alignment of two lines. In other words, thresholds increase linearly with increases in the separation between the stimuli. This phenomenon may be due to various stimulus factors that are usually confounded with separation. In particular, widely separated stimuli must necessarily have one or both stimuli at larger eccentricities than can be achieved for narrowly separated stimuli: two points separated by 4° cannot both fit in the fovea. Consequently, stimulus eccentricity may contribute to the effect of separation. The current study investigates the role of eccentricity by holding it constant and manipulating separation alone. This is done by placing two squares along an imaginary circle centered around fixation. With such an arrangement, the separation can be increased while the eccentricity of each square is held constant. For 0.5° squares, separation discrimination is found to follow Weber’s law even when eccentricity is held constant at 5°. Further measurements at eccentricities of 2 and 18° also follow Weber’s law and for some cases even show similar Weber fractions. Thus eccentricity cannot in general mediate the effect of separation found in localization tasks.
Orientation invariance in two-dot distance discriminations
Authors: Willen J. Douglas , Macleod Donald I. A. , Morgan Michael J.
Published: February 2023

Discrimination thresholds for judgments of dot separation tend to follow Weber’s law. Given the well-documented distortion in comparison of horizontal and vertical distances, we might expect discrimination thresholds for dot separations to be greater when horizontal and vertical distances are compared. Yet we find that, while there is a constant error in comparing a vertical separation to a horizontal, the variance in observer’s judgments does not change appreciably. With an oblique target, observers demonstrated an intermediate constant error but again had similar thresholds. Klein et al.1 find that the increase in threshold with increasing separation does not occur when stimulus eccentricity is held constant, and they suggest that the Weber law increase results from the logarithmic compression of distance from the fovea in the cortical representation. Cortical magnification factors are not, however, the sole contributors to the observed Weber relationship: we found a Weber’s law function when observers were allowed to fixate freely the target stimuli. We propose that the judgment of large distances reflect errors at some level beyond simple cortical scaling and that an orientation invariant representation of distance is formed from necessarily oriented inputs with no significant loss of precision.
Phase cross correlation in Stokes, anti-Stokes generation
Authors: Li Z. W. , Radzewicz C. , Raymer M. G.
Published: February 2023

The degree of phase cross correlation between a fluctuating pump wave and the Stokes wave, which is generated by stimulated Raman scattering, can be determined by observing the spectrum of anti-Stokes light generated by coherent anti-Stokes Raman scattering (CARS). The spectrum of the anti-Stokes light has two distinct features: one that arises from scattering through an off-resonant virtual level and one that arises from resonant scattering through the real level. This provides a method to measure the phase cross correlation between two optical fields of different color. The intensity of the anti-Stokes signal strongly depends on the degree of cross correlation. Depending on the detuning from Raman resonance in the CARS process, the anti-Stokes intensity can be either enhanced or suppressed with respect to the case of uncorrelated fields.1
Director alignment methods for ferroelectric liquid crystal matrix displays
Authors: Bos Phil
Published: February 2023

A method for obtaining good SmC* alignment is described. The method utilizes a high-pretilt surface treatment, where the pretilt angle is of the opposite sign on the two cell surfaces and materials that possess the I–N
*
–A–C
*
phase sequence.
Gaussian beam propagation in nonorthogonal paraxial optical systems
Authors: Andersen Torben B. , Pawlowski Barbara E.
Published: February 2023

The work of Arnaud and Kogelnik1 for propagating Gaussian beams through lenslike media has been generalized to encompass optical systems with nonorthogonal paraxial properties. A 4 × 4 generalized ABCD matrix evaluated along a real chief ray is used to represent the ray transformation through the system. The elements of the ray matrix are determined from real ray data. The Huygens-Fresnel diffraction integral2 for this parabasal ray transformation matrix has been evaluated in closed form. For an input beam with elliptical intensity distribution this yields information about the transversal beam properties at any point in the optical system; in particular, the rotation angles of the transversal intensity and phase distributions, the beam spot radii, and the radii of curvature of the phase fronts are derived. Certain properties for the minimum values of beam parameters are also derived.
Three-dimensional Fourier optics: the linear scalar transfer function
Authors: Kauderer M. H.
Published: February 2023

It was recently shown that there is a formal similarity between 1- and 2-D canonical operators.1 This immediately generalizes all previous results in 2-D Fourier optics to nonorthogonal systems. In particular, the analysis of Nazarathy and Goodman2 goes through as before with no change in the logic. The spatial dispersion relation now expresses p
z
as a function of p
x
and p
y
, where p
i
is the optical direction cosine in the ith direction. This leads to the linear scalar transfer function for Fourier optics in three dimensions. Similar results can be derived in related fields, such as for anisotropic Gaussian-Schell model sources.
Chaotic motion of two ions in a Paul trap
Authors: Hoffnagle John A. , DeVoe Ralph G. , Brewer Richard G. , Reyna Luis
Published: February 2023

Two ions in a rf quadrupole (Paul) trap represent a pair of driven oscillators which are nonlinearly coupled by their electrostatic repulsion. Numerical integration of the equations of motion shows that this system can undergo a transition from periodic to chaotic motion when the Mathieu equation parameter q, which is proportional to the trap voltage, is varied. The transition to chaos entails considerable energy transfer from the rf trapping field to the ions and results in a diffuse hot cloud. Experimental observations of two trapped Ba+ ions show a transition from a compact ordered state to a diffuse cloud when q exceeds ~0.85, as predicted by the calculations; we interpret this as a transition from periodic to chaotic motion. This is the first quantitative agreement between calculations and experiment for a phase transition in a trapped ion system. We note that laser cooling introduces additional control parameters into the equations of motion, allowing an explanation of the melting of ion crystals observed by Diedrich et al.1
Instabilities in self-coupled lasers
Authors: Mandel Paul , Erneux T.
Published: February 2023

We have analyzed an optical system in which two ring lasers are self-coupled. This means that the output of each laser is injected in the other laser. This problem goes back to a work of Spencer and Lamb1 and was recently reinvestigated by Lawandy and Lee2 to produce a Lorenz-type laser with a reduced effective threshold. We have followed the Spencer-Lamb approach and shown that quite a number of different situations can occur. The peculiarity of this system is that despite the fact that the atomic polarization can be adiabatically eliminated, there can remain an intense phase coupling. As a result one can observe periodic output, subharmonic bifurcation sequences, and various forms of chaos of the intensity. In a suitable domain of the control parameters such as the length of the two lasers and the back-coupling of each beam into the other laser, the system has no steady states at all and displays a behavior which is similar to that of a good cavity laser with modulated losses. Therefore, all the variety of multistage periodic and chaotic solutions found in the good cavity laser with modulated losses is recovered in this system.
Practical f/1 infrared gradient-index objective
Authors: zinter J. Robert , Atkinson Leland G. , Mitchell Karin A. , Moore Duncan T.
Published: February 2023

An f/1 objective lens for use in the infrared in the wavelength band from 7.5 to 11.75 μm has been designed using practical infrared gradient-index materials. Designs utilizing both axial and radial gradients have been completed. The resulting lenses are well color corrected and use fewer elements than conventional designs with the same specifications. Benefits of the new designs are lighter weight, easier assembly, and improved performance. A prototype of the axial gradient design is being constructed and tested.
Transient Raman amplification by a train of pulses with alternating phase
Authors: Reintjes J. , Duncan M. D. , Mahon R. , Calame G. , Tankersley L. L.
Published: February 2023

We previously reported both experimental and theoretical results for transient Raman amplification by a train of pulses with identical phase. In that case the Raman amplification can be several orders of magnitude greater than for a single pulse. The increased Raman amplification is due to the phase memory of the Raman medium. When the pulses in either the pump or the seed-Stokes pulse train are given alternating reversed phases, however, the Raman amplification in the train can be reduced dramatically and, in some conditions, even driven below the single-pulse amplification level. Early experiments using eight pulses with a single phase reversal after the fourth pulse confirm a reduction in pulse train amplification. We report results for the Raman amplification obtained by using a train of eight pulses with alternating phase. The alternating phases are created by passing the train of pulses through an electrooptic modulator set for phase modulation and driven by an rf signal of sufficient intensity to achieve a complete phase reversal for adjacent pulses.
Transient Raman amplification in a crossed-beam geometry
Authors: Duncan M. D. , Reintjes J. , Mahon R. , Tankersley L. L.
Published: February 2023

We describe results of experiments in a crossed-beam transient Raman amplifier that examine the spatial quality of the amplified Stokes beam, beam combining properties, and effects related to replication of the crossing pump beams. The experimental measurements were made using 40-ps pulses produced by a mode-locked Nd:YAG laser doubled to 532 nm. The seed Stokes at 683 nm was produced in a self-generator cell filled with hydrogen gas at 25 atm. Both the seed Stokes and the pump beam were spatially filtered before being combined with a variable delay and collimated to have confocal parameters in excess of the 1-m length of the amplifier cell. The pump was split into two beams and crossed in the center of the amplifier cell. An intensified camera was used to record the near-field spatial profiles of the amplified Stokes. These data were then Fourier transformed and compared with measured far-field profiles to indicate the beam quality of the amplified Stokes.
Spatial-frequency channel interactions: the effect of contrast
Authors: Beard Bettina L.
Published: February 2023

Spatial-frequency channel interactions were investigated within a Crawford masking paradigm. Test stimuli were presented before, during, and after mask onset. Virtually no masking effects were seen when test and mask frequencies differed by a factor of 3. The temporal response characteristics of low-spatial-frequency targets were evidenced by transient peaks in the threshold curve near mask onset, while high-spatial-frequency test stimuli showed sustained response characteristics with elevations throughout mask exposure. When mask and test stimuli were the same frequency, maximal masking was obtained. Masking curves for complex masks composed of two components also showed this elevation, but it was not as robust as that obtained when the target and mask were the same frequency. This suggests that the second component had inhibited the effectiveness of the component in the mask, which was identical to the test spatial frequency. Interactions found among the frequency components occurred only at points of maximal masking. For low-spatial-frequency test stimuli, interactions were found near mask onset and were dependent on mask contrast level. High-spatial-frequency test stimuli showed interactions which were independent of mask contrast throughout mask exposure.
Spontaneous mode locking In long-cavity Fabry-Perot xenon lasers
Authors: Tarroja Manuela Fe H. , Sharafi Mohammad , Casperson Lee W.
Published: February 2023

Spontaneous mode locking or self-locking has been shown to be critically dependent on aperture size, excitation current, and the position of the active medium inside the cavity. However, in most previous experimental and theoretical work, a cavity length of not more than a few meters has been used. Our experiments have focused on a 3.51-μm Xe laser with a cavity length of up to 36 m. The characteristics of self-locking have been examined as functions of excitation current, cavity length, and aperture diameter. Unlike the shorter cavity lasers, the long laser remains self-locked for a wide range of excitation current and for various cavity configurations. It is found that the number of pulses per round-trip time increases with increasing cavity length. Furthermore, for a sufficiently long cavity and close to threshold, other pulses appear as ringing of the fundamental 2L/cpulse, where L is the cavity length and c is the speed of light. This behavior is consistent with numerical results based on a semiclassical model for an inhomogeneously broadened laser.
Direct current-excited cw CO2 metal waveguide laser
Authors: Al-Mashaabi Fahad S. , Casperson Lee W.
Published: February 2023

A novel design for a dc-excited cw CO2 metal waveguide laser has been developed in which a slotted hollow cathode also doubles as a metal waveguide for the cavity modes. This design has been implemented in a compact structure that produces up to 1 W of cw 10.6-μm radiation. The discharge characteristics, laser gain, and laser output have been studied as functions of various discharge parameters. The advantages of the new transverse-discharge slotted hollow cathode geometry include low voltage, positive impedance, rugged design, and high optical gain. Overall efficiency is comparable with that of conventional longitudinal CO2 lasers. The output laser modes were very clean low-order Gaussian or Airy function modes.
Experimental studies in ground to orbit laser propulsion
Authors: Rosen D. I. , Rollins C. J. , Gauthier M.
Published: February 2023

Experiments have been conducted in support of a program to develop ground to orbit laser propulsion.1 Optical diagnostics provide temporally and spatially resolved measurements of temperature, opacity, and electron density in laser-induced plasmas for various candidate propellants. Plasma ignition thresholds are determined spectroscopically. Mass loss and impulse coupling measurements have been obtained for various propellants in irradiance regimes appropriate to double-pulse detonation wave rockets driven by a ground-based repetitively pulsed IR laser. Experiments have used two 10-J CO2 TEA lasers. Plans for tests at up to 2 kJ/pulse are also discussed.
Quarterwave optical coatings and Fourier transform techniques
Authors: Bovard Bertrand G.
Published: February 2023

First introduced by Sossi,1 the Fourier transform technique is a powerful tool to design optical filters involving a variable refractive-index layer. A relationship between the transmittance or reflectance and Fourier transform of the refractive-index profile can be established with various degrees of approximation. If the transmittance is specified for a range of wavelengths, the inversion of the Fourier transform yields a variable refractive-index profile. The designer can choose to approximate this profile by a series of homogeneous layers or to keep it as such. The Fourier transform of the logarithmic derivative of the refractive-index profile involves an amplitude term Q(σ) and a phase factor F(σ) where σ is the wavenumber. It is worth emphasizing that this relationship is an approximation. Workers have established several expressions for the Q-function in terms of the transmittance or reflectance. We show that even for the best of the Q-functions, the Fourier transform remains an approximation. The technique is applied to the case of a quarterwave stack such as a mirror (HL)
p
H, and it is shown how the mirror characteristics expected from the Fourier transform technique differ from the actual values obtained using conventional exact matrix calculations.
Study of surface diffusion of CO on Ni(111) by optical second harmonic diffraction
Authors: Zhu X. D. , Rasing T. , Shen Y. R.
Published: February 2023

We have developed a novel technique to study surface diffusion of adsorbed atoms or molecules on substrates. By using laser-induced thermal desorption with two interfering laser pulses overlapping on the Ni(111) surface covered by a monolayer of CO, a 2-D surface grating of CO was formed. The optical second harmonic generation diffracted from the grating was then used to follow the evolution and disappearance of this monolayer grating as the CO molecules diffuse on the surface. The method should allow us to measure the anisotropy of the surface diffusion process. Experimental details and results are presented.
Long baseline optical aperture synthesis in astronomy
Authors: Shao M.
Published: February 2023

Interest in long baseline stellar interferometers has been growing rapidly in recent years. Our current instrument, the third of a series of interferometers, has been operational for 2 years making astrometric and stellar diameter measurements with baselines of 8-12 m. initially, the principal technical problem was phasing the interferometer or equalizing the optical path length in the two arms of the interferometer. At present, the procedure is totally automated, and after the pointing system has acquired the star, the search for stellar fringes usually takes 10 s or less.
Mondrian color constancy, equivalent surround, and simultaneous contrast
Authors: Valberg Arne
Published: February 2023

Land’s recent Mondrian experiments1 are striking demonstrations that colors of objects are largely independent of the wavelength composition of the light incident on them. Attempts to implement this independence in artificial vision systems have renewed interest in color constancy and contrast and the explanation of these phenomena in Retinex theory. We have shown that the color shifts occurring in a Mondrian display are in general much larger than can be tolerated if color constancy were to hold.2 Furthermore, the same color shifts were obtained when the complex Mondrian pattern was replaced by a homogeneous gray field surrounding a test patch. Neither the equivalence of this neutral surround nor the color shifts are correctly predicted by the Retinex algorithm.1 The Land phenomenon of partial cancellation of chromaticity shifts when a change of illuminant reduces to simultaneous contrast, if a spatiochromatic and luminance average over a Mondrian pattern is the same as for a homogeneous surround. Experiments with simultaneous contrast demonstrate that spatial weighting factors are needed in the computation of the combined effect of complex Mondrian patterns.
Surface color constancy: data and models
Authors: Arend Lawrence E.
Published: February 2023

We simultaneously perceive at each visible surface point (at least) a surface color (with several dimensions), an illuminant color, surface orientation, and surface specularity. Perceived surface and lighting variables are a multidimensional function of the past several minutes of chromatic exposure and the current retinal images. Models with outputs that are single-valued functions of image spatial and temporal derivatives (e.g., edge ratio models of Wallach, Land, Horn, Arend) describe sensory processes providing important relational information. However, they are easily shown to be incomplete as models of surface color constancy. Traditional color appearance models are similarly incomplete. This multidimensionality of surface perception was known to early theorists and has been further elaborated by recent human perception and machine vision theorists. The human vision data needed for refinement of computational color constancy models are currently in short supply for both theoretical and methodological reasons. We have recently measured human color constancy using both spatially simple and complex patterns. Our procedure allowed separate measurement of local sensory color (hue, saturation, and brightness) and apparent surface colors (surface chromatic color and lightness). Even in the simplified scenes of our experiments there is a complicated relationship between the sensory color at an image point and the apparent color of the surface perceived on the corresponding sight line.
Prediction of the illuminant’s effect on color appearance
Authors: Brainard David H. , Wandell Brian A.
Published: February 2023

The color constancy hypothesis predicts that the color appearance associated with a vector of cone responses shifts as the visual system adjusts to changes in illumination. We measure color appearance using a classification paradigm1 and verify that changes in illumination do induce shifts in color appearance. Because there are many illuminant spectral power distributions, it is not possible to measure the shifts induced by all of them. If color constancy is to be a useful guide to the prediction of color appearance, the shift induced by any illuminant must be predicted by the shifts induced by a small set of illuminants. We show that when the set of surfaces encountered by the visual system is restricted, the filtering action of the cone spectral responsivities strongly limits the illuminant variation that can be detected at the level of the cone responses. The illuminant variations that are not filtered by the cones form a linear subspace. We test whether the shifts induced by illuminants in this subspace can be predicted from the shifts induced by each of the subspace’s basis illuminants.
Spatial weighting function for induced color contrast
Authors: Zaidi Qasim , Skorupski Anthony
Published: February 2023

The effect of different parts of the surround on the appearance of a central test area was measured by the following method: The observer viewed a central disk surrounded by a radial sine wave: i.e., along any radius through the center, the color of the surround varied sinusoidally in space. As the phase of the sine wave was changed so that it appeared to drift toward the center, the appearance of the center changed cyclically in time. The induced modulation was nulled by adding real temporal modulation to the disk. The amplitude of the nulling modulation was expressed as a function of the spatial frequency of the surround. The spatial weighting function for the induced contrast was obtained by the inverse Fourier transform of this function in those cases where the total induced effect in the central test area was a weighted average of the effect of each pixel of the surround, and the effect of each pixel was proportional to the change in amplitude at that pixel. The validity of these conditions was tested by measuring the effect of the superimposition of surrounds of different spatial frequency. Results are presented for both brightness and chromatic induction.
Elucidation of Langmuir-Blodgett monolayer film microstructure by second harmonic generation
Authors: Mizrahi Victor , Stegeman George I. , Knoll Wolfgang
Published: February 2023

We have elucidated the microstructural properties of a monolayer of a cyanine dye, S120, deposited on a glass substrate by the Langmuir-Blodgett deposition technique. This system is known to form ellipsoidal shaped aggregates of the order of 100 μm in diameter.1 Although these aggregates were previously believed to be isotropically distributed on the water surface of the Langmuir-Blodgett trough and, therefore, on the glass substrate to which they are subsequently transferred, we demonstrate that their distribution function has a two-fold axis of symmetry. This shows up clearly in the orientational dependence of the second harmonic generation (SHG) signal from the film. We provide evidence that this anisotropy originates with an ordering of the elliptical aggregates during the isothermal compression phase on the water surface. By combining the polarization and orientational properties of the SHG signal from the film,2 we deduce for the first time that the individual aggregates have C2 crystal symmetry, in agreement with the proposed brickstone model.1 This was possible even though the laser beam (2.3-mm diameter) averaged over a large number of aggregates and so sampled a distribution of orientations of the aggregates. Had the microscopic distribution been isotropic, determination of the microscopic crystal structure would have been ambiguous. This suggests that a deliberate reduction in the symmetry of an otherwise isotropic distribution could be useful in other systems.
Photorefractive dark conductivity in SBN:60:Cr and SBN:60:Ce,Ca
Authors: Sayano Koichi , Rakuljic George A. , Yariv Amnon , Neurgaonkar Ratnakar R.
Published: February 2023

Previous experiments on photorefractive two-wave mixing and phase conjugation in SBN:60:Ce, Ca and SBN:60:Cr have shown that the two-beam coupling constant Γ increased as the crystals were cooled below room temperature despite the decreasing electrooptic coefficient when the crystal temperature was lowered away from the ferroelectric phase transition at ~70°C. The existing Kukhtarev model only predicts that Γ should decrease with temperature due to the lower electrooptic coefficient as the crystals were cooled away from the phase transition point. This model, however, does not include the dependence of the space-charge electric field on the dark conductivity of the materials. The finite dark conductivity of the material will result in a recombination of the photoionized electrons, which reduces the space-charge field and, therefore, the photorefractive gain. By adding a thermal dark conductivity term σdark proportional to exp(–A/kT), where A is a constant, to the Kukhtarev rate equation, we derived an expression for the steady-state photorefractive gain and response time as Γ = Γ0(1 + σdark/σph)–1 and τ = τ0(1 + σdark/σph)–1, where Γ0 and τ0 represent the low-temperature limit, and σph, the photoconductivity, is a function of total intensity. Using an SBN:60:Cr sample grown at Rockwell International Science Center, we experimentally verified our theoretical predictions.
Polarization properties of enhanced self-diffraction in sillenite crystals
Authors: Marrakchi Abdellatif , Johnson Richard V. , Tanguay Armand R.
Published: February 2023

Self-diffraction effects occur during the photorefractive grating recording process and thereby alter both the modulation depth and phase of the forming grating throughut the volume of the photorefractive medium. A method for calculating the polarization state evolution in photorefractive sillenite crystals (such as Bi12SiO20 and Bi12GeO20) in the presence of such self-diffraction effects has been derived, and representative solutions have been studied. Self-diffraction effects are shown to alter the diffraction process remarkably. For the {K
G
|| 〈001〉} orientation,1 self-diffraction induces significant energy coupling gain; for the {K
G
⊥ 〈001〉} orientation, self-diffraction significantly modifies the polarization states, especially for Doppler-enhanced recording configurations. Striking spatial inhomogeneities are induced in the space-charge field, especially for the {K
G
⊥ 〈001〉} orientation, leading to striations in the space-charge field along the direction of propagation for the case of Doppler-enhanced recording and essentially constant amplitude but corrugated phase fronts for stationary recording. The diffraction properties of the two principal recording configurations prove to be distinctly different, and in part this difference is attributed to fundamental anisotropies inherent in the diffraction process. Finally, the degradation in energy coupling gain with increasing amounts of optical rotation ρd has been calculated for a few representative cases. This degradation can amount to of the order of 50% and must be considered in any holographically based measurements of effective electrooptic coefficients.
Prospects of lossless superconductor-coated waveguides
Authors: Wei David T.
Published: February 2023

Theoretical deviation shows that a superconductor surface has (1) a total reflection for EM radiation, and (2) an evanescence layer without loss. Both properties are promising for waveguiding high density of radiation flux. High T
c
superconductor materials have demonstrated diminishing absorption in the far infrared1 and in the millimeter wave2 experiments. Methods for constructing and testing the superconductor-coated waveguides are proposed. It is believed that for both data and power transmission, waveguiding in a superconducting conduit offers a safe, secure, and EM-interference-free method over a single core wire and deserves our attention.
Anomalous diffraction of a grazing laser beam by a dielectric interface: experimental observation
Authors: Lin Shih-Chun , Hong John
Published: February 2023

In 1977, Kolomeev et al.1 reported observation of anomalous diffusion of a laser beam when incident at a grazing angle to one surface from the 90° vertex and perpendicular to the other surface of a glass right-angle prism. They also provided a theory for the phenomenon. The diffracted light showed a distinct oscillatory distribution, followed the critical angle of total internal reflection with good directivity, and was due to the interface only. Recently we reexamined the phenomenon and thought that the diffraction effect could serve as the basis for designing optical switches, especially in conjunction with nonlaser optical interfaces. Using a CCD linear detector array, we measured the intensity patterns of the transmitted and the diffracted beams of the following light sources: (a) a single-line He–Ne laser; (a) a dual-line Ar+laser; and (c) a broadband xenon-arc lamp, at both glass–air and water–air interfaces. The optical beams, both inside and outside the prism, were visualized. The fact that the diffraction is due to the interface alone was firmly established by perturbing the interface with films of alcohol, contacting dielectric wedges and metal surfaces, and oscillating wires. All these new results are presented. Comparison to the theory is given, and application of this effect to optical switching is described.
Aztec grating: a surface-relief volume diffractive structure
Authors: Cowan James J.
Published: February 2023

The Aztec grating is a deep surface-relief periodic structure having a groove profile that resembles a stepped pyramid. Equally spaced fine vertical steps are superimposed on a coarse horizontal grating. The horizontal grating can be either a linear or honeycomb type. For the zero-order reflection mode the profile comprises a phase-quantized resonant structure for incident light of a given wavelength when the step spacing is a half-wavelength. The bandwidth of the reflected light is inversely proportional to the number of steps. The above behavior is typical of a volume diffractive element. Overcoating a metallized Aztec grating with a dielectric shifts the resonant wave-length to a new value depending on the index of refraction of the dielectric. The structure is suited for replication by metal mastering and molding into plastic.
Modulated-index structures
Authors: Stegeman George I.
Published: February 2023

This tutorial concentrates on how grating (modulated-index) structures are used in thin-film optics. The basic concepts of reflection and transmission through grating structures, consisting either of multiple layers or periodic structures in a film surface, are presented. The use of gratings in linear and nonlinear integrated optics is introduced.
Calculation of the correlation of intensity function for a variable C n 2
Authors: Whitman A. M. , Beran Mark J.
Published: February 2023

We present numerical calculations of the correlation of intensities in a Kolmogorov medium with an altitude dependent

C
n
2

. In addition, asymptotic expressions are derived. These results are obtained by using the two-scale method, which has been previously reported in the literature. The results indicate that the correlation of intensities cannot be approximated by a single phase screen at an arbitrary altitude, as is the case with the scintillation index. We show further that in the perturbation limit the problem can be deconvolved to yield the

C
n
2

dependence on altitude.
Instability of dispersive counterpropagating waves in a Kerr nonlinear medium
Authors: Law C. T. , Kaplan A. E.
Published: February 2023

The temporal instability of counterpropagating beams in a Kerr nonlinear medium due to relaxation is well exploited.1 We demonstrate that another even more fundamental physical parameter, linear dispersion, can lead to instability. We investigate instability in the simplest eigenarrangement2 with the polarization of two counterpropagating beams being linear and parallel to each other. The dynamic equations for forward and backward propagating envelopes E1(z, t) and E2(z, t) are

i

[

(

−
1

)

j
−
1

∂

E
j

/
∂
z
+

v
g

−
1

∂

E
j

/
∂
t

]

−
μ

∂
2

E
j

/
2
∂

t
2

=
−

χ
2

k
0

(

2

I

3
−
j

+

I
j

)

E
j

/
2

, where j = 1, 2 and I
j
= |E
j
|2 is the intensity of the respective wave, μ = ∂2k/∂ω2 is a linear dispersion parameter, χ2 is third-order susceptibility giving rise to nonlinear refractive index, and v
g
is the (linear) group velocity. For large dispersion, unstable modes with lowest frequencies are excited first. The threshold intensity I required to excite these unstable modes diminishes as dispersion increases. In the case of I1 = I2= I and sufficiently large parameter p = χ2k0/L/2, this threshold intensity I is determined by

p
≃
log

(

μ

v
g
2

/
2
L

)

, where L is the total length of the nonlinear medium. More modes are excited when the intensity exceeds this threshold, and their combinations may yield chaotic behavior. The mechanism of the resulting instability can be explained in terms of positive distributed feedback provided by the nonlinear index grating in the situation that the phase speed of small perturbations with slightly different frequencies is altered by linear dispersion.
Dynamics of multiphoton optical resonances of a single cyclotron electron
Authors: Ding Y. J. , Kaplan A. E.
Published: February 2023

A single relativistic electron may exhibit large hysteresis and bistability in its cyclotron resonance at the main frequency (Ω ≈ Ω0, where Ω0 = eH0/m0c is the cyclotron frequency with H0 being a dc magnetic field).1 The microwave cyclotron motion of an electron can also be excited by an optical laser using either biharmonic driving with the optical frequencies ω1 and ω2 so that ω1– ω2 = nΩ, where n is an integer (the so-called cyclo-Raman excitation) or higher-order subharmonics with ω = nΩ.1 So far, only steady-state regimes of oscillations for all these processes have been analyzed in detail. Here we report the results of our study on the dynamic behavior of the electron in large using a phase portrait technique for all the relativistic-related nonlinear effects mentioned above. These results confirm our previously reported results as to the small-perturbation stability of all the steady states and reveal topology of phase portraits of the system for large perturbations very much similar to that of classical anharmonic oscillators.2 The phase portrait for main frequency oscillation consists of two (stable) focal points and one saddle point. The phase portrait for subharmonic oscillation shows the similarity to that for other physical nonlinear systems.2 In biharmonically excited cyclotron oscillations, the phase portrait for the first-order process (n = 1) is qualitatively similar to that for the main frequency, and the higher-order processes have phase portraits essentially similar to those of subharmonic cyclotron excitation.
Nonlinearly induced polarization changes in single-mode fibers
Authors: Lichtman E. , Friesem A. A. , Waarts R. G.
Published: February 2023

We studied both theoretically and experimentally the Kerr-induced polarization changes of a cw wave that propagates through a single-mode bire-fringent fiber together with an amplitude modulated wave. The polarization changes were determined as a function of the angle ϕ between the polarizations of the two input waves and their frequency separation Δf. For example, for a fiber of 300-m length, λ = 514 nm, an input power of the 5-mW modulated wave and Δf = 80 MHz, we found strong polarization fluctuations that are maximum at ϕ = π/4 rad and minimum at ϕ = 0 or π rad. Additional results and their implications concerning coherent communication with frequency division multiplexing are presented.
Complex reflectivity of a corner cube retroreflector
Authors: Scholl Marija S.
Published: February 2023

A corner cube retroreflector, consisting of three mirrors placed at 90° to each other, is used to return the beam in the direction from which it came. Due to the different (complex) reflectivity for the s and p polarization, the corner cube modifies both the phase and polarization state of the incoming light in a nonlinear fashion. In this treatment, the incoming light is described by the direction cosines (k, l, m) and two electric field vectors, all three forming a right triad. An exact analytical expression has been developed for the reflected wave using a vector approach. This expression depends only on the incoming beam parameters described, for example, in the coordinate system defined by the edges of the retroreflector and the angle-dependent reflectivity coefficients of the corner cube reflecting surfaces. Thus the results are suitable for incorporation into standard optical design programs. The magnitude, phase, and direction of the electric field characterizing the incident wave are modified in a nonlinear fashion on its travel through the retroreflector. Additionally, they depend on the direction of the beam travel through the retroreflector. It is also shown that the amplitude reflectivity is bounded by the cubes of the s- and p-reflectivity coefficients. A specific example of a linearly polarized incoming laser beam reflected from the aluminum retroreflector is evaluated. The linearly polarized incoming light becomes elliptically polarized.
Joint PDF of optical scintillation
Authors: Churnside James H. , Erl Noaa
Published: February 2023

We developed a model for the two-point joint probability density function for irradiance fluctuations in the atmosphere. Light was assumed to propagate some distance through refractive turbulence to two points located in a plane perpendicular to the propagation direction. Scattered light from turbulent eddies smaller than the Fresnel zone size is diffracted and assumed to produce a complex field whose real and imaginary parts are jointly Gaussian. This field adds coherently to the unscattered field to produce jointly Rician irradiance fluctuations. The light is also refracted by the larger eddies to produce an additional random modulation factor. The logarithm of this modulation factor was assumed to be jointly Gaussian. In weak turbulence, the result is nearly lognormal and has a single scale size. In strong turbulence, the scattered field has a jointly exponential density function with a small-scale size. The lognormal modulation has a larger scale size, which gives rise to the two-scale covariance function observed in strong turbulence. This model for the probability density function agrees well with recent experimental data.
Detection effects on measurements of atmospheric optical scintillation
Authors: Hill Reginald J. , Churnside James H. , Erl Noaa , Frehlich R. G. , Consortini A , Conforti G.
Published: February 2023

Many aspects of detection systems confound measurements of strongly scintillating irradiance. Recent emphasis on predicting and measuring the moments of irradiance draws attention to extremely intense, improbable, and immeasurable events. We briefly review the effects of amplifier saturation and the attainable number of independent samples. We present recent experimental results on the aperture size and bandwidth needed to simulate a point receiver and instantaneous sampling. The approach to lognormality of aperture-averaged irradiance is noted. We present a history of the use of irradiance moments to verify theoretical models of the probability density of irradiance showing its fall, reemergence, and recent demise.