bluuebunny/crossref_metadata_2025 huggingface dataset:
Brightness estimation supports a two-channel model of achromatic brightness processing
Authors: Drum Bruce
Published: February 2023

Brightness magnitude estimation functions for 3400° white circular increments on a 2700° white 38 cd/m2 background were measured on a modified Tubingen perimeter. The stimuli ranged from 1.7′ to 66′ in diameter, 0.07-1 s in duration, and 0° to 20° in eccentricity. Brightness estimates were obtained at 0.2 log unit luminance intervals from the increment threshold to 1600 cd/m2. A constant-luminance fixation target also served as a reference for the brightness estimates. Although the overall averages can be acceptably fit with power functions, most single-session functions are double-branched, in agreement with Hood and Finkelstein.1 A low-contrast branch first rises steeply with a slope ⪰1, but then asymptotes within 1–2 log units of threshold. A relatively straight high-contrast branch rises out of the low-branch asymptote, with a slope between 0.3 and 0.5. Changes of target size, duration, or eccentricity have little effect on the slope of the high-contrast branch. However, parameter changes that increase threshold can markedly shorten the range and increase the initial slope of the low-contrast branch. The properties of the low-contrast and high-contrast brightness branches are similar to those of the magnocellular and parvocellular retinogeniculate pathways and are consistent with existing brightness matching evidence for a two-channel model of achromatic brightness.2
Reconstruction of objects from simulated photon-limited stellar speckle data using the exponential filtering method
Authors: Black John V. , Walker John G.
Published: February 2023

Results of reconstruction of objects from computer simulated photon limited stellar speckle interferometry data are presented, using the exponential filter method.1 The objects reconstructed are collections of point sources and extended asteroidlike objects. The simulations suggest a limiting magnitude of ~14 for an object consisting of four point sources. This result compares favorably with results obtained using the triple correlation reconstruction method. The data processing involves the calculation of an ensemble average of the autocorrelation of short exposure turbulence degraded images, which have been premultiplied by a linear exponential. A novel adaptation of the traditional photon differencing method for calculating the autocorrelation of a photon limited frame of data is employed. In this adaptation, values placed in the autocorrelation array are no longer restricted to unity but are a product of the values of the linear exponentials at the respective arrival positions of the two photons.
Asymmetric modes in nonlinear multilayered waveguides
Authors: Inguva Ramarao , Rai Jagdish , Bowden C. M.
Published: February 2023

We consider the nature of modes in a multilayered guide with a linear core surrounded by nonlinear cladding layers. Neglecting transverse effects we show that a class of asymmetric modes characteristic of nonlinearities of cladding layers can exist in the guide. Absorption1 is included in the analysis. Explicit numerical results with field profiles are presented. It is found that these modes require a threshold flux density for the waves to propagate. The stability1 of these modes with respect to propagation in the guides is investigated. Potential uses are indicated.
Dynamic instabilities in a laser operating at two different TEM modes
Authors: Franco L. Roso , Corbalan Ramon
Published: February 2023

In some recent experiments1 new kinds of instability have been observed that are possibly related to the competition between two transversal modes. The theoretical study of spatial and temporal dynamics is a numerical problem of tremendous complexity that usually implies a delay between experiments and theory. We present here a model that allows us to describe the dynamics of a laser emitting in two different transversal modes without excessive complexity. The model is based on the assumption that both modes have a fixed spatial profile and the temporal dynamics is driven by the averaged coupling of both fields at each point of the amplifier. We also present results obtained with an oversimplified model that divides the molecules of the active material into three different categories. One is the group of molecules placed in the regions of space where only one mode is present, the other is the group of molecules placed where only the other mode is present, and the third group is the molecules placed in points where both modes are present.
Good cavity laser instabilities
Authors: Mandel Paul , Ruo-Ding Li
Published: February 2023

In an attempt to explain the instability observed by Hillman et al.,1 we have analyzed the domain of parameters in which their dye laser operates. This leads to the consideration of an asymptotic limit for which the slowly varying envelope approximation no longer holds. As a result, the electric field equation must retain its second-order space and time derivatives. This opens new possibilities for the steady states and their stability properties, (i) Even in the case of perfect matching between atomic and cavity frequencies, the laser operating frequency is displaced from the cavity resonance, (ii) The single-mode stable steady state is not necessarily the mode whose frequency is nearest to the atomic transition, (iii) Instability of the steady state can occur without restriction on the laser cavity quality.
Two-color multiphoton ionization and a numerical test of essential states theory
Authors: Roso-Franco L. , Eberly Joseph H. , Rzazewski K.
Published: February 2023

The various essential states (ES) models of multiphoton ionization (MPI) are the only models that permit an analytical solution for the temporal dynamics of MPI. One specific comparison of experimental data1 with ES predictions did not give good agreement. This may have been due to the simplifying assumptions of any essential states model or to the intrinsic difficulties of MPI spectroscopy. One of the important simplifications of ES models, the reliance on the rotating wave approximation (RWA) in the continuum, has recently been overcome.2
Numerical properties of a model atom for multiphoton ionization studies
Authors: Su Q. , Eberly Joseph H. , Javanainen J.
Published: February 2023

We have used a 1-D model atom for a wide range of numerical studies of multi-photon ionization and harmonic generation.1 The model is defined by a very simple one-electron binding potential

V

(
x
)

 
=
 
−
1

1
+

x
2

, but its detailed structural properties are not available in the literature.
High resolution image reconstruction from image plane arrays
Authors: Stark Henry , Oskoui-Fard Peyma
Published: February 2023

When image plane detectors are larger than the blur spot of the imaging optics, postdetection processing is required to reconstruct images with resolution commensurate with the imaging optics. We use the method of convex projections to increase image resolution by using each detector reading as a constraint set and projecting iteratively among these sets. One significant advantage of using convex projections is that the effect of dead detectors is less severe than in algorithms based on least-squares or localized backprojections.1
Resonant optical frequency doubling of injection-locked GaAlAs array output using KNbO3
Authors: Chun M. K. , Goldberg L.
Published: February 2023

Direct frequency doubling of a laser diode output can be an efficient technique for generating coherent blue radiation. Until recently, however, the doubled output power has been low1 due to limited pump power available from GaAlAs laser diodes. To increase the available fundamental power, coupled laser diode arrays, externally injection locked to obtain diffraction-limited operation, have been used2 to achieve 0.72 mW of 421-nm output in a single pass.
Generation and characterization of multiterawatt, high contrast subpicosecond pulses at 1.053 µm
Authors: Patterson F. G. , Perry M. D. , Campbell E. M.
Published: February 2023

Use of the chirped pulse amplification technique1 in large Nd:glass-based amplifier systems is rapidly making possible focused intensities in the 1017–1019-W/cm2 range, allowing a variety of experiments to be performed in a fundamentally new regime. Many of these experiments require not only high peak intensities, but also clean, high contrast pulses. We report here several techniques for generating subpicosecond pulses at 1.053 µm with ultrahigh contrast. One of the disadvantages of the fiber grating compression technique is that the rectangular shape of the spectrum requires 30-cm–1 bandwidth to generate a picosecond pulse. Also, a sine2 pulse shape results with a few percent of the pulse energy contained in pre- and postpulses. A carefully designed Nd:phosphate glass regenerative amplifier has successfully been utilized to shape the spectrum of the chirped pulse to a near-Gaussian profile. Multiterawatt transform-limited pulses of ≈0.9-ps duration and 22-cm–1 bandwidth have been obtained with an intensity contrast ratio >105:1.
Broadband subpicosecond infrared vibrational spectroscopy
Authors: Jedju T. M. , Rothberg L. , Guil-Bert N. R.
Published: February 2023

Broadband (500-cm–1) infrared pulses of 350-fs duration tunable from 2.5 to 5.5 µm have been produced by difference frequency mixing in LiIO3 and used to do subpicosecond time-resolved vibrational spectroscopy.1 Using a synchronized visible pump beam, time-resolved infrared absorption of transient species can be recorded. When the transmitted infrared probe pulses are monitored through a monochromator, vibrational absorption dynamics can be monitored with transform-limited resolution. Evolution of the vibrational spectra of heme proteins after photoexcitation has been studied.
Channel closing and power-law scaling in multiphoton ionization
Authors: Su Q. , Eberly Joseph H. , Greenwood W. G.
Published: February 2023

We present numerical data giving the intensity dependence of the multiphoton ionization rate for two different model atoms. As the intensity increases, the pon-deromotive shift causes peaks in the electron energy spectrum to disappear, one by one, at the threshold. We carefully examine the ionization rate near to and between the intensities at which these ponderomotive channel closings occur. The data have been obtained from ab initio wave functions calculated for 1-D atoms in the presence of intense laser fields. In the first model the potential

V

(
x
)

 
=
 
−
1
/

1
+

x
2

is used to simulate the one-electron binding potential of a typical atom, and in the second model the potential

V

(
x
)

 
=
 
−
exp

(

−
|
x
|

)

/

x
2

+

α
2

is used to simulate the binding potential of the extra electron in the H– negative ion. The first potential is quasicoulombic1 with long tails at large ±x and supports a Rydberg series of bound levels near the continuum limit, while the second potential is short range in nature and supports only a single bound state. Both potentials are enclosed in a box (terminated at x = ±L by rigid walls) that is large enough to support a relatively dense set of positive energy states.
Nonlinear stimulated light scattering in birefringent fibers
Authors: Cappellini G. , Cosentino A. , Trillo S. , Vozzella R. , Wabnitz S. , Daino B.
Published: February 2023

Four-photon mixing processes in optical fibers provide a versatile source for frequency conversion and parametric amplification of lightwaves. In single-mode fibers, phase matching may be controlled by the birefringence of orthogonally polarized modes.1 Whenever intense picosecond pulses are involved in the process, the spectral and temporal characteristics of the emitted radiation exhibit a complex behavior due to the nonlinear nature of the dynamic interaction. We present a fully nonlinear model of polarized light scattering in fibers, including transient contributions to the nonlinear response or Raman scattering. Instabilities may result in the polarization and intensity of the waves, leading to pump power dependent shifts or growth of both immediate2 and higher-order parametric sidebands.
Coherent resonant pulse propagation in a Kerr absorber
Authors: Matulic L. , Torres-Cisneros G. F. , Sanchez-Mondragon J. J.
Published: February 2023

We present a numerical study of the nonsteady state pulse propagation in a resonant inhomogeneously broadened absorber embedded in a dielectric medium with a Kerr-type nonlinearity. We abandon the a priori assumption for the existence of steady state solutions and examine the problem from a more physical point of view, i.e., we study the way in which the pulse actually propagates in the resonant Kerr medium and whether it eventually evolves toward a steady state. We find that the individual atomic dipoles are dephased and that their evolution depends on detunings. We believe that this is the reason for small energy losses which deny the attainment of a truly steady state pulse.
Excitonic saturation intensity in GaAs/AlGaAs quantum well optical modulators
Authors: Fox A. M. , Miller David A. B. , Cunningham J. E. , Henry J. E. , Jan W. Y. , Livescu G.
Published: February 2023

Exciton saturation adversely limits the switching speed of GaAs/AlxGa1-xAs quantum well electroabsorptive optical modulators. We have studied the dependence of the exciton saturation intensity Isx on the strength of the applied field and the height and width of the AlGaAs barriers. Five PIN photodiodes were grown by molecular beam epitaxy with the quantum wells comprising the intrinsic region. The well width was kept nominally constant, but x ranged from 0.2 to 0.4 and the barrier thickness L
b
varied between 31 and 95 Å. We found that Isx increases with applied field, and also depends strongly on x and Lb.For L
b
= 65 Å at zero applied field, Isx increased from 1.3 to 43 kW cm−2 on reducing x from 0.4 to 0.2. Similarly, for x = 0.3 at zero bias, Isx increased from 3.1 to 18 kW cm−2 on reducing Lb from 95 to 31 Å. These results can be explained in terms of a reduction of the carrier lifetime caused by barrier and field dependent tunneling rates. The field dependence of the carrier escape time for the sample with x = 0.31 and Lb = 31 Å was measured by the time-resolved electroabsorption technique.1,2 The measured escape time is ~100 ps at fields below 30 kV cm−1 and decreases to ~10 ps at fields above 60 kV cm−1.
Infrared picosecond pulse correlation measurements using photorefractive beam coupling and harmonic generation in KNbO3 and BaTiO3
Authors: Johnson A. M. , Simpson W. M. , Glass A. M. , Klein Marvin B. , Rytz D. , Trebino R.
Published: February 2023

Recently,1 we reported on the measurement of the coherence properties of 1.06-µm fiber-grating compressed 4-ps duration laser pulses using photorefractive beam coupling in GaAs:Cr. We utilized the conventional pulse width measurement geometry of noncollinear background-free second harmonic generation (SHG). With an SHG crystal the intensity autocorrelation is measured (pulse duration). A photorefractive crystal will result in the measurement of the electric field correlation or coherence function (coherence time). The coherence function is obtained by detecting the fraction of modulated pump beam scattered by the photorefractive index grating into the probe beam as a function of probe delay. Only for the case of a transform limited pulse would the width of the coherence function equal that of the intensity autocorrelation function. This coherence function is equivalent to measuring the pulse spectrum and Fourier transforming it to the time domain. Thus a picosecond optical pulse can be fully characterized by the simple exchange of a photorefractive and SHG crystal.
Optical autoconvolution using photorefractive four-wave mixing
Authors: Chiou Arthur , Yeh Pochi
Published: February 2023

Two-dimensional images with twofold symmetry such as straight lines, ellipses, and circles form an important class of objects in pattern recognition. In this paper we propose and demonstrate a novel optical method based on autoconvolution (or self-convolution) using photorefractive four-wave mixing for the detection of such objects. The 2-D spatial convolution of an image with itelf is equivalent to the correlation of the object with its 180° rotated version. For objects with twofold symmetry, the rotated version is identical to the original object and hence a strong correlation peak results. The basic idea of spatial convolution/correlation using photorefractive four-wave mixing in the Fourier plane had long been demonstrated.1 The novel feature of our method is the capability to perform autoconvolution from a single input pattern.
Ideal transfer characteristic for an optical correlator with a quantized magnitude spatial filter
Authors: Kaura M. A. , Rhodes William T.
Published: February 2023

Optical pattern recognition of targets immersed in noise requires a filter that has a robust response to target distortions as well as one that can be updated in real time using a spatial light modulator (SLM). The phase-with-quantized-magnitude filter (PQMF) retains sufficient magnitude information in addition to phase information to be more resistant to target distortions than phase-only filters. The PQMF algorithm exploits the 2π modularity of phase to select a quantized magnitude value that is closest to the desired magnitude given a SLM with a phase range in excess of 4 πrad.
Construction of bar code systems with a prescribed autocorrelation function
Authors: Yu Francis T. S. , Li Yajun
Published: February 2023

The difficulty encountered in the design of a bar code system for accurate alignment is the quadratic form of the autocorrelation equation. To overcome this difficulty, two different methods have been proposed: One consists of a calculation of all the possible combinations of the bar code modules.1 The other depends on designer's experience about typical structures of bar code systems.2 A new method is proposed on the basis of a choice of the appropriate solution of simultaneous equations obtained from the process of linearization of the autocorrelation equation. Numerical examples are presented to show the usefulness of this new method.
Modeling diffuse reflectance spectra from noncorrelated roughness of double layers
Authors: Ribbing Carl G. , Roos Arne , Bergkvist Mikael
Published: February 2023

A model for light scattering spectra from double layers with uncorrelated roughness is discussed. The model is based on Fresnel multilayer calculations with amplitude reflection coefficients reduced according to the scalar scattering theory and uses the normalized electric field intensity to scale the resulting interface scattering.1 It is demonstrated that the effect of uncorrelated roughness at the two interfaces is to change the electric field pattern in a way which enhances the scattering at wavelengths where the specular interference pattern has a minimum and vice versa. The rms roughness values are considered as parameters and have been used to fit the calculated results to experimental spectra. For the particular case of oxidized copper, which exhibits high scattering levels, the parameters thus obtained show good agreement with roughness values obtained by other techniques.2 In particular we emphasize the importance of using spectral variation to extract information from scattering measurements on this kind of system.
Experimental study of ultra long distance soliton transmission in fiber with loss periodically compensated by Raman gain
Authors: Mollenauer Linn F.
Published: February 2023

By recirculating soliton pulses many times around a closed 42-km fiber loop with loss exactly compensated by Raman gain, we have successfully demonstrated soliton transmission, without electronic regeneration, over distances of up to 6000 km.1,2 (For experimental details, see Ref. 1 or 2.) By using a slight excess of Raman gain, we have also been able to demonstrate adiabatic compression of the originally 55-ps soliton pulses by as much as 4X, with nearly perfect preservation of the ΔtΔf product (0.315) expected for sech shaped pulses. We have also been able to study interaction between closely spaced soliton pairs over thousands of kilometers: In addition to the expected phase-dependent, attractive/repulsive interaction from direct overlap of the pulse tails, we also see a small but significant long range interaction resulting from dispersive wave radiation. Finally, we have also shown that solitons are resistant to broadening from fiber birefringence, but that such polarization dispersion is the source of significant amounts of dispersive radiation. These studies have obvious implications for the development of an all-optical, long distance transmission system.
Performance characteristics of a 64-channel Bragg cell with self-collimating acoustic waves
Authors: Lin Shih-Chun , Boughton R. Scott
Published: February 2023

Multichannel bulk acoustooptic Bragg cells have found wide applications in real time optical signal processing and optical computing. Modern cells are designed for wideband signals and with closely packed transducers. Therefore, interchannel crosstalk and phase tracking capability have become the performance issues of Bragg cells to be used in applications such as the phase array antenna processing and optical computing. In a conventional multichannel Bragg cell, the crosstalk is caused in part by the rf interference and, in part, by the acoustic wave's diffraction spreading. In some typical cells, measurements1 have shown that the crosstalk level is about −30 dB. Recently, a unique 64-channel, GaP Bragg cell, operated at 400 MHz, was fabricated with the self-collimated mode, in that the diffraction spreading was compensated by the self-focusing effect of the acoustic wave to reduce the crosstalk effect. The phase tracking capability was also considered in the design by equalizing the electrical leads. In this paper, we present detailed measurements of the crosstalk and the characterization of the phase tracking performance.
High efficiency light modulation by standing acoustic waves from piezoelectrically induced strain-optic effect in bulk LiNbO3
Authors: Kuti C. , Turi L. , Yan Li , Lee Chi H.
Published: February 2023

The piezoelectrically induced strain-optic (PESO) effect can significantly influence the optical response of linear electrooptic crystals to electric driving fields. By utilizing this effect, low voltage modulators operating at the piezoelectric resonance can be developed.1,2 We have achieved high efficiency modulation of a laser beam using standing acoustic waves generated at piezoelectric resonance in bulk electrooptic crystal with the PESO effect. An 8 × 8-mm aperture, 25-mm long LiNbO3 crystal was chosen in the experiment. The sample was driven by an rf signal from a frequency synthesizer and an rf amplifier. High efficiency modulation on the refractive index by shear acoustic waves was observed in the 10-100-MHz range by using a He-Ne laser. The resonance width (FWHM) is under 10 kHz, confirming the high acoustic Q of the piezoelectric resonant vibrations. For the assembly investigated, the strongest modulation occurred at rf frequency around 50 MHz. A 100% light intensity modulation was achieved at a peak-to-peak driving voltage of 20 V. When the rf frequency was above 100 MHz, the modulation efficiency decreased quickly. However, we can still observe diffraction spots at resonance frequencies as high as 400 MHz. The light modulator based on the PESO effect differs from the conventional acoustooptic modulator. It combines the transducer and the resonance body into one. Thus, construction of the modulator is greatly simplified. With improvements of electric coupling, temperature compensation, and selection of better materials, we expect better performance of the PESO modulators.
Possible photon avalanche in Er:YLiF4
Authors: Kueny Andrew W. , Case William E. , Koch Mark E.
Published: February 2023

We report observations consistent with photon avalanche upconversion1,2 in 5% Er:YLiF4. Intense cw pumping at 558 nm leads to characteristic Er3+ fluorescence at 415 nm. The pump light is chosen to be resonant with a transition between two excited states of the Er3+ ion, but not with transitions from the ground state. In the usual upconversion schemes,3 absorption of pump light by ions in the ground state is a necessary step in the upconversion process and so the wavelength of the pump light must lie in an absorption band of the activated medium. The distinguishing feature of photon avalanche upconversion is the absence of direct optical pumping of ions in the ground state. Instead, ions are excited to an intermediate level by nonradiative transfer of energy from nearby ions in a higher lying level. The optical pumping serves to excite ions from the intermediate level to the higher lying level.
Phase conjugation in SBN fibers
Authors: Hesselink Lambertus , Redfield Steve
Published: February 2023

Recently we developed photorefractive fibers of SBN (Ref. 1) for use in optical data storage and processing applications. It was found that the angular sensitivity of multiple holograms stored in the fiber is four to five times higher than in bulk SBN of the same thickness because of waveguiding. In this paper we expand on the use of fibers for two-beam coupling and phase conjugation. Self-phase conjugation has been observed by directing a single beam down the bore of the fiber. Experimental results as well as theoretical analysis of two-beam coupling and phase conjugation in a fiber are reported.
Improved two-beam coupling gain and diffraction efficiency in bismuth silicon oxoide crystals using a bandpass AR coating
Authors: Karim Z. , Kyriakakis C. , Tanguay A. R.
Published: February 2023

Bismuth silicon oxide (Bi12SiO20, BSO) is widely used as a photorefractive material for two-beam coupling (TBC) gain and holographic grating recording applications.1 As is the case for most photorefractive materials, BSO suffers from several inherent reflectivity induced problems due to its high index of refraction (n = 2.61 at 514 nm). Multiple internal reflections of the writing beams interact among themselves to write transmission gratings with periods at or near the primary grating period. Furthermore, interference of the incident beams with the internally reflected beams gives rise to additional reflection gratings. Both types of interaction combine to reduce the overall grating modulation strength and to increase the potential for crosstalk and cross coupling, particularly for the case of multiplexed holograms. Reducing these problematic multiple reflections by means of a simple double layer bandpass antireflection coating2 increases the TBC gain coefficient and the diffraction efficiency by increasing the effective grating modulation depth. The writing sensitivity of the crystal (as expressed by the grating rise time) and the dependence of the diffraction efficiency on the incident beam polarization were also investigated for both uncoated and AR coated crystals.
Parallel algorithms with irregular interconnections and optical technology
Authors: Lu Dau-Tsuong , Paturi Ramamohan , Esener Sadik , Lee Sing H.
Published: February 2023

We consider the implementation of fast parallel algorithms. We make the case that irregular interconnections result in faster parallel algorithms; consequently, optical technologies that can support such randomlike interconnections are preferable. We demonstrate this by selecting a problem which has very fast parallel algorithms when irregular interconnections are used, and by suggesting certain optical technologies that can support such irregular interconnections for implementing these fast parallel algorithms. (On the other hand, it is well known that any technology that can only support regular and planar interconnections cannot achieve the same speedup.) The problem is that of approximately halving a sequence of numbers. This problem comes up as a natural subproblem in the optimal sorting algorithm of Ajtai, Komlos, and Szemeredi.1 We use probabilistic means to construct the interconnections necessary for the approximate halver problem and certify their properties by computing their spectrum. We also show that the interconnection pattern underlies several other fast parallel algorithms such as sorting, selection, load balancing, distributed consensus, and error-correcting codes. We then suggest that POEM (programmable optoelectronic multiprocessor) technology is capable of implementing such randomlike interconnections.
Optical flow computation by voting through spatial frequency channels
Authors: Schyns Philippe G. , Bulthoff Heinrich H.
Published: February 2023

Following Bulthoff et al.,1 we have designed a scheme that computes an optical flow field using independent measures of motion. The independent motion measurements are computed in parallel in different spatial frequency channels for all possible displacements. An area-based winner-take-all scheme leads to different voting distributions in each channel. These partial estimates of motion are integrated into a more robust optical flow field using an additional voting step through the channels. Provided an adequate architecture, the implementation of this scheme would retain the highly parallel characteristic of the original voting algorithm (Bulthoff et al., 1989). Results obtained from sequences of synthetic images indicate that the additional voting step compensates for false motion estimates in the independent channels. Our scheme shows how a better approximation of the optical flow could emerge from partial estimates. It is also physiologically plausible because it accounts for the fact that motion tuned cells in cortical area V1 are also spatial frequency tuned.
Experimental distinctions between population inversion gain and hyper-Raman gain in xenon
Authors: Rankin M. , Davis J. , Giranda C. , Squicciarini M. , Bobb L. C.
Published: February 2023

Three-level systems in which the middle level has opposite parity from the remaining levels can develop optical gain on the excited-state transition when a two-photon resonant field couples the ground state with the uppermost state. Both population gain, which leads to amplified spontaneous emission, and hyper-Raman gain, which leads to stimulated hyper-Raman scattering, occur.1 Evidence for both types of process appears in two-photon-excited bidirectional emission in atomic xenon.2 This paper presents and compares threshold scaling characteristics, temporal behavior, directional energy distribution patterns, and tuning behavior as evidence for the gain mechanism in xenon. Near threshold, clear evidence for population inversion gain exists. Above threshold, axial emission energy exceeds off-axial (spontaneous emission) energy by a factor which increases with excitation energy. This excess of energy indicates the presence of hyper-Raman and/or four-wave mixing gain.
Experimental study of transient spontaneous Raman scattering
Authors: Duncan M. D. , Reintjes J. , Mahon R. , Tankersley Law Rence L.
Published: February 2023

The quantum nature of the source of stimulated Raman scattering and its effect on the output of Raman self-generators have been investigated in a number of studies.1 Quantum theories of stimulated Raman scattering predict that its source is equivalent to spontaneous Raman scattering. We report on experimental investigations of spontaneous Raman scattering in the transient limit. In this regime the quantum theories predict that the spontaneous Stokes radiation produced in a single pass generator with Fresnel number 1 reproduces the amplitude and phase variations of the pump radiation, with the possible exception of a constant phase offset that can vary randomly from one laser pulse to the next. We have investigated the temporal behavior of spontaneously generated Stokes radiation from hydrogen using second harmonic 40-ps pulses from a mode-locked Nd:YAG laser. Our initial results indicate that the spontaneous Stokes pulse follows the pump pulse in its intensity profile, even though its duration is less than the material response time by a factor of 2.
Low loss, wideband, and strictly single-mode dispersion-flattened fibers
Authors: Safaai-Jazi A. , Lu L. J.
Published: February 2023

Most designs for dispersion-flattened fibers reported in recent years are either bend sensitive1 or are not truly single-mode at certain wavelengths of interest.2 We propose a new class of dispersion-flattened fibers which support only one mode for λ > 1.28 μm, provide a total dispersion of <2 ps/km·nm over the 1.32-1.7-μm wave length range, and have a fundamental mode cutoff larger than 1.85 μm indicating that they suffer low bending losses. Furthermore, these fibers may be designed such that the chromatic dispersion vanishes at wavelengths in the neighborhood of 1.33 and 1.55 μm which correspond to the low attenuation windows of glass fibers. The proposed fibers have a step-index profile and consist of a central core, five inner claddings, and an outer cladding. The index differences are kept below 0.8% to ensure low losses due to Rayleigh and stress-induced scattering. An objective function is used to optimize the index profile with respect to (1) desired minimum dispersion, (2) wavelengths of zero dispersion, and (3) width of the dispersion-flattened window.
Pump replication in stimulated Raman scattering
Authors: Menyuk Curtis R. , Hilfer G. , Reintjes J.
Published: February 2023

Theoretical work1 has indicated that four-wave mixing is a possible source of pump replication in experiments1,2 that have been carried out at the Naval Research Laboratory. In these experiments, several off-axis pump beams supply power through the Raman effect to a centrally propagating Stokes beam.
Color image correlation using reflection hologram
Authors: Wu Shudong , Mayers Andy , Yu Francis T. S.
Published: February 2023

The use of color information in image recognition has important applications in machine vision. Based on the high wavelength selectivity of reflection holograms, holographic filters for multiple wavelengths that can be independently written- in and read-out are proposed for correlation of color images. The correlator can work either in joint-transform correlator form or in VanderLugt filter form. For dynamic operation, a color LCTV can be used as the input spatial light modulator, a piece of photorefractive crystal as the recording medium, and a white light laser with three wavelengths as the coherent light source. The optical system is achromatic and the thickness of the crystal should be less than the focal depth of the Fourier transform lens. Experimental demonstrations and theoretical analysis of the correlator are presented.
Phase preserving amplitude stripping by photorefractive two-beam coupling
Authors: Biernacki Anne Marie , Wong Carlton , Cronin-Golomb Mark , Gonsalves Robert A.
Published: February 2023

Optical implementation of iterative phase retrieval algorithms (such as the Gerchberg-Saxton algorithm) for reconstruction of phase information from amplitude or intensity measurements is of interest for application to such fields as atmospheric and astronomical imaging, crystallography, and electron microscopy.1,2 In a proposed system employing successive signal iterations through a coherent optical processor in a cavity, unwanted amplitude information builds up on the phase-information bearing beam. Saturating an amplified signal can compensate for this. When two beams are coupled in a photorefractive crystal such as BaTiO3, amplifications of the signal beam can occur at the expense of pump beam depletion. At saturation of the amplitude gain, signal amplitude variations are clipped, preserving phase information. Current work includes characterizing amplitude stripper performance, examining the gain curve calculated from the input and output signals, and looking for maximum saturation sufficient for amplitude stripping relative to the coupling coefficient. Future work includes inserting the amplitude stripper into an optical phase retrieval system and evaluating its effectiveness.
Optical and mechanical properties of mixed composition infrared thin film materials
Authors: Sankur H. , Gluck N. S. , Gunning W.
Published: February 2023

Synthesized materials obtained by physical mixing are often used to generate de sired optical and mechanical properties. These properties often differ from the average of the properties of individual materials that make up the mixture.1 In this paper we discuss the optical, structural, and mechanical properties of several composite IR material systems. SrF2 films have a micro-structure with pronounced columnar grains that results in rough morphology and optical scatter. Films of pure ZnSe and mixtures of ZnSe with up to 70-vol % SrF2 have very smooth morphologies. Films with alternating material layers, on the other hand, do not become smooth even in the ZnSe layers. Codeposition results in strongly oriented crystalline films where both ZnSe and SrF2 crystalline phases are observed, indicating rapid segregation during deposition. In general, mixing reduces the crystalline grain size. Mixed intermediate composition films have crystalline orientations that differ from those of the pure components. Films rich in one material suppress the crystallinity of the other material and impose the preferred orientation of that material on the composite film. Intrinsic stress of the fluorides is small and tensile whereas the stress in thermally evaporated ZnSe is compressive. The stress in the composite films changes abruptly at 70% SrF2 composition from compressive to low tensile.
Enhanced acoustooptic diffraction using nonlinear optical feedback
Authors: Vachss Frederick , Mcmichael Ian
Published: February 2023

It is well known that the photoelastic coupling between sound waves and refractive index variations in an acoustooptic device is electrostrictive in nature.1 The presence of an electrostrictive nonlinearity in a medium, however, allows optical electric fields to influence the strain field within the medium. Thus an acoustooptic Bragg deflector operated at sufficiently high optical powers will exhibit a coupling from the optical fields back into the original acoustic wave. We present the theory of this optical feedback and derive the conditions in which it may be used to enhance or diminish the diffraction efficiency of an acoustooptic beam deflector.
System characterization of the Hamming apodized acoustooptic Bragg cell
Authors: Pieper R. , Poon T.-C.
Published: February 2023

An acoustooptic system can be characterized in terms of the spatial impulse response or a transfer function which relates the angular plane wave spectrum of the output light field to that of the input light field. A general methodology for calculating the acoustooptic impulse response and transfer function is briefly explained. This methodology is then specifically applied to the case of acoustooptic cells with and without Hamming apodization of the sound field. Results obtained are compared to analytical expressions for the conventional Bragg cell for typical ranges of the Klein-Cook Q parameter1 and typical sound intensities. Implications of the results relating to beam distortion2 in the scattered light field are discussed.
Optically cascadable folded perfect shuffle
Authors: Haney M. W. , Levy J. J.
Published: February 2023

The folded perfect shuffle1 (FPS) was proposed to overcome the space-bandwidth product limitations of 1-D optics. The approach is based on 2-D raster formatting of the input data and an off-axis magnifying imaging system which overlays and shifts the data to the proper format. Pixel interlacing is achieved via subpixel masking, shifted input elements, or shifted imaging lenses. The interconnection patterns comprising a multistage network generally must be cascadable to preclude the need for active interstage regeneration. In a cascadable network both output and input have the same intensity, format, array and pixel dimensions, and beam angles. The previous FPS approach does not meet these criteria. We propose a cascadable implementation of the FPS, based on the original off-axis imaging system, using imaging lenses to provide the proper pixel placement. A 2-D lenslet array is added to correct for the magnification of each data pixel, and a 2-D microprism array generates the1 proper beam angles so that light will not walk out of the system. The architecture has no fundamental light loss, and every stage is identical and possesses lenslet and prism arrays that can be fabricated, in large sizes (e.g., 256 × 256), with current technology.
Optical properties of photorefractive gratings
Authors: Saxena Ragini , Vachss Frederick , Yeh Pochi
Published: February 2023

A dynamic hologram can always be updated to allow for any movement of the optical components (like the drift in the alignment of the optical component over time). This makes the dynamic hologram very useful in optical signal processing. We have studied the diffraction efficiency that can be obtained from dynamic photorefractive gratings as a function of the intensity of the read beam and the direction of readout. For small two-beam coupling constants, the diffraction efficiency is independent of the intensity of the read beam as long as it is weaker than or equal to the intensity of the write beams. For large two-beam coupling constants, there seems to be an optimum value of the intensity of the read beam for which large diffraction will occur, after which it eventually falls to zero with increasing read beam intensity due to erasure of the grating being read. The maximum value of the diffraction efficiency of the read beam can be 50% or 100%, depending on the sign of the two-beam coupling constant. This change of sign corresponds to readout from the two different input ports for equal intensities of the write beams and demonstrates the nonreciprocal nature of the photorefractive gratings—a diffraction efficiency that is dependent on which input port the photorefractive grating is being read from. This is because from one readout direction, the grating written by the read beam and its diffracted beam reinforces the grating being read, thus leading to high diffraction efficiencies. Readout from the other direction results in the two gratings being out of phase, leading to a lower diffraction efficiency in this case.
Ion beam deposition of aluminum and silicon oxynitride coatings
Authors: Al-Jumaily G. A. , Mooney T. A. , Cardone P. A.
Published: February 2023

There is an increasing interest in the nitride and oxynitride coatings of aluminum and silicon for optical and microelectronic applications. Silicon nitride as well as aluminum nitride have indices of refraction of 2.1 in the visible region of the spectrum. Silicon oxide and aluminum oxide have indices of 1.45 and 1.65, respectively. Therefore the index of refraction of the coating can be varied by varying the ratio of oxygen to nitrogen in the coating.1 Important applications are in the fabrication of graded-index optical components such as graded-index antireflection coatings and rugate filters. The ability to tailor the index of refraction has been realized with application of ion beams to the deposition process of optical coatings. Thin films of nitrides, oxynitrides, and oxides of aluminum and silicon have been deposited using ion assisted deposition and ion beam sputtering. Experimental results illustrating the effects of deposition conditions on the optical properties of coatings are presented.
Novel techniques for ray tracing in inhomogeneous media
Authors: Stone Bryan D. , Forbes G. W.
Published: February 2023

Tracing rays through inhomogeneous media generally involves numerically integrating a differential equation. Accuracy doubling,1 which takes advantage of the extremal property of a ray (namely, Fermat's principle) in a novel way, provides a framework for deriving new higher-order integration schemes from existing integration schemes. These accuracy doubled integration schemes will, in general, be of twice the order as the scheme from which they are derived. This technique has been applied to a commonly used fourth-order Runge-Kutta scheme introduced by Sharma et al.2 In the context of inhomogeneous ray tracing, however, the form of the differential equation makes it possible to derive a more efficient fourth-order Runge-Kutta scheme on which to apply the techniques of accuracy doubling. The resulting scheme is eighth order, but it requires fewer computations to achieve a given accuracy than the eighth-order scheme derived from Sharma's method. It so happens that, to derive these accuracy doubled schemes, the underlying Runge-Kutta scheme must be modified to provide differential ray information about the base ray being traced. A method of determining this differential ray information is presented, and this procedure has uses outside the context of accuracy doubling.
Beam distortion due to carrier refraction in ZnSe at 532 nm
Authors: Wu Y. Y. , Hagan D. J. , Van Stryland E. W. , Soileau M. J.
Published: February 2023

We report intensity-dependent self-defocusing in polycrystalline ZnSe via beam distortion measurements using 30-ps frequency-doubled Nd:YAG laser pulses. The dominant mechanism is nonlinear refraction due to carriers created by two-photon absorption (2PA). This sequential X(3):X(1) process is equivalent to a fifth-order process. The numerical fitting to the experimental results gives the refractive index change per carrier of 3.3 × 10−21 cm3 using a 2PA coefficient of 5.5 cm/GW.1 We interpret these measurements in combination with DFWM and Z-scan (a method for n2 measurements) experimental results,2 where a fast third-order followed by a slower fifth-order nonlinearity are detected. The magnitude of the third-order nonlinearity given by the Z-scan experiment was n2 = −3.5 × 10−11 esu. At low irradiance levels where the third-order nonlinearity governs the nonlinear process, this value is too small to cause any observable change in the beam shape. For this reason, we can conclude that only the carrier induced nonlinearity is important in passive semiconductor optical limiters.1 While this highlights the insensitivity of beam distortion measurements to phase distortions of less than λ/4, the technique remains useful for quantifying large refractive nonlinearities, such as those required for optical limiting.
Two wavelength operation of a nonlinear fiber loop mirror
Authors: Nelson B. P. , Nayar B. K. , Blow K. J.
Published: February 2023

The nonlinear fiber loop mirror has been shown to be a useful device for demonstrating nonlinear optical switching. Following on from the theoretical work1 a number of experimental results have been published. These have included long pulse (quasi-cw) switching and recently complete pulse switching using solitons.2 Examples were single source but it is possible to use a source at one wavelength to switch a low power signal of another wavelength and this is the situation we have investigated. The switching power is provided by a mode-locked Nd:YAG laser at 1.32 µm and the switched signal is from an F-center laser operating at 1.53 µm. We present results showing up to four switching cycles of the 1.53-µm source, the number of cycles being eventually limited by the onset of Raman generation. This arrangement of the nonlinear loop mirror represents a method of modulating one signal with another merely by using a coupler and a length of optical fiber.