bluuebunny/crossref_metadata_2025 huggingface dataset:
Content addressable memory with smooth transition and adaptive thresholding
Authors: Lee K. S. , Farhat Nabil H.
Published: February 2023

An optical implementation of the Hopfield model of neural nets may offer advantages over a microelectronic implementation because of the parallelism and massive interconnectivity of optical systems. As the number of stored patterns increases, the elements of the memory or the synaptic matrix will assume values spread over a wider range. This means wider range of grey levels in the optical mask required. For ease of optical implementation, quantization into a bipolar binary (BB) mask or even into a unipolar binary (UB) mask would be desirable. We examine the use of UB masks with adaptive thresholding and a smooth or graded neuron response. For the UB mask a nonzero thresholding value must be adopted. Since the given information is contained in bits of zeros and ones, one-half of the input energy serves quite well as an adaptive sharp thresholding level. As each iteration proceeds, the weighted projection of the current iterate is applied to an adaptive sharp thresholding unit. The result is multiplied by a relaxation parameter, of value < 1, and is combined with previous iterate and is finally limited to [0,1]. In this way we realize a smooth transition of states of the neural net effectively. The smooth transition method is also related to relaxation techniques used in solving systems of linear equations. Results of digital simulation and experiment are presented for a 32 neuron system. These are shown, as far as storage capacity and error correction are concerned, to compare favorably to the performance of a multivalued mask where merely sharp thresholding is used.
Bragg cell processors for complete Fourier transforms
Authors: Pancott B. , Reeve C. D.
Published: February 2023

Bragg cell acoustooptic processors have been used to find the complete Fourier transforms of signals using various architectures. Two dual-Bragg cell systems which implement the multiply-convolve-multiply algorithm have been used to obtain the real and imaginary parts of the continuous Fourier transform of various signals. The results are compared with the theoretically calculated transforms, and the performance of the processor is evaluated. The possibility of obtaining a complete Fourier transform using only a single Bragg cell is considered.
Gabor functions and textural segmentation
Authors: Turner Mark R.
Published: February 2023

This paper investigates the applicability of Gabor functions to textural segmentation. Gabor functions are sinusoidal plane waves in 2-D Gaussian envelopes. The choice of parameters characterizing the geometry of an individual Gabor function affects its spatial extent as well as orientation and spatial frequency tuning. Daugman has indicated that these functions belong to a class of filters having optimal joint resolution in the 2-D space and 2-D frequency domains. They are, therefore, appropriate filter choices for tasks which require selective measurement in these domains. Textural segmentation appears to be one of those tasks. A set of Gabor functions of different frequencies and orientations is applied by computer program to images containing regions of different texture. This process produces a kind of localized and orientation selective frequency spectrum of various fields in the image. The program then attempts to delineate the boundaries of the textured regions by identifying spectrum differences between these fields. Gabor functions are effective in distinguishing between many of the textures used in psychophysical studies differing in first- or second-order statistics. Additional textures in which the difference is related to some aspect of the collinearity of the texture elements have also been tried with promising results.
Associative memories: a survey
Authors: Kohonen Teuvo
Published: February 2023

Various constructs named associative or content-addressable memories have existed for about thirty years, and they are extensively used in computer technology, especially as fast buffer memories. The role of associative memories as the basic storage medium of large relational data bases, or for the storage and handling of text has been much less significant than initially expected. Two reasons for this are (1) the still relatively high price of the associative memory elements, (2) theoretical problems in the description of associations. The cost problem could be solved soon by extra-large-scale integration of the electronic circuits or possibly by new optical methods. On the other hand, natural experiences, for example, the sensory data from imaging devices, are seldom describable by discrete relations. A genuine associative memory ought to take into account and be able to represent the statistics of the occurrence of various items. This task falls into the category of pattern recognition problems. A natural solution would be a massively parallel, collectively operating computer resembling the biological brain. These aspects and the basic technologies of associative memories are surveyed in this presentation.
Adapation mechanisms of color appearance
Authors: Larimer James
Published: February 2023

Natural illumination varies over a wide range of intensities and colorations during the course of the day. Monocular experiments which briefly present spots of light to a light-adapted observer have revealed at least two adaptation mechanisms to deal with these changes. One mechanism adjusts the sensitivity of the visual system and the other tends to cancel out coloration in the illuminant. Both mechanisms help to maintain the constant color appearance of surfaces viewed in these varying illumination conditions. Measurements made with the opponent colors equilibria, lights that are neither red nor green (i.e., lights that are neutral for the red/green opponent color mechanism) or lights that are neither yellow nor blue (i.e., lights that are neutral for the yellow/blue opponent color mechanism), reveal the profound color appearance effects of these adaptation mechanisms. The sensitivity mechanism measured with superthreshold lights will be related to sensitivity changes measured at threshold. The canceling effect will be shown to depend on the spatial properties of the display near threshold, clear borders reduce the canceling effect.
Cr-doped SrAIF5 tunable laser
Authors: Lai Shui T. , Jenssen H. P. , Gabbe D.
Published: February 2023

We have demonstrated room temperature laser operation in Cr-doped SrAIF5. In a 20-cm long nearly concentric resonator a 1.2-cm long crystal was longitudinally pumped with the 647.1-nm line from a krypton-ion laser. The laser was operated in a quasi-cw chopped mode. Stable repetitive relaxation oscillations are observed in the laser output. With output mirror transmission ≃ 0.5%, the output is 60 mW at 932 nm with 2 W of absorbed power at 647.1 nm and the threshold is 1.12 W. A tuning range of 852-947 nm has also been achieved with a single element birefrigent tuner. The relatively high laser threshold indicates a high-loss in our presently available sample. From the laser power I/O plot, a round-trip loss of 5% in our 1.2-cm long sample has been calculated. Such loss is not uncommon among laser crystals in their early stage of crystal growth.
Strong polarization sensitive electroabsorption in GaAs/AIGaAs quantum-well waveguides
Authors: Weiner J. S. , Miller D. A. B. , Chemla Daniel S. , Damen T. C. , Gossard A. C. , Wiegmann W. , Wood T. H. , Burrus C. A.
Published: February 2023

We report the first measurements of strong perpendicular field electroabsorption (quantum-confined Stark effect) in GaAs/AIGaAs quantum wells for light propagating parallel to the plane of the layer. The samples consisted of two quantum wells embedded in a GaAs/AIGaAs superlattice, clad with layers of GaAs to form a leaky waveguide. The superlattice was doped as a p-i-n diode, which was reverse biased to apply an electric field perpendicular to the quantum-well layers in the intrinsic region. For incident polarization parallel to the plane of the layers both the heavy hole (hh) and light hole (lh) excitons are seen in absorption. With perpendicular polarization, which is only accessible in the waveguide geometry, only the lh exciton appears. In both geometries the absorption edge shows a marked shift to lower energy with increasing field. In the perpendicular polarization the lh exciton is still resolvable at a field of 2.2 X 105 V/ cm, at which point it has shifted by 40 meV. For a 150-µm long sample we obtain a modulation depth of 10 dB. We have also demonstrated for the first time optical bistability due to the self-electrooptic effect in a waveguide. The advantage of this geometry lies in the strong (20:1) on/off ratio obtainable. By using the appropriate biasing we have also demonstrated other self-electrooptic effects such as self-linearized modulation and optical level shifting similar to those previously demonstrated in MQWs at normal incidences. These effects have application to integrated optics and polarization sensitive devices.
Thermooptic and thermal expansion coefficients of nonlinear crystals isomorphic to KH2PO4
Authors: Hoefer Carolyn S. , Deshazer Larry G.
Published: February 2023

The sixteen isomorphs of potassium dihydrogen phosphate (KDP) are very useful in state-of-the-art nonlinear optical devices. Proper design of these crystals in high power laser applications requires knowledge of their thermal characteristics. The thermooptic coefficients α
n
= n−1dn/dT and the thermal expansion coefficients α
L
= L−1dL/dT were measured for thirteen of these isomorphs (KDP, KD*P, ADP, AD*P, CDA, CD*A, ADA, AD*A, RDP, RD*P, RDA, RD*A, KDA). A double interferometric technique using Fizeau and Twyman- Green interferometers was developed to measure these thermal coefficients. As a sample was heated, fringes from each interferometer shifted as a result of the changing optical path length in the sample. Even though both fringe patterns were functions of α
L
and α
n
, the two patterns changed with temperature at different rates. α
L
and α
n
were determined from the measurement of these two fringe rates by the simultaneous solution of the two equations that describe the phase shifts for the two interferometers. The dispersion in α
n
was measured for all thirteen isomorphs between 441 and 1152 nm. Data for the KDP crystal were fit to a theoretical prediction based on a two-resonance Sellmeier fit of the refractive index.
Calculating machines based on content addressable memories
Authors: Caulfield H. John
Published: February 2023

A variety of calculating machines including Turing machines (TMs) and cellular array processors (CAPs) appear to need optical content addressable memories (CAMs) for high speed and/or high flexibility. In all cases auxiliary memories (the tape for TMs and the array for CAPs) are needed as well. Because the requirements on them are not severe, these auxiliary memories need not be optical. This use of optical CAMs permits extremely long and complex operations to be carried out quite rapidly. In the simplest scheme binary input maps are converted into new binary input maps through CAMs. The particular program is fixed. Data input is by pulsed light addressing of acoustooptic cells with a data flow pattern chosen to minimize temporal complexity. In future systems, flexible, more complex or even adaptive CAMs will prove useful.
All-optical associative holographic memory with feedback using phase conjugate mirrors
Authors: Dunning G. J. , Marom E. , Owechko Y. , Soffer B. H.
Published: February 2023

We propose to use a hologram as a storage medium in an all-optical feedback configuration which includes phase conjugate mirrors. The associative properties of holograms have been suggested earlier by Gabor. Addressing the system by a partial or distorted version of the stored data generates the complete version most closely associated with that input. The phase conjugate mirrors are used to form an optical resonator, containing the hologram, with threshold adjustment such that only the strongest reconstructed image and its corresponding reference beam are fully reconstructed. Because a hologram can be decomposed into a set of single holographic gratings, we demonstrate that a phase conjugate resonator can operate with multiple intracavity gratings. The resonator consisted of the phase conjugator, BaTiO3, an intracavity hologram of two superimposed gratings, and two output couplers normal to each diffracted beam. The resonator oscillates between both output couplers unless there is a different loss in each path. By adjusting the loss and therefore the threshold, either leg of the resonator could be made to oscillate. To replace permanent holograms, the use of photorefractive materials for real-time adaptive associative memories is also presented.
Mechanisms for chromatic discrimination in the macaque LGN
Authors: Valberg A. , Seim T. , Lee B. B.
Published: February 2023

Combinations of responses of some types of spectrally-opponent sustained cell of the macaque lateral geniculate nucleus (LGN) may be related to equidistant color space. Response curves of such cells to stimuli of different luminance ratios and wavelengths show striking similarities to opponent coordinates of the new SVF color difference formula. 1 Mathematical simulation of the responses of these cells to a variety of color stimuli is possible through computation of cone excitations and subsequent sums and differences of cone signals. When the response functions thus obtained for cells are used to replace the coordinates of the SVF diagram, depending on the choice of cells, the distributions of equiluminous stimuli with the same sensory differences from an achromatic stimulus approximate ellipses or circles about the white point, and loci of constant hue approximate straight lines. This indicates possible roles of these cell types for color scaling in primates. Improved uniformity may be obtained by including more LGN cell types with best responses to other luminance ratios and other directions of color space, the combination of responses probably occurring at a later stage.
Laser cooling and confining of atoms
Authors: Hollberg L. , Chu Steven , Bjorkholm John E. , Cable Alex , Ashkin A.
Published: February 2023

Using six optical beams (+ and − directions on three orthogonal axes) tuned slightly to the red of resonance it is possible to cool sodium atoms to a few hundred microkelvin. The cooling occurs because the laser radiation pressure creates a damping force which reduces the atomic velocities to a rms value of ~60 cm/s, determined by random scattering. In this optical molasses, the laser beams propagate in all directions and the light acts as a viscous photon fluid which damps the atomic velocity. The resulting motion of the sodium atoms is diffusive until the atoms reach the edge of the laser beams where they escape. The process of atomic confinement by optical molasses can be modeled as a random walk in a viscous medium with a boundary. For a spherical volume of 0.2 cm3 the resulting confinement time is ~0.1 s for half of the atoms to escape. Direct time-of-flight measurement of the atomic temperature gives ~240 μK, which agrees with predictions of quantum limited cooling by resonance radiation pressure. With confinement times of 0.1 s and temperatures of ~240 μK this method provides a source of atoms sufficiently slow for even the highest resolution optical spectroscopy, collision studies, or optical traps.
Optical properties of graphite pellets
Authors: Querry Marvin R.
Published: February 2023

Many pellets were prepared seperately from Dixon KS-2, HPN-2, 200-10, and Asbury Micro 260 graphite powders which were compressed between polished stainless steel rams at a pressure of 0.67 GPa for 30 min. The most specular reflective pellet surface thus formed from each of the four powders was then selected for further investigation. The mass/unit volume of the pellets was in the 2.02 ± 0.06-g/cm3 range compared with 2.23 g/cm3 for crystalline graphite. Near-normal incidence reflectance spectra R(v) of the four pellets were obtained throughout the 180-50,000-cm-1 wave-number region. Kramers-Kronig methods applied to R(v) provided complex refractive indices N(v)= n(v)+ ik(v) and dielectric functions ϵ(v) = ϵ1(v) + iϵ2(v). The E1
u
in-plane and A2
u
out-of-plane infrared active vibrational modes showed maxima in the k(v) spectra at 1584 and 864 cm-1, respectively. These modes were also quite evident in the first derivative of the reflectance spectra. Spectral values of R(v), N(v), and ϵ(v) are presented in graphical form. Differences and similarities to spectra for crystalline graphite are also presented.
Photodissociation dynamics of cluster anions
Authors: Castleman A. W. , Albertoni C. R. , Marti K. , Hunton D. E. , Keesee R. G.
Published: February 2023

Studies of the dissociation dynamics of cluster ions provide insight into the process of energy disposal for mass-selected species. Detailed investigations of the photodissociation of the cluster anions (SO2)2− and CO3−(H2O)n (0 ⩽ n ⩽ 3) have been accomplished for mass-selected ion species using an intracavity dye laser pumped with an argon-ion laser. In the case of CO3−, the unhydrated parent ion is observed to have a bound electronic excited state, through which absorption of a second photon proceeds to a repulsive state leading to the ejection of O−. There are two possible mechanisms for CO3− hydrate dissociation: one is a repulsive and the second a predissociative mechanism. In both, cluster dissociation is initiated by the same 2A1←2B1 transition from the ground to a weakly bound excited state of the core ion and leads to the loss of all water molecules within the time of observation. In the photodissociation of CO3−, CO3−(H2O)1,2,3, and (SO2)2− considerable excess energy is partitioned into relative translation of the photoproducts. Through studies of energy release in (SO2)2− with photons of parallel and perpendicular polarization, evidence has been obtained that the lifetime of the complex preceding photodissociation is less than a rotational period. The implications of the findings are discussed In terms of phase space theory.
End-pumped Ti:sapphire oscillators: design and modeling
Authors: Eggleston John M. , DeShazer L. G. , Kangas K. W.
Published: February 2023

Ti:sapphire oscillators longitudinally pumped by doubled Nd:YAG laser radiation are a useful source of tunable near-IR radiation. Unfortunately the design and modeling of these oscillators involve considerations which are different from standard flashlamp pumped storage laser systems. We present the resonator design considerations to avoid resonator optics damage at low fluences and the material/optical kinetics required to achieve a detailed fit between experimental and numerical results. Modeling parameters include Ti:sapphire kinetics, gain switch optical kinetics, bandwidth, and a comparison of 1-D, 2-D, and 3-D physical optics modeling of the optical mode.
Effect of intracavity birefringent elements on mode characteristics of solid-state laser resonators
Authors: Newstein M. , Lin K. , Liu K. C. , Rhoades M. , Cohen Martin G.
Published: February 2023

It is well known that the addition of intracavity birefringence elements in solid-state laser resonators can lead to significant modification of the mode characteristics when thermal birefringence in the laser rod is significant. We have developed a theoretical model of the birefringent laser to analyze this class of problems and have used it to derive the spatial and polarization properties of a cw-pumped Nd:YAG laser operating with intracavity birefringent elements for frequency doubling. In particular, the effects of a type II SHG crystal (KTP) and a fundamental quarterwave plate in a configuration described previously have been studied and compared with the experimental results.
Atmospheric isoplanatic angle measurements
Authors: Walters Donald L. , Stevens Kurt B.
Published: February 2023

Four generations of atmospheric isoplanatic angle sensors have recently evolved. An apodized 20-cm aperture lightweight low-cost instrument has been developed that achieves accurately weighted isoplanatic angle measurements with 1-s time resolution. Comparing data from four different locations in the northern hemisphere, at least three general isoplanatic angle categories are evident. Frequently we observe rapid, 10 s to several minutes, 200–400% variations in the atmospheric isoplanatic angle. The location of the atmospheric jet stream is the most significant forcing function.
Spectral line profile of turbulent gas
Authors: Gao Z. , Qian J.
Published: February 2023

A simple theoretical model is proposed to determine the spectral line profile of a turbulent gas. It is shown that the spectral line profile of a turbulent gas is still a Voigt profile. Its Lorentzian component is hardly influenced by the turbulent motion.
Laser beam propagation through turbid media using small-angle approximation for radiative transfer equations
Authors: Lou Shu-Hsiang , Kuga Yasuo , Tsang Leung , Ishimaru Akira
Published: February 2023

Laser beam propagation through fog or cloud is a problem of considerable interest. Although numerical solutions of the radiative transfer equation are available for the case of plane wave incidence, only very few numerical solutions are calculated for the beam wave case because of the formidable amount of computation time required. The small-angle approximation has been shown to be useful for lightwave propagation in turbulent media where the variance of index of refraction fluctuations is small. However, its applicability to turbid media with discrete scatterers is not clear because of the much larger contrast in the index of refraction. The small-angle approximation is used to calculate the beam wave solution through foggy atmospheres at visible wavelengths. The Henyey-Greenstein phase function is used to describe the scattering characteristics of the particles. The mutual coherence function is first calculated and then the intensity and the specific intensities are calculated. Numerical results are illustrated as a function of optical thickness and the mean cosine of scattering angles. They are compared with available exact numerical solutions of the transport equations and are also used to explain the results observed in laboratory experimental data.
Multimicroprocessor computer systems in optics
Authors: Shoemaker Richard L.
Published: February 2023

The commercial availability of powerful 32-bit microprocessors provides an unprecedented opportunity for researchers to construct inexpensive but very powerful multiprocessor computer systems that are optimally designed for specific computational tasks. However, to obtain good performance from such systems it is essential that both the processor interconnection architecture and the system software be appropriate for the problem. Two multiprocessor system designs being developed at the Optical Sciences Center illustrate the potential of such systems. The first is an image- data-driven dynamically reconfigurable multiprocessor using thirty Motorola 68000 microprocessors interconnected by multiple global data buses. This system is fully operational and is being used to develop machine vision systems capable of very high speed biomedical image acquisition and image understanding. The second system is still in the design phase and will be used for image reconstruction using simulated annealing. It consists of an array of sixty-four Inmos Transputers with fourway or six-way nearest-neighbor interconnections.
Multiple focused image hologram model
Authors: Kostuk Raymond K. , Goodman Joseph W. , Hesselink Lambertus
Published: February 2023

A model describing the image irradiance of two images produced by a multiple grating reflection type focusing element is developed. The gratings may be formed simultaneously or sequentially, and the recording medium can change thickness during processing. In the algorithm, the spot size and location of the image are first calculated by ray tracing from each surface grating in the multiplexed grating element. The efficiencies of these rays are then computed with a multiple grating model which allows for coupling between the fields in the grating volume. Finally, the efficiency of each ray is weighted by a factor corresponding to the intensity profile of the reconstruction beam. The sum of these weighted rays per unit area in the image plane determines the relative image irradiance. A comparison is then made between the image irradiance produced by experimental multiple focused elements and the calculated irradiance pattern. The model predicts the relative profile and the size of the irradiance patterns.
Optical limiting observed in GaAs/AlGaAs multiple quantum well cladded waveguides
Authors: Mueller Craig T. , Kanter H. , Vernon F. L.
Published: February 2023

Optical limiting in a waveguide cladded with a nonlinear medium has been predicted to occur when the refractive index of the cladding approaches the effective index of a guided mode. To date, this effect has only been demonstrated experimentally for the TE1 mode in a glass waveguide cladded with a nonlinear liquid crystal, MBBA. It has been suggested that as the intensity is increased the TE0 mode will first exhibit a localized maximum, and then degenerate into a surface polariton guided at the interface. We report here the first measurements of optical limiting of the te0 mode in GaAs/AlGaAs MQW cladded waveguides. The waveguide structure consists of a 0.4-μm thickness of a GaAs/Ga0.94Al0.06 As (100A/100A) superlattice grown by molecular beam epitaxy on a 1.2-μm GaAs/Ga0.6Al0.4As (100A/100) MQW substrate layer. At milliwatt power levels the output power of the TE0 mode polarized parallel to the MQW layers saturates and then falls off with increasing input power. The TM0 mode polarized perpendicular to the MQW layers does not show these effects. Low power absorption coefficients for the TE0 mode range from 150 cm–1 at 864 nm to 20 cm–1 at 875 nm. Assuming a simple theory of the intensity-dependent waveguide cutoff, a nonlinear index of n2 = 1 × 10–6 cm2/W is predicted.
Small-size low-loss Ti:LiNbO3 waveguides for optimum device performance at 1.3 μm
Authors: Suchoski Paul G. , Horton T. , Ramaswamy R. V.
Published: February 2023

Previous efforts have concentrated on adjusting the Ti:LiNbO3 fabrication parameters to achieve optimal coupling to single-mode fibers. Such waveguides exhibit fairly large mode size and are not suitable for efficient modulator applications. By methodical investigation, we have achieved small-mode size low-loss waveguides at 1.3 μm. A series of 2-cm long single-mode waveguides with titanium thicknesses of 500, 600, 700, 800, 900 and 1100Å and widths ranging from 2 to 4 μm were fabricated on the z surface of LiNbO3 by diffusing in a wet O2 environment for 5 and 6 h at 1025°C. The waveguides were then polished and characterized at 1.3-μm wavelength by measuring the fiber-waveguide throughput loss and the optical near-field patterns. The propagation loss in the guides was isolated from the other loss mechanisms by subtracting the mode mismatch loss between the channel waveguide and the single-mode fiber (which was evaluated numerically) and the Fresnel losses. The results indicate that the smallest possible waveguide (Ti strip width of 4 μm and thickness 800 Å, t = 6 h) at 1.3 μm has a 1/e intensity full width and depth of 3.8 and 2.7 μm, respectively, for the TM mode. The TE mode profile is considerably larger. This guide exhibits a propagation loss of 0.2 dB/cm for both TE and TM modes and a mode mismatch loss with a Corning single-mode fiber (7-μm 1/e intensity diameter) of 1.81 dB/face for TM modes. It is expected that this large mode mismatch loss can be eliminated by using a suitable tapered transition.
Annular apodizer for low sensitivity to defocus and spherical aberration
Authors: Ojeda-Castaneda Jorge , Andres P. , Diaz A.
Published: February 2023

For many practical uses in microelectronics, robotics, microscopy, and medical imaging, it is desirable to increase the focal depth and to decrease the influence of spherical aberration. It has been known for some years that annular apertures can be used for this purpose; in particular, some scanning optical microscopes exploit quite well the characteristics of annular apertures.
Optical method to locate the centroids of nonoverlapping objects in a scene
Authors: Freeman Mark O. , Saleh Bahaa E. A.
Published: February 2023

Matched filtering techniques have the important property that they can detect an object within a scene containing many other objects. However, when the target class contains statistical variations, matched filtering does not perform well. On the other hand, feature extractors for statistical pattern recognition usually operate properly only when the scene contains one object at a time, thus necessitating segmentation if the scene contains many objects.
Intensity covariance of waves propagating in a random medium with a Kolmogorov spectrum and an Inner scale of turbulence
Authors: Frehlich R. G.
Published: February 2023

The intensity covariance of a point source of waves propagating in a random medium is calculated using the asymptotic theory and a Kolmogorov refractive-index spectrum with an innerscale cutoff. In strong scattering, the variance is only a function of one parameter. The variance and small correlation scale are both larger than predicted for zero-inner scale. The small-scale structure is controlled by irregularities near the receiver while the large-scale structure is controlled by irregularities near the transmitter. The theoretical predictions are compared to experiment. Higher terms in the asymptotic theory are required in order to adequately predict intensity covariance. Full saturation of intensity fluctuations (variance of unity) is unlikely for laser propagation in reasonable atmospheric conditions. A similar analysis can be applied to the case of a Gaussian beam. The variance of intensity from a Gaussian beam is equal to the point source case when the transmitter aperture is smaller than the Fresnel distance and the beam diameter at the receiver plane is larger than the effective scattering region. The plane wave case is approximated when the transmitter aperture is larger than the scattering region and the radius of curvature is larger than the propagation distance. When the laser beam is focused onto the receiver plane, there is a regime where the variance is independent of the incident beam dimensions.
Bidirectional ring laser model: stability analysis and numerical solutions
Authors: Abraham Neal B. , Lippi G. L. , Mandel P. , Zeglache H.
Published: February 2023

Linear stability analysis of the bidirectional ring laser model of Khanin gives an analytic expression for instabilities. We describe the derivation of the model and indicate several oversights which have led to errors in models recently discussed in the literature. We find that there is a critical value of detuning necessary for instabilities that is of the order of the relaxation oscillation frequency and that there is a limited range of excitation values over which the instability occurs. The instabilities may be observed asymptotically close to the laser threshold for large detuning, when the field decay rates are large compared with the population inversion decay rates and when there is some difference in the decay rates for the fields in the two directions. Asymptotic limits for the boundaries of the instability region are presented. Bistability as well as instabilities appear depending on the degree of difference in the two field decay rates. Causes of such asymmetries in decay rates in real ring lasers are discussed. Time-dependent solutions show several different kinds of pulsation depending on the degree of detuning and excitation. For small detunings the laser periodically switches direction of operation with nearly complete suppression of the counterpropagating beam for relatively long periods of time. For larger detunings the solutions are more periodic and correspond to anticorrelated modulations of lasing in both directions simultaneously.
Effect of the aperture-lens separation on the focal shift in large f/No. systems
Authors: Wenzel Robert G.
Published: February 2023

The classical Lommel intensity distribution in the vicinity of the focus of a uniform spherical wave diffracted by a circular aperture is found to be in error in large f/No. systems. In particular, the point of maximum intensity is not found at the geometrical focus but is shifted toward the aperture. We consider the more general problem of a uniform spherical wave which is diffracted by a circular aperture and then focused by a lens. The Huygens-Fresnel integral is solved to obtain the axial intensity distribution following the lens. The result is where G (z) is the geometrical intensity (obtained in the absence of diffraction) and F (z) is the number of Fresnel half-period zones at z. The migration of the intensity maximum from the geometrical focus, known as the focal shift, is found to be dependent on the aperture–lens separation. Defining this separation to be z0 and the focal length to be f, we find that, for z0 < f, the focal shift is toward the lens and, for z0 > f, it is away from the lens. When z0 = f there is no focal shift even for small aperture radii, and the classical result is retrieved.
Fiber-coupled Interferometric Imaging array
Authors: Roddier F.
Published: February 2023

Interferometric image reconstruction with long base-line telescope arrays requires both a good u,v plane coverage and a good signal-to-noise ratio (SNR). This implies a redundant configuration of telescopes with adaptive wavefront corrections. We propose to build an experimental array of small movable telescopes with adaptive tilt compensation to be used in the red and near IR. Fiber optics is considered as a way to combine all the beams together. The array will be easy to expand and reconfigure providing a test bench for future earth-based and space interferometers. Two possible schemes are considered for beam recombination. In one scheme the output pupil is nonredundant and linear in one direction with spectral dispersion in the orthogonal direction. In the other scheme the output pupil is a scaled version of the input pupil. Theoretical SNR is computed in both cases assuming either photon noise or additive noise. The first scheme is shown to give the highest SNR for bright sources whereas the second scheme gives the highest SNR for faint sources. Fiber optics allow flexible beam recombination.
Fiber-optic electric field sensor
Authors: Lawson Christopher M.
Published: February 2023

A novel type of fiber-optic electric field sensor has been demonstrated. This device was fabricated by fusing piezoelectrically sensitized fibers into a fiber-optic Mach-Zehnder interferometer testbed. Quadrature stabilization for the interferometer test-bed was provided by an active homodyne scheme. The fibers were sensitized to an electric field by embedding them in a piezoelectric material that expands and contracts in response to the electric field. The resulting pressure waves modulate the optical path length through the sensitized fiber, which modulates the output of the fiber-optic interferometer. Two designs were used to sensitize fibers to electric fields. In the first design, unjacketed fibers were embedded in a piezoelectric lithium borosilicate (LBS) glass plate. In the second design, fibers were embedded in a composite PZT material. One of the LBS sensitized fibers and three of the PZT sensitized fibers were tested in the fiber-optic interferometer test-bed. The signal to noise of each sensor was mapped out vs frequency, with a reference electric field applied across the sensors. Future efforts will be devoted to directly coating an unjacketed fiber with a thin LBS coating.
Pulse distortion in single-mode optical fibers: chirped pulses
Authors: El-Halafawy Farag Z. , El-Badawy El-Sayed A. , El-Gammal Mohamed A. , Aly Mostafa H.
Published: February 2023

The theory of pulse distortion in single-mode optical fibers is extended to include laser sources that suffer a linear frequency sweep (chirp) during the duration of the pulse. The chirp is manifested as a variable frequency shift during the pulse. We have studied the soliton transmission in inhomogeneous optical fibers with W-tailored refractive index of the form1 where R is the fiber radius and α and m are the controlling parameters.
Metal waveguide modeling of tapered graded-index media
Authors: Zandi Bahram , Casperson Lee W.
Published: February 2023

Graded-index media having quadratic radial variations of the index of refraction have several well-known advantages for transmission of optical signals. Tapered graded-index waveguides are also of interest as waveguide couplers, power concentrators, and image size reducers. Unfortunately, the experimental study of such z-dependent media may be difficult. Even If well-characterized tapered media could be constructed, the measurement of the parameters of a microscopic propagating beam would not be straightforward. We describe a metal waveguide technique for inexpensive modeling of tapered graded-index fibers. One finds that in some respects the modes of a certain class of concave curved metal waveguides are mathematically equivalent to the modes of quadratic index media. The focusing profiles (curvatures) of these waveguides can be made arbitrarily weak, so that even at optical frequencies the modes may be large and easily observed. We have constructed several tapered metal waveguides using a variety of materials and corresponding to a variety of taper functions. The performance characteristics and applications of these devices are discussed, and the results are compared to theoretical expectations.
Highly damage-resistant cavity mirrors in vacuum to extreme UV high power lasers
Authors: Sasaki W. , Kurosawa K. , Uehara Y. , Takeuchi K. , Yoshida K. , Fujiwara E. , Kato Y. , Yamanaka M. , Yamanaka C. , Okamoto H.
Published: February 2023

Optical damage is a serious problem in highpower operation of short-wavelength lasers. Using a high-power argon excimer laser with a MgF2 output mirror of 70% transmittance, we are investigating the feasibility of various materials for the cavity mirrors. SiO2 glass, Si single crystal, and Mo single crystal have great potential in short-wavelength lasers. At 126 nm, the measured reflectivities were 80% for MgF2/AI of a conventional mirror, 14% for SiO2, 20% for Si, and 17% for Mo. These values were different from the reflectivities calculated by reported optical constants. The scattering loss due to surface roughness might cause significant degradation in their performance especially in this spectral region. A MgF2/AI mirror damaged at ~1 MW of the output power, and SiO2 led to ~5-MW performance but with a slightly damaged surface. Si and Mo allowed the high-power operation more than 3 MW without any surface damage. Since Mo keeps high reflectivity down to 60 nm, it is feasible for the cavity mirror of extreme ultraviolet lasers.
Nonlinearities in glass for subpicosecond optical switches
Authors: Friberg S. R. , Smith P. W.
Published: February 2023

Ultrarapid (subpicosecond) optical switching and signal processing elements should find important applications in future communication and computing systems. Glasses with fast (< 100-fs) electronic third-order susceptibilities could be used to fabricate ultrarapid switches. These nonlinearities, described by the nonlinear index coefficient n2, are compared with slow thermal index changes for various glasses. A degenerate four-wave mixing (DFWM) technique using a 250-ns train of 80-ps pulses from a mode-locked Q-switched Nd:YAG laser was used to measure n2. Thermal index changes were obtained by comparison of the DFWM signal from initial and final pulses of the pulse train. Results for leaded glasses agree with previous reports, with n2 of SF-59 equal to 19% of that of CS2. Titanium glasses, e.g., Hoya FDS-90, have an n2 that is 7 % that of CS2, and n2 does not increase with linear index as for leaded glasses. The observed thermal index changes are small, as expected for very low absorption coefficients. We define a figure of merit to help compare nonlinear materials for use in optical switches. It is the ratio of the (fast) nonlinear index change necessary to produce switching to the accompanying (slow) thermal index change. We show that some optical glasses appear to have higher figures of merit than any other nonlinear optical material. We conclude with a discussion of some possible configurations for glass all-optical switches.
Observation of nonlinear transmission prior to laser damage in ZrO2
Authors: Mansour N. , Soileau M. J. , Van Stryland E. W.
Published: February 2023

We report direct measurements of nonlinear transmission prior to laser-induced breakdown in a wide band gap dielectric. Various workers have suggested that bulk laser-induced breakdown is due to avalanche breakdown induced by multiphoton absorption. A detailed quantitative description of the generation of the initial conduction band electrons and the dynamics of the breakdown process is not yet available. A major obstacle to the development of a quantitative description of the damage process has been the inability of researchers to measure the onset of nonlinear absorption (e.g., multiphoton absorption) which initiates the avalanche process. In this work we observe pronounced, reversible nonlinear transmission prior to bulk breakdown. The sample studied was cubic zirconia (ZrO2) stabilized with yttria (Y2O3) which has an UV transmission cutoff at 330 nm. Our measurements were made at 532 nm which is within the range where two-photon absorption is possible. The observed nonlinear transmission prior to breakdown is consistent with the interpretation that damage in this material is due to two-photon absorption-induced avalanche breakdown.
Lau effect with cross gratings
Authors: Cheng Yih-Shyang
Published: February 2023

A grating, when illuminated by a quasi-monochromatic point source, can form self-images downstream. With a periodic, spatially incoherent source illumination, when the self-images due to all the source points coincide, a high contrast gratinglike structure is formed. In this paper, the Fresnel approximation is used to calculate the diffraction pattern of the object. The resulting intensity pattern is essentially the correlation between the intensity distribution of the spatially incoherent source and the scaled intensity distribution of the Fresnel diffraction of the object. By suitably choosing the plane of observation and the source structure to object structure ratio, a high contrast cross-gratinglike structure can be obtained. Al though the image pattern is the correlation between the source structure and the scaled object structure, the magnification ratio between the self-image and the source structure is the same as that in conventional imaging. We can then consider the self-image to be the image of the source structure, imaged by the object grating. When the illuminating source is imaged to the right of the object grating, the relationship between the self-image and the new source is found to be the same as before.
Design of a novel millimeter wave to infrared lightguide
Authors: Bagby Jonathan S. , Smith Charles V.
Published: February 2023

A novel hollow, multilayered O-type dielectric lightguide is proposed for use at millimeter wave, submillimeter wave, and near-infrared frequencies. In this frequency range closed-pipe metallic waveguides and optical fibers exhibit prohibitive material losses. In contrast to conventional light-guides, energy is not guided by surface waves in this structure; instead, guidance is accomplished in the air-core region by reflection of incident radiation from the inner air-dielectric interface. This results in a minimization of the energy confined to the lossy dielectric material. The number of layers used, layer thicknesses, and layer composition are chosen to maximize reflection at the inner interface for a prescribed range of incidence angles, in close analogy to the process used in design of dichroic laser cavity mirrors. Fabrication techniques perfected in construction of optical fibers can be readily adapted to this new configuration. Numerical results demonstrating feasibility with realistic material parameters are presented. Applications of such a guide in construction of optical devices such as transversal filters are discussed.
First-order Image surface of nonaxially symmetric optical systems
Authors: Eckhardt Stephen K.
Published: February 2023

The standard first-order refraction equation has been rewritten in vector notation for application to nonaxially symmetric optical systems. This makes it possible to map several object points through the system and therefore visualize the image of the entire object plane. This image is a bounded curved surface. Given the shape of the image surface, it is possible to quickly evaluate a configuration of mirrors and/or lenses for application to a given design problem. Several designs are discussed, including some systems in which third-order theory predictions are seriously misleading. If time permits, techniques for determining the amount of keystone and anamorphism present will also be discussed.
Stereoscopic visual acuity for bending motion in 3-D space
Authors: Lappin Joseph S. , Craft Warren D. , Payne Theodore J.
Published: February 2023

Human stereoscopic vision may provide precise information about 3-D structure. Fundamental theoretical questions concern the specific geometric properties of monocular stimulation that carry visual information for stereopsis. Recent evidence on the stereoscopic acuity for moving patterns indicates that perceived 3-D structure derives from disparities between the monocular optical fields, rather than from disparities between retinal positions of individual points, and from the self-congruence of spatiotemporal form.1 The spatial structures in previous experiments, however, were rigidly moved. The present experiment tested the necessity for rigid motion. Optical patterns consisted of four dots (initially collinear and equally spaced—30 min of arc) undergoing bending motion in a plane tilted in depth. The task was to detect the depth displacement of a single dot from this continually changing curve. Observers detected displacements < 10-sec of arc disparity as accurately for bending curves as for stationary straight lines. Thus, stereoscopic acuity derives from the self-congruence of even nonrigidly moving form.
Generalized radiance of Lambertian sources
Authors: Winston R. , Ning X.
Published: February 2023

During the last two decades several attempts have been made to formulate the foundations of radiometry. Numbers of definitions of radiance (spectral intensity) have been proposed in terms of various kinds of second-order field correlation function. It has been shown that even although none of these definitions satisfies all the conditions that traditional radiometry requires, most of them do agree with traditional radiometry in the small-wavelength limit.
Optical architecture implications of parallel computation models
Authors: Jenkins B. Keith , Giles C. Lee
Published: February 2023

We describe the use and implications of parallel models of computation in the design of optical architectures. The models covered include shared memory models and graph/network models1; these models can be universal machines. Shared memory models consist of a set of processing elements operating in parallel and communicating with a global memory in an extremely general, contention-free manner; these models provide for fast computation of parallel algorithms. An architecture derived from a shared memory model is introduced and includes a set of electronic or optical processors, an optical reconfigurable interconnection network that permits fan-in, and some form of optical memory that permits contention-free simultaneous reads. Graph/network models consist of a set of processing elements with fixed interprocessor connections. Fine grained systems are considered. Fundamental differences between electronic and optical implementations of graph/network models are discussed; these stem from differences in assumptions on the processors and on the network degree and topology. Extensions to reconfigurable interconnections are also considered.