bluuebunny/crossref_metadata_2025 huggingface dataset:
InGaAsP (1.3 μm) vertical-cavity lasers using GaAs/AlAs mirrors
Authors: Dudley J. J. , Ishikawa M. , Miller B. I. , Babic D. I. , Mirin R. , Jiang W. B. , Shimizu M. , Bowers J. E. , Hu E. L.
Published: February 2023

We demonstrate room temperature optically pumped operation of a 1.3 μm vertical-cavity laser (VCSEL) on a GaAs substrate. The 1.3 μm InGaAsP/ InP double heterostructures (DHs) were fused to GaAs/AlAs mirrors at 650°C in hydrogen. The larger refractive index ratio of GaAs to AlAs makes these mirrors an attractive alternative to mirrors made of InP/InGaAsP. Also, current can be injected through the GaAs /AlAs mirror into the InP DH. The second mirror is made by depositing a Si/SiNx dielectric stack. The wafer is optically pumped by using a mode-locked Nd:YAG laser with an 80 MHz repetition rate and 80 ps pulses. The room temperature threshold was 20 mW for a 16 μm diameter input spot. The laser operated in a single longitudinal mode at 1.283 μm up to twice threshold.
Green thunderstorms
Authors: Bohren Craig F.
Published: February 2023

Green thunderstorms are observed occasionally. Only the severest
thunderstorms are green. With one exception, they have received no
scientific attention, experimental or theoretical. Fraser suggested
that a thick thunderstorm provides a dark backdrop. Greenness is a
consequence of reddened sunlight illuminating selective scatterers
along the observer’s line of sight. Thus a thunderstorm is not green,
it is a black backdrop for green airlight near sundown. An alternative
explanation, more in accord with my observations, is that green
thunderstorms may be a consequence of the intrinsic blueness of clouds
because of absorption by pure water, liquid or ice. Most clouds are so
thin that the light transmitted by them is not markedly colored
because of absorption. Only the most massive clouds, large both
vertically and horizontally, are thick enough to color incident
sunlight upon transmission. If that incident light is reddened, the
transmitted light can be perceptually green.
Drop composition effects on simulation of vertical profiles of extinction and backscatter coefficients in very low stratus clouds and subcloud regions
Authors: Kilmer Neal H. , Rachele Henry
Published: February 2023

A microphysics model developed by the authors simulates vertical profiles of drop size distribution, relative humidity, temperature, drop concentration, and liquid water content. Drop size distribution profiles simulated by this microphysics model were used with Mie formulations in computing profiles of extinction and backscatter coefficients for several wavelengths. The drops were assumed to be spheres and to exhibit complex indices of refraction defined for two cases: (1) pure water and (2) a combination of pure water and material from condensation nuclei in accordance with a mixture rule. The adequacy of assuming case 1 was tested by comparing results with those calculated for case 2. Extinction and backscatter coefficient profile differences between the two cases were often insignificant (for the wavelengths considered) for engineering applications. This result is significant for computing, since much less computer time is required to compute only one set of Mie efficiency factors per wavelength and then reuse them many times, as can be done when the drops are treated as pure water.
Compact interferometer using a laser diode
Authors: Jutamulia S. , Fujita A. , Toyoda S. , Niwa H.
Published: February 2023

A fiber end must be optically flat and perpendicular to the fiber axis to produce high coupling efficiency in a fiber connection. The cleave quality of the fiber end is usually inspected by using a Michelson interferometer. The interferogram is visually inspected by a microscope. The disadvantage of this equipment is that the whole system is bulky and not portable. We improve the system by using a compact laser diode emitting coherent light at 670 nm with 5 mW of power. Since the optical power is sufficiently high, the interferogram can be directly projected onto a screen. Therefore, we can observe the interferogram without using a microscope. This makes the system very compact and portable.
Eye-safe lidar at 2.09 μm
Authors: Overbeck Jay A. , Duncan Bradley , McManamon Paul , McCracken Scott
Published: February 2023

A 2.09 μm eye-safe laser radar is being built that will have a coherent and an incoherent mode of operation. The trade-offs between coherent and incoherent detection methods will be investigated. In coherent detection a single pulse can be used for both velocity and range measurements. The problem that arises in coherent detection is that accurate velocity measurements require a long pulse and the longer the pulse the worse the range resolution. Another problem that exists in coherent detection is the longer the coherence length of the pulse the more degradation in the signal at the receiver caused by speckle. In an incoherent system velocity measurements can be made with two successive range measurements. The pulse length used can be shortened, increasing the range resolution and decreasing the degradation caused by speckle. This system will be used to compare the theoretical predictions with actual measurements as well as to examine the practical implementation of the different detection techniques.
Eye-safe lidar system for boundary layer aerosols
Authors: Gimmestad G. G. , Patterson E. M. , Roberts D. W.
Published: February 2023

An eye-safe lidar system has been developed for the purpose of monitoring the vertical distribution of boundary layer aerosols in Atlanta, Georgia, as part of an intensive series of air pollution measurements to be conducted during the summer of 1992. The system has a Raman-shifted Nd:YAG source laser at 1.54 μm wavelength operating at 4 Hz pulse repetition frequency with a pulse energy of 60 mJ. In order to cover an expected range of variation in boundary layer thickness from 50 m to 4 km, the system has two receivers, one with a 46 mm aperture and a 105 mm focal length and another with a 150 mm aperture and a 300 mm focal length. Both use InGaAs p-i-n diode detectors. The system will be used in a semi-automated mode to monitor diurnal variations in the vertical distribution of aerosols, from which boundary layer thicknesses will be inferred.
A model for the limits of stereopsis at low spatial frequencies
Authors: Kontsevich Leonid , Tyler Christopher
Published: February 2023

Studies of upper and lower stereoscopic thresholds demonstrate that at 2.5 c/deg the thresholds abruptly change their behavior; below this frequency they fit power functions of wavelength (e.g., power 1 for static depth discrimination task and 1/2 for dynamic case). Dichoptic spatial frequency tuning studies have been interpreted as revealing a set of narrowband channels in this low frequency domain. However, data on the monocular masking of narrowband stereoscopic targets indicate that the sensitivity of channels below 2.5 c/deg is negligible. We therefore made the alternative assumption that there are channels only above 2.5 c/deg and that the rise of thresholds for low frequencies is a result of the drop in the effective contrast response of the 2.5 c/deg channel at low frequencies. The dichoptic tuning functions for low frequencies would also then be explained by contrast effects. The dichoptic difference in mean spatial frequency, which is varied for the tuning experiments, produces a difference in effective contrast between the eyes, which, in turn, causes the observed deterioration in sensitivity. Based on these ideas we propose a model that provides a consistent explanation for a large set of experimental data.
Optical exchange-bypass switch with multilevel mirrors
Authors: Athale R. A. , Daliltojary S.
Published: February 2023

An optical 2×2 exchange-bypass switch that can be coupled with an optical perfect shuffle network is a critical element in building an optical multistage interconnection networks. We describe a design for optical exchange-bypass switch based on a modified Mach-Zender interferometer (MZI). The mirror in one arm is replaced by a patterned multi-level mirror that reverses the spatial ordering of adjacent channels. A multielement liquid crystal switch in conjunction with polarizing beam splitters and combiners determines which data-channel pairs will be sent along the exchange arm of the MZI. We describe the design and fabrication of the multilevel mirror and present experimental results for an eight-channel exchange-bypass switch.
Demonstration and assessment of a cellular logic image processor using arrays of symmetric self electro-optic effect devices
Authors: Wakelin Suzanne , Tooley Frank A. P.
Published: February 2023

A circuit has been constructed that is based on a cellular logic image processor (CLIP) architecture. It uses two Self electro-optic effect device (S-SEED) arrays as logic devices, optically connected in a loop with a 1-D nearest neighbor interconnect. The circuit is programmable in NAND and NOR and can implement image processing algorithms on a 16 × 8 dual rail image input with a spatial light modulator. Construction and assessment of this basic circuit is to allow investigation into generic issues in optical and mechanical design of parallel optical information processors. This will allow the implementation of more complex circuit design by using application specific devices and various local and non-local interconnection schemes. Analysis of the circuit performance and tolerances will permit an optical implementation using larger arrays. Addressing these issues is important before the complexity of this type of system can be increased.
Characterization of interfaces by using surface analysis and x-ray diffraction techniques
Authors: Slaughter Jon M.
Published: February 2023

Improving the performance of x-ray multilayer structures hinges on accurate determination and control of the details of interfaces between material pairs. Surface analysis tools, such as RHEED, LEED, Auger, XPS, AFM, and STM, are appropriate for the study of interfaces on a sub-nanometer scale. We have studied the initial stages of growth and interface formation in a variety of material pairs that are used in multilayer x-ray optics. Examples of such single-interface studies will be given, demonstrating the relationship between the growth mode and interface quality. In multilayers, interface roughness may vary throughout the stack and may have a component that is correlated from layer to layer. Specular and non-specular x-ray diffraction measurements and transmission electron microscopy were used to characterize these roughness effects in Mo/Si mirrors for soft x-ray lasers. These data show varying amounts of correlated roughness depending on the sample preparation.
Optical fabrication tolerances for synchrotron beamline optics
Authors: Lewotsky Kristin , Harvey James E.
Published: February 2023

Recent advances in optical fabrication techniques have resulted in a resurgence of activity in the field of x-ray and extreme ultraviolet (EUV) imaging systems. However, traditional optical design and analysis techniques (geometrical ray tracing) are woefully inadequate for predicting the performance of high resolution imaging systems at these very short wavelengths. Diffraction effects of grazing incidence optics and small angle scattering effects due to residual optical fabrication errors will frequently dominate geometrical design errors in the degradation of image quality. Surface scattering theory must be implemented to model the image degradation effects of residual surface irregularities over the entire range of relevant spatial frequencies. These include small angle scattering effects due to mid spatial frequency surface errors falling between the traditional figure and finish specifications. Performance predictions can then be presented parametrically to provide some insight into the optical fabrication tolerances necessary to meet a particular image quality requirement.
ABCD law for partially coherent Gaussian light propagating through first-order optical systems
Authors: Bastiaans Martin J.
Published: February 2023

It is well known that the paraxial propagation of spherical waves through Luneburg’s first-order optical systems is governed by a bilinear relationship—the so-called ABCD law—which relates the curvature of the incoming wave with the curvature of the outgoing wave. It is also well known that this ABCD law applies to completely coherent Gaussian light if we describe such light by its complex curvature. Moreover, it has been shown that the same relationship applies to partially coherent Gaussian light, at least in the one-dimensional case. In this paper we study the behavior of partially coherent Gaussian light in the two-dimensional (or, generally, n-dimensional) case, and we find the conditions under which an ABCD law can again be formulated. It is shown that in such n-dimensional cases an ABCD law holds only for partially coherent Gaussian light for which the matrix of second-order moments of the Wigner distribution function is proportional to a symplectic matrix. Furthermore, it is shown that this is the case if we are dealing with a special kind of Gaussian Schell-model light, for which the real parts of the quadratic forms that arise in the exponents of the Gaussians are described by the same real, positive definite symmetric matrix.
Propagation of Gaussian Schell-model beams in dispersive and absorbing media
Authors: Wang W. , Wolf E.
Published: February 2023

The spectral properties and the coherence properties of Gaussian Schell-model beams propagating in dispersive and absorbing media are discussed. Unlike in free space, the ratio of the transverse spatial coherence length to the beam width is found to increase on propagation. Consequently, the beam becomes spatially more coherent as it propagates. Numerical results for the spectrum and for the degrees of spectral and of temporal coherence of the field are presented for some selected values of the beam parameters and for several values of the propagation distance. Propagation in a gain medium is also briefly discussed.
Polarization aberrations in three dimensions
Authors: Hansen Eric W. , Jiang Hong
Published: February 2023

To some degree, all optical systems affect the polarization of light passing through them. When this instrumental polarization adversely affects the performance of the system (e.g., the classic Maltese cross artifact in the polarizing microscope), it is said to possess polarization aberrations. Using the polarizing microscope as a model system, we present in this paper an analysis of polarization aberration effects upon 3-D image formation. We have derived analytic expressions for the 3-D point spread function and compared them with experimental observations. We also discuss how one scheme for correcting polarization aberrations performs in 3-D.
Visible vertical-cavity surface-emitting lasers
Authors: Bryan R. P. , Schneider R. P. , Lott J. A. , Olbright G. R.
Published: February 2023

We report the operation of visible-light (670-630 nm) vertical-cavity surface-emitting lasers (VCSELs) in a photopumped structure. Visible VCSELs are a promising breakthrough for visible laser technology because of their surfacenormal operation, nonastigmatic output beams, ease of fabrication of closely spaced 1- and 2-dimensional arrays, and ease of integration with electronic and photonic devices. To date, operation of VCSELs has been limited to wavelengths greater than 770 nm. We have extended this range into the visible, where numerous applications exist, including ultrafast holographic memory, visible laser projection displays, plastic fiber-optic communications, and many applications for which He-Ne lasers are currently employed. The visible VCSEL structure was grown by using low-pressure metal-organic vapor phase epitaxy. The active region consists of InGaP strained quantum wells and InAlGaP graded barriers. We characterized and optimized the optical properties, strain effects, and band offsets of the materials as a function of growth and structure parameters with low temperature photoluminescence and photoluminescence excitation spectroscopies. Distributed Bragg reflectors reflecting at visible wavelengths were grown in both the AlGaAs and InAlGaP material systems. Room temperature photopumped lasing was achieved with a very low threshold power, comparable to our best AlGaAs/GaAs near-infrared VCSELs. We have achieved photopumped lasing in both doped and undoped structures at wavelengths ranging from 670 to 630 nm. We are currently engaged in the fabrication of visible VCSEL diodes and will discuss this work. These results demonstrate the viability of visible VCSEL technology, opening the door for a broad range of new visible-light optoelectronic applications.
Depth in anticorrelated random dot stereograms (RDS)
Authors: Cogan A. I. , Lomakin A. J.
Published: February 2023

Global depth seen in densely seeded RDS is not perceived in anticorrelated RDS (opposite sign of brightness contrast on corresponding loci) whether the dots are at rest or in motion, as extant data show. We report that global, disparity-dependent depth appears in anticorrelated RDS that are flashed repetitively with an interocular delay. During the delay, a dot array in one eye was paired with a gray frame in the other eye and presumably could interact with the negative afterimage of the contralateral dot array. In anticorrelated RDS with 8 arcmin dots at 10% to 50% density and a 15-ms flash, robust depth was produced by interocular delay on the order of 45-75 ms. In correlated (otherwise identical) RDS, depth deteriorated sharply with delays larger than 45 ms. The range of delays producing depth was dependent on flash duration, dot size, and dot density. Surprisingly, anticorrelated RDS displayed continuously showed disparity-dependent depth at low (1 %) dot density provided the dots were larger than some 2 arcmin; as dot density was increased depth became harder to see, but depth became visible when such RDS were flashed with an interocular delay. Our results suggest that all stereoscopic depth signals emerge exclusively from same-sign binocular cortical filters.
Cascaded model of neural networks: a new approach to pattern recognition
Authors: Zhang Yanxin X. , Mu Guogang G.
Published: February 2023

The cascaded model of neural networks proposed in this paper is a new approach to pattern recognition. The motivation comes mainly from the fact that the human brain is functionally subdivided into smaller parts. The model consists of some subnets that are functionally complementary and hierarchical, and therefore can enhance the performance of the entire system. Furthermore, the learning process of the cascaded model is carried out separately in each subnet, where the learning rule is much simpler. Therefore, the difficulty of subjecting the model to convergence on a local minimum, as is often the case in multi-layered neural networks, can be avoided. The design of a cascaded system is somewhat similar to that of an optical or electronic system composed of cascaded components. As an example, a 3-D target classifier for four kinds of aircraft with arbitrary spatial orientation is demonstrated based on the cascaded model. Computer simulation and experimental results have shown the model can correctly classify input patterns, both inside and outside the training sets, with rotational invariance.
An efficient holographic collimator for 820 nm laser diodes
Authors: Aharoni A. , Goodman J. W. , Amitai Y.
Published: February 2023

High-quality collimation and focusing of laser diode beams is hindered by their asymmetric divergence and elliptical beam section. These result from the large aspect ratio of the source. In gain-guided diode lasers the beam also suffers astigmatism due to the lensing effects of the nonuniform current porofile. A holographic correcting and collimating element for such diodes is desirable for its low mass-production cost and small dimensions. Efficient holograms cannot, however, be recorded directly with the infrared wavelengths of diode lasers, and normally large aberrations and low efficiencies are obtained when recording with shorter wavelengths. Using a recursive holographic method1 we recorded a hologram with blue light that collimates a 820 nm diode beam with low aberrations and high efficiency. This element corrects for the diode's astigmatism and ellipticity and achieves near-diffraction-limited collimation. The design can also alleviate effects due to diode wavelength instability.
The design and fabrication of a lenslet array with multilevel phases
Authors: Chin Kuo-fan , Yan Yingbai , Wang Xu , Wu Minxiang
Published: February 2023

The lenslet array has been widely used in many fields; it is especially important component in optical computing. A CAD program is designed for optimizing and designing the parameters of the designed array. Multiple phase levels are used for enhancing the optical effiency. Stepper and etching techniques are used for fabricating the array. A micro-interferometer under computer control is developed for testing the phase structure of the lenslets. Both 16 × 16 and 32 × 32 arrays with eight phase levels have been produced and will be presented. The application of the lenslet array for performing a binary image algebra processing will also be presented.
Calculation of the linewidths of heavily modulated waveguide grating filters
Authors: Magnusson R. , Wang S. S.
Published: February 2023

Waveguide gratings are spatially modulated structures possessing a higher average refractive index than the adjacent layers on either side. Under certain conditions that are given in the limit of small modulation by elementary slab waveguide theory, coupling of external diffracted waves with the characteristic modes of the waveguide takes place. This produces sharp resonances where efficient switching of light energy among waves occurs. This effect may be useful for filtering. For small modulation, the calculated linewidths are found to be extremely narrow. Larger linewidths are desirable for many potential applications as well as for experimental studies of these resonance phenomena. These can be achieved by increasing the modulation strength of the waveguide grating. In this paper calculated results for the linewidths and line shapes of experimentally realizable waveguide grating filters are presented. Calculations for planar sinusoidal gratings with modulation amplitude up to 10% of the average permittivity show that linewidths of the order of 100 pm in wavelength can be obtained with well shaped symmetric lines. Increasing the modulation further tends to corrupt the shape of the line. Calculated spectral linewidths and shapes are presented for both planar and surface-relief waveguide grating structures.
Femtosecond wave-packet dynamics and spectroscopy of the cesium dimer Cs2
Authors: Rodriguez G. , Eden J. G.
Published: February 2023

Ultrafast pump-probe experiments have been applied to the study of vibrational wave-packet dynamics in the excited states of the cesium dimer. Pumping the C(1П
u
)←X(1Σg+) band at several wavelengths between 627 and 650 nm produces a vibrational wave packet with an oscillation period of 600 fs. The wave packet is detected by photoionizing the excited molecule with a time delayed probe pulse. The wave packet oscillations persist for approximately 5 ps, and their decay is attributed to predissociation of the C state. Numerical quantum calculations using the FFT spectral method have been performed on the excited state surface to simulate the time evolution of the wave packet density. Experimental results and simulations for these processes will be presented.
Feasible solution in image restoration, reconstruction, and reproduction
Authors: Trussell H. J.
Published: February 2023

A feasible solution is one that satisfies all a priori constraints known about the characteristics of the true solution. Classical signal estimation methods rely on optimizing a particular cost function. Set theoretic methods can allow for the possibility of a multiplicity of solutions. The sets are defined directly from the known properties of the physical signal that is to be estimated. This is in contrast to the use of penalty functions or regularization parametersto produce qualitative properties. The set theoretic methods range from the simple alternating projection methods to fuzzy set methods. Projection is only one way of finding a feasible solution. Fuzzy set methods use vector space optimization techniques. For nonconvex sets or convex sets whose projection operator is extremely complicated, random search methods may be used to find a point in the intersection of the sets. Examples of the various methods will be presented along with a discussion of the relative advantages and disadvantages of the methods.
Single radiating atoms and coherence effects
Authors: Hegerfeldt Gerhard C.
Published: February 2023

Advances in trap technology have made feasible experiments on single, or a few, atoms and ions. Photon counts from a single atom may exhibit new features that are washed out in ensembles. Wilser and I recently proposed a theoretical approach based on repeated gedanken measurements, which was then also independently proposed by Dalibard et al. and by Carmichael. The optical Bloch equations are not used but rather follow as a consequence. The approach is applied to a three-level V-system with small separation of the upper levels having parallel dipole moments. Macroscopic dark periods are predicted for such a laser-driven system, although none of the levels is metastable as in the Dehmelt system. The reason is a coherence effect. For particular values the dark period may become arbitrarily long. A similar result holds for A-systems. This explains the observed dark resonances in ensembles of such systems by increasingly long dark periods of individual atoms. The approach is then applied to atoms driven by incoherent light, and nonrate equations with off-diagonal terms are derived. It is shown that these may lead to coherence effects.
Historical perspective on integrated optics
Authors: Taylor Henry F.
Published: February 2023

Integrated optics got its start in the late 1960's with the suggestion that circuits based upon optical waveguides could perform key functions in communications and signal processing systems. As the optical analog to integrated electronics, the term "integrated optics" implies that advantages in cost, performance, and reliability can be realized from interconnecting a multiplicity of optical components on a common substrate. During the 1970's, numerous waveguide materials and fabrication techniques were investigated. After the discovery that metal- indiffused waveguides are readily produced in lithium niobate, this substrate emerged as the favored one for fabrication of modulation and switching devices. Such devices were commercialized by several manufacturers during the 1980's. Over the past few years, a number of companies have been developing multifunction lithium niobate chips for gyroscope application. Semiconductor-based "integrated optoelectronics" and "integrated photonics" have emerged as important research fields, with the hope of combining lasers and photodetectors with traditional guided-wave components on a common substrate. During the 1990's, we can expect increasing application of lithium niobate modulators, switches, and wavelength multiplexers in communications systems. Practical uses for semiconductor photonic circuits can also be anticipated in the near future.
Diode pumped upconversion laser with 100 mW output
Authors: Stephens R. R. , McFarlane R. A.
Published: February 2023

An upconversion laser pumped at 797 nm has produced 100 mW output at 551 nm with an overall IR to visible conversion efficiency of more than 5%. A high power, narrow band, multi-element semiconductor diode laser system was used to longitudinally pump the upconversion laser crystal in a manner that provided an efficient matching of laser and pump mode geometries and a spectral linewidth that closely matched the absorption width of the active rare earth ion at the operating temperature. The maximum pump power delivered to the upconversion crystal was 1.75 W. The monolithic laser crystal was a 5 mm long cylinder of Er5%:YLiF4 with dielectric reflectors applied to each end, and was cooled to 48 K. The absorbed pump power was 88% of the incident pump power. The system oscillated on two components of the 4S3/2-4I15/2 transition, at 550.97 nm (air) and 551.25 nm (air). A net IR-to-visible conversion efficiency of 5% was achieved. At the higher powers a slope efficiency of 8% has been demonstrated. Higher output power, improved efficiency, and better beam quality can be achieved with a better mode-match and an improved resonator design.
Room temperature tunable cw Cr: Forsterite laser
Authors: Snell K. J. , Labranche B. , Galarneau P.
Published: February 2023

Room temperature, tunable cw operation of a Cr:Forsterite laser has been obtained by using Nd:YAG laser pumping. A 10 mm long, 3 mm diameter, AR-coated Cr:Forsterite rod (Mediscience Technology, Inc.) was longitudinally pumped in a 10.4 cm long hemispherical cavity consisting of a flat total reflector and a 10 cm radius output coupler. The Nd:YAG pump was focused through the rear mirror and onto the rod face by using a 10 cm focal length lens. A thermoelectric cooler was used to actively cool the rod. In fixed wavelength operation with a 1.1 %T output coupler, the laser threshold was 1W, the slope efficiency was 11.4%, and an output power 190 mW was obtained with 2.5 W of absorbed pump power. Modulating the pump with a 10% duty cycle increased the slope efficiency to 14.9% and output power to 250 mW. Lasing was centered at 1.24 μm with a line width of 4.8 nm. By using a single plate biréfringent tuner, the laser was tuned from 1.19 to 1.33 μm with a pump power of 2.5 W and a single 1.1 %T output coupler. A maximum output power of 125 mW was obtained at 1.22 μm, and the linewidth was reduced to 1.3 nm. To our knowledge, the tuning range of 134 nm is the largest yet obtained at room temperature.
Crystal growth, spectroscopy, and laser properties of Yb, Er, and Nd doped materials
Authors: Allik T. H. , Merkle L. D. , Chai B.H.T.
Published: February 2023

Recent results on new solid state laser materials that are pumpable with AlGa As and InGaAs laser diodes will be discussed. For laser applications near 1.06 μm, Nd-doped KYF4 boules (5 cm long by 2 cm diameter) have been grown by the top-seeded solution method with concentrations as high as 3% while maintaining excellent optical quality. The broad absorption spectra near 800 nm and the long (>500 μs) fluorescence lifetime of the 4F3/2 laser state in this host are attractive features over both YAG and YLF for Q-switch applications. Polarized optical spectra, fluorescence lifetimes, Judd–Ofelt analysis, and laser parameters (cross sections) will be presented. In addition, several crystalline and glass hosts doped with Er and Yb/Er are being investigated for eye-safe emission near 1.5 μm. Spectroscopic, energy transfer, and laser investigations have been carried out, and examples will be given.
Two stages of nonlinearity in texture processing
Authors: Victor Jonathan
Published: February 2023

The visual system readily discriminates certain black-and-white textures whose spatial frequency spectra (or autocorrelations) are identical. This implies that preattentive texture discrimination is based on local features, and not just global spatial frequency content. The psychophysi­cal distinguishability of such isodipole textures is paralleled by an evoked-potential component elicited by texture interchange. Psychophysical and evoked-potential measures of discrimination among a library of isodipole texture pairs force rejection of computational models based on symmetry, information content, or a single nonlinear stage. Models with two nonlinear stages, which consist of local nonlinear subunits whose outputs combine in a spatially specific and cooperative fashion, are consistent with experimental observations. In order for such models to maintain sensitivity and specificity over a wide range of contrasts, modulation of one or both nonlinear stages by local contrast is required. This model structure is consistent with features of connectivity within striate cortex. A spatiotemporal analog of this model may account for joint extraction of Fourier and non-Fourier motion.
Characteristics of complex channels (second-order processes) in perceived texture segregation
Authors: Graham N. , Sutter A. , Venkatesan C.
Published: February 2023

Aspects of perceived texture segregation suggest the existence of complex channels. A complex channel consists of two linear-filtering stages separated by a rectification-type nonlinearity, a structure like that hypothesized for a complex cortical cell. We tested this proposed structure by having human observers rate the degree of segregation between two textures, each composed of the same two types of elements but in differing arrangements (checkerboard vs. striped arrangements). The two element types were Gabor patches of the same spatial frequency and orientation but differing in size (number of cycles at half-maximal contrast). A tradeoff between contrast and size was found, approximating that predicted for Gabor-patch elements by complex channels (analogous to that predicted for square elements by simple first-order channels—Sutter, Beck, and Graham, 1989, Perception and Psychophysics). Elaborations of the model may be necessary, however. Further, we investigated complex-channel characteristics including (i) bandwidths of first- and second-stage filters and (ii) mapping rules specifying first-stage-second-stage combinations of preferred spatial frequencies and orientations.
Effects of γ-irradiation on photorefractive barium titanate
Authors: Moore T. R. , Boyd R. W.
Published: February 2023

Experiments show that irradiating photorefractive BaTiO3 with γ-rays can increase the photorefractive efficiency of some crystals. We measured the two-beam coupling gain as a function of grating period for two crystals, irradiated both crystals with γ-rays from a 60Co source, and repeated the two-beam coupling gain measurements. This sequence was repeated for irradiation doses totaling 105, 106, 107, and 108 rad (water). Results of the measurements showed no change in one crystal. The second crystal showed a marked improvement in photorefractivity; however, this increase saturated at an irradiation of 107 rad. All of our results are explained by a model that assumes dual photoactive species in BaTiO3, only one of which is affected by the γ-irradiaion.
Nonlocal adiabatic elimination
Authors: Jakobsen P. K. , Ru P. , Moloney J. V.
Published: February 2023

It is often the case that when studying laser models one is confronted with the problem of stiffness: some of the variables in the problem vary much faster than others. An example of this is standard semiconductor laser models where the time scales in the problem vary from femtoseconds to nanoseconds. For such systems it is a reasonable approach to try to eliminate the fastest variables and only retain them as slaved to the slow variables in the problem. The method of choice in the optics community for realizing the slaving has mostly been standard adiabatic elimination in which the fast variables are slaved by setting the corresponding time derivative to zero and solving for the fast variables. This method has met with reasonable success, but it has been shown that it tends to remove physically relevant instabilities from the reduced mathematical description. For laser models described by ordinary differential equations an application of centremanifold techniques has been shown to retain more of the structure of the full problem. For laser models described by partial differential equations, as is the case if multimode transverse or longitudinal dynamics are studied, the application of centremanifold techniques has so far met with mixed success as it has tended to produce an equation with a singular time behavior. We have developed a new method of adiabatic elimination that is very easy to implement, that is well behaved, that is, free of unphysical instabilities and that reproduces all of the instabilities and dynamics contained in the full system.
Optics research and development at Rockwell Power Systems
Authors: Rotge James R.
Published: February 2023

Current optics research and development at Rockwell Power Systems is described. Gradient index optical rods are being explored for use in compact, rugged imaging and transfer systems. Experiments and theoretical analyses are addressing the feasibility of fiber-coupled, long-baseline optical interferometers. Imaging-related efforts include an optical smoother for partial removal of amplitude fluctuations in holographic-type atmosphere distortion correction; techniques for producing passive generalized-Hough-transform filters for shape detection; development of interferometric filters for reducing scattered light noise in astronomical telescopes; and a laboratory demonstration of real-time correction of effects of a thin phase distorter for an object illuminated with natural light. A unique imaging radiometric pyrometer is being developed for measuring as a function of time the spatial distribution of temperatures over a hot surface. Blackbody radiation of the surface is imaged and transmitted by means of optical fibers. The design and construction of diode-pumped solid state lasers includes Nd:YLF, Nd:YAB, and Tm:YALO. End pumping is used for efficiency and side pumping is used for high power. An in-house ability to mount laser-diode bars to microchannel coolers is being developed.
Enhanced storage with binary phase associative memories
Authors: Khoury J. , Gianino P. , Kane J. , Woods C.
Published: February 2023

Most holographic associative memories share a similar architecture, which is essentially a correlator with a retroreflecting thresholding device. The thresholding device performs two functions: it reduces cross correlation components, and it sharpens the correlation peak so that it is more like a δ function. In order to enhance the storage capabilities of the binary phase-only associative memory demonstrated in our previous work, these two operations must be improved. We have examined two techniques for this purpose; the first is de blocking, which improves the signal to noise ratio, and the second is cascading the BPOF with a Laplacian filter, which enhances the correlation peak and reduces the noise. Our results show that de blocking can enhance the storage capability by a factor of 4 and Laplacian filtering enhances the storage capability by a factor of 6.
Spectral measurement of timing jitter in gain-switched semiconductor lasers
Authors: Leep David A. , Holm David A.
Published: February 2023

We calculate the power spectrum of a periodically gain-switched semiconductor laser whose output pulses have amplitude fluctuations and timing jitter that are uncorrelated from pulse to pulse. Our spectrum is in agreement with analogous calculations for correlated jitter in the low-noise limit. Thus rms values of uncorrelated jitter can be measured with subpicosecond sensitivity from spectrum analyzer data. We compare our results to a numerical simulation and experimental measurements.
Smart o-ring fiber pressure sensors
Authors: Bouzid A. , He Z. , Abushagur M. , Workman G. , Kuston S. , Rodriguez P.
Published: February 2023

Fiber sensors are being proposed for the measurement of diverse environmental parameters. In this paper we are proposing the use of fiber sensors in the monitoring of the pressure in the o-ring of the Space Shuttle boosters. Both single fiber and interferometric systems are presented. The measurement of both pressure and temperature will be discussed, and novel techniques for measuring these quantities independently will be presented. Theoretical analysis and experimental results will be compared. Both multi-wavelength and heterodyning methods are going to be introduced and tested. Sensitivity and dynamic range of the system will be analyzed.
Comprehensive device simulation via solution of the 2-D Helmholtz equation
Authors: Hadley G. Ronald
Published: February 2023

The simulation of photonic circuit elements, such as diode lasers, waveguides, couplers, and turning mirrors, has in the past depended to a great extent upon the use of the paraxial beam propagation method. This method, although easy to use and efficient in terms of computer resources, is entirely inadequate to model many of the devices now being fabricated, because of the restricting assumptions employed in its derivation. In order to overcome these limitations, we solve the 2-D (or 3-D) harmonic Helmholtz equation prior to the introduction of restrictions, resulting in a formulation that includes the following three important features: (1) treatment of adjacent regions of widely different indices of refraction, (2) wide-angle (non-paraxial) beam propagation, and (3) reflection. The solution is obtained by using an iterative technique in order to minimize the required storage. Although the method is numerically intensive at present, future progress in solution techniques coupled with the advent of larger and faster computers is expected to make this method more attractive for a variety of applications. We illustrate the capability of this technique by solving several problems of interest, including energy loss resulting from propagation through a large-angle Y-guide splitter, reflectivity of a facet-terminated waveguide, and the performance of waveguides etched with both first and second order Bragg gratings.
Investigation of influence of grating shapes on diffracted beam profiles
Authors: Bao Y. , Verber C. M. , Kenan R. P.
Published: February 2023

The influence of grating shapes on the diffracted and the transmitted beam profiles for right-angle guided-wave diffraction was investigated both numerically and experimentally. Numerically, by using the first order 2-D coupled mode equations and the finite-difference method, the diffracted and the transmitted beam distributions from corrugated surface-relief gratings with different shapes were investigated both for weak and strong index modulations. Experimentally, different shaped photoresist gratings were fabricated on BK7 ion-exchanged single-mode glass waveguides, which were then covered with a layer of Nile-blue-dye-doped PMMA. The fluorescent beam profile revealed by dye:PMMA was recorded by an optical multichannel analyzer. Both numerical and experimental results show that square or rectangular gratings, which result in a distorted profile for the diffracted beam, are not suitable for right-angle diffraction and that a truncated shape can improve both diffracted and transmitted beam profiles significantly. About 50% diffraction effciency was achieved by using a shaped grating 300 μm wide.
The sliding-frequency guiding filter: an improved form of soliton jitter control
Authors: Mollenauer Linn F. , Gordon James P. , Evangelides Stephen G. , Lichtman Eyal , Neubelt M. J.
Published: February 2023

We describe a simple, strictly passive technique, fully compatible with extensive WDM, that allows for a very large increase in bit-rate per channel in trans-oceanic soliton transmission.
Outer product expansion of the reference function in an optical correlator
Authors: Athale R. A. , Astor M. E.
Published: February 2023

An arbitrary image, when considered as a matrix, can be expressed as a weighted sum of rank 1 (outer product) matrices. The singular value decomposition of a matrix leads to a compact representation of an image that is optimum in the least squares sense. In this paper we combine the general concept of a truncated outer product expansion of a reference image with optical correlators. We investigated analytically and via computer simulations the effects of the truncation on the auto and cross correlations and sidelobes. Since the outer product expansion can be easily implemented optically by using orthogonally oriented 1-D spatial light modulators, this technique has the potential of eliminating the requirement of 2-D devices for representing the reference function in an optical correlator. Additionally, the truncated outer product expansion of the reference function can potentially lead to an order of magnitude reduction in the data storage requirements. Optical architectures for combining the outer product processor with an optical image correlator will be described.
Moving object correlator
Authors: Anderson R. J. , Sharp E. J. , Wood G. L. , Clark W. W. , Salamo G. J. , Neurgaonkar R. R.
Published: February 2023

Cross talk is observed in a double-phase conjugator and utilized to construct a moving object correlator. Separate objects are placed in each arm of a double-phase bridge phase conjugator by using a strontium barium niobate cerium doped crystal. During the transient time of grating or bridge formation, cross talk is observed in each arm of the double-phase phase conjugator. After a time corresponding to the characteristic photorefractive response time of the crystal, the cross talk is observed to disappear. If one of the objects in either arms is suddenly moved, however, a signal representing the correlation between the object in each arm can be observed in the correlation plane of the other arm. Consequently, as long as the object in one arm of the double-phase phase conjugator is moving, the signal representing the correlation between the two objects can be observed in the others arm's correlation plane. If the moving object becomes stationary for a time longer than the photorefractive response time, however, the correlation signal disappears. In this way, only the correlation signal between objects moving in one arm and stationary reference objects in the second arm are observed.
Instabilities in semiconductor lasers with optical feedback
Authors: Mørk J. , Tromborg B. , Mark J.
Published: February 2023

A semiconductor laser with optical feedback is a simple and cheap system that displays a wealth of interesting nonlinear dynamics. Feedback can be utilized for linewidth narrowing, but unless precisely controlled, or carefully eliminated, it often degrades the laser properties, as exemplified by the coherence col lapsed state. In the talk, we will present experimental and theoretical results on the various instabilities found in semiconductor lasers at different levels of feedback. Starting at the lowest feedback levels, we discuss the occurence of mode jumping and show that it can be quantitatively explained by a simple analytical potential model. The model also explains the enhanced single mode behavior as the feedback level is increased. However, above a certain feedback level, relaxation oscillations become undamped, and a transition to chaos (coherence collapse) through quasiperiodicity interrupted by frequency locking is observed. In the transition region, we also observe competition between different attractors belonging to the same external cavity mode, and the existence of three-frequency quasiperiodic attractors is predicted theoretically. At still higher feedback levels and at low bias currents, low-frequency intensity fluctuations appear, which are shown to be explained in terms of bistability and dynamic formation of bistability.
Analog-optical techniques for directly acquiring directional gradients of images
Authors: Storrs M. , Mehrl D. J.
Published: February 2023

Acquisition of directional derivatives, invaluable in edge detection and edge enhancement, is tradition ally accomplished by use of various digital filtering techniques. These techniques in variably constitute high-pass spatial filters, making them noise sensitive. Attempts to suppress noise, e.g., by low-pass filtering, increase computation and compromise spatial resolution. An optical scanning technique for directly acquiring various spatial derivatives of two dimensional images is described. The technique employs spatiotemporal modulation of a focused scan beam, in conjunction with synchronous electrical detection, to acquire directional derivative information in a relatively noise-free fashion. The technique also allows spatial resolution to be electronically controlled in a dynamic fashion. Proof-of-principle results are presented. We also discuss an extension of this technique to tracking and identifying moving objects.