bluuebunny/crossref_metadata_2025 huggingface dataset:
Novel interferometric method for NLC film thickness measurement
Authors: Pan Ru-Pin , Chen S. M. , Pan Ci-Ling
Published: February 2023

Accurate determination of the thickness of nematic liquid crystal (NLC) films is essential in the study of the physical proper ties and use of these films. We demonstrate a novel interferometric method in which the birefringence of the NCL material is utilized. A probe laser polarized at an angle 45° off the incident plane is focused onto the sample at an arbitrary angle θ. The NLC film induces an optical path difference between the o-ray and the e-ray components of the transmitted light, and interference fringe spacing is used to calculate film thickness. This method is not affected by the substrate; it can be used to measure standard cells as well as NLC films with a free surface, i.e., NLC films with one surface exposed to the air.
Electrically tunable liquid crystal wave plate in the infrared
Authors: Sharp R. C. , Resler D. P. , Hobbs D. S. , Dorschner T. A.
Published: February 2023

We report phase retardance measurements of a liquid crystal (LC) based, electrically tunable wave plate as a function of applied voltage and incident optical intensity at 10.6 µm. Utilizing Stokes and Mac-Cullaugh ellipsometry, we distinguish twist effects of the LC director with respect to substrate surfaces from tilt effects off the surface. Thermal changes in birefringence determine device performance at intensities up to 900 W/cm2 and not optically induced reorientation effects. The LC wave plate utilized a 25-µm thick parallel aligned layer of E7. Phase retardance and output polarization state azimuth angle were measured as a function of optical intensity. Absence of any statistically significant variation in measured azimuth angle as a function of the incident optical field strength demonstrates that the observed loss of phase retardance with increasing optical intensity is not the result of optically induced reorientation effects. Based on the 1-D theory of Deuling,1 we have calculated the phase retardance of our LC cells up to 50°C. Agreement between theory and experiment clearly indicates that the dominant factor in the loss of retardance with increasing optical intensity is attributable to d(Δn)/dT.
Paraxial light propagation in strongly and weakly confining semiconductor rib waveguides
Authors: Yevick David , Rolland Claude , Hermansson Bjorn
Published: February 2023

We have found that the results of various 3-D numerical models based on the Fresnel equation for forward wave propagation in semiconductor rib waveguides depend considerably on the value chosen for the reference refractive index n0. This constant should optimally not deviate greatly from the effective indices of the guided modes. To illustrate, we analyze a weakly confining Y-junction structure with a small rib height where excellent agreement between our 3-D calculations and the 2-D effective index method is attained only if n0 is selected according to the above criterion. We then display the variation in the computed losses of strongly guiding Y-junctions with changes in n0.
Programmable optical binary threshold logic implementation via optoelectronic bistable circuit
Authors: Liu Shutian , Li Chunfei , Wu Jie
Published: February 2023

Integrated optoelectronic logic circuits are promising for use in optical computing. Much attention has been paid to demonstrate binary and multiple-valued optical logic implementation using optoelectronic circuits. In this paper we present what we believe to be the first demonstration of optical binary threshold logic using a modified optoelectronic PD LED bistable circuit (BILED).
Relative-weight ambiguities in phase-retrieval problems
Authors: Trebino Rick , Barker Charles E. , Kostenbauder Adnah G.
Published: February 2023

Many important diagnostic techniques yield the magnitude, but not the phase, of the Fourier transform of a response, and hence, may not determine the response uniquely. Even when a specific model function is known to approximate the response well, determining the parameters of the model uniquely may not be possible. Consider, for example, the commonly used temporal response, the sum of two exponentials: h
α
(t) = θ(t){A exp(−t/τ
f
) + B exp(−t/τ
s
)}, where θ(t) is the unit step function, τ
f
and τ
s
are fast and slow time scales, A ≡ α/τ
f
, B = (1 − α)/τs, and α is the relative weight. Knowledge of only the magnitude of the Fourier transform of hα(t) allows two distinct values of α to result from a curve fit (even in the absence of noise). We have seen this ambiguity in frequency domain experiments measuring the ultrafast optical Kerr transient of carbon disulfide using a nonlinear optical technique. The ambiguity can be removed by reconfiguring the experiment to allow some coherent, quadrature phase background to add to the Fourier transform of hα(t) before the magnitude is taken. Only one scan is necessary. For more general impulse responses consisting of sums of two arbitrary functions, we have derived expressions for (1) the condition in which relative weight ambiguity occurs and (2) the spurious solution that results. A corollary to these results reveals a general condition for determining whether coherent background removes a relative weight ambiguity. We also consider the sum of N exponentials, where we show that as many as 2N−1 solutions for the relative weights can exist, depending on whether real or complex weights are assumed. Finally, we derive expressions for the amount of quadrature phase background necessary to definitively remove relative weight ambiguities (that is, remove even local minima in the fitting function) in sum-of-two-exponential impulse responses.
Electrooptical implementations of the alternating projection neural network
Authors: Smith Alan T. , Walkup John F.
Published: February 2023

We examine two incoherent electrooptical neural network processors. Both schemes use optical matrix-vector multiplication with feedback to model the alternating projection neural network (APNN). In the first, the Stanford matrix-vector multiplier architecture using a photographic negative as the spatial light modulator (SLM) was used. Feedback was accomplished with a detector array leading to a light emitting diode (LED) array whose intensities represent the state of the neurons. In the second architecture, active SLMs allow for the updating of the neuron interconnection matrix in real time. A Semetex magnetooptic SIGHTMOD encodes the matrix onto a light beam which is projected onto the write side of a Hughes liquid crystal light valve. Light from the LED array is reflected from the read side and the modulated intensity is detected and fed back to update the LEDs. Convergence for both implementations is assured if the stored data meet certain requirements1 and if the thermal dark current is electrically subtracted after each iteration. Although their computing power and accuracy are currently limited by the physical size of the components and the SLM display resolution, respectively, the high operating speed of the electrooptical systems make them worth studying.
Real time associative holographic memory with liquid crystal electrooptical switches
Authors: Xu Kebin , Xu Haiying , Yuan Yang , Yu Youlong , Xu Yuhuan , Zhu Deri
Published: February 2023

The optical implementations of associative memory based on various thresholding and feedback schemes have been demonstrated recently.1,2 We here show a real time parallel optical associative memory system implemented with a KNSBN:Co crystal as recording medium and liquid crystal electrooptic switched as a reflective thresholding device to provide binary thresholding response. In our experiment, two original images, HIT and XXY, are stored in KNSBN:Co crystal at the Fourier plane through interfering with angular multiplexed reference beams. When the system is addressed by a half of one of the stored objects, the detecting beam split from its autocorrelation beam is higher than the thresholding power of 14.7 µW of the liquid crystal electrooptic switch, but the detecting power of its cross-correlation beam is lower than the thresholding power. The autocorrelation peak corresponding to the partial input switches the liquid crystal electrooptic switch into the transmissive state. Switching the liquid crystal electrooptic switch allows the counterpropagating beam to read out the hologram and a complete image is reconstructed. Thus, the no iteration system demonstrates the ability to retrieve the correct memory element without crosstalk.
Character recognition using a trinary associative memory
Authors: Cherri A. K. , Awwal Abdul Ahab S. , Karim Mohammad A.
Published: February 2023

A trinary (or ternary) neuron representation for Hopfield's associative memory1 has been recently found to be more effective in associative recall. The trinary associative memory overcomes the discrepancies associated with the unipolar binary and bipolar binary representations of neurons.2 In the trinary neuron representation, the known portion of the partial input is represented in bipolar binary and the unknown portion is represented by a string of 0s. In this work, the trinary associative memory is used to process 1-D as well as 2-D stored images such as those encountered in character recognition applications.
Guided-wave electrooptic field sensor
Authors: Riedl-Bratengeyer E. , Horn-Bachner D. , Guttler G.
Published: February 2023

A guided-wave electrooptic phase modulator, coupled with single-mode fibers, offers great potential in broadband electromagnetic field sensing. The device can be highly sensitive and the use of nonelectrically conducting components guarantees minimum perturbation of the field being measured. A Ti:LiNbO3 phase modulator consisting of a straight single-mode channel waveguide, 20 mm in length, and of coplanar electrodes has been developed (λ = 0.83 μm). The electrodes were connected to a dipole antenna. An electric field sensor causes a change of the refractive index along orthogonal crystal axes and thus alters the polarization state of an optical carrier propagating along the waveguide. The polarization modulation is transferred into intensity modulation by means of a high birefringent fiber acting as a polarizer. For best sensitivity and linearity the modulator was biased by controlling the input polarization state thus requiring no metallic leads. This was accomplished by a dedicated polarization controller which alters the birefringence of a low birefringent input fiber via the elastooptic effect. The voltage required to switch the light from on to off was 2.9 V.
Chromatic opponency through indiscriminate connections to cones
Authors: Lennie Peter , Haake P. William , Williams David R.
Published: February 2023

A midget (P) ganglion cell in or close to the macaque's fovea has a receptive field in which the center is probably driven by a single cone. Thus, even if the surround drew indiscriminately on signals from all classes of cone, such a receptive field would be chromatically opponent. We have used a computer simulation to analyze the chromatic properties of P-cells with receptive fields constructed by drawing on a mosaic of R, G, and B cones, and allowing single or multiple cones to contribute to the center, without selection of the classes of cones that contribute to center and surround. For each synthetic cell we calculated the spectral sensitivity and also the weights that it attached to signals from the different classes of cone, and we compared the distribution of weights obtained from a population of such cells with the distribution obtained by Derrington, Krauskopf, and Lennie [J. Physiol. 357, 241 (1984)] in their analysis of the chromatic properties of P-cells in LGN.
Future of manufacturing with optical microlithography
Authors: Knight Colin W. T.
Published: February 2023

Optical microlithographic technology has been used for the manufacture of semiconductor devices for over 30 years. It has shown remarkable technological stamina and has repeatedly confounded those who have predicted its demise as the feature sizes used in semiconductor devices have been decreased.
Nonlinear modulation of light in integrated optic Mach-Zehnder modulators
Authors: Walpita L. M. , Wang S. C.
Published: February 2023

Modulation of light in integrated optic Mach-Zehnder modulators is usually achieved in the linear region of the intensity voltage characteristics and therefore the effects of nonlinearities are very small. However, under large signal operation, nonlinearities become prominent. Such nonlinearities cause optical intensity to change.
Circuit design approach for high extinction integrated optic modulators and switches
Authors: Koai Kwang T. , Su Shing-Fong
Published: February 2023

A circuit design approach using cascaded structures for integrated optic modulators and switches with high extinction ratios is described and analyzed. The extinction ratios are improved by a multiplication effect and can be exceeding 30 dB for polarization independent devices with no individual voltage tuning. The trade-offs are more chip area consumption, more optical insertion loss, and a slower device response speed. These trade-offs can be minor for nonhigh speed applications, such as photonic cross-connect systems, where the switch control and crosstalk are more important issues.
Pump modulation of close-coupled, LNP cube lasers
Authors: Lingvay L. S. , Dixon G. J.
Published: February 2023

Current modulation of the pump diode laser in a lithium neodymium tetraphosphate (LNP) close-coupled cube (C2 cube) laser1 provides a convenient way to generate a regular train of submicrosecond long pulses without changing the device geometry. Spike-mode oscillation in external mirror LNP lasers at a maximum repetition rate of 200 kHz has been previously described by Kubodera and Otsuka.2 We report the observation of single frequency, spike-mode oscillation at frequencies near 1 MHz from C2 cube lasers. For substantial modulation amplitudes (>20%) at an average cw pump power approximately two times the lasing threshold, we have observed stable spiking over a frequency range of 590-940 kHz. A peak power enhancement (peak spike power divided by cw power at the same average input power level) of > 10 was obtained at 590 kHz. Resonant relaxation oscillation with reduced amplitude has also been observed at 2.2 MHz.
Electrical considerations of high average power pulsed lasers
Authors: McDuff Glen
Published: February 2023

Many power conditioning aspects not considered or ignored in low average power pulsed lasers can complicate and degrade performance and operation in higher power systems. With electrical to optical conversion efficiencies of the order of a few percent, to achieve an output of several hundred watts requires many tens of kilowatts input power. In the case of high repetition rate machines, the associated high rms currents accentuate the effect of component equivalent series resistance (I2R) and design of interconnecting conductors. If high average power is achieved by increasing the peak pulse power, mechanical stress, component mounting, and layout become critical design issues. These effects greatly impact the cost, longevity, reliability, availability, and maintainability of the laser system.
Applications of terahertz beams
Authors: Grischkowsky D. , Fattinger C. , Van Exter Martin , Keiding Soren R.
Published: February 2023

Using our newly developed techniques of terahertz optics,1 we have generated well-collimated beams of freely propagating subpicosecond electromagnetic pulses. By comparing the high speed response of different receiver-transmitter combinations, we have optimized the system so that the terahertz pulses are detected with subpicosecond resolution and with signal-to-noise ratios of more than 1000:1. With this terahertz source, time domain spectroscopic measurements of single crystals of sapphire, quartz, MgO, and high resistivity silicon have provided the most accurate characterization of their terahertz dielectric properties to date. When the terahertz beams propagate through as little as 20 cm of laboratory air, a 50-ps duration oscillatory structure appears after the main subpicosecond pulse. This structure is caused by the absorption and dispersion of several strong resonance lines in water vapor. Using a controlled ambient atmosphere for the terahertz beam path, we have made the most accurate measurements to date of the absorption strengths of the nine strongest lines of water vapor in the spectral region from 0.2 to 1.5 THz.
Chirped grating analysis based on successive conformal mappings
Authors: Kok Yon-Lin
Published: February 2023

Binary chriped gratings have been empirically found to demonstrate the focusing properties of a lens by many researchers but without rigorous theoretical analysis. This paper presents a theoretical analysis of plane wave diffractions by an arbitrarily chirped binary grating. The solution techniques include successive conformal mappings (SCM) and the vector coupled wave theory. The grating, being perfectly conducting, is uniform in one direction but the grating period varies slowly in the other (the grating vector) direction. Under these assumptions, the orthogonal decomposition of any electromagnetic fields in terms of the fast and the slow polarization is valid. Furthermore, the nonperiodic electromagnetic boundary conditions which have been introduced by the chirped grating profile are mapped into periodic ones with three conformal mappings: (1) the Schwarz-Christoffel transformation, (2) the linear fractional transformation, and (3) the natural logarithmic transformation. This newly mapped domain can be modeled as a smooth, conducting surface loaded by dielectrics with a periodic varying index. The Helmholtz wave equation is treated in this transformed domain where the conventional coupled wave analysis is exploited. As the SCM techniques were previously used for solving periodic grating diffraction problems,1 this work extends its validity to nonperiodic grating structures.
Geometric transformations using binary optics
Authors: Zweig David A. , Power Michael P. , Mchugh Thomas J. , Logue James E.
Published: February 2023

Geometric transformations have been an area of continued research for the past fifteen years. A wide variety of applications, including scale rotation invariant pattern recognition and diffraction plane sampling, have been proposed. Recent advances in binary optics allow the construction of geometric transform elements which demonstrate a high diffraction efficiency and a large space-bandwidth product. Binary optics refers to computer-generated holograms made by multiple mask lithography. For a single mask (as is conventionally used in computer-generated holography) two possible phase levels are available. This leads to a maximum diffraction efficiency of 40%. Using N masks, 2
N
−1 phase values are possible with a diffraction efficiency given by [sin(π/N)/ (π/n)]2. (A binary optic constructed using three masks has a diffraction efficiency approaching 95%) To demonstrate the performance of a binary optic transformer, an element is constructed which generates a logarithmic polar coordinate transformation: each point (x, y) on the input plane is mapped to a point [ln(x2 + y2)1/2, tan−1(y/x)] on the output plane.
Influence of the eye on contrast sensitivity for different luminance levels
Authors: Losada M. A. , Santamaria J. , Navarro R.
Published: February 2023

We analyze the influence of the optical imaging quality of the eye on contrast threshold perception using both physical and psychophysical data for the same human subject. We have completely characterized the optical quality of human eyes by means of a hybrid optical-digital method and a phase retrieval algorithm that allows us to obtain the point spread function (PSF) and the wave aberration of an individual eye. From the wave aberration, we can calculate the 2-D modulation transfer function (MTF) in any conditions of luminance which are directly related to the pupil size, defocusing, etc. We have also obtained 2-D contrast sensitivity functions (CSF) for the same subjects with a forced-choice staircase method for different values of the average luminance of the test. The link between both kinds of data is a 2-D model for the theoretical determination of threshold contrast that considers the different stages of the early visual process. In this way we are able to evaluate the relative influence of both the optical and neural stages on contrast sensitivity for any desired luminance level, defocusing, etc.
Achromatic constancy and adaptation level
Authors: Sewall Laura , Wooten Billy
Published: February 2023

Theories of lightness constancy are frequently based on luminance ratio or adaptation level explanations. In a previous study, we found lightness constancy as a function of invariant contrast relations under sustained viewing, but not for brief stimulus durations. We hypothesized that the observed constancy occurred either as a result of adaptation to the overall stimulus luminance and/or due to the constant luminance ratios. In this study, we measured achromatic constancy in varying adaptation conditions. Stimuli consisted of center test regions with contiguous surrounds presented in Maxwellian view. Ten center/surround ratios were used in each of three test luminance level conditions and in six adaptation conditions. Subjects judged either the lightness or brightness of the center test region. We found constant lightness judgments, with constant ratios, independent of adaptation level. However, brightness judgments varied when the center luminance was slightly below or above the surround luminance.
Fiber optic bathyphotometer
Authors: Patton R. J. , Wanser K. H.
Published: February 2023

A laboratory prototype bathyphotometer for the measurement of bioluminescence in the ocean has been developed and tested. The instrument is designed to collect the light emitted by bioluminescent organisms stimulated by turbulent flow in the measurement chamber. The main features of the design include a mirrored measurement chamber and a totally internally reflecting light funnel which are used to efficiently channel the light into an optical fiber. The goal of the design effort is to develop an expendable instrument which eliminates costly and power consuming electronics in situ, concentrating these components on the deployment platform (e.g., PMT detectors). The optical components must collect light from a distributed, noncollimated source and reduce it to an area and distribution compatible with the size and numerical aperture of the collecting fiber. For a nonimaging system, it can be shown that light tapers (i.e., cones) provide the maximum capability for light collection. A comparison of various cone types and shapes is made including mirrored and solid transparent cones. A computer program has been developed to analyze the light collecting properties of the optical components including the reflecting tube, lenses, cones, and fibers. In particular, the full ray path solution for off-axis and/or distributed sources has been treated, i.e., nonparaxial and nonmeridional (skew) ray paths are calculated. The results of this analysis are discussed. The experimental measurement chamber allows water to be pumped through a grid at its entrance to measure the bioluminescence of organisms introduced to the flow.
Model based image restoration of underwater scenes
Authors: Blaker J. Warren , Morelli Michael V.
Published: February 2023

Underwater images suffer degradation as a result of the noise introduced by the water column between the object and the imaging device. This noise includes such problems as suspended particle scattering and unequal spectral attenuation. These result in a significant degradation of the image. The problem is compounded by the requirement of artificial illumination. Model based attacks on this problem are presented with particular attention being given to illumination models.
Lasers and pulse power
Authors: Guenther A. H.
Published: February 2023

There is no question but that the development of electric discharge or e-beam excited lasers, whether they be CO2 in the infrared or excimers in the ultraviolet, have made laser technologists more aware of the important role high voltage and pulse power plays in laser development. However, except for employing lasers to synchronize electrooptical diagnostics of laser-material interaction or other such studies, the laser community, by and large, is not aware of the myriad of unique advantages offered by lasers to the pulse power technologist—advantages that stem from their noninvasive nature. Whether it be to initiate high voltage (>5-MV) discharges in many classes of material or types of device from vacuum to superconductor and from thyratron to semiconductor, lasers have as well been used to open or break conduction through resonant interaction with gas species (by controlling gas kinetics), i.e., optogalvanic processes. They are frequently used to determine the concentration and spatial distribution of dynamic plasma components. Today, pulse power on a chip is becoming advanced as laser diode output transported through optical fibers is used to trigger bulk photoconductor switches at the multi-kV, multi-kA range from femtosecond response. Lasers are also employed frequently with fiber optics as electrical diagnostics whether it be for mapping electric field distributions or accurately determining temporal waveforms by the Pockels effect or for current by the Faraday effect, all with an RFI insensitivity unmatched by contact probes. But perhaps the greatest application is in modular systems such as frozen-wave generators, the precision triggering of phased arrays, or multimodule particle beam fusion devices. The scope of use is expanding, not only because of the surgical application and sensitivity of laser devices but because of their inherent safety by remote or noncontact application.
Grating outcoupling from large area rib waveguide arrays fabricated on GaAs/AlGaAs by selective ion beam milling
Authors: Rastani K. , Tanguay A. R. , Frateschi N. C. , Kapre R. , Madhukar A.
Published: February 2023

The development of large area rib waveguide arrays on GaAs, with low crosstalk and low propagation losses, is essential for monolithically integrated advanced optical signal processors.1In addition, surface gratings for outcoupling from such arrays are important to access the third dimension of large area integrated optical devices for additional processing capability.1,2Here, we report the fabrication of large area rib waveguide arrays with high surface uniformity (660 elements, 8µm wide, 0.6µm high, 3 mm long, with 2-µm separations) on GaAs/Al0.3Ga0.7As slab waveguides grown by molecular beam epitaxy (MBE), through photolithographic definition and selective argon-ion etching (beam current density ≃0.1 mA/cm2at 300 V). In addition, we have fabricated uniform surface gratings (2- and 4-µm periods, 0.2-µm peak-to-peak, and 1-mm long) on such rib waveguide arrays through a second photolithographic step and ion beam milling process. End-fire coupling of a Nd:YAG laser beam (1.06µm) into individual waveguides was conducted to excite their guided modes revealing good mode confinement and low cross-talk. In addition, surface gratings were employed to successfully outcouple the guided modes into the third dimension of the rib waveguide arrays.
Pattern recognition using a magnetooptic spatial light modulator
Authors: Kirsch James C.
Published: February 2023

The U.S. Army Missile Command has recently funded the development of driver electronics to allow the Semetex magnetooptic spatial light modulator (MOSLM) to be operated at video frame rates and addressed with a standard RS170 video from a camera. The driver electronics consists of a single PC board which fits in an IBM XT. The video signal from a camera is fed to the board, binarized, and sampled down to a 128 × 128 image and then output to the MOSLM. The binarization threshold as well as the sampling frequency can be set in the controller software for the board. The first field of a video frame is binarized, sampled, and written to the MOSLM. The scene is printed and displayed during the second video field and the process repeats itself with the next frame.
Fourier plane dynamic range requirements for binary nonlinear joint transform correlators
Authors: Javidi Bahram , Ruiz Jack S.
Published: February 2023

Binary nonlinear joint transform correlators have been shown to produce remarkable results in correlation-based signal detection.1 The binarization is simply reducing the joint power spectrum information at the Fourier plane to a + 1 or a – 1 (1 bit/ pixel) according to a predetermined threshold value. An important question that arises is the effect of the quantization of the joint power spectrum before binarization of the power spectrum. Since digitization of the joint power spectrum means limiting the joint power spectrum to a fixed amount of levels, it is necessary to determine the minimum number of quantization levels. We show that the minimum number of quantization levels for digitizing the joint power spetrum has a significant effect on the performance of the binary nonlinear joint transform correlator. If the joint power spectrum is quantized with an insufficient number of levels, the binarization of the joint power spectrum will result in extremely poor correlation performance.
Laser pumped 3-µm Ho:YAlO laser
Authors: Rabinovich W. S. , Bowman S. R. , Feldman B. J.
Published: February 2023

The potential medical uses for 3-µm lasers has spurred a search for efficient high power solid state sources. Holmium laser operation has been demonstrated using flashlamp pumping in a number of different hosts where the lowest thresholds have been observed using holmium doped yttrium aluminum perovskite (YAlO3). Emission in YAlO3 has been reported at 2.92 and 3.02 µm on the 5I6–5I7 transition. We have utilized intracavity laser pumping with the 1.08-µm line of the Nd:YAlO3 laser which has resulted in good efficiency and the observation of a new laser line at 2.85 µm. The intracavity pumping scheme was adopted because of the low absorption (~8% per pass) of the 1.08-µm pump by the Ho:YAlO3 rod. A Ho:YAlO3 rod was placed inside the cavity of a flashlamp pumped Nd:YAlO3 laser whose cavity losses aside from the rod are low. Since the rod is the dominant loss in the Nd:YAlO3 cavity it absorbs most of the pump light after many passes. A separate 3-µm cavity cocked at a slight angle from the 1.08-µm cavity also contains the ho:yalo3 rod. Emission was observed at 2.92 µm as well as on a new line at 2.85 µm. The 2.85-µm line was extremely sensitive to atmospheric water absorption and ran well only when the laser was purged with dry nitrogen. In a nonoptimized cavity we observed slope efficiencies of ~4% relative to the 1.08-µm pump and a maximum output energy of 8 mJ.
Interactive Fourier analysis on IBM PC-compatible computers
Authors: Abrams Mark C.
Published: February 2023

While the techniques of Fourier analysis and signal processing lend themselves to the development of computer programs for data reduction, relatively few programs offer a full complement of algorithms specialized to spectroscopy. Programs for interactive Fourier analysis should provide an environment in which various techniques can be compared and in which data can be displayed and manipulated in both Fourier domains. The decomp spectral decomposition program is designed for the reduction of Fourier transform spectra and focuses on interactive graphic display and Fourier analysis of data. For experiments that require the maximum achievable accuracy or must coax a modicum of signal from noise the DECOMP package offers something more than black-box spectrum analysis. Techniques of Fourier analysis and signal processing are demonstrated, and an IBM PC-compatible computer is available for hands-on demonstrations.
Spectroscopic determination of Cr3+ site distribution in mullite glass ceramics
Authors: Liu H. , Knutson R. , Jia W. , Yen W. M.
Published: February 2023

Previous work has revealed the existence of high field site (HFS) Cr3+ ions in the mullite glass ceramics which relax through R-line emission from the excited 2E state. Recent studies of the anomalous spectral behavior of this material have suggested the existence of CR3+ ions at disordered low field sites (LFS) which emit broadband 4T2 emission at longer wavelengths. In this paper we report the results of excitation spectra monitored at the R-line or broadband 4T2 emission wavelengths which have allowed us to determine the site distribution. These measurements show that the maxima of the HFS 4T1 and 4T2 absorption bands lie ~1000 cm−1 higher in energy and have a bandwidth ~1500 cm−1 narrower than the overall absorption bands of the entire site distribution. From these data we conclude that ~10% of the ions are still in LFS in the mullite glass ceramic sample after heat treatment; radiative energy transfer exists from HFS to LFS ions resulting from the absorption of HFS R-line emission of LFS ions; the Dq value of the HFS ions increases with the degree of heat treatment. Lifetime measurements reveal a site dependence of the luminescence decay. The lifetime of the HFS R-line emission varies from 6.5 to 1.5 ms as the excitation wavelength is scanned from 532 to 670 nm, and the lifetime of the LFS 4T2 emission varies from 280 to 50 μs as the excitation wavelength is scanned from 563 to 743 nm.
High efficiency optical interconnects in dichromated gelatin
Authors: Taghizadeh M. R. , Robertson B. , Restall E. J. , Walker A. C.
Published: February 2023

We report on the design and fabrication of high diffraction efficiency, high damage threshold space-variant and space-invariant optical fan-out and interconnects in dichromated gelatin (DCG). We have fabricated a range of high efficiency (>90%) lenslet arrays for the visible and near infrared with near diffraction-limited operation capability and excellent uniformity (better than 2%). In addition we have been successful in combining the flexibility in design of the computer-generated holograms with high diffraction efficiency of DCG to fabricate compact holographic elements which can perform several functions simultaneously. One such device is capable of fan-out, light focusing, and spectral filtering of the input beam. By employing various pre- and postprocessing techniques we have increased the optical damage threshold of these holographic optical elements (HOEs) to >100 W cm−2. This is of particular importance when large fan-out (>1 → 100 × 100) components are required to power large arrays of optical logic gates.
Binary phase grating beam splitters for a very large array generation
Authors: Taghizadeh M. R. , Turunen J. , Vasara A , Westerholm J.
Published: February 2023

To exploit the full potential of parallelism of light in optical processing systems it is of paramount importance to have optical components which can efficiently generate arrays of >100 × 100 beams and/or images. In this paper we report on fabrication of very large binary phase grating beam splitters, better known as Dammann gratings. These space-invariant fan-out elements are the essential components when solving alloptical digital logic based 2-D and 3-D problems. Using very advanced computational techniques such as simulated annealing and downhill greedy algorithm, we have been successful in calculating the structure of fan-out elements as large as 200 × 200. The calculated structures have been plotted using electron-beam lithographic techniques with resolution of 0.1 μm. These computer-generated amplitude holograms, to our knowledge, are the largest fan-out elements fabricated to date. These amplitude masks have been copied in a range of materials including thin films of metals, dielectrics, photoresists, and bleached silver halide holographic plates to create phase elements. This results in an increase in the 2-D diffraction efficiency of the elements from 15% to >65%.
Some optical phenomena: a video
Authors: Goodman Douglas S.
Published: February 2023

A video tape has been prepared showing a variety of optical phenomena, most of which require only modest equipment. One purpose of this work is to remind ourselves of the value and enjoyment of observation. It is also meant as a partial antidote to our drowning in mathematical analysis and computer simulations.
Laser-heated pedestal growth: a viable technique for laser material evaluation studies
Authors: Chang R. S. F. , Sengupta S. , Shaw L. B. , Djeu N.
Published: February 2023

The key to an effective laser material evaluation program is a ready supply of crystals at a reasonable cost. For most studies a crystal rod size of 1-mm diameter and 10-mm length is sufficient. More important for material optimization is the number of crystals with different dopant concentrations. Bulk growth techniques would not be appropriate for this task. This paper presents the laser-heated pedestal growth technique as a fast and cost-effective way to grow small laser crystals. Rare earth doped garnets of good crystal quality are now routinely grown at our facility. Lifetime measurements showed good agreement with those of bulk crystals. Tm:YAG is one of the dopant/host combinations studied as a function of dopant concentration. At high thulium concentration, excitation in metastable 3F4 ions can migrate over a large number of sites in the lattice and be quenched by impurities and defects in the crystal. Only a slight decrease in lifetime was observed in our crystals, same as for bulk grown crystals. This attests to the crystal quality and demonstrates the viability of this growth technique for material optimization and comprehensive study of energy pathways in laser crystals.
Perceived contrast of vertical-plus-horizontal gratings
Authors: Nasanen Risto , Rovamo Jyrki
Published: February 2023

Perceived contrast of the sum of a vertical and a horizontal grating of the same spatial frequency and contrast was estimated at threshold and suprathreshold contrast levels. The gratings with circular apertures and equiluminous surrounds were generated using computer graphics. Contrast thresholds were estimated by a two-alternative forced-choice method. Above threshold, contrast perception was studied using contrast matching. The contrast of a single component grating was matched with the perceived contrast of the two-component grating. At threshold, there was hardly any summation of the two grating components, as has been previously reported. However, above threshold there was summation: The perceived contrasts of the single component and the vertical-plus-horizontal gratings were equal when their rms contrasts (the standard deviation of the contrast waveform) were equal.
Pi-4 is field additive under large and small spot conditions
Authors: Schirillo James , Reeves Adam
Published: February 2023

David Stockman (1983) measured Stiles's Pi-4 color mechanism using tiny (3-min of arc) test flashes on tiny (7-8-min of arc) fields, and reported better isolation of the MW cone fundamental than Stiles had originally obtained with 1° tests on large fields. However, David Foster (1981) found spectral sharpening in the detection of tiny spots on combinations of large and tiny fields, consistent with R/G opponent site involvement in this case. We reasoned that if such opponency was influencing Stockman's results, red and green fields might be subadditive (they are additive in Stiles's: Reeves, 1987) in his conditions. We used either a 19- or 200-ms 3.6-min of arc test (530 nm) on either 8.6-min of arc or 10° fields (481 and 622 nm). Results for two observers show additivity (+/−0.06 log units) at ten times absolute threshold, as long as adaptation is complete (6 min).
Gas detection by diode laser spectroscopy
Authors: Davies D. B. , Bone S. A. , Johnson S. A.
Published: February 2023

Some photoacoustic cells have been evaluated for use with diode lasers, and their performance compared with that obtained from the usual transmission methods.
Atmospheric extinction effect on remote chemical sensing
Authors: Rosen David L. , Gillespie James B.
Published: February 2023

Laser-induced fluorescence (LIF) is sometimes used for lidar because of the sensitivity of LIF spectra to chemical composition. Because of aerosols, background fluorescence, and quenching associated with the boundary layer, most LIF lidar studies have been restricted to simple gases above the troposphere. We performed a computer simulation to show the feasibility of applying UV fluorescence lidar to the chemical analysis of aerosols and hard targets in the troposphere. We applied factor analysisrank annihilation (FARA) techniques to a numerically generated excitation-emission matrix (EEM) of a mixture of amino acids. The EEM of the mixture was modified to include atmospheric extinction. The calculated concentrations of the amino acids were greatly reduced by extinction. This shows that the LIF spectrum must be deconvolved from the atmospheric extinction spectrum to properly interpret UV fluorescence lidar returns.
Grating phase in mutually pumped phase conjugators
Authors: Christian William R. , Saxena Ragini , Mcmichael Ian
Published: February 2023

Much attention is focused on the mutually pumped phase conjugators (MPPCs). Their unique ability to operate with mutually incoherent pumping beams makes them ideal candidates for applications in communications, laser locking, fiber optic gyros, and optical information processing. In the simple physical picture of MPPCs, the conjugate beams result from gratings that form between each pump beam and amplified scattering from that pump beam. Therefore, one might expect that the phase of the grating is determined by the relative phase between the pumping beams and their scattering. That is, the scattering acts as a seed beam that interferes with the pumping beam to determine the phase of the grating. The situation is complicated by the possibility of moving gratings, multiple reflections, and the sharing of holograms written simultaneously by both pump beams and their corresponding scattering. The problem of MPPC driven by an external seed beam was analyzed by Kroli- kowski et al.1 They predicted temporal instabilities for low intensity seed beams and stable solutions for high intensity seed beams. However, it has not been demonstrated (at least experimentally) that a seed beam can control the phase of the diffracted beam without destroying its conjugate nature. To investigate this question, we have set up a MPPC with a variable (phase and amplitude) seed beam and an interferometer to measure the relative phase between an incident and diffracted beam, and thus the phase of the grating. The same interferometer is used to determine whether the seed beam is influencing the wavefront of the diffracted beam and therefore destroying the conjugation process. This effect might be used as a nonmechanical means of biasing phase conjugate interferometers.
Two-wave mixing of optical signals in the plasma medium
Authors: Tataronis J. A. , Papen G. C.
Published: February 2023

Amplification resulting from two-wave mixing of optical signals occurs in nearly degenerate or, equivalently, transient conditions, subject to the restriction that the response of the nonlinear material to the incident wave is not instantaneous. Recent work has focused on Kerrlike materials.1 In the present study, we explore the time dynamics of two-wave mixing in a Kerrlike (Δn ∝ I), but nonlocal, plasma where the required delay process is induced either by a simple collision mechanism or by Landau damping. Maxwell's equations are coupled to the appropriate governing equations of the plasma, followed by linearization based on a strong undepleted pump wave and simplification by the slowly varying envelope approximation. Two plasma models are explored: (i) a warm collisional plasma, and (ii) a Vlasov plasma to simulate the effects of Landau damping. In each case, the resulting equations are solved via Laplace transform techniques. In the presence of a delay mechanism, the two-wave mixing geometry produces spatial amplification of an applied probe wave in transient (or nearly degenerate) conditions. An enhancement of the plasma response occurs when a moving intensity grating, propagating at a velocity of a natural mode of the plasma, is generated. This enhancement of two-wave mixing can be considered a form of stimulated Brillouin scattering.
Interferometry using pulsed recording of holograms in lithium niobate
Authors: Hafiz A. , Magnusson R.
Published: February 2023

Double-exposure Fourier transform holographic interferometry using a pulsed, long coherence length Nd:YAG laser is reported. The recording medium is pure and iron doped lithium niobate. Interferometry experiments with λ1 = 532 nm only and λ1 = 532 nm with λ2 = 1064 nm simultaneously present (two-photon recording) have been conducted. Pulsed recording of holograms in undoped crystals is possible with acceptable results but low diffraction efficiency. The diffraction efficiency and the sensitivity are greatly enhanced by simultaneous presence of the 1064-nm line as expected with the holograms permanently stored in the crystals. These recording techniques are applied to recording and visualization of transient aerodynamic flow field phenomena. Good results are also obtained in heat transfer studies from electronic chips.
Second harmonic generation in atomic hydrogen
Authors: Marmet L. , Hakuta K.
Published: February 2023

Second harmonic generation (SHG) in free atoms is one of the unsolved problems in nonlinear optics. The most probable mechanism is due to Stark mixing between nearby excited states through an electric field caused by charge separation through the process of three-photon ionization. To test the mechanism, atomic hydrogen appears to be the best candidate because of its well-known electronic structure and large dipole moments of its excited states. In this work we report on the observation of SHG in atomic hydrogen in the Lyman-α region. Pulsed dye laser radiation was doubled and tuned to 243 nm for excitation of the 2s ← 1s transition at 121.6 nm by a two-photon resonance process. A cw atomic beam was used with hydrogen atoms generated in a dc discharge of H2 gas. The laser beam was focused ~1 mm below the nozzle where the hydrogen atom density was estimated to be 1013 cm−3. The second harmonic radiation was strongly enhanced by adding an external dc field of up to 10 kV/cm. Stark splitting was clearly observed at higher fields.
Modeling of optically pumped self-frequency doubling
Authors: Lin J. T. , Chong S. W.
Published: February 2023

Self-frequency doubling (SFD) nonlinear crystals have been recently studied for the generation of green light in optically pumped miniature systems.1 In the case of end pumping, the system involves three-wave coupling of the pump, the fundamental, and the second harmonics. For the conventional intracavity doubling, the nonlinear loss due to the doubling has been treated as an effective outcoupling of the fundamental. In the present system of SFD, however, beam propagations are coupled within the same crystal which should be treated simultaneously.
Thermal design of optics for ALS beam lines
Authors: Digennaro Richard
Published: February 2023

This paper describes the heat loads and thermal design for optical components on synchrotron radiation beam lines at the Advanced Light Source (ALS). To accommodate the high heat loads produced by insertion device sources and still fully exploit the unique properties of the ALS, directly cooled mirror and diffraction grating substrates are used for minimizing thermal distortion of the optical surfaces.