bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi-object joint transform correlation with preprocessing
Authors: Alam M. S. , Karim M. A.
Published: February 2023

A joint transform correlator (JTC) is suitable for real-time operation because it does not require a complex filter.11 The performance of a classical JTC can be enhanced greatly by using a binary JTC2 where the joint power spectrum (JPS) is binarized by applying a threshold before the final inverse Fourier transform operation. However, JPS binarization involves significant computation time for determining the threshold for binarization. Also, false alarms are produced with multiple objects in the input scene. In this paper, we propose and investigate the multi-object joint transform correlation where the input joint image is first preprocessed by applying the Roberts edge enhancement operator before applying the joint transform operation. The main advantage of this scheme is that the preprocessing operation can be performed long before the image is introduced in the input plane, thereby eliminating any detrimental effect on the operational speed of the JTC. The proposed pre-processing technique is found to yield significantly high autocorrelation peaks while avoiding the false alarms associated with other JTCs.
The role of the gain medium in semiconductor laser instabilities
Authors: Hu Y. Z. , Chow W. W. , Koch S. U.
Published: February 2023

We will show in this paper that the relevant semiconductor gain medium properties giving rise to spatial and temporal instabilities are the differential gain and refractive index, dG/dN and dδn/dN, respectively. The latter is often replaced by the antiguiding or linewidth enhancement factor, α= K0(dδn/dN)/(dG/dN), where K0 is the laser field wave vector in vacuum. A purpose of our paper is to point out the relationships among G,dG/dN, dδn/dN, and α. Owing to these relationships, the common practice of treating these quantities as independent parameters in the modeling of semiconductor lasers is inconsistent. On the other hand, the experimentally obtainable combinations of G, dG/dN, dδn/dN, and αcover a very wide range of situations. This is because one has the choice of bulk and quantum well gain media, and for a quantum well medium, present growth techniques make available a wide range of well widths and strains. This flexibility makes the semiconductor laser an excellent tool for studying laser instabilities. We will use a gain model, which contains the many-body carrier-carrier interactions and the band mixing effects due to quantum confinement and strain, to illustrate the different experimental conditions reachable with a semiconductor laser. As an example, we will present the results of our investigation on the behavior of a vertical cavity surface emitting laser (VCSEL) in the presence of an injected optical field, where we observed relaxation oscillation, sub-harmonic bifurcation, and chaos under certain operating conditions.
Chaotic behavior of an AR-coated semi-conductor laser in an external cavity
Authors: Li Hua , McInerney John G.
Published: February 2023

The first experimental observation of chaotic behavior to our knowledge is reported for an external cavity semi-conductor laser with AR-coating on one laser facet (residual reflectivity <0.1 %) and an intracavity étalon. As the cavity is detuned, the laser oscillates in a narrow single external cavity mode (linewidth less than 2 MHz, which is limited by the resolution of the equipment) or in multiple external cavity modes, depending on the cavity detuning from the resonance center of the étalon. In the latter situation, undamped relaxation oscillation and beating between external cavity modes are observed in the intensity noise spectrum. Quasiperiodicity and chaos occur sequentially by changing the cavity detuning, interrupted by frequency locking. The optical spectra show that symmetry breaking takes place as instability occurs. Similar phenomena have been observed as the pump current is changed. The mechanism for the instability in this system is postulated to be the inter action between the beating of multiple cavity modes and the undamped relaxation oscillation in the nonlinear laser medium.
Noise immunity of threshold-decomposition morphological image processing
Authors: Tasto Joseph L. , Rhodes William T.
Published: February 2023

One of the biggest problems facing optical signal processing is the low accuracy of most optical processors. Threshold-decomposition image processing overcomes this problem to a large degree in the performance of nonlinear filtering operations, such as morphological and ranked-order filtering, on gray-scale images. Threshold decomposition is a technique by which an input gray-scale image is decomposed into a series of binary cross sections, each corresponding to the input thresholded at a different level. The binary cross sections are individually convolved with a binary kernel and are subjected to a thresholding/hard-limiting operation (the key to the noise immunity), and the results are summed to produce a filtered gray-scale image. The robustness of this process to system noise implies that fast, parallel optical processing technology can be combined with digital electronics without degrading the overall system accuracy. The implementation of such an opto-electronic system would consist of an emitter (e.g., laser diode) array, a photodetector array, an incoherent optical convolver, and digital electronics. We analyze the immunity to noise generated by spatial variations of emitter and photodetector arrays in the performance of the morphological operations of median filtering, dilation, and erosion.
Progress in FET-SEED-based smart pixels
Authors: Anthony P. J.
Published: February 2023

Several demonstrations of switches based on symmetric self-electro-optic-effect devices (S-SEEDs) have shown the advantages of using free-space optics for handling high interconneclion density at high bandwidth. The S-SEEDs were fabricated by using standard GaAs processing, but with MBE grown multiple quantum wells, to produce large arrays of devices that switch with picojoule energies. Since the S-SEEDs function solely as optical detectors and modulators, problems of reliability and heat generation of the optical power are removed from the chip. After several generations of S-SEEDs, progress stalled on further improvements in switching energy and therefore processing capacity. Smart pixels, cells containing optical inputs and outputs with some electronic processing localized to the cell, offered the possibility of extending the switching energy progress curve and enhancing the functionality of each cell of an array. FETs have been fabricated with S-SEEDs in compatible processes and have resulted in improved optical switches. There are tradeoffs, however, inherent (such as decreased density, suboptimized performance of both electronic and optical devices, and complexity of processing) that need to be considered in the design. Progress made in several generations of FET-SEED-based smart pixels and predictions of future trends will be presented.
A Monte Carlo method for the determination of optical properties of biological materials
Authors: Schafer S. A. , Powell R. C.
Published: February 2023

Biological materials are efficient scatterers of optical radiation, making direct measurement of their optical properties difficult. Analytical and semi-analytical approximations to the relevant scattering mechanisms are frequently used to model the propagation of light in these materials. Availability of high-speed personal computers has made the technique of Monte Carlo simulation a useful alternative to analytical approximations for measurement and prediction of the response of biological materials to optical irradiation. We describe a combined experimental and Monte Carlo method with which the optical parameters of thin tissue samples may be rapidly and accurately determined from measurements of reflected and transmitted light intensity. We also present the results of application of this method to laser-tissue interaction projects in our laboratory, for example, to predict the distribution of light within tissues during laser hyperthermia and photo-dynamic therapy.
Quantitation and localization of scattering and absorption properties of small tissue volumes by time and frequency domain spectroscopy
Authors: Chance B. , Wang N. G. , Kang K. , Bo H. , Weng J.
Published: February 2023

The quantitation of μ
a
and μ
s
′ in suspensions of intracellular organelles identifies the principal scatterers and allows a comparison with whole cells. Small quantities of the specimens or small biological objects are available (a few grams), and the assumption of semi-infinite boundaries for photon diffusion in highly scattering media are violated. Immersing the sample in a semi-infinite scattering medium of matched μ
a
/μ
s
′ affords a solution to this problem. A match is assumed when no change of μ
a
and μ
s
′ in the external medium is observed when the sample is immersed into the optical field. Suspensions of mitochondria give μ
a
≤ 0.04 cm-1 and μ
s
′= 15 cm-1. Ascites tumor cells give μ
a
≤ 0.003 cm-1 and μ
s
′ = 12 cm-1. Localization of such small objects by interference of coherent waves of photon density1 employs a four element phased array with a sensitivity at the null of>20°/mm at 200 MHz in a medium of μ
s
′ = 10cm-1. This localized beam is used to detect the small changes of μ
a
and μ
s
′ characteristic of small breast tumors or localized absorbers/scatterers in the brain as in Alzheimers disease.
UV laser interaction with biological tissue: ablation mechanisms
Authors: Oraevsky A. A. , Tittel F. K. , Jacques S. L.
Published: February 2023

Mechanisms of UV laser ablation of biological tissue (aorta wall), which include thermal, mechanical, and photochemical effects, will be reported. Tissue optical properties were determined, followed by a quantitative study of laser energy transformation channels into heat generation, photochemical reactions, fluorescence, and reflectance. The role of photochemical decomposition was found to be minor (~1%) in comparison with thermal effects (~80%). The relative contribution of thermal explosion and mechanical stress in the mechanism of tissue ablation by means of XeCl and Nd:YAG laser pulses was investigated. Time-resolved acoustic signal detection and luminescence spectroscopy of ablation products with nanosecond resolution was utilized. Absolute pressure values in the irradiated tissue were measured. The peak pressure amplitude in tissue can reach several hundreds of atmospheres at threshold fluence by short laser pulse ablation, which is substantially higher than that in optically homogeneous aqueous solution. This observation indicates the important role of tissue microchromophore centers in the ablation process.
Mechanism of biliary stone fragmentation with the Ho:YAG laser
Authors: Schafer S. A. , Durville F. M. , Jassemnejad B. , Bartels K. E. , O'Hare C. M. , Tytle T. , Powell R. C.
Published: February 2023

We have previously demonstrated that the flashlamp-pumped Ho:YAG laser is an attractive alternative to the FDA-approved pulsed dye laser for gallstone lithotripsy. We report here that the mechanism of fragmentation for the Ho:YAG laser differs substantially from that for the dye laser. Fragmentation occurs as a three-step process: The laser first melts an area on the surface of the stone. The molten material is then ejected into the surrounding aqueous ambient, where it rapidly cools and recrystallizes. Finally, the newly formed crystallites are swept away by macroscopic motion of the surrounding liquid, the result of rapid expansion and contraction of a vapor bubble produced by the laser pulse. We show that absorption of the laser light by both the stone and the water is required for efficient fragmentation. This is in contrast to lithotripsy with the dye laser, where fragmentation is believed to be initiated by a shock wave.
Undersea applications of erbium-doped fiber amplifiers
Authors: Bergano Neal S.
Published: February 2023

Transoceanic cable transmission systems are on the verge of a capacity increase, the magnitude of which has not been equaled since the change from analog coaxial cable to digital light-wave systems. The next generation of undersea cable transmission systems will use erbium-dopedfiber amplifiers (EDFAs) in the repeaters to boost signals periodically as they travel across the world's oceans. The first optical amplifier systems will increase the transmission capacity four to eight fold over the most advanced regenerator systems now in use today. The first generation of undersea optical amplifier systems will go into service in the 1995-1996 time frame. These systems will use the NRZ modulation format at bit rates up to 5 Gb/ s, with dispersion shifted fiber that is matched to the operating wavelength. The feasibility of this type of system has been demonstrated with circulating loop experiments with transmission distances in excess of 21,000 km and with long straight experiments of lengths up to 4500 km. The technique of measuring the bit error rate in a circulating loop, which we first reported one year ago, has proven to be quite useful in the design of long-haul optical amplifier systems. With this technique we have demonstrated transmission distances of 14,000 km and 21,000 km at 5 Gb/s and 2.4 Gb/s data rates for a maximum bit rate distance product of 71 Tb km/sec. These results showed that 5 Gb/s all-optical EDFA transmission systems are capable of achieving transoceanic distances at very low error rates.
Uses for EDFAs in high speed, high sensitivity all-optical networks
Authors: Laming R. I. , Zervas M. N. , Richardson D. J. , Payne D. N. , Chernikov S. V.
Published: February 2023

The noise of an efficient EDFA is characterized, and it is shown that at optimum gain efficiency the EDFA exhibits an increased NF. In addition, with the conventional (co-directionally pumped) configuration it is virtually impossible to obtain a combination of high gain (>30 dB) and quantum-limited NF. Incorporating an isolator in the EDFA overcomes these problems, and we have demonstrated an amplifier with 51 dB gain and 3.1 dB NF for only 45 mW of pump power. Constant power optical amplifiers are discussed, and we show that simply inducing a differential loss between pump and signal in the EDFA achieves an optical power limiting action with a >30 dB dynamic range. Experimental results are given. We report the generation of a high-purity 70-Gbit/s cw soliton train. The technique is based on the nonlinear propagation and compression of a dual-frequency beat signal, amplified with an erbium-doped power amplifier and propagated through a dispersion decreasing fiber.
Adaptive optical systems based on spiring type magnetostrictive actuator and their uses
Authors: Apollonov V. V. , Chetkin S. , Kijko V. , Savransky A. , Vdovin G.
Published: February 2023

The systems for correcting high-intensity IR laser radiation phase distortions based on efficient and precision devices for linear displacement are discussed. The experimentally grounded mechanical systems studied, with two or three degrees of freedom made on the basis of the high precision displacement devices, are presented.
Measurement of the coating weight of diffuse coated media
Authors: Bauer George T.
Published: February 2023

The transmittances of coated paper and base paper were measured in a narrow wavelength range over which the absorption of the coating was high and the base paper's absorption was low. The effect on the accuracy of the large spatial variations of the reflectance of the paper was eliminated by the application of a highly reflective, partially transparent plate. The plate was placed between the radiation source and the paper. Because of the high reflectance of the plate, most of the radiation that was transmitted by the plate was pumped through the paper. Based on a model of diffuse transmittance of weakly absorbing coated media, it was shown that there was a nearly linear correlation between coating weight and relative transmittance. After experimental confirmation, the results were used to develop an apparatus for the measurement of coating weights during the paper coating process. Two measuring units were installed along the paper pass for measuring the transmittance of the base paper and of the coated paper, respectively. The units were interfaced with a computer that was programmed to calculate the coat weight and display the result on monitors.
Unavoidable aberrations in the image of a small volume
Authors: Walther Adriaan
Published: February 2023

A finite volume cannot be imaged perfectly unless the angular magnification is unity throughout. If, therefore, a lens must be designed that forms a high quality image of a finite volume, it is prudent first to determine whether the required image quality can be obtained in principle. Mock ray tracing is often preferable to analytic techniques to answer such questions. I will show that the amount of programming involved can be quite modest, mainly because modern computing equipment makes numerical efficiency less important. Results for two cases will be presented. In the first case a high numerical aperture afocal system must form an enlarged image of a spherical volume, with a prescribed minimum Strehl definition. In the second case similar requirements must be met for a spherical object volume copied at nominally one-to-one magnification by a lens with a specified focal length. Where possible the numerical and analytical results will be compared.
Vorticity and suppressed heating in optical molasses
Authors: Marcassa L. , Oria M. , Surdutovitch G. , Lafyatis G. , Zilio S. , Bagnato V.
Published: February 2023

We present results of numerical simulation that demonstrated the existence of long-lived optical molasses (super-molasses). The model is based on the presence of a vortex force, which is mainly due to the misalignment and the nonuniform transverse profiles of the laser beams. A computer simulation shows an increase in the molasses storage time, which qualitatively agrees with previous experimental observations. A few different aspects and explanations arising from this model are discussed.
Laser spectroscopy of aromatic and organometallic radicals
Authors: Miller Terry A.
Published: February 2023

Laser induced fluorescence techniques can be used to obtain rotationally resolved electronic spectra of moderately large aromatic and organometallic radicals cooled to <2 K in a supersonic free jet expansion. Aromatic organic radicals studied include cyclopentadienyl, benzyl, triphenyl methyl, and substituted derivatives thereof. Organometallic radicals studied include Cd and Zn monomethyl as well as group IIA and IIB derivatives of cyclopentadienyl, methylcyclopentadienyl, and pyrrolyl radicals.
Resonance Raman study of the allyl radical excited states
Authors: Getty James D. , Liu Xianming , Kelly Peter B.
Published: February 2023

The characterization of the ground and excited electronic states of the allyl radical is important to the understanding of free radical chemistry. Resonance Raman spectroscopy can provide detailed information on the allyl radical excited state dynamics through the intensities of the ground state normal modes. Previous resonance Raman studies have examined the promotion of the valence a
2
electron to the valence b1 orbital, the 2A2 → 2B1 transition. The intensities of the Raman spectra at 224 nm indicate initial excited state dynamics consistent with a disrotary photoisomerization of the allyl radical to form the cyclopropyl radical. Resonance Raman spectroscopy has been utilized to examine the nature of the weakly allowed transitions between 235 nm and 250 nm. Rydberg states have been predicted to lie in this energy range. Analysis of the Raman spectra revealed enhancement in the fundamental, overtones, and combinations of the non-totally symmetric modes v9 and v12. Intensity in the fundamentals of these non-totally symmetric modes is evidence for B-term Raman scattering and hence vibronic coupling. The sharp resonances in the enhancement of the Raman spectra simplify the assignment of the overtone and combination tones ground state vibrational frequencies. Schemes for the observed vibronic coupling and ground state anharmonic couplings will be presented.
Formation and desorption of hydroxyl radicals during Pt-catalyzed oxidation
Authors: Mooney C. E. , Anderson L. C. , Lunsford J. H.
Published: February 2023

Heterogeneously catalyzed reactions often involve reactive radical intermediates that are produced on the surface and subsequently desorb into the gas phase. In this work, laser-induced fluorescence (LIF) has been used to measure OH radical desorption from a catalytically active polycrystalline platinum wire during the oxidation of methane and hydrogen. The temperature range of the wire was 900 K to 1300 K, and reactant partial pressures were kept between 1 and 20 mtorr to minimze any gasphase reactions. Under these conditions the surface reactions and energetics involving the hydroxyl radical can be studied for Pt-catalyzed oxidation by observing the LIF intensity when changing reaction conditions. The concentration of gas-phase OH radicals depends not only on the concentration of surface hydroxyl radicals but also on the apparent activation energy for OH desorption. This activation energy was found to be inversely related to oxygen coverage, varying from ~33 kcal/mol under oxygen-rich conditions to ~55 kcal/mol under oxygen-depleted conditions. The results indicate that the breaking of the Pt-OH is the rate limiting step for the appearance of OH radicals in the gas phase.
Evidence of excess noise from a Raman generator in a multipass cell
Authors: Battle P. R. , Wessel J. G. , Carlsten J. L.
Published: February 2023

Previous investigations of Raman amplification in a multipass cell showed that a plane-wave transient theory matched the experimental data. In this work the output energy of a Raman amplifier was measured when no input was present. To model the output we initially used a fully quantum mechanical plane wave theory, which was scaled to take into account both the focusing and the multiple passes with the noise source being the usual one photon per mode. While the theory adequately describes the exponential growth for large gain, for gz < 10 there is a substantial deviation between the theory and experiment. To account for this excess noise the Stokes output is modeled by using a set of non-power-orthogonal modes. These modes arise naturally from the non-Hermitian nature of the gain guided process. It is the inherent non-power-orthogonality of these modes that can lead to excess noise, as noted first by Petermann. We find that the output from this model matches qualitatively the observed experimental output.
A low pump power fiber seeded Raman capillary system
Authors: Selker M. D. , Lawandy N. M. , Driscoll T. J.
Published: February 2023

The conversion efficiency of stimulated Raman scattering (SRS) is limited by the fact that the process builds up from spontaneous emission noise. The system used to overcome this limiting effect utilizes nonlinear frequency conversion in a glass fiber to produce a seed for the SRS, and it uses a capillary tube to confine the pump light in the gas. The experimental set-up includes a cell containing CH4 at 560 1b/in2, pumped at 532 nm by using a mode-locked, Q-switched, and frequency doubled Nd:YAG laser. With an average power of approximately 50 mW of the pump light split off to the fiber to produce the seed, a change in the intensity of the first Stokes line at ~630 nm was clearly visible. This corresponded to a conversion efficiency of 22% with only 40 mW of pump power at a Q-switch rate of 450 Hz. At higher powers a sizable anti-Stokes emission at 467 nm was also generated. The large conversion efficiency of this unoptimized system demonstrates the feasibility of SRS for obtaining multiple wavelengths with diode pumped laser systems.
Motion characterization and identification of microbes using a hybrid LDV technique
Authors: Zheng B. , Pleass C. M. , Ih C. S.
Published: February 2023

A hybrid three-axis laser Doppler velocimeter (LDV) system has been set up in our laboratory. The system can monitor the motion of microbes in a larger volume and in a nearly natural environment. A computer is used to collect and process the data only when one microbe swims within the measuring volume. We can then obtain a FFT spectrum that represents exactly the motion signature of this particular microbe. The ambiguity caused by multiscattering among two or more microbes can be avoided. By using this new system and FFT spectrum analysis, a feature vector related to the size of a microbe, its average translational velocity, its rotation or wobbling, and its flagellum motility is obtained. Such a vector is a good criterion for distinguishing different species based on a weighted average maximum likelihood algorithm. We have successfully identified several species under a controlled environment. It thus becomes possible to automatically identify known microbes. In addition, this system can be used to discover or search for unexpected microbes in a situation where the concentration is low.
Perceptual evaluation of JPEG compression for medical dynamic image sequences
Authors: Whiting James S. , Eckstein Miguel , Einav Shmuel , Eigler Neal
Published: February 2023

Lossless compression achieves only modest data reduction in noisy medical image sequences such as digital coronary angiograms. Lossy compression achieves greater reductions, but diagnostic performance criteria for use of lossy compression in dynamically displayed image sequences have yet to be established. We performed a quantitative perception study to measure low contrast feature detectability within moving simulated arterial segments inserted frame by frame into actual human cardiac cine radiographs to produce dynamic displays with realistic quantum and structure noise. Thirty-frame angiograms were displayed in loop format at 15 fps. Three subjects made 200 observations of the original images and at each of three levels of JPEG compression in a four-alternative forced-choice experiment. Compressions of 6:1, 12:1, and 18:1 resulted in average relative detectability of 102%, 95%, and 64%, respectively, compared to the original images. Only the detectability loss at 18:1 reached statistical significance at the p<0.05 confidence level in this study. We conclude that observer performance techniques are able to quantify specific changes in actual diagnostic information. Our preliminary data suggest that detection of low contrast objects in coronary angiograms is not degraded for JPEG compression ratios of 12:1 or less.
New dynamic display significantly improves feature detection in coronary angiograms
Authors: Whiting James S. , Eckstein Miguel , Honig David , Eigler Neal
Published: February 2023

The goal of this study was to improve detectability of low contrast features within moving coronary arteries in dynamically displayed cineangiograms. We compared performance and decision times between the standard display, in which the artery moves with respect to a nearly stationary background of patient structure noise, and a "stabilized" display, where each image of the sequence is digitally shifted so that the artery is fixed at the center of the screen and the background moves. We performed a four-alternative forced-choice study to measure detectability of spherical voids within cylindrical segments (simulating arteries) mathematically projected into human coronary angiograms to produce images with realistic quantum and structure noise. For both display types the resulting 32-frame cine loops were displayed at 7.5, 15, and 32 frames per second. Four subjects made 320 observations for each display type at each frame rate. There was no time limit, but time for each response was recorded. Results show that the stabilized display at 32 fps improved detectability (d′) over the best standard display (moving artery at 8 fps) by an average of 50%. Furthermore, the optimized display reduced the average decision time by a factor of 3. We conclude that this display may be of significant value in coronary angiography and in similar visual detection tasks in dynamic displays.
Ultrafast switching in AlGaAs waveguide devices
Authors: Aitchison J. S. , Al-Hemyari K. , Ironside C. N. , Villeneuve A. , Stegeman G. I. , Kennedy G. T. , Grant R. , Sibbett W.
Published: February 2023

Theoretical modeling of the nonresonant nonlinearity in semi- conductors has predicted an enhancement in n2 in the spectral region of half the band gap. We have fabricated and tested a range of nonlinear switching devices in AlGaAs. The material contained 18% Al, sufficient to shift the fundamental band gap to λ
g
= 0.75 μm and, consequently, the half band gap to 1.5 μm. Thus, working in the 1.55 μm spectral region allowed purely refractive effects to be observed, without the detrimental effects of two-photon absorption. We have experimentally observed ultrafast switching in directional couplers and integrated asymmetric Mach–Zehnder interferometers. The switching speed was <100 fs, with peak powers in the 200 W range and the experimentally determined values for n2 in the region of 1 × 10-13 cm2 W-1. Because of the nonresonant nature of the nonlinearity, devices had a high throughput at 30%. We will present a range of experimental data on the materials and devices tested.
Low-loss ultrafast all-optical switching in a GaAIAs integrated interferometer at half the band gap
Authors: Al-Hemyari K. , Aitchison J. S. , Ironside C. N. , Kennedy G. T. , Grant R. , Sibbett W.
Published: February 2023

Recent work on the nonresonant refractive optical nonlinearity in semiconductors has shown that the optimum operating photon energy is below half the band-gap energy. The nonlinearity is ultrafast, and there is relatively low loss under these operating conditions. We have employed the nonresonant refractive nonlinearity in an all-optical switch based on an integrated asymmetric Mach–Zehnder interferometer fabricated in Al0.18Ga0.82As optical waveguides. The device has been demonstrated in the 1.55 μm region employing ultrashort pulses of around 330 fs from a KCl:Tl color center laser. The device requires a π phase shift for more than 80% switching, which is achieved at input intensities of 3.92 GW/ cm2 in the waveguide; this corresponds to an intensity dependent refractive index n2 of 5.86 × 10-14 cm2/W. For the 330 fs pulse employed in the operation of the switch, the switching energy is approximately 100 pJ, although the absorbed energy through the residual two photon absorption is ≈5 pJ.
Femtosecond intracavity dispersion measurements
Authors: Knox W. H.
Published: February 2023

In femtosecond mode-locked lasers, dispersion plays a critical role in the mode-locking physics. With the advent of new broad bandwidth solid-state gain media that could support pulses of less than 10 fs duration in principle, dispersive effects are becoming more important and represent a barrier for further progress. Previously, dispersion in individual cavity elements has been measured with femtosecond accuracy. In this talk, we discuss a new technique for measurement of intracavity dispersion in operating mode-locked femtosecond lasers. Using a frequency-domain technique, we measure the frequency dependent group delay with ±0.65 fs accuracy. We discuss limits imposed by the high order dispersion in such lasers and also extend the technique to cover intracavity spectroscopy and nonlinear effects.
Generation of pulses as short as 33 fs from tunable Cr-doped lasers
Authors: Rizvi N. H. , French P. M. W. , Taylor J. R.
Published: February 2023

Ti:sapphire will largely supercede organic dyes in ultrafast laser applications. We report two tunable lasers continuing this trend that may also eventually supplant the (ultrafast) color-center laser. The n2 of the Ti:sapphire medium has been used to provide both amplitude modulation, through self-focussing, and phase modulation that, together with GVD, produces further pulse compression. These mode-locking techniques are generally applicable to vibronic solid-state laser media. Cr3+:LiSrAIF6 is an attractive alternative to Ti:sapphire—owing to its greater storage capacity, lower threshold, and ability to be diode-pumped. We report what is to our knowledge the first 488-nm-pumped cw Cr3+:LiSrAIF6 laser. This laser has been passively mode locked and, after the GVD of the system is optimized, yields pulses as short as 33 fs for only 500 mW pump power. We anticipate that similar techniques will be applied to new solid-state lasers tuning from 1.1 to 1.6 μm—the domain of cryogenic color-center lasers. Cr4+:YAG is a stable room-temperature laser that may be pumped at 1.06 μm and can lase from 1.35 to 1.56 μm. Our initial experiments on the first cw mode-locked Cr4+:YAG laser incorporating an acousto-optic modulator produced detector-limited pulses of <50 ps duration at 1.44 μm at up to 500 MHz.
A model for spatial interactions among contrast sensitive mechanisms
Authors: Cannon Mark W.
Published: February 2023

Recent research demonstrating that the perceived contrast of a small central grating patch can be strongly influenced by the presence of another grating in an annular surround implied the presence of two types of lateral interaction networks, one excitatory and one inhibitory. The present paper describes the development of a model for these networks, under the conditions where both center and surround contain gratings of the same spatial frequency and orientation. Two different interconnection networks were studied. In the feed-forward system, the gain of each member of a 2-D array of contrast sensitive mechanisms is adjusted by the weighted sum of the stimulus inputs to adjacent mechanisms. In the feedback system, the gain of each member of the array of contrast sensitive mechanisms is adjusted by a weighted sum of the outputs of adjacent mechanisms. Simulations indicate that psychophysical data can be accounted for only by the feed-back type network and that individual differences in suppression and enhancement effects can be accounted for by minor differences in the shapes of the spatial weighting functions.
Globally optimal filters
Authors: Juday Richard D.
Published: February 2023

A filter calculating algorithm is presented. The algorithm produces filters that are analytically optimal. That is, we show that no filter exceeds this algorithm's performance for the stated metric, noise and signal models, and SLM limitations. It is suitable to a wide variety of performance metrics, including signal-to-noise ratio and correlation sharpness. It explicitly accommodates colored additive input noise of known power spectral density and also detector plane additive noise that may be intensity dependent. The modulator on which the filter is expressed must be known, though (aside from the restriction that it be finite) it is entirely arbitrary. The SLM may be complex, coupled, phase-only, amplitude-only, binary, ternary, or otherwise discrete. It need not be spatially uniform in its action. Finite contrast ratio is accommodated. The algorithm requires a search over no more than two parameters, and conditions that may restrict the search are also presented.
Horner efficiency of phase-only and binary phase-only filters
Authors: Davis Jeffrey A. , Kane John J. , Cottrell Don M.
Published: February 2023

The Horner efficiencies of the phase-only filter (POF) and the binary phase-only filter (BPOF) used for optical pattern recognition are discussed relative to both the filter formulation algorithm and the geometry of the object. Both the POF and BPOF transmit 100% of the energy incident on the filter. The POF sends 100% of this energy to the correlation, while the BPOF only sends 40%. However, in each case, the shape of the correlation peak is strongly affected by the structure of the object. The correlation narrows, causing the peak value to increase as the number of edges in the object increases. The peak value can reach nearly 100% under optimum conditions. Computer simulations are shown for both filters. Experimental results are obtained by using the BPOF written onto the magneto-optic spatial light modulator.
Composite filter for reducing target illumination angle dependence
Authors: Hanson A. C. , Buzzard Robert D. , Kirsch James C. , Loudin Jeffrey A. , Gregory Don A.
Published: February 2023

It has been found that binary phase only filters made of real-world scenes are sensitive to changes in the target illumination angle. The illumination angle dependence for several cases will be measured, and a novel technique for reducing this illumination angle sensitivity will be introduced. A filter composed of sections of several filters is used to reduce scene illumination dependence. These filters are made with the target illuminated from different angles, and the resultant filter is more tolerant to changes in target illumination. The filter construction technique as well as results obtained when such a filter is used will be discussed.
Binary image classification based on symmetries in the correlation distribution
Authors: Lu Jianyi , Lin T. W. , Gregory Don
Published: February 2023

Both the intensity of correlation peaks and the correlation distribution1 have been utilized in a binary pattern recognition process to obtain information on the contour of the input object. In this paper, the symmetry properties of the correlation distribution are investigated and introduced as a parameter in classifying input objects. An example of identifying alphabetical letters from this parameter is presented. A horizontal bar and a vertical bar are chosen as selected features in the correlation process. Based on the symmetric properties of the correlation distribution, different categories can be classified. Further identification of individual letters in the same class can be achieved by the comparison of correlation peak intensities. The proposed approach drastically reduces the number of references required, compared to some conventional filtering methods, in identifying input objects from a given set of images. In addition, the technique presented shows promising performance on noise tolerance and scale invariance.
Error diffusion binarization methods for joint transform correlators
Authors: Inbar H. , Marom E. , Konforti N.
Published: February 2023

Optical pattern recognition techniques based on the optical joint transform correlator (JTC) scheme are attractive because of their simplicity. Recent improvements in spatial light modulators (SLMs) increased the popularity of the JTC, providing a means for real time operation. Using a binary SLM for the display of the Fourier spectrum first requires binarization of the joint power spectrum distribution. Although hard clipping (HC) is the simplest and most common binarization method used, we suggest to applying error diffusion (ED) as an improved binarization technique. The performance of a binary JTC, whose input image is considered to contain additive white noise, is investigated. Various ways for nonlinearly modifying the joint power spectrum prior to the binarization step, based on either ED or HC techniques, are discussed. These nonlinear modifications aim at increasing the contrast of the interference fringes at the joint power spectrum plane, leading to better definition of the correlation signal. Mathematical analysis, computer simulations, and experimental results are presented.
Global shape models for optical sectioning microscopy
Authors: Joshi S. , Miller M. I. , McNally J. G. , Grenander U. , Amit Y.
Published: February 2023

Movement of biological cells is crucial to a wide range of physiological processes. This has motivated the development of 3-D flexible models of living biological cells in slime molds undergoing shape changes during locomotion, imaged by using optical sectioning fluorescence microscopy. While living cells are strongly structured, they are not rigid and therefore exhibit high variability. A fundamental task is construction of models that incorporate both variability and structure in a mathematically precise way. For this we use the global shape models of Grenander. In this approach an ensemble of biological shapes are represented in terms of their typical structure via the construction of templates, and their variabilités are represented by the definition of probabilistic transformations applied to the templates. The transformations form groups and are applied so that while a rich family of shapes may be generated from a single template, the global properties of the templates are maintained. We take a Bayesian approach, with the optics of the sectioning microscope modeled as in Preza et al. and with the CCD camera modeled as a non-homogeneous Poisson counting process. The algorithms are implemented on a massively parallel SIMD architecture.
A method for synthesizing phase-only composite filters based on the extended interpattern association model
Authors: Yu Francis T. S. , Zhou Hanying , Chao Tien-Hsin
Published: February 2023

A technique of extending the interpattern association (IPA) model to construct a phase-only composite filter is given. By using the logical operation rules on a set of Fourier spectral (i.e., training set), the special and the common phase features of the training set can be determined. Then, by applying the extended IPA model on the special and the common phase features, a large capacity phase-only composite fiiter can be constructed. Computer simulation results from using this model are given, and the information capacity of the extended IPA composite filter is calculated.
Adaptation of quantization noise to image details in electronic halftoning
Authors: Scheermesser T. , Broja M. , Bryngdahl O.
Published: February 2023

Halftoning techniques are applied to convert graytone images into binary distributions. The optimization problem consists of achieving an acceptable compromise between graytone and detail resolution. By using iterative techniques it is possible to observe constraints in both the image and its spectrum. The possibility of controlling the spectrum during the binarization is used to influence the quantization noise. It will be shown how manipulation of the amplitude and phase in the spectrum can be used to enhance details and edges of the image, i.e., to conform the binarization noise to characteristics of the graytone image.
Maximum entropy restoration of ISAR images.
Authors: Bajkova A. T. , Frieden B. Roy
Published: February 2023

ISAR (interactive synthetic aperture radar) images are formed by Fourier transformation of complex frequency-space data. These generally suffer from noise and can also be degraded by under-sampling due to deliberate frequency skipping. Such a problem of incomplete information is typically well handled by the concept of maximum entropy. However, to apply the approach one has to force maximum entropy in a complex function. This was accomplished as follows (ATB). Each part (real or imaginary) is regarded as the difference between two associated, positive-only functions. The latter are forced to not overlap, through the action of a tuning parameter. Then the overall entropy is the sum of the ordinary entropies of the associated functions. This approach was adapted (BRF) into a MAP estimation algorithm where the entropy steps are absorbed into a prior probability law and the likelihood law allows for Gaussian noise. Because the exp(·) imaging kernel is separable in x and y,the 2-D processing problem can be implemented as a sequence of 1-D problems, first row-wise and then column-wise. An ordinary Newton-Raphson procedure solves for the unknown Lagrange multipliers defining each 1-D problem. ISAR image examples will be shown.
Texture segmentation by threshold decomposition filtering
Authors: McKeekin M. Sue , Rhodes William T.
Published: February 2023

An optoelectronic system is described that can segment a natural scene into regions of different texture characteristics, independent of the illumination levels in those regions. The system consists of a cascade of a thresholding hard-limiting spatial light modulator with controllable threshold, a bank of coherent spatial filters designed for specific textures, a square-law operation (coherent-to-incoherent conversion), a low-pass incoherent spatial filter, and a final thresholding-hard-limiting operation. The scheme works because thresholding at the local texture median produces a binary distribution that preserves considerable texture-related information and that has maximum energy per unit area in non-zero spatial frequencies. In addition, that energy per unit area is the same, independent of the specific texture, implying that only the distribution of spectral energy is different and that selective filtering can be used to discriminate in favor of a given texture and against others. The basic system and its underlying theory of operation will be discussed, and the results of numerical experiments will be presented.
Triplator—signal processor based on a rotational shearing interferometer
Authors: Rosen Joseph , Bixon Yael , Silvera Ezra , Shamir Joseph
Published: February 2023

A Fourier transform (FT) hologram of an object illuminated by spatially incoherent light can be recorded by a rotational shearing interferometer. Reconstruction in a coherent 2-f system leads to an integral operation, involving three complex functions, which we shall call triplation and define by the relation The triplation has some interesting characteristics. For example, its FT is also a triplation involving the FTs of the original functions. Other theoretical aspects of this novel process will be discussed, and experimental implementation will be presented for the case in which two of the functions involved are complex spatial functions, while the third is the coherence function of the illuminating source.
Turbulent patterns in wide gain section lasers
Authors: Moloney J.V. , Jakobsen P.K. , Wenden S.G. , Newell A.C.
Published: February 2023

We will report on a close analogy between turbulent convection in large aspect ratio fluids and traveling wave turbulent patterns in lasers with wide gain sections. In contrast to the fluid case, exact traveling wave solutions can be easily derived for the Maxwell-Bloch equations, including diffraction effects. As in the fluid case, we can identify regions in physical parameter space where stable traveling wave and turbulent patterns can coexist. For a two-level laser, the stable traveling wave region (Busse balloon) extends down to the first laser threshold. However, the Busse balloon for a Raman laser may lie well above the instability threshold or may extend down to the first laser threshold, depending on the sign of the intermediate level detuning from the pump. The physical manifestation of these stable traveling wave solutions is a controllable off-axis far-field emission of the laser. The onset of turbulent patterns beyond the first threshold in the Raman laser is caused by a sideband instability occurring at right angles to the underlying traveling wave (roll) solution. This instability causes the spontaneous nucleation of defects from finite amplitude solutions.
Control of chaos in a multimode laser system
Authors: Colet Pere , Gills Z. , Murphy T.W. , Roy Rajarshi
Published: February 2023

A Nd:YAG laser with an intracavity frequency doubling crystal displays chaotic dynamics for certain ranges of operating parameters. It has been experimentally shown that dynamical control of such a laser system is possible and that a variety of periodic waveforms can be stably generated through an appropriate feedback technique. Computations performed on a numerical model establish that unstable periodic orbits can be stabilized by this procedure. The nature of the stabilization process is studied, and transients from chaotic dynamics to periodic waveforms after application of the control signal are examined experimentally and numerically. The distribution of power among the modes of the laser and the synchronization of this process by the control signal are studied. These investigations demonstrate the dynamical control of a chaotic optical system.