bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental observation of topological defects in a laser
Authors: D'Angelo E. , Green C. , Izaguirre E. , Tredicce J. R.
Published: February 2023

Recently, it was proposed that topological defects are the cause of the symmetry breaking in transverse intensity distribution of lasers.1 We use a CO2 laser with variable Fresnel number to study experimentally the transition from symmetric to asymmetric patterns. This process of spontaneous symmetry breaking leads to the appearance of standing and traveling waves in the azimuthal direction.2 Here, we show experimentally that the symmetry breaking process is followed by the appearance of topological defects moving around the center of the beam. We detect defects both by intensity measurements and interferometric techniques. Interferometric techniques allow the observation of the position of the defect and the phase of the field around the defect. We also discuss the effects of the presence of defects on intensity and field cross-correlation measurements, and how they induce different length scales in the system.
Nonlinear refraction from χ(2):χ(2) in KTP
Authors: DeSalvo R. , Sheik-Bahae M. , Hagan D. J. , Van Stryland E. W. , Stegeman G. I. , Vanherzeele H.
Published: February 2023

We describe the cascading of two second-order nonlinearities [χ(2):χ(2)] to produce an effective third-order nonlinearity in KTP.1 This nonlinearity appears as either an irradiance dependent loss or phase distortion on the incident fundamental beam. The loss mechanism is simply the depletion of the fundamental due to second-harmonic generation. For low conversion efficiencies the loss of the fundamental is proportional to irradiance and appears as an effective Im[χ(3)] (i.e., as two-photon absorption). The refractive part of the nonlinearity [i.e., Reχ(3)] is less well known and usually ignored since it occurs off phase matching where ΔkL ≠ 0. This effect gives rise to either a phase advance or phase delay depending on the sign of ΔkL. In the experiment, we use a hydrothermally grown KTP sample of length L = 1 mm cut for type II SHG of 1064 nm and monitor the induced phase change of the incident fundamental beam using the Z-scan technique.2 The sign of the nonlinear refractive index changes sign as the angle tuned KTP sample is rotated through phase matching in accordance with theory. The experiment gives a maximum effective nonlinear refractive index n2≃±1 × 10-11 esu (±2 × 10-14 cm2/W) for an angle detuning of ±5°. This index scales with L and (deff)2 where we measure deff(KTP) = 3.1 pm/V.
Measurement of reduced nonlinearity in proton exchanged and annealed proton exchanged LiNbO3 by surface second-harmonic generation
Authors: Laurell F. , Roelofs M. G. , Hsiung H.
Published: February 2023

Proton exchanged (PE) and annealed proton exchanged (APE) LiNbO3 waveguides have recently been used for frequency doubling in the form of Cerenkov radiation or by quasiphase matching.1 The efficiency observed was much larger than predicted in the Cerenkov case but much lower with quasiphase matching. However, both these results can be explained by a strongly reduced nonlinearity in the waveguide.1 Areduction of the nonlinearity after PE has been reported,2 but it is not sufficiently large to explain the efficiencies obtained in Cerenkov radiation and quasiphase matching. We have used surface SHG with a frequency doubled Nd:YAG laser running at 532 nm to measure the nonlinearity of PE LiNbO3 and, in contrast to the previous reports, found an almost complete reduction of the nonlinearity. This is in much better agreement with conversion efficiencies reported.1 The short absorption length of the SH (266 nm) guarantees a surface selective measurement in our case, which the previous measurements did not ensure. The successive restoration of the nonlinearity with annealing has also been measured and the implications on nonlinear optical devices using PE and APE waveguides are discussed.
Nonlinear susceptibility of proton exchanged LiNbO3
Authors: Bortz M. L. , Lim E. J. , Fejer M. M.
Published: February 2023

Proton exchanged waveguides are often used in quasiphase-matched nonlinear devices in LiNbO3. Conversion efficiencies are often significantly smaller than the theoretical values, indicating the need for accurate measurements of the nonlinear susceptibility of proton exchanged LiNbO3. Reflected second harmonic generation (SHG) from interfaces can be used to measure the nonlinear susceptibility of a film on a nonlinear substrate; however, the air–film and film–substrate interfaces both contribute to the total reflected SH signal. Generation of harmonic wavelengths above the film absorbtion edge will discriminate against the film–substrate interface reflection, so that the reflected harmonic will only be sensitive to the nonlinear properties of the film. We report measurements of the d33 nonlinear coefficient of proton exchanged LiNbO3 using reflected SHG of 266-nm radiation and find that upon proton exchange d33 is reduced to ≈1% of the bulk LiNbO3 value. This result disagrees with previous reports of ≈50% reduction for doubling 1.06-µm radiation.1,2 A discussion of the discrepancy among these measurements along with additional measurements of d33 as a function of annealing are presented.
Comparison of various thresholding schemes of the joint transform power spectrum
Authors: Yu Francis T. S. , Tanone Aris , Wu Shu-dong , Tam Eddy C. , Gregory Don A.
Published: February 2023

A joint transform power spectrum (JTPS) usually has a very high dc component; an attempt to record the whole intensity spectrum produces a low-pass effect that results in a wider correlation peak. There are various thresholding schemes to improve the performance, such as binarization of the JTPS, or other methods encountered in practical situations, such as the hard clipping effect from the limited dynamic range of the square law detector or simply the dc blocking scheme that blocks the dc part of the JTPS. To find an optimum thresholding scheme, we have compared the performances of all these thresholding methods, using various inputs imbedded in an additive Gaussian noise environment. Our computer simulation results show that the dc blocking method is comparable in performance with that of the binarization method and is much simpler from the experimental point of view.
Multiobject correlation using a joint Fourier transform correlator
Authors: Alam M. S. , Karim M. A.
Published: February 2023

A joint Fourier transform correlator1 has been found to be reasonably successful in noise-free single target detection. One of the recent studies2 has already considered the effects of fringe binarization of multiobject correlation when the objects were all positioned along a horizontal axis. Our current work extends the scope of Ref. 2 by studying the effects of fringe binarization when multiple objects are arbitrarily introduced into the input scene. Furthermore, both in-class and out-of-class recognition is studied simultaneously for quantifying the degree of correlation discrimination.
Holographic implementation of crossover networks
Authors: Wang J. M. , Sawchuk A. A.
Published: February 2023

A crossover network is a variation of a multistage interconnection network. Its regular interconnection scheme is appropriate for optical implementation by space invariant systems. A crossover network requires two kinds of permutation called pass-through and crossover connections. One implementation of the crossover network uses different prism arrays in each path of a two-path imaging system.1 We propose here a holographic architecture, which can perform pass-through and crossover connections in a single path. The architecture is based on a Gabor-typo hologram. When the input illuminated by a collimated beam is imaged at a Gabor hologram plane (or the input is placed directly at the hologram plane) that has a focus F, it will propagate in two directions. The zero-order image array of the hologram arrives at the input plane of next stage without distortion after propagating a 2F distance. This performs the pass-through connection. At the same time, a first-order image array is diffracted by the hologram and focused at point F. After crossing over each other at F, the crossover connection can be performed at the input plane of next stage.
Spatial structures of solitons in stimulated Raman scattering
Authors: Suzuki Mitsutoshi , Midorikawa Katsumi , Tashiro Hideo
Published: February 2023

After a π phase shift in the initial Stokes field had been predicted to initiate soliton formation in the depleted pump field during stimulated Raman scattering, a spontaneously generated π phase shift was certainly observed in the actual Stokes wave. Practically, the amplitude of this anomalous pump reversal forming the soliton often varies from shot to shot, and its spatial profile during the stable propagation in the media has not been known so far. Recently, diffractive soliton decay has been theoretically investigated,1 however, the fluctuating amplitude makes it difficult to observe experimentally the transverse intensity distribution. In this study, we investigated the reversal composed of various transverse modes and its decay due to transverse phase shift using 2-D and semiclassical calculations with a π phase shift in the initial Stokes field. In conclusion, the spatial profile can be explained on the assumption that transverse-mode coupling occurs even in the soliton formation. If multiple-mode reversal should occur with a low-quality pump beam, it would decay rapidly because of the transverse phase shift during propagation.
Electrooptic fiber optic switches having low crosstalk
Authors: Meadows M. R. , Sullo N. , Chase H. , Swann W.
Published: February 2023

Fiber optic switches that use liquid crystals to switch all the unpolarized light from multimode optical fibers have been known for some time. The operation of one type of switch uses the liquid crystal as a variable retarder that controls the polarization of light, which is separated into and recombined from orthogonal linear components by a polarizing beam splitter cube.1 This combination of components results in a fully symmetric 2 × 2 switch, but its crosstalk is limited to worse than -20 dB, by even the best available dielectric polarizing beam splitter films. This short comingseverely limits the utility of such switches no matter what liquid crystal is used. We describe several switches that use these two components in new combinations to achieve crosstalk performance that can theoretically approach -40 dB. Our prototypes of the simpler switches exhibit crosstalk levels of -27 dB or better and insertion loss at about -3 dB. Since we used ferroelectric liquid crystals, they switch quickly between states with a rise time of 35 ps in response to 30 Vdc while requiring 1 nA of holding current. Our prototypes have operated at either 633 nm or 830 nm, but operation at other wavelengths should be possible.
Fully numerical solution of photorefractive phase conjugation of pulses
Authors: Diaz-Torres L. A. , Sanchez-Mondragon J. J. , Torres-Cisneros G. E.
Published: February 2023

We present a full numerical solution of phase conjugation of pulses in photorefractive media by four-wave mixing, solving the linearized Kukhtarev equations.1 The procedure avoids analytical reflectivity and its complexity as a required intermediate step by directly producing final data, in either the stable and unstable regimen.1 We compare our conjugated pulse solutions with the analytical cases available,1 and display additional information such as the spatial and temporal behavior of the space-charge field inside the conjugator, the transmitted pulse, and the reflectivity that plays now such an informative role. The results agree with the theoretical and experimental results of earlier work and corroborate some predictions2 concerning multiple internal reflections.
Theory of stimulated Mandelshtam-Brillouin scattering in running conditions in a inhomogeneous scattering medium
Authors: Benenson Z. M. , Yakovleva T. V.
Published: February 2023

The paper considers the theory of stimulated Mandelshtam-Brillouin scattering (SMBS) in running conditions, i.e., with the length of the scattering medium significantly exceeding the spatial length of the pump pulse. In such conditions the pump pulse runs through the medium, and stimulated scattering of the pump wave into the Stokes wave takes place from different layers of the medium.1 An investigation of the process is carried out for both homogeneous and inhomogeneous media. The strict expressions are obtained from the Green function and the Stokes wave field. The spectrum and the coefficient of increment of the Stokes irradiation are analyzed. It has been shown that the effect of modulation of the Stokes irradiation frequency takes place due to stationary periodic inhomogeneities of the refractive index in a medium. This effect occurs also from periodic alterations of the value of light fiber cross-section. A computer simulation of the process of running SMBS confirms the theory. Practical possibilities are discussed concerning the usage of running SMBS for remote probing of a medium’s inhomogeneities and for some other applications.
Phase conjugate resonators and bistabilities in BaTiO3
Authors: Dokhanian Mostafa , Putcha Venkateswarlu , Sekhar P. Chandra
Published: February 2023

We report our observations of optical bistabilities and hysteresis1 in photorefractive passive phase conjugate resonators with BaTiO3, caused by grating erasure effects between two resonators in three different configurations. Bistable oscillations have been observed between two semilinear oscillators, two ring resonators, and also between a ring and a semilinear resonator. In the experimental arrangement used for bistability studies, a single-mode argon ion laser beam (4880 Å) entered the crystal in a face appropriate for self-pumped phase conjugation. However, the orientation is kept in such a way that no self-pumped phase conjugation is observable if none of the resonators is in operation. In this geometry, bistability and hysteresis were observed in the signal intensity of one resonator as the reflectivity of the variable beam reflector in the second resonator was increased and then decreased in a cycle. Bistability was also seen in the signal intensity of a single resonator as a function of the reflectivity of the external mirror.
Calculation on non-collinear phase match for biaxial crystal in three-wave interaction
Authors: Yao Jianquan , Miles Richard B.
Published: February 2023

Some biaxial crystals have been shown to have good nonlinear qualities; for example, large effective nonlinear coefficient (such as KTiOPO4 and KNbO3), ultraviolet transparency (such as LiB3O5), large acceptance angle and temperature (such as KTP). A number of papers have presented the wide use of biaxial crystals in optical frequency conversion. Based on our previous work,1 the expressions for noncollinear phase matching conditions of some biaxial crystals are now presented and then numerically calculated for some biaxial crystals such as KTP and LBO.
Stabilization of an argon laser with a phase-conjugate mirror
Authors: Gay David , McCarthy Nathalie
Published: February 2023

It was shown that the fluctuations of the output power of a dye laser are principally due to the noise of the argon pump laser.1 A noise reduction of the pump laser will therefore improve the stability of the dye laser. We have been able to experimentally improve the signal-to-noise ratio of a cw multilongitudinal-mode argon laser without power loss. This reduction is achieved by reinjecting a small fraction of the output pump laser beam back into the laser cavity as in the experiment of Beaud.2 A photorefractive BaTiO3 crystal is used as a phase-conjugating mirror to form the external cavity. The advantages of this mirror over a conventional mirror is its self-alignment and the ease with which the feedback it produces can be measured. Even a very small feedback leads to an appreciable noise reduction. The low frequency noise components are reduced by up to 15 dB and the frequency components of the spectrum of the longitudinal-mode beating are attenuated by factors reaching 46 dB. The reduction is obtained with an external cavity shorter or longer than the laser cavity between 5 and 10 cm. The optimum stabilization occurs when the difference between the cavity lengths is ~8 cm.
Optical routing element using semiconductor nonlinear optical waveguides
Authors: Sarma J. , Shore K. A. , Canal F. , Dios F.
Published: February 2023

An assessment is given of the characteristics of coupled nonlinear rib-structure optical waveguides for utilization in an optical coupled guide cross-point switch (CGCPS). The CGCPS is effective for the practical design of electrically controlledcompact optical switches.1 The novel concept in the CGCPS is to effect optical switching at the junctions between orthogonally arranged input and output channels by creating a parallel optical power tap only at the junction between the input channel and the required output channel. Using III–V semiconductor optical waveguides this arrangement has been shown2 to permit the fabrication of compact switching elements suitable for multiport switching networks. In the present work, the novel features of the CGCPS design are combined with the properties of semiconductor nonlinear optical waveguides to achieve optically controlled signal routing. The analysis procedure adopted here represents an extension of our earlier studies of both isolated and coupled nonlinear optical waveguides. In this way we have been able to give consideration to the engineering requirements of an all-optical cross-point switch.
Polarized images of radiation scattered from objects with randomly rough surfaces
Authors: Chang-Hyuk  , Zeringue Kyle J.
Published: February 2023

We have developed a theoretical model for scattering of polarized radiation from randomly rough surfaces using the vector Kirchhoff integral equation. Shadowing of neighboring roughness facets is taken into account. The theoretical results are compared with experimental results by O’Donnel and Mendez1 who performed scattering experiments with samples of controlled roughness. Our theoretical results agree well with the experiment and reproduced depolarization and backscattering as observed by the experiment for highly rough surfaces. The theoretical model is incorporated into a finite element signature model to analyze polarized images of objects illuminated by active or passive radiation sources. Studies of intensity distributions and polarization states of objects with different roughness are performed for various illumination and detection angles and for various incident beam polarizations. It is found that images of scattered intensity are insensitive to the polarization state of the incident beam, but sensitive to the surface roughness and optical constants of the surface. Polarization images are sensitive to the incident polarization state as well as the surface roughness and optical constants.
High-efficiency even- numbered spot array generators
Authors: Walker Sonya L. , Morrison Rick L.
Published: February 2023

Phase gratings are important components of prototype free-space digital optical processing systems. They generate spot arrays to illuminate optical processing devices and there by facilitate the transfer of information between modules. Three criteria for judging spot array generation systems are (i) the match of the spot array formate to that of the optical logic array (e.g., odd vs even numbered formats), (ii) the relative spot intensity uniformity, and (iii) the efficiency of coupling light into the desired spots. Earlier work has demonstrated improvements in the first two issues.1The issue of efficiency requires examination in anticipation of high speed or very large multichannel systems. The efficiencies of simple binary phase gratings ranges from ~70-85% for 1-D arrays. By extending designs to multiple phase levels the efficiency is significantly increased. These multilevel gratings are fabricated by repeating steps in the fabrication process.
IR-UV double resonance studies of collision-induced vibrational energy transfer in acetylene
Authors: Orr B. J. , Chadwick B. L.
Published: February 2023

Time resolved IR-UV double resonance spectroscopy enables state-selective kinetic studies of rotationally resolved vibrational energy transfer in gas phase acetylene-d2 (C2D2). As a prime example, C2D2/Ar collisions are found to equilibrate laser-induced populations in the 4151 vibrational level with the neighboring 42 and 52 levels, prior to significant transfer to the lower 41 and 51 levels. This vibrational bottleneck effect is attributable to the high efficiency of quasielastic intramolecular V−V energy transfer relative to V−T,R processes. In the contrasting case of C2D2 self-collisions, quasielastic intermolecular V−V energy transfer relieves the bottleneck. Relevant second-order rate constants have been measured, and selectivity with regard to angular momenta and state symmetries identified. Other related experiments aim to demonstrate ways in which intramolecular perturbations contribute to collision-induced rovibrational energy transfer, with potential for enhancement of laser-induced photochemistry. Novel experimental strategies, including Raman−UV double resonance1 and implementation of a narrowband optical parametric oscillator,2 are also being used in this context.
Frequency-agile rf notch filter using photorefractive two-beam coupling
Authors: Hong John H. , Chang Tallis Y.
Published: February 2023

In modern signal processing environments the ability to excise certain frequencies from a given rf signal, known as notch filtering, is a useful function. Programmability of the notch frequencies is a crucial capability in many applications, which at the same time is difficult using conventional electronic methods. Hybrid electrooptical (EO) methods have been proposed and demonstrated where spatial light modulators are used in conjunction with an acoustooptic (AO) device to perform the desired function via Fourier optics. In this paper, we describe a new approach using photorefractive two-beam coupling with EO modulators. Two-beam coupling is a non-linear optical phenomenon where two coherent beams of light can exchange energy in a photorefractive medium.1 Our system uses a generalization of this effect for temporally modulated beams which has been theoretically characterized elsewhere.1 Our system uses a two-beam coupling apparatus in which one beam (the pump) is modulated with the received signal to be filtered and the other (the probe) is modulated with synthesized sinusoids at the notch frequencies. The modulation is performed with either AO or EO devices. The two-beam interaction causes those portions in the pump beam with the same frequency content as those present in the probe beam to become almost completely diverted into the probe beam direction so that the pump beam that emerges from the photorefractive crystal is nearly devoid of those temporal frequencies. This beam is then directed into a heterodyne detector apparatus to recover the notch filtered signal.
Reconstruction of a gray level image from the autocorrelation function with insufficient or noisy constraint
Authors: Awwal A. A. S. , Birt T. , Klemer D. , Flahive J. , Belloto J.
Published: February 2023

For optical pattern recognition or triple correlation use, it is often necessary to obtain an improved image of the input to attain strong correlation. One way to do this is to perform associative retrieval using the incomplete data into an associative memory. The other possibility is to retrieve data from the autocorrelation function result with some approximate constraint. The problem then becomes a phase retrieval problem used for remote sensing, wavefront sensing or crystallography. Simulation results demonstrating effects of incompleteness of the data constraint with the rate of convergence is illustrated. Possible use in building or enhancing the associative recall process is explored.
Apodized apertures for beam profile formation and avoiding hard-edge Fresnel diffraction ripples
Authors: Lukishova Svetlana G.
Published: February 2023

Shaping of the spatial intensity profiles by apodized apertures (AAs) is necessary for many practical problems in laser physics and their applications1,2: avoiding hard-edge Fresnel diffraction effects in the beam cross-section by rounding the uniform spatial intensity profile at the edges; generation diffraction-limited beams without side lobes, diminishing beam divergence and even sometimes increasing output energy in single-mode lasing by using AAs inside the resonator; increasing the second-harmonic conversion efficiency; compensation of the large scale in homogeneities in beam cross-section, e.g., elimination of pumping inhomogeneity in active elements; formation of a time-profiled pulse by electrooptical deflectors; experiments of interaction of laser radiation with matter, which require Gaussian or super-Gaussian spatial intensity profiles; optical processors, etc. In this paper four types of AA for lasers are presented, namely: induced absorption, photodestruction (including photooxidation) and thermochemical reduction AA work in UV, visible, near and partially mid-IR regions. The working principle of frustrated total internal reflection AAs is valid for the large spectral region including UV, visible, IR, submillimeter and millimeter spectral bands. AA fabrication techniques are considered and the results of investigations both outside resonator AA and inside AA in beams of 0.342-, 0.63-, 1.315-, 2.79-, 2.94-, 5.6-, and 10.6-μm lasers are discussed in comparison with other types of AA.
Energy upconversion in neodymium-doped sodium borosilicate glass
Authors: Reddy B. R. , Ololo Anthony , Venkateswarlu Putcha
Published: February 2023

Energy upconversion was known to occur in rare earths by ion–ion interactions and sequential two-photon excitation processes.1 Recently there has been a renewed interest in these studies after the development of upconverted lasers.2 The phonon spectrum of the host lattice influences are not only the resonant fluorescence quantum efficiency but also that of upconverted fluorescence and hence the laser efficiency. So far upconverted laser action was seen only in single crystals. To extend these studies to other materials we prepared sodium borosilicate glasses doped with neodymium. All the prominent absorption peaks of Nd3+ are identified in this host. When the D and E levels are resonantly excited upconverted violet fluorescence is detected from the L levels of Nd3+ in the glass host.
Experiment with the storage capacity and shift invariance of trinary associative memory for character recognition
Authors: Ahmed F. , Awwal A. A. S. , Chen P.
Published: February 2023

Character recognition by a trinary associative memory (TAM) neural network model is proposed. All the twenty-six letters of the English alphabet are stored in the trinary memory. The proposed scheme will then be able to recognize a character from its partial input. The dot product of the partial input with all the stored patterns is calculated as a measure of discrepancy from the desired pattern. Zero thresholding and arithmetic mean thresholding and some other statistical thresholding methods are then applied to select the desired output. The convergence of the recall procedure of the TAM network depends upon the storage representation and thresholding mode. So from the simulation run, an optimum threshold measure is found that establishes efficient character recognition. Shift invariance is also examined by simulation.
Comparison of brightness and contrast induction
Authors: McCourt Mark E.
Published: February 2023

Lateral spatial interactions in the visual nervous system are commonly adduced to account for a variety of simultaneous contrast (induction) phenomena with regard to the perception of both brightness1 and spatial contrast.2 The relationship of these two systems of lateral interaction has not been clarified. Available evidence suggests that the lateral interactions governing perceived contrast embody a form of multiplicative contrast gain control.2 Evidence is presented that the lateral interactions governing brightness perception are similarly the result of multiplicative luminance-gain control, in addition to a subtractive (zero-offset) process. Utilizing the grating induction paradigm1 in association with a contrast-matching procedure, the inducing grating spatial frequency was varied across a range of eight octaves (0.0625-16.0 c/d), encompassing a clear transition in the form of lateral interactions from ones based on a luminance-gain control mechanism to those based on a contrast-gain control mechanism.
Stark shift/broadening by stimulated emission
Authors: Dressler Edward , Davis Jon P.
Published: February 2023

Previous experimental work by our group1 has shown that the two-photon excitation of the 6p[1/2,0] state of xenon has a fairly sharp threshold for the onset of amplified spontaneous emission (ASE) from the 6p[1/2,0] state to the excited 6s state. The excitation probability has the expected quadratic dependence on excitation intensity below the ASE threshold but an unexpected linear dependence above threshold. We hypothesize that the reduced excitation probability results from an ac Stark shift/broadening of 6p[1/2,0] state by the ASE field. We have numerically solved the coupled Maxwell-Bloch equations with and without the relevant Stark terms. The ASE threshold predicted by the numerical solutions occurs at an intensity about an order of magnitude higher than the experimental value. Because of uncertainties in some of the experimental parameters this difference is not unreasonable. Solution of the equations without Stark terms gives quadratic excitation dependence above the ASE threshold. Including the Stark terms reduces the dependence to a 1.2 power law. The change to a 1.2 power occurs ~50% above ASE threshold, whereas the experimental change occurs almost right at threshold.
Continuous-wave laser strategy of probing NO at 220 nm for flow measurements
Authors: Di Rosa Michael D. , Hanson Ronald K.
Published: February 2023

Nitric oxide is a species intrinsic to hypersonic flight, and the capability of nonintrusively monitoring gasdynamic properties of NO could prove useful in experimental studies of wings, inlets or combustors. Accessing both the (0,0) and (1,0) band of NO A ← X with a cw ring dye laser is now possible due to recent wavelength extensions to 210 nm. Furthermore, modifying a ring dye laser for rapid-tuning1 allows scanning the laser frequency 80 GHz repetitively at a rate of 4 kHz. This paper investigates the use of rapid-tuning in single-point fluorescence detection NO A ← X transitions for simultaneous measurements of velocity, temperature, and pressure within an underexpanded free jet. The 80-GHz scan width encompasses two NO rotational lines that, by intentional selection, have differing temperature dependences. Because of the Doppler shift, the laboratory-frame resonance frequency depends on the velocity component of the flow along the beam direction. Imparting a relative Doppler shift between acquired spectral features was accomplished by dividing the UV output into two separate paths that pass through the flow at different angles. Fluorescence signals from these two beams are collected from nearly coincident 1-mm3 probe volumes along the jet centerline. Modeling the acquired spectral lines with Voigt profiles yields the temperature, pressure, and Doppler shift from which the velocity is directly computed.
Collisional quenching of NO A2Σ (v′ = 0) by NO, CO2, Ar, and He in a shock tube
Authors: Gray Jeffrey A. , Paul Phillip H. , Durant Joseph L.
Published: February 2023

Because NO is formed in a variety of combustion processes, concentration measurements using laser-induced fluorescence (LIF) have been widespread. Accurate measurements are needed at high temperatures to validate flame models, yet the fluorescence quantum yield, an essential factor relating optical signals to concentration, has previously been estimated using only quenching rates determined primarily at lower temperatures.1,2 We report rate coefficients for the deexcitation of NO (A2Σ, v′ = 0, N′ = 17) by NO, CO2, Ar, and He at temperatures more representative of combustion environments. Output pulses (~10 ns) from a Nd:YAG-based laser system excited the A-X (0,0) R1(17) transition of NO, which was nanometrically seeded in unreactive gas mixtures and then heated by incident shock waves. Filtered fluorescence in the (0,3) band was detected perpendicular to the laser beam and the gas flow and was recorded using a transient digitizer. Fluorescence decay rates obtained using different partial pressures of NO and NO/CO2 in Ar or He/Ar allow us to extract collisional quenching rate coefficients at temperatures between 900 and 2200 K. These rates, together with additional rates at room temperature, are interpreted using collision models and discussed in terms of application to flame diagnostics using LIF.
Near-field observations of a strip grating illuminated with a plane monochromatic wave
Authors: Franco Paulo R. E. , Naqvi S. Sohail H.
Published: February 2023

Microscopy, in general, results from conventional lens-based optical instruments. These instruments, as a matter of principle, work in the far-field region and are limited in their resolution by the wavelength of the illuminating source. Relying on the near-field description of the surface being imaged, a new generation of lensless microscopes is being developed.2 for solution of the electric-field distribution in the case of an arbitrarily polarized plane wave obliquely incident on a metallic strip, we can obtain the distribution of these electromagnetic fields. The theoretical effect of variation of certain parameters, such as surface resistivity, and how close we are to the object under observation, defined by the observation height hob, allow us to obtain valuable information about the near-field region. Variation in the observed width of the strip is apparent when we vary the surface resistivity as well as when we vary hob.
Interface refinement method for two-material optical multilayer coatings
Authors: Dobrowolski J. A. , Kemp R. A.
Published: February 2023

The numerical design of optical layer systems can be subdivided into two main approaches: optimization of a starting design with a performance that is close to that desired, or the use of a thin film synthesis method in which the starting design is either optional or is quite rudimentary. Both approaches rely heavily on refinement methods. The efficiency of various refinement methods have been compared recently;1 so have the relative speeds of different thin film synthesis methods.2 In this latter study it has been shown that for several of the synthesis methods the initial speed advantage was lost during the refinement stage. However, for the technologically important case of multilayers composed of two materials only it is possible to devise refinement methods that are very fast. The existence of such procedures impacts on the optimum choice of the synthesis method. In the interface refinement method described substantial time savings are affected through the minimization of the evaluations of the elements of the layer matrices.
Reflective quarterwave and halfwave retarders based on light interference in birefringent thin films on metal substrates at normal incidence
Authors: Azzam R. M. A. , Cekic A. Ender
Published: February 2023

Transmissive quarterwave retardation plates have been successfully fabricated recently using birefringent thin films (~3 μm thick) obtained by oblique-incidence deposition of metal oxides on glass slides.1 The desired retardance is acquired in one-way transmission through the film at normal incidence and the effect of (incoherent) multiple-beam interference was apparently negligible. In this paper light interference in a birefringent thin film deposited on a reflecting (typically metallic) substrate for normally incident coherent illumination is considered. The object is to design birefringent mirrors that function as quarterwave and halfwave retarders in normal-incidence reflection. For a given substrate at a given wavelength, the constraint on the birefringence, average refractive index, and thickness of the film so that quarterwave or halfwave retardation is attained at normal incidence is determined. A 2-D Newton-Raphson method is utilized to find all possible solutions. The sensitivity of a given design (i.e., the deviation from exact quarterwave or halfwave retardation) is calculated for given perturbations of the film parameters.
Self-mode-locking in Ti:Al2O3 lasers
Authors: Piche Michel , McCarthy Nathalie , Salin Francois , Squier Jeff
Published: February 2023

The recent observation1,2 of self-mode-locking in Ti:Al2O3 lasers has shown that solid-state lasers can generate femtosecond pulse trains with an average power exceeding the watt level. At such a power level, it can be estimated that the nonlinear phase shift due to the Kerr nonlinearity in the laser rod can be as high as unity; hence, self-focusing from the spatial variation of this phase shift is likely to modify the beam shape to favor the generation of short pulses. In this paper, we describe the operation of such lasers using a numerical model, where propagation, self-focusing, and gain reshaping are treated exactly, and an approximate ray matrix model which assumes Gaussian beams. For appropriate resonator parameters, the average power increases with laser intensity, a situation favoring self-mode-locking; the beam shape is then modified and the saturated gain profile is such that a cw signal falls below threshold. At large intensities, the output power drops and beam filamentation may develop. Experiments have revealed that the output power and beam size are different in the mode-locked and cw regimes.
Effect of junction rounding on the propagation characteristics of Y-junction waveguides
Authors: Subramanian P. , Wai P. K. A. , Menyuk C. R. , Hawkins R. J.
Published: February 2023

The beam propagation method1 is used to stimulate the propagation of light through Y junctions. Such structures have been extensively studied in the past, but the effect of junction rounding on the propagation characteristics has not been considered before. For photolithographically patterned, dry etched Y junctions, junction rounding usually leads to the branching point of the junction being 1–20 μm away from the position where it would have been if it were sharp. This work describes the effect of junction rounding on the propagation characteristics of symmetric and asymmetric Y junctions. The total power in each arm and the power in the fundamental mode is plotted as a function of propagation distance for different junction angles, input field distributions, and extents of junction rounding. We find that junction rounding has a surprisingly small effect on the propagation characteristics, so that it will have almost no detrimental impact on the experiments.
Compact InGaAs/lnAIAs quantum well waveguide polarization splitter
Authors: Jacobovitz G. R. , Zucker J. E. , Jones K. L. , Chang T. Y. , Sauer N. J.
Published: February 2023

Guided-wave polarization splitters are important for many optical signal processing uses including coherent polarization-diversity receivers. For monolithic integration with other optoelectronic devices and compatibility with optical fiber communications at 1.5 μm wavelengths, splitter components that are compact and based on the InP material system are preferred. In this work we demonstrate an InP-based TE/TM splitter with 380-μm active length that can provide better than -10-dB crosstalk.
Scanning microscopy by vibrating knife edge corner: 2-D aspects
Authors: Korpel Adrian , Samson Scott
Published: February 2023

Recently we reported results of 1-D beam profiling with a scanning, vibrating knife edge and its implications for near-field optical microscopy.1 We now report on the implementation of this idea in two dimensions. A knife edge corner is made to vibrate in two orthogonal directions with frequencies of 1000 Hz and 1100 Hz and amplitudes w1 and w2, respectively. A photodiode downstream from the knife edge detects the mixing signal at 100 Hz and records its amplitude, while the vibrating knife edge corner scans the optical field. A gray scale recording of this amplitude yields an image of the field with (electronically variable) resolution w1 × w2. In this paper we show images achieved with this mixing technique and with another more sensitive one that uses 2-D scanning with a 1-D vibrating knife edge, combined with computer postprocessing.
X-ray coherence tomography
Authors: Aye Tin M. , Jannson Tomasz , Hirsh Jay
Published: February 2023

A radically new method of tomographic image recording based on reduced coherence interferometry is presented. It has potential for 3-D x-ray imaging use. Previous attempts to produce 3-D x-ray imaging usually involved, in the case of computer assisted tomography,1 2-D projection images and heavy use of computers for data processing and display, thus reducing spatial resolution. Recent attempts to produce x-ray holograms2 have met shortcomings because of the poor temporal coherence of x-ray lasers. The new method, which is based on holographic recording using radiation with very short temporal coherence length,3,4 can overcome the above major drawbacks, while still preserving the advantages of both tomography and holography. Moreover, the method can provide a continuum of high spatial resolution images of object slices in a single recording.
Iterative discrete on-axis encoding of computer generated holograms optimized for generation of large spot arrays
Authors: Stack Jared D. , Feldman Michael R.
Published: February 2023

The iterative discrete on-axis (IDO) method1 is a computer generated hologram (CGH) encoding algorithm that employs iterative optimization routines such as simulated annealing. The IDO method has been used for the optimization of large 2-D even numbered, spot arrays for optical interconnect applications. Even numbered spot arrays have several performance advantages including low sensitivity to fabrication errors, and reduced computational complexity. Binary and multiple phase level CGHs have been designed and fabricated. Intensity and/or phase of the individual output pixels can be controlled. Methods to decrease the computation time, similar to the recursive mean squares error function described in Ref. 2, have been developed for this use. Diffraction efficiencies of up to 84% have been achieved for generating spot arrays with 100–1024 points.
Complex-valued error diffusion for off-axis computer generated holograms
Authors: Eschbach Reiner , Fan Zhigang
Published: February 2023

Error diffusion has proved to be a valuable tool in the calculation of computer generated holograms. Real-valued error diffusion has been used in calculating the transmission functions real-valued holograms with off-axis reconstructions. Complex-valued error diffusion has been restricted to use with complex-valued holograms. We demonstrate the use of a complex-valued error diffusion algorithm on real-valued hologram data to achieve larger flexibility in the shaping of the noise spectrum. We show a simple scheme of weights selection that results in an almost noise-free region centered at an arbitrary location in the frequency domain.
Design of an astigmatism-correcting holographic collimator for gain-guided diode lasers
Authors: Amitai Y. , Aharoni A. , Goodman J. W.
Published: February 2023

The astigmatism and asymmetrical divergence of gain-guided diode laser beams are major disadvantages in many uses. These characteristics are caused by asymmetry in the two traverse beam profiles: in the plane of the active junction confinement is effected by the electric current profile, whereas perpendicular to the junction the beam is trapped within the much narrower waveguide dimension. A holographic correcting and collimating element for such diodes is desirable for its low mass-production costs and small dimensions. Unfortunately, efficient holograms cannot be recorded directly with the infrared wavelengths of diode lasers. Large aberrations and reduced efficiencies are expected if the hologram would be recorded at shorter wavelengths. Previously1 we presented a method for recording high-efficiency low-aberration collimators for spherical light sources with large recording/readout wavelength shifts. Here we generalize that method for astigmatic light sources. We record with nonspherical waves to compensate for wavelength shift aberrations and correct for the source’s astigmatism. Suitable wavefronts are derived from spherical holograms by a judicious choice of their readout and recording geometries.
Flashlamp pumped, passively mode-locked Cr:LiSrAIF6 laser
Authors: Hagan D. J. , Xia T. , Mansour K. , Gilchrist G. B. , Chai B. H. T.
Published: February 2023

Cr3+-doped LiSrAlF6 (LISAF), with a wide tuning range and an upper state lifetime of ≃65 µsec, offers the possibility of a broadly tunable flashlamp pumped ultrafast laser source. Previous results on a flashlamp pumped LISAF laser have shown multimode laser operation from 790 to 1010 nm.1 Diode pumped laser operation with a low threshold power (7.5 mW) using a 670-nm diode laser pump source has also been reported.2 Here, we report preliminary investigations of passive mode-locking of LISAF with 4% Cr3+ doping using a low repetition rate, pulsed helical discharge xenon flashlamp pump source. Mode-locking is achieved using a saturable absorber in a flowing dye cell or a filter glass. Preliminary streak camera measurements show pulses of 230-psec (FWHM) duration in Q-switched pulse trains containing approximately twenty pulses at a wavelength around 850 nm using IR-140 dye in solution in methanol. The wavelength was controlled using a three-plate birefringent filter. It is believed that the relatively long pulsewidth is due to the long (≃1 nsec) recovery time of the IR-140 dye. This long lifetime is common to most saturable absorbers in this wavelength region and hence represents a problem for passive mode-locking of solid-state lasers. Mode-locking was also achieved using the Schott Glass infrared filter RG-830; however, preliminary results show that this yields even longer pulses. Consequently, we are currently studying alternative mode-locking shemes, including self-mode-locking, and the use of negative feedback by introduction of a two-photon absorber into the cavity.3 The results of our investigations of such mode-locking schemes are reported.
Effects of four-wave mixing on the noise characteristics of semiconductor lasers
Authors: Gary George R. , Agrawal Govind P.
Published: February 2023

It is well known that four-wave mixing (FWM) couples the longitudinal modes of a semiconductor laser.1 Such a coupling, although not commonly included in the laser rate equations, can significantly affect the noise characteristics. Both interband and intraband contributions to FWM should be considered. The interband contribution is generally believed to be negligible because of a relatively slow response time (≈1 ns) and a large mode spacing (≈100 GHz). The intraband contribution has its origin in the nonlinear gain whose response is governed by the intraband relaxation time (≈ 0.1 ps). We study the effects of such FWM by numerical simulation of the appropriately modified stochastic rate equations. The equations incorporate not only FWM but also the coupling through self- and cross-saturation.