Showing papers on "Wavelength published in 1992"

The interaction of Lamb waves with defects

Abstract: The interaction of individual Lamb waves with a variety of defects simulated by notches is investigated using finite-element analysis, and the results are checked experimentally. Excellent agreement is obtained. It is shown that a 2-D Fourier transform method may be used to quantify Lamb wave interactions with defects. The sensitivity of individual Lamb waves to particular notches is dependent on the frequency-thickness product, the mode type and order, and the geometry of the notch. The sensitivity of the Lamb modes a/sub 1/, alpha /sub 0/, and s/sub 0/ to simulated defects in different frequency-thickness regions is predicted as a function of the defect depth to plate thickness ratio and the results indicate that Lamb waves may be used to find notches when the wavelength to notch depth ratio is on the order of 40. Transmission ratios of Lamb waves across defects are highly frequency dependent. >

High-resolution fibre-grating based strain sensor with interferometric wavelength-shift detection

Abstract: A technique for the detection of dynamic strain induced wavelength shifts in fibre Bragg grating sensors is reported that is based on an unbalanced-interferometer wavelength discriminator, which is capable of subnanostrain resolution (0.6 ne/√(Hz) at 500 Hz) sensing.

Four-wave mixing in an optical fiber in the zero-dispersion wavelength region

Abstract: Fiber four-wave mixing (FWM) in the zero-dispersion wavelength region is described. The phase-matching characteristics are studied in the wavelength region where the first-order chromatic dispersion is zero. The results show that the phase-matching condition is satisfied and FWM light is efficiently generated at particular combinations of input light wavelengths. It is also shown that the deviation of the zero-dispersion wavelength along the fiber length plays an important role in FWM behavior. >

A passive wavelength demodulation system for guided-wave Bragg grating sensors

Abstract: A novel, passive, and self-referencing wavelength detection system (WDS) that measures the wavelength of the narrowband back-reflected spectrum of guided-wave Bragg gratings is described. This letter also reports on the use of such a detection system with fiber-optic Bragg gratings used as absolute strain sensors. The wavelength detection system demonstrated a 1% strain resolution of the total strain measurement range. >

A comparative study of scattering, intrinsic, and coda Q−1 for Hawaii, Long Valley, and central California between 1.5 and 15.0 Hz

Abstract: A new method recently developed by Hoshiba et al. (1991) was used to separate the effects of scattering Q−1 and intrinsic Q−1 from an analysis of the S wave and its coda in Hawaii, Long Valley, and central California. Unlike the method of Wu [1985], which involves integration of the entire S wave energy, the new method relies on the integration of the S wave energy for three successive time windows as a function of hypocentral distance. Using the fundamental separability of source, site, and path effects for coda waves, we normalized the energy in each window for many events recorded at many stations to a common site and source. We plotted the geometric spreading-corrected normalized energy as a function of hypocentral distance. The data for all three time windows were then simultaneously fit to Monte Carlo simulations assuming isotropic body wave scattering in a medium of randomly and uniformly distributed scatterers and uniform intrinsic Q−1. In general, for frequencies less than or equal to 6.0 Hz, scattering Q−1 was greater than intrinsic Q−1, whereas above 6.0 Hz the opposite was true. Model fitting was quite good for frequencies greater than or equal to 6.0 Hz at all distances, despite the model's simplicity. The small range in energy values for any particular time window demonstrates that the site effect can be effectively stripped away using the coda method. Though the model fitting generally worked for 1.5 and 3.0 Hz, the model has difficulty in fitting the whole distance range simultaneously, especially at short distances. Despite the poor fit at low frequency, the results generally support that in all three regions the scattering Q−1 is strongly frequency dependent, decreasing proportional to frequency or faster, whereas intrinsic Q−1 is considerably less frequency dependent. This suggests that the scale length of heterogeneity responsible for scattering is at least comparable to the wavelength for the lowest frequencies studied, of the order of a few kilometers. The lithosphere studied in all three regions can be characterized as a random medium with velocity fluctuation characterized by exponential or Gaussian autocorrelation functions which predict scattering. Q−1 decreasing proportional to frequency or faster. For all frequencies the observed coda Q−1 is intermediate between the total Q−1 and expected coda Q−1 in contrast with theoretical results for an idealized case of uniform distribution of scatterers and homogeneous absorption which predict that coda Q−1 should be close to the intrinsic Q−1. We will discuss possible causes for this discrepancy.

Laser cooling of trapped ions in a standing wave.

Abstract: Laser cooling of trapped ions in a standing- and running-wave configuration is discussed theoretically. The ions are assumed to be spatially localized on the scale provided by the wavelength of the laser (Lamb-Dicke limit). A master equation for the center-of-mass distribution of the ion is derived for a multilevel system, and explicit results are presented for two- and three-level systems and harmonic trapping potentials. For the two-level system located at the node of the standing wave, we find final temperatures that are a factor of 2 lower than the limit for a running wave and cooling rates that do not saturate with the laser intensity. At the point of maximum gradient of the standing wave, blue detuned cooling is found that is analogous to the Sisyphus cooling of free atoms. For a three-level system we compare our results with those of Wineland, Dalibard, and Cohen-Tannoudji [J. Opt. Soc. Am. B 9, 32 (1992)].

Capillary rollers and bores

Abstract: A very intriguing phenomenon, which throws much light on the small-scale structure of the sea surface, is the occurrence of “parasitic capillaries” on the forward face of moderately short gravity waves, especially those with wavelengths 5 to 50 cm; see Figure la. These capillary waves were first studied experimentally by Cox (1958). Evidently their existence depends on the fact that a gravity wave and a much shorter capillary wave may have the same phase speed. The dynamical theory of the generation of parasitic capillaries has been developed by Longuet-Higgins (1963), Crapper (1970) and Ruvinsky et al. (1981, 1985, 1991). This so far takes into account only the first-order effects of viscous damping.

Instabilities of shallow dynamic thermocapillary liquid layers

Abstract: Results of experiments with thermocapillary flow in shallow liquid layers heated from the side are presented. The fluid has Prandtl number 17 and the main configuration investigated is an annular gap to avoid side‐wall effects. The liquid depth d was d≤3.00 mm to have negligible buoyancy effects. Various instabilities have been observed. At a Marangoni number M≂6⋅102, a transition to steady multicellular flow occurred. The convection cells are longitudinal rolls embedded in the main flow all rotating in the same direction. At M≂3⋅103, a transition of the steady multicellular flow to time‐dependent flow states (t) was observed. Two different t‐flow states have been identified by thermocouple measurements and by visualization of the dynamic‐free surface deformations of oscillatory flow. Both t states can be described by disturbances in the form of traveling waves. A short‐wavelength t state with small surface deformations and with waves traveling in azimuthal direction is the preferred mode for d≤1.4 mm. A long‐wavelength t state with larger surface deformations and with waves traveling in radial and in azimuthal directions is preferred for d≥1.4 mm. The stability diagram, wavelength, frequency, and phase speed of both t states are presented and the findings in comparison to an already existing theory by Smith and Davis [J. Fluid Mech. 132, 119, 145 (1983)] are discussed.

Beat-Wave Excitation of Plasma Wave and Observation of Accelerated Electrons

Abstract: The simultaneous observations of a beat-wave-excited plasma wave and of energetic electrons are reported. A plasma wave is excited when the beat-wave frequency of a laser with two wavelengths 10.6 and 9.57 μm equals the plasma frequency. The Stokes sideband measurement gives a wave amplitude δn/n 0 of ∼5%. Plasma electrons with the energies of more than 10 MeV are observed at the resonant density

Apparatus and method for spectroscopic analysis of scattering media

Abstract: Apparatus and method for spectroscopic analysis of scattering media. Subtle differences in materials have been found to be detectable from plots of intensity as a function of wavelength of collected emitted and scattered light versus wavelength of excitation light.

Infrared glucose sensor

Abstract: Systems and methods for non-invasive blood analysis for the presence of glucose are disclosed in which blood is illuminated at a plurality of discrete wavelengths selected from the near infrared spectrum. Measurements of the intensity of transmitted or reflected light at such wavelengths are taken, and an analysis of transmittance or reflectance ratios for various wavelengths is performed. Changes in the ratios can be correlated with specific near infrared (IR) absorption peak for glucose at about 1600 nm +/-15 nm which varies with concentration of the analyte (the data wavelength) and the a second (reference) wavelength being sufficiently removed from the first so that measurements of light absorption at this second wavelength are relatively insensitive to the concentration of the analyte and yet the second wavelength is sufficiently close to the first wavelength to minimize interference from scattering effects and the like. Typically, the window bracketing these closely spaced wavelengths will be less than about 300 nm and preferably less than about 60 nm wide and, in some instances, more preferably less than about 30 nm wide.

High-power strained-layer InGaAs/AlGaAs tapered traveling wave amplifier

Abstract: High power, nearly diffraction‐limited cw performance has been obtained from a traveling wave amplifier, fabricated in a strained‐layer InGaAs/AlGaAs laser structure, with a laterally tapered gain region and with a cavity‐spoiling feature to prevent laser oscillation. The input beam diffracts as it propagates, efficiently filling the tapered active region. For input optical power of 85 mW from a Ti:sapphire laser, total cw output of 1.44 W has been achieved with 1.28 W in a central lobe with width less than 1.2 times the diffraction limit at 977 nm wavelength. Only 15 mW of power incident on the amplifier was sufficient to provide 1 W output into the central lobe.

Optical modulating element and electronic apparatus using it

Abstract: A photomodulation element which uses a color variable filter to obtain the light of a desired wavelength and a liquid crystal element. Light entering the photomodulation element from one side passes through the liquid crystal panel and a second polarizer. The distribution of the transmission light intensity about the wavelength is nearly close to the normal distribution centered on a specific wavelength determined on the basis of the retardation of the liquid crystal panel. Based on the half-value width W necessary in this distribution, the retardation of the panel is determined and the number of the liquid crystal elements is determined so as to obtain a bandwidth which corresponds to the color of the light desired. The retardation of the light crystal is changed by a voltage regulating circuit so that the transmission light intensity distribution at the wavelength of the transmission light of the liquid crystal element is maximized at the desired wavelength λo so that the wavelength of the light leaving the photomodulation element may be selected to provide the desired light.

Non-invasive blood analysis by near infrared absorption measurements using two closely spaced wavelengths

Abstract: Systems and methods for non-invasive blood analysis are disclosed in which blood is illuminated at a plurality of discrete wavelengths selected from the near infrared spectrum. Measurements of the intensity of transmitted or reflected light at such wavelengths are taken, and an analysis of transmittance or reflectance ratios for various wavelengths is performed. Changes in the ratios can be correlated with specific near infrared (IR) absorption peak for the analyte which varies with concentration of the analyte (the data wavelength) and the a second (reference) wavelength being sufficiently removed from the first so that measurements of light absorption at this second wavelength are relatively insensitive to the concentration of the analyte and yet the second wavelength is sufficiently close to the first wavelength to minimize interference from scattering effects and the like. Typically, the window bracketing these closely spaced wavelengths will be less than about 300 nm and preferably less than about 60 nm wide and, in some instances, more preferably less than about 30 nm wide.

High performance optical wavelength shifter

Abstract: A broadband (>50 nm) wavelength shifter capable of switching multigigabit data between optical frequencies in the 1.5 μm region is presented. The device, based on gain saturation in a semiconductor optical amplifier, is tunable, has gain, and is nearly polarisation insensitive. Data degradation after wavelength shifting is negligible.

Scanning-tunneling optical microscopy: a theoretical macroscopic approach

Abstract: A theoretical model for calculating the images obtained in scanning-tunneling optical microscopy is proposed. We calculate the intensity detected by a small spherical tip above a regular glass lattice illuminated in total internal reflection. The model is based on a macroscopic approach. We show that the resolution is limited neither by the wavelength nor by the decay length of the evanescent wave but that it is determined by the tip–sample distance and by the signal-to-noise ratio. We also discuss the quality of the images. In general, the intensity profile does not reproduce the sample profile. We analyzed two kinds of filtering that can deform the true profile. We also show that for a small sample period a strong signal is obtained only in TM polarization.

Distributed resonant cavity light beam modulator

Abstract: An optical apparatus consisting of a laser for producing a coherent polarized beam of electromagnetic radiation of a preselected wavelength. A substrate of silicon has a first transparent cover layer for receiving the polarized beam substantially normally incident thereto, and a second transparent guide layer for receiving the polarized beam from the first layer and for supporting at least one resonant mode. The first and second layers have a preselected index of refraction and a grating is interposed between them, having a grating period less than half the preselected wavelength. The layers and grating interact to produce a standing wave resonance by Bragg reflection. A control obtains a resonance wavelength in the guide layer equal to the predetermined radiation wavelength and thereby high reflectance to modulate the polarized beam. The grating can be optimized for angular bandwidth by controlling the grating's Fourier components either in a single layer or in two grating layers which can also include a termination.

Diffraction of cylindrical and plane waves by an array of absorbing half-screens

Abstract: Multiple forward diffraction past an array of many absorbing half-screens whose separation is large compared to wavelength is examined. Starting with the physical optics approximation for half-planes that are equally spaced and of equal height, the field incident on successive edges is represented by a multidimensional Fresnel integral, which is then expanded into a series of functions studied by Boersma (1978). When the angle of incidence with respect to the plane containing the edges is small, each edge is in the transition region of the previous edge, which precludes the use of the geometrical theory of diffraction and related asymptotic theories. The solution obtained applies for incidence either from above or below the plane containing the edges, and is especially suited to the case of near-grazing incidence. This method of solution allows for numerical evaluation of a large number of half-screens and shows how the multiple diffracted fields are influenced by the physical parameters. Both incident plane waves and incident cylindrical waves can be treated. >

Development and analysis of electric field sensor using LiNbO/sub 3/ optical modulator

Abstract: The sensitivity of an electromagnetic field sensor which uses a LiNbO/sub 3/ electrooptical crystal and an optical-fiber link is improved by using a Mach-Zehnder interferometer, whose half-wave voltage is about 4 V at 1.3- mu m wavelength, and a YAG laser pumped by a laser diode whose output power is 25 mW. The resulting frequency response is about flat from 100 Hz to 300 MHz, and the minimum detectable electric field strengths are 0.22 mV/m at 50 MHz and 0.079 mV/m at 750 MHz. The variation of the sensitivity with the frequency and element length are analyzed using the moment method, and the calculated results agree with the measured results. The measurement of the cross-polarization of the sensor indicates that this property is similar to that of a dipole antenna. The improved sensor can measure an electromagnetic impulse whose peak value is larger than 10 V/m and whose width is wider than 5 ns. >

Optical properties of thin semicontinuous gold films over a wavelength range of 2.5 to 500 μm

Abstract: The optical reflectance and transmittance of percolating gold films close to the metal-insulator transition were measured over an extended wavelength range: from 25 to 500 \ensuremath{\mu}m It is shown that the inhomogeneous nature of such films controls the optical properties even at such long wavelengths as 500 \ensuremath{\mu}m, where the typical grain size is 10 nm Effective-medium theory is shown to be invalid close to the percolation threshold even at 500 \ensuremath{\mu}m Comparison of the measured data of all the samples with the scaling model of Y Yagil et al [Phys Rev B 43, 11 342 (1991)] yields excellent agreement over the entire wavelength range In particular, the short length scale determined by the anomalous diffusion relation is shown to be the relevant length scale for the optical measurements This scaling model for the optical properties of such films is thus experimentally justified, both qualitatively and quantitatively

Perfluorinated polyimide synthesis

Abstract: A polyfluorinated polyimide that has T g over 260°C and a high optical transparency over the entire optical communication wavelengths was prepared from 1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride and tetrafluoro-m-phenylenediamine. The polymer had 10% weight loss temperature of 501°C

Nonlinear water waves at a submerged obstacle or bottom topography

Abstract: Nonlinear diffraction of low-amplitude gravity waves in deep water due to a slightly submerged obstacle is studied experimentally in a wave channel and theoretically. The obstacle is either a circular cylinder or a rectangular shelf. The incoming waves (with wavelength λ) undergo strong nonlinear deformations at the obstacle when the wave amplitude is finite. An infinite number of superharmonic waves are then introduced to the flow. Their wavelengths far away from the obstacle are λ/4, λ/9, λ/16,…, due to the dispersion relation being quadratic in the wave frequency. The superharmonic wave amplitudes grow with increasing incoming wave amplitude up to saturation values. They are found to be prominent at the obstacle's lee side and vanishingly small at the weather side. The second- and third-harmonic wave amplitudes are, surprisingly, in several examples found to be comparable to the incoming wave amplitude. Up to 25% of the incoming energy flux may be transferred to the shorter waves. The theoretical model accounts for nonlinearity by the Boussinesq equations in the shallow region above the obstacle, with patching to linearized potential theory in the deep water. The theory explains both qualitatively and quantitatively the trends observed in the experiments up to breaking.

Spectral properties of lasing microdroplets

Abstract: The spectral behavior of 15.3-μm-diameter Rhodamine 6G in water solution droplets was studied. Microdroplet lasing is known to occur simultaneously at many discrete wavelengths, each corresponding to one of many possible spherical cavity resonances. We show that lasing takes place on several mode orders at once. Modes of a given order were found to form a bell-shaped spectral cluster of typically 4–6 resonance lines having consecutive principal mode numbers. Clusters of different mode orders appear somewhat displaced spectrally from one another, with lowest-order clusters shifted to the red. This multiplicity of lasing modes is accounted for by spatial hole-burning effects. The relative lasing intensities of the differing mode orders are explained by an output coupling theory that considers the gain enhancement that is due to cavity quantum electrodynamic effects. An upper limit of 108 for the Q of a nondegenerate cavity mode was estimated from the data.

Turbulence and Internal Waves at the Equator. Part I: Statistics from Towed Thermistors and a Microstructure Profiler

Abstract: High correlations between turbulent dissipation rates and high-wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the Equatorial Undercurrent An extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high-wavenumber internal waves In the low Richardson number (Ri) thermocline below the mixed layer but above the core of the Equatorial Undercurrent, and when winds were moderate and steadily westward, it was found that: • the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band (corresponding to 150–250-m zonal wavelength) of internal waves; • both turbulence and internal waves varied diurnally—hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a