Showing papers on "Group velocity published in 1992"

Dispersion cancellation in a measurement of the single-photon propagation velocity in glass.

Abstract: We have demonstrated for the first time that single photons in glass travel at the group velocity, and have observed a novel, nonlocal dispersion-cancellation effect. We used a two-photon interferometer in which a conjugate pair of photons produced in parametric fluorescence travel separate paths and are detected in coincidence after being recombined at a beam splitter. A piece of glass was placed in the path of one of the photons, and a variable delay was adjusted to precisely compensate for it. The single-photon propagation time was thus measured to within approximately 4 fsec.

Dispersion cancellation and high-resolution time measurements in a fourth-order optical interferometer

Abstract: Group-velocity dispersion (GVD) places a fundamental limitation on the resolution possible when measuring the propagation time of short optical pulses through dielectric media. We show that due to a nonlocal, quantum-mechanical effect, a surprising cancellation can occur in fourth-order interference, allowing the time interval between photons emitted in spontaneous parametric down-conversion to be measured without significant degradation of the resolution due to the spreading of the individual photon wave packets. This may also prove useful for measuring the higher-order contributions to GVD

Self-focusing of light pulses in the presence of normal group-velocity dispersion.

Abstract: The influence of normal group-velocity dispersion on the self-focusing of light pulses is numerically studied. Temporal splitting of the field envelope is observed when the critical power is exceeded along with diffraction of the spatially sharpened central part of the pulse. Dispersion increases considerably the self-focusing threshold for short pulse durations.

Nonlinear 1D laser pulse solitons in a plasma

Abstract: A class of exact one-dimensional solutions for modulated light pulses coupled to electron plasma waves in a relativistic cold plasma is investigated. The solutions are in the form of isolated envelope solitons and the nonlinear relationship between their group velocity, amplitude, and frequency are discussed. Numerical results are presented for intense pulses propagating close to the velocity of light; such pulses are of great interest from the point of view of particle and photon accelerators.

Harmonic excitation of mantle Rayleigh waves by the 1991 eruption of Mount Pinatubo, Philippines

Abstract: An unusually long (at least two hours) seismic wave train having periods of about 230 sec was recorded at many worldwide seismic stations during the major eruption of Mount Pinatubo in the Philippines on June 15, 1991. This wave train exhibits two sharp spectral peaks at 228 and 270 sec. The group velocity, phase velocity, and the particle motion of this wave train indicate that it is a Rayleigh wave. The most probable excitation mechanism is acoustic coupling of atmospheric oscillations that were set off by continuous thermal energy flux from the volcano. The two spectral peaks correspond to the characteristic periods of acoustic and gravity modes of the Earth's atmosphere. The magnitude of the vertical single force equivalent to the acoustic coupling is 1.6×10^(17) dynes over a frequency band of 0.003 to 0.01 Hz. The results suggest the possibility of using acoustically coupled Rayleigh waves for detection, characterization and quantification of volcanic eruptions. Acoustic coupling of the atmosphere and the solid Earth provides a unique seismic source with long duration.

Stochastic Gyroresonant Electron Acceleration in a Low-Beta Plasma. I. Interaction with Parallel Transverse Cold Plasma Waves

Abstract: The gyroresonance of electrons with parallel transverse cold plasma waves is considered, and the Fokker-Planck equation describing the evolution of the electron distribution function in the presence of a spectrum of turbulence is derived. A new resonance which produces a divergence in the Fokker-Planck coefficients is identified; it results when the electron is in gyroresonance with a wave that has a group velocity equal to the velocity of the electron along the magnetic field. Under the assumption of a power-law spectral density, the Fokker-Planck coefficients are calculated numerically, and their complicated momentum and pitch-angle dependence, as well as the influence of various approximations to the dispersion relation, gyroresonance condition, and spectral density are discussed. It is found that there is no resonance gap at any pitch angle as long as the full gyroresonance condition is used and waves propagating on both directions are present.

Method of obtaining the yawing velocity and/or transverse velocity of a vehicle

Abstract: A method for obtaining the yawing velocity ω and the transverse velocity vy of a vehicle so that these values can be employed, for example, in regulating the vehicle system, with the values being determined with the aid of simple sensors. Measured values are obtained for the transverse acceleration ay and the steering angles δ of the two axles, and the yawing velocity ω and the vehicle's transverse velocity vy are estimated therefrom in a state estimator.

Heterodyne pump - probe technique for time-domain studies of optical nonlinearities in waveguides.

Abstract: We describe a novel pump–probe technique in which pump and probe pulses are collinear and have parallel polarizations but are still distinguishable. This new technique is especially useful for femtosecond time-domain studies of waveguides. We present experimental results for bulk (V-groove) semiconductor optical amplifiers at 1.5 μm and compare them with results for orthogonally polarized pump–probe measurements. The comparison demonstrates differences that are due to anisotropy of the group velocity as well as of the waveguide nonlinearity.

Method and apparatus for processing signal data to form an envelope on line

Abstract: An apparatus for generating a velocity spectrogram is described. The apparatus includes a transducer which is positionable within a vessel. The transducer receives a time-varying Doppler signal which contains information related to fluid velocity values within the vessel. A Fourier transformation device processes the time-varying Doppler signal to generate a sequence of spectra. Each spectrum corresponds to a segment of the time-varying Doppler signal and defines a set of velocities and their corresponding spectral values. A device is used to identify the instantaneous spectral peak velocity within each spectra. The peak velocity corresponds to the highest velocity within a spectra which has a spectral value above a defined threshold value which is related to the background noise level. The sequence of spectral peak velocities is plotted on a visual interface device to form an instantaneous spectral peak velocity waveform.

Kink solitons and optical shocks in dispersive nonlinear media.

Abstract: The generalized nonlinear Schr\"odinger equation governs wave propagation in nonlinear dispersive media by including the effects of group-velocity dispersion, self-phase-modulation, stimulated Raman scattering, and self-steepening. This equation is shown to have solitary-wave solutions that correspond to an optical shock front moving at the group velocity. The properties of such kink-type solitary waves are discussed.

Close to collinear acousto-optical interaction in paratellurite

Abstract: First results on the investigation of close to collinear coustooptical interaction in paratellurite single crystals are presented Anisotropic Bragg diffraction of light on ultrasound in TeO2 with a length of interaction of up to several centimeters is examined A peculiarity of the interaction is optical beam propagation in the crystal collinearly with group velocity of ultrasound It is shown and verified experimentally that the interaction is characterized by an extremely high selectivity of scattering Some applications of the examined regime of diffraction are discussed

Shear-wave velocity measurement of soils using Rayleigh waves

Abstract: A modified version of the spectral analysis of surface waves (SASW) equipment and analysis procedure has been developed to determine in situ shear-wave velocity variation with depth from the ground...

Simulations to demonstrate reduction of the Gordon–Haus effect

Abstract: The superposition of spontaneous emission noise on a train of soliton pulses produces a random change of the center frequency of the soliton spectrum that causes a change of the group velocity of individual solitons, which in long-light-wave systems translates into a random jitter of the position of the pulses at the receiver. This phenomenon is known as the Gordon-Haus effect. If uncontrolled, the Gordon-Haus effect sets a definite limit on the permissible data rate or on the length of soliton-based light-wave systems. Recently Kodama and Hasegawa [Opt. Lett. 17, 31 (1992)] have shown that the Gordon-Haus effect can be suppressed by placing filters along the fiber that reduce the frequency jitter and the concomitant group-velocity changes. We demonstrate the reduction of the Gordon-Haus effect by computer simulations.

Analytical formulas for the resonant frequency changes due to opening apertures on cavity walls

Abstract: Based on the perturbation method, the resonant frequency changes fue to apertures on a cavity wall have been investigated, and analytical formulas have been derived and compared with numerical and experimental results. The dispersion relation of a periodic disk-loaded slow wave structure, which relates the group velocity explicitly to the shapes and sizes of coupling apertures, is established and verified. These explicitly expressed analytical dispersion relations established in this paper will not only serve as useful tools in the design of forward (electrically coupled) and backward wave (magnetically coupled) linear accelerators, but also show a clear picture of relevant physical processes.

High-speed coplanar transmission lines

Abstract: The authors present measurements of picosecond pulse propagation on coplanar strip transmission lines for which speed (i.e., group velocity), as well as phase and amplitude information are measured. Electrode effects are studied using transmission line loops 1 mm in diameter with cumulative propagation distances as long as 6 cm. The intrinsically low dielectric constant of coplanar-air transmission lines is shown to result in high signal speed and low attenuation. The authors use the data to verify the applicability of a model based largely on empirical formulae for radiation loss (geometry) and conductor absorption. The model is then used to identify optimal design criteria (for a given bandwidth) and is extended to the case of small dimensions. >

Femtosecond time domain measurements of group velocity dispersion in diode lasers at 1.5 mu m

Abstract: The authors have measured the group velocity dispersion of bulk V-groove semiconductor lasers and multiple quantum well lasers operating at a wavelength near 1.5 mu m. The data yield group velocity dispersions in the range from -0.63 to -0.95 mu m/sup -1/ and indicate that material dispersion is the dominant factor in the diodes. Cross-correlation traces of transmitted femtosecond pulses confirm the measured values of dispersion. >

Effects of group velocity dispersion on self/cross phase modulation in a nonlinear Sagnac interferometer switch

Abstract: Time-resolved numerical analysis of a nonlinear Sagnac interferometer switch (NSIS) reveals that the combined effects of group velocity dispersion (GVD), self phase modulation, cross phase modulation, and pump-probe walk-off seriously degrade switching performance when the soliton number N of the pump pulse is under 5. This means that the peak power of short pump pulses cannot be reduced to less than the critical value at N>5 to prevent the effect of GVD. This restriction is more severe for pump pulses in the anomalous dispersion region than for those in the normal dispersion region because of higher-order soliton compression. System designs for time-division demultiplexers that use NSISs and picosecond pump pulses generated by a laser-diode coupled to erbium-doped fiber amplifiers are discussed. It is found that 1:32 demultiplexing from 160 to 5 Gb/s and 1:8 demultiplexing from 80 to 10 Gb/s with a switching contrast of more than 60 are possible using diode-laser-pumped 1- and 2-ps pump pulses, respectively. >

Deep structure of the Iberian Peninsula determined by Rayleigh wave velocity inversion

Abstract: SUMMARY A rigorous study of velocity dispersion of surface waves generated by teleseismic events propagating across the Iberian Peninsula and traversing main geological units, has been carried out from a set of selected analogue data, as digital records have only become available recently. Dispersed seismic signals have been obtained over a period of 16 years, between 1967 and 1982, at the five Iberian stations having long-period instruments. In our study, we have considered many earthquakes thus obtaining a fairly good path coverage of most of the peninsula for two-station Rayleigh wave velocity measurements. In all cases, the approach azimuths of the wavefronts were carefully checked. Several digital filtering techniques have been employed to remove the effects of multipathing and modal contamination, and to isolate the fundamental mode from Rayleigh wavetrains. Thus, we have obtained good estimates for both phase and group velocities. A time-variable filter has reduced the influence of noise and removed higher mode interference. Multiple filtering is then used to compute group velocity. Frequency-domain Wiener deconvolution is used to compute the interstation phase velocity. The determined average Rayleigh wave velocities reveal differences in the propagation conditions of the seismic energy across the peninsula. A mapping of velocities for various periods of reference, together with a mapping of errors in velocity, are the basis for obtaining the Rayleigh wave velocity distribution in the peninsula. Theoretical 2-D layered earth models are obtained by joint inversion of phase and group velocity dispersion curves using the stochastic inverse operator. In our inversion scheme, we use velocities corrected for anelastic effects. Finally, a 3-D mapping of S velocity is performed. This study shows important regional features of the deep structure of Iberia; we see small lateral inhomogeneities and also two low-velocity layers: one with shear velocities usually ranging from 4.23 to 4.31 km s-1 directly under the Moho, and another, the asthenosphere, with a negative velocity gradient for depths between 81 and 181 km, terminated at the bottom by a sharp discontinuity.

Acousto-optic tunable filter

Abstract: An electronically tunable optical filter utilizing noncollinear acousto-optic interaction in an acoustically anisotropic, optically birefringent crystal. The directions of optical and acoustic waves are chosen so that the optical ray is collinear with the group velocity of the acoustic wave. The collinear beam configuration provides increased spectral resolution and reduced drive power.

Theory of relativistic Backward Wave Oscillators operating near cutoff

Abstract: Backward-Wave Oscillators utilize a high-current electron beam to produce high-power, coherent radiation in the centimeter and millimeter wavelength regime Under certain voltage and beam current operating conditions, a Backward-Wave Oscillator (BWO) can operate near the upper edge of the transmission band where the group velocity of the electromagnetic wave goes to zero In this regime, the cold structure dispersion relation can be approximated as a quadratic function of the wavenumber A theoretical model similar to those presented in [1–3] has been developed to describe the operation of the device in this regime We solve a self-consistent set of equations to describe the slow evolution of the envelope of the radiation field and the relativistic motion of the particles along a strong magnetic field Included in the theoretical model are the effects of DC and AC space charge, and velocity spread in the beam Numerical calculations of the starting current are performed and compared with an analytic expression for the starting current derived by assuming a fixed field profile

Chromatic dispersion effects in coherent absolute ranging.

Abstract: Multiple-wavelength interferometers and chirped-frequency coherent laser radars use the variation of interferometric phase as a function of wavelength to measure optical path lengths. Since these instruments generally include a number of dispersive components, the group-velocity index must be included in the path-length calculation. A simple experiment is performed to show that neglecting first-order chromatic dispersion can lead to measurement errors of the order of 1% of the distance.

Sound velocity measurement based on guided acoustic-wave Brillouin scattering

Abstract: The guided acoustic-wave Brillouin scattering (GAWBS) technique is applied to acoustic velocity estimation for single-mode fibers. The GAWBS measurement can be carried out with a simple experimental setup. This technique is used to measure the fluorine (F) concentration dependence of the shear wave velocity. As a result, it is clarified that the sound velocity of the shear wave decreases as F concentration increases. Moreover, the shear wave velocities obtained with this method are in good agreement with the values obtained for bulk glass by elastic property measurements. >

Eckhaus instability for traveling waves

Abstract: The mechanism by which a one-dimensional pattern of traveling waves changes wavelength (i.e., the Eckhaus instability) is studied in a binary fluid mixture. Long-lived transient phase modulations connect states of uniform wave number. The dynamics of wave-number increases and wave-number decreases are found to be qualitatively different due to a strong dependence of the group velocity on wave number. Relevant theoretical models are discussed.

Fundamental aspects of pulse phase-locked loop technology-based methods for measurement of ultrasonic velocity.

Abstract: A new instrument based on a constant frequency pulse phase‐locked loop concept has been developed to accurately measure the ultrasonic phase velocity in condensed matter. Measurements of the sound velocity in ultrapure water are reported in which both damped and undamped transducers are used with the instrument together with reflectors of various thicknesses placed in the sound propagation path. An analysis of measurements made with the new instrument and similar measurements, taken under identical experimental conditions, using a popular variable frequency pulsed‐phase‐locked loop instrument is reported. Uncertainties in both measurement systems are analyzed and discussed. A method for measuring inherent phase shifts, not addressed by previous investigators, within the variable frequency pulsed phase‐locked loop system and a derivation of the equations that govern the overall use of variable frequency systems using phase‐sensitive comparisons are presented. The effects of a finite pulse length on the measurements of phase velocity in dispersive media are addressed in detail.

The characteristics of spiral waves in an axially rotating pipe

Abstract: An experimental study of the instability of a flow in an axially rotating pipe is performed by means of LDV and flow visualization technique. It is found that the axial velocity of the rotating pipe flow fluctuates like a sine wave at first, then its fluctuating pattern assumes a somewhat sawtooth wave form as a spiral wave appears, which is predicted by means of linear and nonlinear stability analysis. At a certain rotation rate, the amplitude of the velocity fluctuations amounts to 30% of the axial velocity. At the down-stream section, another fluctuating component appears in the velocity, which interferes with the initially appearing component, then the fluctuation becomes one with broad-band spectral components. There is a close analogy between this spectral evolution and that of a Taylor-Couette flow. Deformation of the velocity distribution is obtained from the velocity fluctuating pattern and its phase, and the structure of the spiral wave is considered. The strength, azimuthal wavenumber and angular velocity of the spiral wave obtained from the velocity data are confirmed by flow visualization. The change of pressure loss in the rotating pipe is compared with the case without rotation.

LDA-Measurements of Mean Velocity, RMS-Values and Higher Order Moments of Turbulence Intensity Fluctuations in Flow Fields with Strong Velocity Gradients,

Abstract: : The influence of the LDA-measuring volume size on the mean velocity, rms-values of velocity fluctuations and higher order statistical moments for the velocity data was investigated experimentally. The measurements are compared with calculation, simulating the data statistics for different volume sizes. A simple relationship to correct for the effect of the LDA-measuring volume size is proposed for measurements of rms-values of turbulent velocity fluctuations. (Author)

Dispersion of hypersonic velocity and damping at diffuse phase transition in PbMg1/3Nb2/3O3

Abstract: Dispersion of the acoustic anomalies for hypersonic phonons studied by Brillouin scattering has been found in region of a typical diffuse ferroelectric phase transition in PbMg1/3Nb2/303 Both the velocity dip and the damping maximum centred on the mean Curie temperature are connected with fluctuation contributions. The value of anomalies is decreased by dispersion at high frequencies. The relaxation contribution dispersion leads to the appearing of an ordinary shifted maximum of damping and the stretching of velocity step when measuring at Brillouin frequencies.

Extended states of nonlinear traveling-wave convection. I: The Eckhaus instability

Abstract: I describe experiments on one-dimensional states of nonlinear traveling-wave convection in a narrow annular cell. Spatially uniform states are found to be stable within a band of wave numbers whose width grows approximately as the square root of the distance above a saddle-node Rayleigh number. Inside the band, I have measured the static properties of the traveling waves, including their response to spatial inhomogeneities in the Rayleigh number. Outside the stability band, traveling-wave states become unstable to temporally growing modulations of the spatial wave-number profile that propagate through the system at the group velocity of the underlying traveling waves. If allowed to grow, this Eckhaus instability leads, via a subcritical bifurcation, to spatiotemporal defects in which pairs of rolls are created or annihilated. In contrast with the action of the Eckhaus instability in stationary pattern-forming systems, the recurrent appearance and propagation of defects leads to long, erratic transients in which the system may or may not be driven back into the stable wave-number band.