Search results for "Group velocity"
showing 10 items of 36 documents
Local Rossby Wave Packet Amplitude, Phase Speed, and Group Velocity: Seasonal Variability and Their Role in Temperature Extremes
2020
AbstractTransient Rossby wave packets (RWPs) are a prominent feature of the synoptic to planetary upper-tropospheric flow at the midlatitudes. Their demonstrated role in various aspects of weather and climate prompts the investigation of characteristic properties like their amplitude, phase speed, and group velocity. Traditional frameworks for the diagnosis of the two latter have so far remained nonlocal in space or time, thus preventing a detailed view on the spatiotemporal evolution of RWPs. The present work proposes a method for the diagnosis of horizontal Rossby wave phase speed and group velocity locally in space and time. The approach is based on the analytic signal of upper-troposphe…
Modulational instability and generation of self-induced transparency solitons in resonant optical fibers
2009
International audience; We consider continuous-wave propagation through a fiber doped with two-level resonant atoms, which is described by a system of nonlinear Schrodinger-Maxwell-Bloch (NLS-MB) equations. We identify the modulational instability (MI) conditions required for the generation of ultrashort pulses, in cases of both anomalous and normal GVD (group-velocity dispersion). It is shown that the self-induced transparency (SIT) induces non-conventional MI sidebands. The main result is a prediction of the existence of both bright and dark SIT solitons in the anomalous and normal GVD regimes.
Noncontact Air-Coupled Guided Wave Ultrasonics for Detection of Thinning Defects in Aluminum Plates
2001
Ultrasonic guided waves are gaining increasing attention for the inspection of platelike and rodlike structures. At the same time, inspection methods that do not require contact with the test piece are being developed for advanced applications. This paper capitalizes on recent advances in the areas of guided wave ultrasonics and noncontact ultrasonics to demonstrate a superior method for the nondestructive detection of thinning defects simulating hidden corrosion in thin aluminum plates. The proposed approach uses micromachined gas (air)-coupled capacitive transducers for the noncontact generation and detection of guided plate waves. Interesting features in the dispersive behavior of select…
Measurement of the group velocity of light in sea water at the ANTARES site
2012
The group velocity of light has been measured at eight different wavelengths between 385 nm and 532 nm in the Mediterranean Sea at a depth of about 2.2 km with the ANTARES optical beacon systems. A parametrisation of the dependence of the refractive index on wavelength based on the salinity, pressure and temperature of the sea water at the ANTARES site is in good agreement with these measurements.
Measuring guided waves in long bones: Modeling and experiments in free and immersed plates
2005
Guided waves, consistent with the A0 Lamb mode, have previously been observed in bone phantoms and human long bones. Reported velocity measurements relied on line fitting of the observed wave fronts. Such an approach has limited ability to assess dispersion and is affected by interference by other wave modes. For a more robust identification of modes and determination of phase velocities, signal processing techniques using the fast Fourier transform (FFT) were investigated. The limitations of FFT because of spatial resolution were addressed to improve the precision of the measured modes. An inversion scheme was developed for determining the plate thickness from the measured velocity. Experi…
Nonlinear frequency conversion in a birefringent microstructured fiber tuned by externally applied hydrostatic pressure.
2013
We studied vector frequency conversion in externally tuned microstructured fibers for applications as a novel, nonlinear fiber-optic sensor. We investigated both experimentally and numerically a possibility of shifting vector and scalar modulation instability gain bands by pressure-induced changes in the linear properties of a microstructured fiber. Our results show that polarization-dependent vector nonlinear processes sensitive to variation of fiber group velocity difference (group birefringence) exhibit a clear advantage for pressure-sensing applications compared with scalar nonlinear processes only sensitive to group velocity dispersion changes. Analytical predictions and numerical simu…
Temperature independence of birefringence and group velocity dispersion in photonic crystal fibres
2004
Experimental results are presented for the dependence of the dispersion and the birefringence of a highly birefringent photonic crystal fibre with temperature. It is shown that, unlike conventional optical fibres, where temperature induces stress regions between the different materials present in their structure, photonic crystal fibres exhibit no dependence with temperature of these optical properties owing to the single material nature of their structures.
Low-Temperature Coherent Thermal Conduction in Thin Phononic Crystal Membranes
2016
In recent years, the idea of controlling phonon thermal transport coherently using phononic crystals has been introduced. Here, we extend our previous numerical studies of ballistic low-temperature heat transport in two-dimensional hole-array phononic crystals, and concentrate on the effect of the lattice periodicity. We find that thermal conductance can be either enhanced or reduced by large factors, depending on the the lattice period. Analysis shows that both the density of states and the average group velocity are strongly affected by the periodic structuring. The largest effect for the reduction seen for larger period structures comes from the strong reduction of the group velocities, …
Near-field characterization of Bragg mirrors engraved in surface plasmon waveguides
2004
International audience; Surface plasmon waveguides (SPW's) are metal ridges featuring widths in the micrometer range and thicknesses of a few tens of nanometers. A focused ion beam has been used to carve microscatterers into gold SPW's and the near-field distributions around these microstructures are observed by means of photon scanning tunneling microscopy (PSTM). On the basis of near-field images, we show that a finite length periodic arrangement of narrow slits can reflect a surface plasmon mode propagating along a SPW. The reflection efficiency of the micrograting is found to depend upon the number of slits, the period of the grating, and the incident wavelength. The optimum reflection …
Frequency tunable polarization and intermodal modulation instability in high birefringence holey fiber
2009
International audience; We present an experimental analysis of polarization and intermodal noise-seeded parametric amplification, in which dispersion is phase matched by group velocity mismatch between either polarization or spatial modes in birefringent holey fiber with elliptical core composed of a triple defect. By injecting quasi-CW intense linearly polarized pump pulses either parallel or at 45 degrees with respect to the fiber polarization axes, we observed the simultaneous generation of polarization or intermodal modulation instability sidebands. Furthermore, by shifting the pump wavelength from 532 to 625 nm, we observed a shift of polarization sidebands from 3 to 8 THz, whereas int…