9,702 research outputs found
Phase mapping of ultrashort pulses in bimodal photonic structures: A window on local group velocity dispersion
The amplitude and phase evolution of ultrashort pulses in a bimodal waveguide structure has been studied with a time-resolved photon scanning tunneling microscope (PSTM). When waveguide modes overlap in time intriguing phase patterns are observed. Phase singularities, arising from interference between different modes, are normally expected at equidistant intervals determined by the difference in effective index for the two modes. However, in the pulsed experiments the distance between individual singularities is found to change not only within one measurement frame, but even depends strongly on the reference time. To understand this observation it is necessary to take into account that the actual pulses generating the interference signal change shape upon propagation through a dispersive medium. This implies that the spatial distribution of phase singularities contains direct information on local dispersion characteristics. At the same time also the mode profiles, wave vectors, pulse lengths, and group velocities of all excited modes in the waveguide are directly measured. The combination of these parameters with an analytical model for the time-resolved PSTM measurements shows that the unique spatial phase information indeed gives a direct measure for the group velocity dispersion of individual modes. As a result interesting and useful effects, such as pulse compression, pulse spreading, and pulse reshaping become accessible in a local measuremen
Full field inversion in photoacoustic tomography with variable sound speed
Recently, a novel measurement setup has been introduced to photoacoustic
tomography, that collects data in the form of projections of the full 3D
acoustic pressure distribution at a certain time instant. Existing imaging
algorithms for this kind of data assume a constant speed of sound. This
assumption is not always met in practice and thus leads to erroneous
reconstructions. In this paper, we present a two-step reconstruction method for
full field detection photoacoustic tomography that takes variable speed of
sound into account. In the first step, by applying the inverse Radon transform,
the pressure distribution at the measurement time is reconstructed point-wise
from the projection data. In the second step, one solves a final time wave
inversion problem where the initial pressure distribution is recovered from the
known pressure distribution at the measurement time. For the latter problem, we
derive an iterative solution approach, compute the required adjoint operator,
and show its uniqueness and stability
Search for astro-gravity correlations
A new approach in the gravitational wave experiment is considered. In
addition to the old method of searching for coincident reactions of two
separated gravitational antennae it was proposed to seek perturbations of the
gravitational detector noise background correlated with astrophysical events
such as neutrino and gamma ray bursts which can be relaibly registered by
correspondent sensors. A general algorithm for this approach is developed. Its
efficiency is demonstrated in reanalysis of the old data concerning the
phenomenon of neutrino-gravity correlation registered during of SN1987A
explosion.Comment: 29 pages (LaTeX), 4 figures (EPS
Characteristic Evolution and Matching
I review the development of numerical evolution codes for general relativity
based upon the characteristic initial value problem. Progress in characteristic
evolution is traced from the early stage of 1D feasibility studies to 2D
axisymmetric codes that accurately simulate the oscillations and gravitational
collapse of relativistic stars and to current 3D codes that provide pieces of a
binary black hole spacetime. Cauchy codes have now been successful at
simulating all aspects of the binary black hole problem inside an artificially
constructed outer boundary. A prime application of characteristic evolution is
to extend such simulations to null infinity where the waveform from the binary
inspiral and merger can be unambiguously computed. This has now been
accomplished by Cauchy-characteristic extraction, where data for the
characteristic evolution is supplied by Cauchy data on an extraction worldtube
inside the artificial outer boundary. The ultimate application of
characteristic evolution is to eliminate the role of this outer boundary by
constructing a global solution via Cauchy-characteristic matching. Progress in
this direction is discussed.Comment: New version to appear in Living Reviews 2012. arXiv admin note:
updated version of arXiv:gr-qc/050809
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