88,143 research outputs found
Characterization of causes of signal phase and frequency instability Final report
Characteristic instabilities in phase and frequency errors of reference oscillator
Current Density Imaging through Acoustically Encoded Magnetometry: A Theoretical Exploration
The problem of determining a current density confined to a volume from
measurements of the magnetic field it produces exterior to that volume is known
to have non-unique solutions. To uniquely determine the current density, or the
non-silent components of it, additional spatial encoding of the electric
current is needed. In biological systems such as the brain and heart, which
generate electric current associated with normal function, a reliable means of
generating such additional encoding, on a spatial and temporal scale meaningful
to the study of such systems, would be a boon for research. This paper explores
a speculative method by which the required additional encoding might be
accomplished, on the time scale associated with the propagation of sound across
the volume of interest, by means of the application of a radially encoding
pulsed acoustic spherical wave
Sine-Gordon breathers generation in driven long Josephson junctions
We consider a long Josephson junction excited by a suitable external
ac-signal, in order to generate control and detect breathers. Studying the
nonlinear supratransmission phenomenon in a nonlinear sine-Gordon chain
sinusoidally driven, Geniet and Leon explored the bifurcation of the energy
transmitted into the chain and calculated a threshold for the
external driving signal amplitude, at which the energy flows into the system by
breathers modes. I numerically study the continuous sine-Gordon model,
describing the dynamics of the phase difference in a long Josephson junction,
in order to deeply investigate the "continuous limit" modifications to this
threshold. Wherever the energy flows into the system due to the nonlinear
supratransmission, a peculiar breather localization areas appear in a parameters space. The emergence of these areas depends on the damping
parameter value, the bias current, and the waveform of driving external signal.
The robustness of generated breathers is checked by introducing into the model
a thermal noise source to mimic the environmental fluctuations. Presented
results allows one to consider a cryogenic experiment for creation and
detection of Josephson breathers.Comment: 8 pages, 3 figure
Stabilized high-power laser system for the gravitational wave detector advanced LIGO
An ultra-stable, high-power cw Nd:YAG laser system, developed for the ground-based gravitational wave detector Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory), was comprehensively characterized. Laser power, frequency, beam pointing and beam quality were simultaneously stabilized using different active and passive schemes. The output beam, the performance of the stabilization, and the cross-coupling between different stabilization feedback control loops were characterized and found to fulfill most design requirements. The employed stabilization schemes and the achieved performance are of relevance to many high-precision optical experiments
Photon noise in a random laser amplifier with fluctuating properties
We study fluctuations of the number of photocounts measured by an ideal
photodetector illuminated by light scattered in an amplifying disordered
medium, below the threshold for random lasing. We show that the variance of
fluctuations and their correlation function carry information about fluctuating
properties of the medium. A direct link is established between the fluctuations
of the number of photocounts due to the amplified spontaneous emission (ASE)
and the dimensionless conductance g of the medium. Our results suggest a
possibility of probing amplifying disordered media by analyzing statistics of
their ASE, without illuminating them from outside by a probe beam.Comment: 14 pages, 9 figure
Noise control by sonic crystal barriers made of recycled materials
A systematic study of noise barriers based on sonic crystals made of
cylinders that use recycled materials like absorbing component is here
reported. The barriers consist of only three rows of perforated metal shells
filled with rubber crumb. Measurements of reflectance and transmittance by
these barriers are reported. Their attenuation properties result from a
combination of sound absorption by the rubber crumb and reflection by the
periodic distribution of scatterers. It is concluded that porous cylinders can
be used as building blocks whose physical parameters can be optimized in order
to design efficient barriers adapted to different noisy environments
Quantum Theory of Flicker Noise in Metal Films
Flicker (1/f^gamma) voltage noise spectrum is derived from finite-temperature
quantum electromagnetic fluctuations produced by elementary charge carriers in
external electric field. It is suggested that deviations of the frequency
exponent \gamma from unity, observed in thin metal films, can be attributed to
quantum backreaction of the conducting medium on the fluctuating field of the
charge carrier. This backreaction is described phenomenologically in terms of
the effective momentum space dimensionality, D. Using the dimensional
continuation technique, it is shown that the combined action of the photon heat
bath and external field results in a 1/f^gamma-contribution to the spectral
density of the two-point correlation function of electromagnetic field. The
frequency exponent is found to be equal to 1 + delta, where delta = 3 - D is a
reduction of the momentum space dimensionality. This result is applied to the
case of a biased conducting sample, and a general expression for the voltage
power spectrum is obtained which possesses all characteristic properties of
observed flicker noise spectra. The range of validity of this expression covers
well the whole measured frequency band. Gauge independence of the power
spectrum is proved. It is shown that the obtained results naturally resolve the
problem of divergence of the total noise power. A detailed comparison with the
experimental data on flicker noise measurements in metal films is given.Comment: 20 pages, 2 tables, 2 figure
SPIDER: a balloon-borne CMB polarimeter for large angular scales
We describe SPIDER, a balloon-borne instrument to map the polarization of the
millimeter-wave sky with degree angular resolution. Spider consists of six
monochromatic refracting telescopes, each illuminating a focal plane of
large-format antenna-coupled bolometer arrays. A total of 2,624 superconducting
transition-edge sensors are distributed among three observing bands centered at
90, 150, and 280 GHz. A cold half-wave plate at the aperture of each telescope
modulates the polarization of incoming light to control systematics. Spider's
first flight will be a 20-30-day Antarctic balloon campaign in December 2011.
This flight will map \sim8% of the sky to achieve unprecedented sensitivity to
the polarization signature of the gravitational wave background predicted by
inflationary cosmology. The Spider mission will also serve as a proving ground
for these detector technologies in preparation for a future satellite mission.Comment: 12 pages, 6 figures; as published in the conference proceedings for
SPIE Millimeter, Submillimeter, and Far-Infrared Detectors and
Instrumentation for Astronomy V (2010
- …