308 research outputs found
Self-attraction effect and correction on three absolute gravimeters
The perturbations of the gravitational field due to the mass distribution of
an absolute gravimeter have been studied. The so called Self Attraction Effect
(SAE) is crucial for the measurement accuracy, especially for the International
Comparisons, and for the uncertainty budget evaluation. Three instruments have
been analysed: MPG-2, FG5-238 and IMPG-02. The SAE has been calculated using a
numerical method based on FEM simulation. The observed effect has been treated
as an additional vertical gravity gradient. The correction (SAC) to be applied
to the computed g value has been associated with the specific height level,
where the measurement result is typically reported. The magnitude of the
obtained corrections is of order 1E-8 m/s2.Comment: 14 pages, 8 figures, submitted to Metrologi
A nonlinear detection algorithm for periodic signals in gravitational wave detectors
We present an algorithm for the detection of periodic sources of
gravitational waves with interferometric detectors that is based on a special
symmetry of the problem: the contributions to the phase modulation of the
signal from the earth rotation are exactly equal and opposite at any two
instants of time separated by half a sidereal day; the corresponding is true
for the contributions from the earth orbital motion for half a sidereal year,
assuming a circular orbit. The addition of phases through multiplications of
the shifted time series gives a demodulated signal; specific attention is given
to the reduction of noise mixing resulting from these multiplications. We
discuss the statistics of this algorithm for all-sky searches (which include a
parameterization of the source spin-down), in particular its optimal
sensitivity as a function of required computational power. Two specific
examples of all-sky searches (broad-band and narrow-band) are explored
numerically, and their performances are compared with the stack-slide technique
(P. R. Brady, T. Creighton, Phys. Rev. D, 61, 082001).Comment: 9 pages, 3 figures, to appear in Phys. Rev.
Correction due to finite speed of light in absolute gravimeters
Correction due to finite speed of light is among the most inconsistent ones
in absolute gravimetry. Formulas reported by different authors yield
corrections scattered up to 8 Gal with no obvious reasons. The problem,
though noted before, has never been studied, and nowadays the correction is
rather postulated than rigorously proven. In this paper we make an attempt to
revise the subject. Like other authors, we use physical models based on signal
delays and the Doppler effect, however, in implementing the models we
additionally introduce two scales of time associated with moving and resting
reflectors, derive a set of rules to switch between the scales, and establish
the equivalence of trajectory distortions as obtained from either time delay or
distance progression. The obtained results enabled us to produce accurate
correction formulas for different types of instruments, and to explain the
differences in the results obtained by other authors. We found that the
correction derived from the Doppler effect is accountable only for of
the total correction due to finite speed of light, if no signal delays are
considered. Another major source of inconsistency was found in the tacit use of
simplified trajectory models
Comparison between two mobile absolute gravimeters: optical versus atomic interferometers
We report a comparison between two absolute gravimeters: the LNE-SYRTE cold
atoms gravimeter and FG5#220 of Leibniz Universit\"at of Hannover. They rely on
different principles of operation: atomic and optical interferometry. Both are
movable which enabled them to participated to the last International Comparison
of Absolute Gravimeters (ICAG'09) at BIPM. Immediately after, their bilateral
comparison took place in the LNE watt balance laboratory and showed an
agreement of 4.3 +/- 6.4 {\mu}Gal
Perturbations of the local gravity field due to mass distribution on precise measuring instruments: a numerical method applied to a cold atom gravimeter
We present a numerical method, based on a FEM simulation, for the
determination of the gravitational field generated by massive objects, whatever
geometry and space mass density they have. The method was applied for the
determination of the self gravity effect of an absolute cold atom gravimeter
which aims at a relative uncertainty of 10-9. The deduced bias, calculated with
a perturbative treatment, is finally presented. The perturbation reaches (1.3
\pm 0.1) \times 10-9 of the Earth's gravitational field.Comment: 12 pages, 7 figure
Searching for periodic sources with LIGO
We investigate the computational requirements for all-sky, all-frequency
searches for gravitational waves from spinning neutron stars, using archived
data from interferometric gravitational wave detectors such as LIGO. These
sources are expected to be weak, so the optimal strategy involves coherent
accumulaton of signal-to-noise using Fourier transforms of long stretches of
data (months to years). Earth-motion-induced Doppler shifts, and intrinsic
pulsar spindown, will reduce the narrow-band signal-to-noise by spreading power
across many frequency bins; therefore, it is necessary to correct for these
effects before performing the Fourier transform. The corrections can be
implemented by a parametrized model, in which one does a search over a discrete
set of parameter values. We define a metric on this parameter space, which can
be used to determine the optimal spacing between points in a search; the metric
is used to compute the number of independent parameter-space points Np that
must be searched, as a function of observation time T. The number Np(T) depends
on the maximum gravitational wave frequency and the minimum spindown age
tau=f/(df/dt) that the search can detect. The signal-to-noise ratio required,
in order to have 99% confidence of a detection, also depends on Np(T). We find
that for an all-sky, all-frequency search lasting T=10^7 s, this detection
threshhold is at a level of 4 to 5 times h(3/yr), where h(3/yr) is the
corresponding 99% confidence threshhold if one knows in advance the pulsar
position and spin period.Comment: 18 pages, LaTeX, 12 PostScript figures included using psfig.
Submitted to Phys. Rev.
Quantum noise in laser-interferometer gravitational-wave detectors with a heterodyne readout scheme
We analyze and discuss the quantum noise in signal-recycled laser
interferometer gravitational-wave detectors, such as Advanced LIGO, using a
heterodyne readout scheme and taking into account the optomechanical dynamics.
Contrary to homodyne detection, a heterodyne readout scheme can simultaneously
measure more than one quadrature of the output field, providing an additional
way of optimizing the interferometer sensitivity, but at the price of
additional noise. Our analysis provides the framework needed to evaluate
whether a homodyne or heterodyne readout scheme is more optimal for second
generation interferometers from an astrophysical point of view. As a more
theoretical outcome of our analysis, we show that as a consequence of the
Heisenberg uncertainty principle the heterodyne scheme cannot convert
conventional interferometers into (broadband) quantum non-demolition
interferometers.Comment: 16 pages, 8 figure
Sagnac Interferometer as a Speed-Meter-Type, Quantum-Nondemolition Gravitational-Wave Detector
According to quantum measurement theory, "speed meters" -- devices that
measure the momentum, or speed, of free test masses -- are immune to the
standard quantum limit (SQL). It is shown that a Sagnac-interferometer
gravitational-wave detector is a speed meter and therefore in principle it can
beat the SQL by large amounts over a wide band of frequencies. It is shown,
further, that, when one ignores optical losses, a signal-recycled Sagnac
interferometer with Fabry-Perot arm cavities has precisely the same
performance, for the same circulating light power, as the Michelson speed-meter
interferometer recently invented and studied by P. Purdue and the author. The
influence of optical losses is not studied, but it is plausible that they be
fairly unimportant for the Sagnac, as for other speed meters. With squeezed
vacuum (squeeze factor ) injected into its dark port, the
recycled Sagnac can beat the SQL by a factor over the
frequency band 10 {\rm Hz} \alt f \alt 150 {\rm Hz} using the same
circulating power kW as is used by the (quantum limited)
second-generation Advanced LIGO interferometers -- if other noise sources are
made sufficiently small. It is concluded that the Sagnac optical configuration,
with signal recycling and squeezed-vacuum injection, is an attractive candidate
for third-generation interferometric gravitational-wave detectors (LIGO-III and
EURO).Comment: 12 pages, 6 figure
Noise reduction in gravitational wave interferometers using feedback
We show that the quantum locking scheme recently proposed by Courty {\it et
al.} [Phys. Rev. Lett. {\bf 90}, 083601 (2003)] for the reduction of back
action noise is able to significantly improve the sensitivity of the next
generation of gravitational wave interferometers.Comment: 12 pages, 2 figures, in print in the Special Issue of J. Opt. B on
Fluctuations and Noise in Photonics and Quantum Optic
Searching for periodic sources with LIGO. II: Hierarchical searches
The detection of quasi-periodic sources of gravitational waves requires the
accumulation of signal-to-noise over long observation times. If not removed,
Earth-motion induced Doppler modulations, and intrinsic variations of the
gravitational-wave frequency make the signals impossible to detect. These
effects can be corrected (removed) using a parameterized model for the
frequency evolution. We compute the number of independent corrections
required for incoherent search strategies which use stacked
power spectra---a demodulated time series is divided into segments of
length , each segment is FFTed, the power is computed, and the
spectra are summed up. We estimate that the sensitivity of an all-sky search
that uses incoherent stacks is a factor of 2--4 better than would be achieved
using coherent Fourier transforms; incoherent methods are computationally
efficient at exploring large parameter spaces. A two-stage hierarchical search
which yields another 20--60% improvement in sensitivity in all-sky searches for
old (>= 1000 yr) slow (= 40 yr) fast (<=
1000 Hz) pulsars. Assuming 10^{12} flops of effective computing power for data
analysis, enhanced LIGO interferometers should be sensitive to: (i) Galactic
core pulsars with gravitational ellipticities of \epsilon\agt5\times 10^{-6}
at 200 Hz, (ii) Gravitational waves emitted by the unstable r-modes of newborn
neutron stars out to distances of ~8 Mpc, and (iii) neutron stars in LMXB's
with x-ray fluxes which exceed . Moreover,
gravitational waves from the neutron star in Sco X-1 should be detectable is
the interferometer is operated in a signal-recycled, narrow-band configuration.Comment: 22 Pages, 13 Figure
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