49 research outputs found
Fundamental Limitations of Cavity-assisted Atom Interferometry
Atom interferometers employing optical cavities to enhance the beam splitter
pulses promise significant advances in science and technology, notably for
future gravitational wave detectors. Long cavities, on the scale of hundreds of
meters, have been proposed in experiments aiming to observe gravitational waves
with frequencies below 1 Hz, where laser interferometers, such as LIGO, have
poor sensitivity. Alternatively, short cavities have also been proposed for
enhancing the sensitivity of more portable atom interferometers. We explore the
fundamental limitations of two-mirror cavities for atomic beam splitting, and
establish upper bounds on the temperature of the atomic ensemble as a function
of cavity length and three design parameters: the cavity g-factor, the
bandwidth, and the optical suppression factor of the first and second order
spatial modes. A lower bound to the cavity bandwidth is found which avoids
elongation of the interaction time and maximizes power enhancement. An upper
limit to cavity length is found for symmetric two-mirror cavities, restricting
the practicality of long baseline detectors. For shorter cavities, an upper
limit on the beam size was derived from the geometrical stability of the
cavity. These findings aim to aid the design of current and future
cavity-assisted atom interferometers.Comment: 11 pages, 12 figure
The Influence of Dual-Recycling on Parametric Instabilities at Advanced LIGO
Laser interferometers with high circulating power and suspended optics, such
as the LIGO gravitational wave detectors, experience an optomechanical coupling
effect known as a parametric instability: the runaway excitation of a
mechanical resonance in a mirror driven by the optical field. This can saturate
the interferometer sensing and control systems and limit the observation time
of the detector. Current mitigation techniques at the LIGO sites are
successfully suppressing all observed parametric instabilities, and focus on
the behaviour of the instabilities in the Fabry-Perot arm cavities of the
interferometer, where the instabilities are first generated. In this paper we
model the full dual-recycled Advanced LIGO design with inherent imperfections.
We find that the addition of the power- and signal-recycling cavities shapes
the interferometer response to mechanical modes, resulting in up to four times
as many peaks. Changes to the accumulated phase or Gouy phase in the
signal-recycling cavity have a significant impact on the parametric gain, and
therefore which modes require suppression.Comment: 9 pages, 11 figures, 2 ancillary file
Thermal modelling of Advanced LIGO test masses
High-reflectivity fused silica mirrors are at the epicentre of current
advanced gravitational wave detectors. In these detectors, the mirrors interact
with high power laser beams. As a result of finite absorption in the high
reflectivity coatings the mirrors suffer from a variety of thermal effects that
impact on the detectors performance. We propose a model of the Advanced LIGO
mirrors that introduces an empirical term to account for the radiative heat
transfer between the mirror and its surroundings. The mechanical mode frequency
is used as a probe for the overall temperature of the mirror. The thermal
transient after power build-up in the optical cavities is used to refine and
test the model. The model provides a coating absorption estimate of 1.5 to 2.0
ppm and estimates that 0.3 to 1.3 ppm of the circulating light is scattered on
to the ring heater.Comment: 14 pages, 9 figure
Optical suppression of tilt-to-length coupling in the LISA long-arm interferometer
The arm length and the isolation in space enable the Laser Interferometer Space Antenna (LISA) to probe for signals unattainable on the ground, opening a window to the subhertz gravitational-wave universe. The coupling of unavoidable angular spacecraft jitter into the longitudinal displacement measurement, an effect known as tilt-to-length (TTL) coupling, is critical for realizing the required sensitivity of
picometer
/
â
Hz
. An ultrastable interferometer test bed has been developed in order to investigate this issue and validate mitigation strategies in a setup representative of LISA and in this paper it is operated in the long-arm interferometer configuration. The test bed is fitted with a flat-top beam generator to simulate the beam received by a LISA spacecraft. We demonstrate a reduction of TTL coupling between this flat-top beam and a Gaussian reference beam via the introduction of two- and four-lens imaging systems. TTL coupling factors below
±
25
Ό
m
/
rad
for beam tilts within
±
300
Ό
rad
are obtained by careful optimization of the system. Moreover, we show that the additional TTL coupling due to lateral-alignment errors of elements of the imaging system can be compensated by introducing lateral shifts of the detector and vice versa. These findings help validate the suitability of this noise-reduction technique for the LISA long-arm interferometer
Comparing ultrastable lasers at 7âĂâ10â17 fractional frequency instability through a 2220âkm optical fibre network
Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. Here, we report a comparison of two geographically separated lasers, over the longest ever reported metrological optical fibre link network, measuring 2220 km in length, at a state-of-the-art fractional-frequency instability of 7âĂâ10â17 for averaging times between 30âs and 200âs. The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre. Based on the characterisation of the noise in the lasers and optical fibre link network over different timescales, we investigate the potential for disseminating ultrastable light to improve the performance of remote optical clocks
Search for Tensor, Vector, and Scalar Polarizations in the Stochastic Gravitational-Wave Background
The detection of gravitational waves with Advanced LIGO and Advanced Virgo has enabled novel tests of general relativity, including direct study of the polarization of gravitational waves. While general relativity allows for only two tensor gravitational-wave polarizations, general metric theories can additionally predict two vector and two scalar polarizations. The polarization of gravitational waves is encoded in the spectral shape of the stochastic gravitational-wave background, formed by the superposition of cosmological and individually unresolved astrophysical sources. Using data recorded by Advanced LIGO during its first observing run, we search for a stochastic background of generically polarized gravitational waves. We find no evidence for a background of any polarization, and place the first direct bounds on the contributions of vector and scalar polarizations to the stochastic background. Under log-uniform priors for the energy in each polarization, we limit the energy densities of tensor, vector, and scalar modes at 95% credibility to Ω0T<5.58Ă10-8, Ω0V<6.35Ă10-8, and Ω0S<1.08Ă10-7 at a reference frequency f0=25 Hz. © 2018 American Physical Society
First narrow-band search for continuous gravitational waves from known pulsars in advanced detector data
GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2
We describe the observation of GW170104, a gravitational-wave signal produced by the coalescence of a pair of stellar-mass black holes. The signal was measured on January 4, 2017 at 10â¶11:58.6 UTC by the twin advanced detectors of the Laser Interferometer Gravitational-Wave Observatory during their second observing run, with a network signal-to-noise ratio of 13 and a false alarm rate less than 1 in 70â000 years. The inferred component black hole masses are
31.
2
+
8.4
â
6.0
M
â
and
19.
4
+
5.3
â
5.9
M
â
(at the 90% credible level). The black hole spins are best constrained through measurement of the effective inspiral spin parameter, a mass-weighted combination of the spin components perpendicular to the orbital plane,
Ï
eff
=
â
0.1
2
+
0.21
â
0.30
. This result implies that spin configurations with both component spins positively aligned with the orbital angular momentum are disfavored. The source luminosity distance is
88
0
+
450
â
390
â
â
Mpc
corresponding to a redshift of
z
=
0.1
8
+
0.08
â
0.07
. We constrain the magnitude of modifications to the gravitational-wave dispersion relation and perform null tests of general relativity. Assuming that gravitons are dispersed in vacuum like massive particles, we bound the graviton mass to
m
g
â€
7.7
Ă
10
â
23
â
â
eV
/
c
2
. In all cases, we find that GW170104 is consistent with general relativity