783 research outputs found
A 6D interferometric inertial isolation system
We present a novel inertial-isolation scheme based on six degree-of-freedom
(6D) interferometric sensing of a single reference mass. It is capable of
reducing inertial motion by more than two orders of magnitude at 100\,mHz
compared with what is achievable with state-of-the-art seismometers. This will
enable substantial improvements in the low-frequency sensitivity of
gravitational-wave detectors. The scheme is inherently two-stage, the reference
mass is softly suspended within the platform to be isolated, which is itself
suspended from the ground. The platform is held constant relative to the
reference mass and this closed-loop control effectively transfers the low
acceleration-noise of the reference mass to the platform. A high loop gain also
reduces non-linear couplings and dynamic range requirements in the
soft-suspension mechanics and the interferometric sensing
High dynamic range spatial mode decomposition
Accurate readout of low-power optical higher-order spatial modes is of
increasing importance to the precision metrology community. Mode sensors are
used to prevent mode mismatches from degrading quantum and thermal noise
mitigation strategies. Direct mode analysis sensors (MODAN) are a promising
technology for real-time monitoring of arbitrary higher-order modes. We
demonstrate MODAN with photo-diode readout to mitigate the typically low
dynamic range of CCDs. We look for asymmetries in the response our sensor to
break degeneracies in the relative alignment of the MODAN and photo-diode and
consequently improve the dynamic range of the mode sensor. We provide a
tolerance analysis and show methodology that can be applied for sensors beyond
first-order spatial modes
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
Feasibility of near-unstable cavities for future gravitational wave detectors
Near-unstable cavities have been proposed as an enabling technology for
future gravitational wave detectors, as their compact structure and large beam
spots can reduce the coating thermal noise of the interferometer. We present a
tabletop experiment investigating the behaviour of an optical cavity as it is
parametrically pushed to geometrical instability. We report on the observed
degeneracies of the cavity's eigenmodes as the cavity becomes unstable and the
resonance conditions become hyper-sensitive to mirror surface imperfections. A
simple model of the cavity and precise measurements of the resonant frequencies
allow us to characterize the stability of the cavity and give an estimate of
the mirror astigmatism. The significance of these results for gravitational
wave detectors is discussed, and avenues for further research are suggested.Comment: 11 pages, 8 figure
Coating-free mirrors for high precision interferometric experiments
Thermal noise in mirror optical coatings may not only limit the sensitivity of future gravitational-wave detectors in their most sensitive frequency band but is also a major impediment for experiments that aim to reach the standard quantum limit or cool mechanical systems to their quantum ground state. We present the design and experimental characterization of a highly reflecting mirror without any optical coating. This coating-free mirror is based on total internal reflection and Brewster-angle coupling. In order to characterize its performance, the coating-free mirror was incorporated into a triangular ring cavity together with a high quality conventional mirror. The finesse of this cavity was measured using an amplitude transfer function to be about F≃4000. This finesse corresponds to a reflectivity of the coating-free mirror of about R≃99.89%. In addition, the dependence of the reflectivity on rotation was mapped out
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
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