Imperfect cavity mirrors in the arms of an interferometric gravitational-wave (GW) detector can result in the generation of unwanted higher order modes that are close to or at resonance in the ...cavity. This can lead to many undesirable effects such as increased roundtrip losses or degraded contrast defect. Polishing mirrors to fulfill the requirements of 2nd generation GW interferometers is extremely challenging. For the use of LG33 modes in 3rd generation detectors it would be necessary to improve polishing by an order of magnitude. In this article we present a novel technique for the in situ correction of cavity mirror surface figures by central heating residual aberration correction. This system targets and corrects specific features of the mirror surface figure such as to reduce scattering from the fundamental cavity mode to specific higher order modes by more than an order of magnitude. We demonstrate, by simulation, the feasibility of such a system and give examples of its application to 2nd and 3rd generation GW interferometers.
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High-reflectivity fused silica mirrors are at the epicentre of today's 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 1.5-2.0 ppm and estimates that 0.3 to 1.3 ppm of the circulating light is scattered onto the ring heater.
The production of a primordial stochastic gravitational-wave (GW) background by processes occuring in the early Universe is expected in a broad range of models. Observing this background would open a ...unique window onto the Universeʼs evolutionary history. Probes like the cosmic microwave background (CMB) or the baryon acoustic oscillations (BAO) can be used to set upper limits on the stochastic GW background energy density for frequencies above 10−15 Hz. We perform a profile likelihood analysis of the Planck CMB temperature anisotropies and gravitational lensing data combined with WMAP low- polarization, BAO, South Pole Telescope and Atacama Cosmology Telescope data. We find that at a 95% confidence level for adiabatic initial conditions, which improves over the previous limit by a factor 2.3. Assuming that the primordial GW has been produced by a network of cosmic strings, we have derived exclusion limits in the cosmic string parameter space. If the size of the loops is determined by gravitational back-reaction, string tension values greater than ∼4 × are excluded for a reconnection probability of 10−3.
An asymmetry in radii of curvature of the mirrors in the arms of an interferometric gravitational-wave detector can degrade the performance of such a detector. In addition, the non-perfect mirror ...surface figures can excite higher order modes if the radii of curvature are close to higher order mode degeneracy. In this paper, we present a novel technique for changing the radii of curvature of arm cavity end mirrors by Central Heating Radius of Curvature Correction. This system was installed in the Virgo experiment in Cascina and proved to be an efficient, non-invasive solution with a large dynamic range. We present how the interferometer was tuned using such a system in order to obtain the best duty-cycles and sensitivity achieved with Virgo to date.
Control noise is a limiting factor in the low-frequency performance of the LIGO gravitational wave detectors. In this paper we model the effects of using new sensors called HoQIs to control the ...suspension resonances. We show if we were to use HoQIs, instead of the standard shadow sensors, we can suppress resonance peaks up to tenfold more while simultaneously reducing the noise injected by the damping system. Through a cascade of effects this will reduce the resonant cross-coupling, allow for improved stability for feed-forward control, and result in improved sensitivity of the detector in the 10-20 Hz band. This analysis shows that local sensors such as HoQIs should be used in current and future detectors to improve low-frequency performance.
High-quality optical resonant cavities require low optical loss, typically on the scale of parts per million. However, unintended micron-scale contaminants on the resonator mirrors that absorb the ...light circulating in the cavity can deform the surface thermoelastically, and thus increase losses by scattering light out of the resonant mode. The point absorber effect is a limiting factor in some high-power cavity experiments, for example, the Advanced LIGO gravitational wave detector. In this Letter, we present a general approach to the point absorber effect from first principles and simulate its contribution to the increased scattering. The achievable circulating power in current and future gravitational-wave detectors is calculated statistically given different point absorber configurations. Our formulation is further confirmed experimentally in comparison with the scattered power in the arm cavity of Advanced LIGO measured by in-situ photodiodes. The understanding presented here provides an important tool in the global effort to design future gravitational wave detectors that support high optical power, and thus reduce quantum noise.
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