A. Heptonstall

The GEO 600 gravitational wave detector

The GEO 600 laser interferometer with 600 m armlength is part of a worldwide network of gravitational wave detectors. Due to the use of advanced technologies like multiple pendulum suspensions with a monolithic last stage and signal recycling, the anticipated sensitivity of GEO 600 is close to the initial sensitivity of detectors with several kilometres armlength. This paper describes the subsystems of GEO 600, the status of the detector by September 2001 and the plans towards the first science run.

Design and development of the advanced LIGO monolithic fused silica suspension

The detection of gravitational waves remains one of the most challenging prospects faced by experimental physicists. One of the most significant limits to the sensitivity of current, and future, long-baseline interferometric gravitational wave detectors is thermal displacement noise of the test masses and their suspensions. Suspension thermal noise will be an important noise source at operating frequencies between approximately 10 and 30 Hz, and it results from a combination of thermoelastic damping, surface loss and bulk loss associated with the suspension fibres, and weld loss from their attachment. Its effects can be reduced by minimizing thermoelastic loss and optimizing pendulum dilution factor via the appropriate choice of geometry of the suspension fibre and attachment geometry. This paper will discuss the design and fabrication of a prototype of the fused silica suspension stage for use in the advanced LIGO (aLIGO) detector network, analysing in detail the design of the fused silica attachment pieces (ears), together with the suspension assembly techniques. We also present a full thermal noise analysis of the prototype suspension, taking into account for the first time the precise shape of the actual fibres used, and weld loss. We shall demonstrate the suitability of this suspension for installation into aLIGO.

Update on quadruple suspension design for Advanced LIGO

We describe the design of the suspension systems for the major optics for Advanced LIGO, the upgrade to LIGO—the Laser Interferometric Gravitational-Wave Observatory. The design is based on that used in GEO600—the German/UK interferometric gravitational wave detector, with further development to meet the more stringent noise requirements for Advanced LIGO. The test mass suspensions consist of a four-stage or quadruple pendulum for enhanced seismic isolation. To minimize suspension thermal noise, the final stage consists of a silica mirror, 40 kg in mass, suspended from another silica mass by four silica fibres welded to silica ears attached to the sides of the masses using hydroxide-catalysis bonding. The design is chosen to achieve a displacement noise level for each of the seismic and thermal noise contributions of 10^(−19) m/√Hz at 10 Hz, for each test mass. We discuss features of the design which has been developed as a result of experience with prototypes and associated investigations.

Quadruple suspension design for Advanced LIGO

In this paper, we describe the conceptual design for the suspension system for the test masses for Advanced LIGO, the planned upgrade to LIGO, the US laser interferometric gravitational-wave observatory. The design is based on the triple pendulum design developed for GEO 600—the German/UK interferometric gravitational wave detector. The GEO design incorporates fused silica fibres of circular cross-section attached to the fused silica mirror (test mass) in the lowest pendulum stage, in order to minimize the thermal noise from the pendulum modes. The damping of the low-frequency modes of the triple pendulum is achieved by using co-located sensors and actuators at the highest mass of the triple pendulum. Another feature of the design is that global control forces acting on the mirrors, used to maintain the output of th ei nterferometer on a dark fringe, are applied via a triple reaction pendulum, so that these forces can be implemented via a seismically isolated platform. These techniques have been extended to meet the more stringent noise levels planned for in Advanced LIGO. In particular, the Advanced LIGO baseline design requires a quadruple pendulum with afi nal stage consisting of a 40 kg sapphire mirror, suspended on fused silica ribbons or fibres. The design is chosen to aim to reach a target noise contribution from the suspension corresponding to a displacement sensitivity of 10 −19 mH z −1/2 at 10 Hz at each of the tes tm asses. PACS number: 0480N

The status of GEO 600

The GEO 600 laser interferometer with 600m armlength is part of a worldwide network of gravitational wave detectors. GEO 600 is unique in having advanced multiple pendulum suspensions with a monolithic last stage and in employing a signal recycled optical design. This paper describes the recent commissioning of the interferometer and its operation in signal recycled mode.

Invited article: CO2 laser production of fused silica fibers for use in interferometric gravitational wave detector mirror suspensions.

In 2000 the first mirror suspensions to use a quasi-monolithic final stage were installed at the GEO600 detector site outside Hannover, pioneering the use of fused silica suspension fibers in long baseline interferometric detectors to reduce suspension thermal noise. Since that time, development of the production methods of fused silica fibers has continued. We present here a review of a novel CO(2) laser-based fiber pulling machine developed for the production of fused silica suspensions for the next generation of interferometric gravitational wave detectors and for use in experiments requiring low thermal noise suspensions. We discuss tolerances, strengths, and thermal noise performance requirements for the next generation of gravitational wave detectors. Measurements made on fibers produced using this machine show a 0.8% variation in vertical stiffness and 0.05% tolerance on length, with average strengths exceeding 4 GPa, and mechanical dissipation which meets the requirements for Advanced LIGO thermal noise performance.

Increased Brownian force noise from molecular impacts in a constrained volume.

We report on residual-gas damping of the motion of a macroscopic test mass enclosed in a nearby housing in the molecular flow regime. The damping coefficient, and thus the associated thermal force noise, is found to increase significantly when the distance between the test mass and surrounding walls is smaller than the test mass itself. The effect has been investigated with two torsion pendulums of different geometry and has been modeled in a numerical simulation whose predictions are in good agreement with the measurements. Relevant to a wide variety of small-force experiments, the residual-gas force noise power for the test masses in the LISA gravitational wave observatory is roughly a factor 15 larger than in an infinite gas volume, though still compatible with the target acceleration noise of 3 fm s(-2) Hz(-1/2) at the foreseen pressure below 10(-6) Pa.

Finite element modelling of the mechanical loss of silica suspension fibres for advanced gravitational wave detectors

Detection of gravitational waves remains one of the most challenging problems faced by experimental physicists. One of the most significant limits to the sensitivity of current, and future, long-baseline interferometric gravitational wave detectors is thermal displacement noise of the test masses and their suspensions. Detector suspension thermal noise will be an important noise source at operating frequencies between approximately 10 and 30 Hz, and results from a combination of thermoelastic damping, surface and bulk losses associated with the suspension fibres. However its effects can be reduced by minimizing the thermoelastic loss and optimization of pendulum dilution factor via appropriate choice of suspension fibre and attachment geometry. This paper will discuss finite element modelling and associated analysis of the loss in quasi-monolithic silica fibre suspensions for future advanced gravitational wave detectors.

A new torsion pendulum for testing the limits of free-fall for LISA test masses

On-ground verification of the precision with which a test mass can be in perfect free-fall, without any stray forces, is among the most challenging aspects of preparing for LISA and LISA Pathfinder. This study aims at improving the sensitivity in torsion pendulum measurements of the stray forces arising in the interaction between a test mass and the capacitive position sensor that surrounds it. Measurements are performed with pendulum suspensions based on both tungsten and higher quality factor uncoated fused silica torsion fibers. The results achieved with the fused silica pendulum establish more stringent upper limits on the excess force noise attributable to the sensor—at a level that roughly coincides with the LISA Pathfinder flight goal around 1 mHz. Additionally, these measurements demonstrate a force sensitivity improvement over what can be achieved with thermal noise-limited tungsten over a wide range of frequencies, with significant further improvements still possible.

Investigation of mechanical dissipation in CO2 laser-drawn fused silica fibres and welds

The planned upgrades to the LIGO gravitational wave detectors include monolithic mirror suspensions to reduce thermal noise. The mirrors will be suspended using CO2 laser-drawn fused silica fibres. We present here measurements of mechanical dissipation in synthetic fused silica fibres drawn using a CO2 laser. The level of dissipation in the surface layer is investigated and is found to be at a similar level to fibres produced using a gas flame. Also presented is a method for examining dissipation at welded interfaces, showing clear evidence of the existence of this loss mechanism which forms an additional component of the total detector thermal noise. Modelling of a typical detector suspension configuration shows that the thermal noise contribution from this loss source will be negligible.