C. I. Torrie

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.

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.

Damping parametric instabilities in future gravitational wave detectors by means of electrostatic actuators

It has been suggested that the next generation of interferometric gravitational wave detectors may observe spontaneously excited parametric oscillatory instabilities. We present a method of actively suppressing any such instability through application of electrostatic forces to the interferometers test masses. Using numerical methods we quantify the actuation force required to damp candidate instabilities and find that such forces are readily achievable. Our predictions are subsequently verified experimentally using prototype Advanced LIGO hardware, conclusively demonstrating the effectiveness of our approach.

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.

Design and prototype tests of a seismic attenuation system for the advanced-LIGO output mode cleaner

Both present LIGO and advanced LIGO (Ad-LIGO) will need an output mode cleaner (OMC) to reach the desired sensitivity. We designed a suitable OMC seismically attenuated optical table fitting to the existing vacuum chambers (horizontal access module, HAM chambers). The most straightforward and cost-effective solution satisfying the Ad-LIGO seismic attenuation specifications was to implement a single passive seismic attenuation stage, derived from the seismic attenuation system (SAS) concept. We built and tested prototypes of all critical components. On the basis of these tests and past experience, we expect that the passive attenuation performance of this new design, called HAM-SAS, will match all requirements for the LIGO OMC, and all Ad-LIGO optical tables. Its performance can be improved, if necessary, by implementation of a simple active attenuation loop at marginal additional cost. The design can be easily modified to equip the LIGO basic symmetric chamber (BSC) chambers and leaves space for extensive performance upgrades for future evolutions of Ad-LIGO. Design parameters and prototype test results are presented.

An investigation of eddy-current damping of multi-stage pendulum suspensions for use in interferometric gravitational wave detectors

In this article we discuss theoretical and experimental investigations of the use of eddy-current damping for multi-stage pendulum suspensions such as those intended for use in Advanced LIGO, the proposed upgrade to LIGO (the US laser interferometric gravitational-wave observatory). The design of these suspensions is based on the triple pendulum suspension design developed for GEO 600, the German/UK interferometric gravitational wave detector, currently being commissioned. In that detector all the low frequency resonant modes of the triple pendulums are damped by control systems using collocated sensing and feedback at the highest mass of each pendulum, so that significant attenuation of noise associated with this so-called local control is achieved at the test masses. To achieve the more stringent noise levels planned for Advanced LIGO, the GEO 600 local control design needs some modification. Here we address one particular approach, namely that of using eddy-current damping as a replacement or supplement to active damping for some or all of the modes of the pendulums. We show that eddy-current damping is indeed a practical alternative to the development of very low noise sensors for active damping of triple pendulums, and may also have application to the heavier quadruple pendulums at a reduced level of damping.

A high throughput instrument to measure mechanical losses in thin film coatings

Brownian thermal noise generated by mechanical losses in thin film coatings limits the sensitivity of gravitational wave detectors, as well as several high precision metrology experiments. Improving the sensitivity of the next generation of gravitational wave detectors will require optical coatings with significantly reduced mechanical losses. In this paper, we describe a system that we developed to measure the mechanical loss angle of thin film coatings deposited on fused silica substrates. The novelty of this system resides in the capability of parallel measurement of up to four samples and the ability to simultaneously probe all the resonant modes of each sample. This high throughput measurement system allows the exploration of a large number of deposition and material parameters, which can be tuned to achieve low loss coatings.

Coming clean: understanding and mitigating optical contamination and laser induced damage in advanced LIGO

The cleanliness of optical surfaces is of great concern as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) project transitions from installation to operation at full power. More particulates than expected were observed on and near the core optics as a result of assembly and installation work, prompting a re-evaluation of longheld contamination control practices. Even low particulate levels can potentially damage the fused silica optics and reduce overall interferometer sensitivity. These risks are mitigated from a combination of the following approaches: quantifying the extent of the contamination, identifying its sources, improving practices to reduce the generation of particulates, introducing a non-contact in-situ cleaning technique for suspended optics in air, qualifying cleanliness levels against induced damage, and developing methods for remotely measuring and cleaning suspended optics under vacuum. While significant progress has been made in understanding and mitigating contamination, and thus, protecting the optics from losses and damage, there is still more work to be done to reach ultimate performance requirements.

Design of a tuned mass damper for high quality factor suspension modes in Advanced LIGO

We discuss the requirements, design, and performance of a tuned mass damper which we have developed to damp the highest frequency pendulum modes of the quadruple suspensions which support the test masses in the two advanced detectors of the Laser Interferometric Gravitational-Wave Observatory. The design has to meet the requirements on mass, size, and level of damping to avoid unduly compromising the suspension thermal noise performance and to allow retrofitting of the dampers to the suspensions with minimal changes to the existing suspensions. We have produced a design satisfying our requirements which can reduce the quality factor of these modes from ∼500 000 to less than 10 000, reducing the time taken for the modes to damp down from several hours to a few minutes or less.