[en] We present two options for length sensing and control of a three-mirror coupled cavity. The control of the first cavity uses amplitude or single sideband modulation and phase modulation in combination with a beat-frequency demodulation scheme, whereas the control scheme for the second cavity incorporates phase modulation and single demodulation. The theoretical and experimental performance is discussed as well as the relevance to a research programme to develop interferometric techniques for application in future interferometric gravitational wave detectors

[en] The control of coupled cavity systems, as employed in interferometric gravitational wave detectors, depends to a large extent on the design and optimization of sensing systems that can correctly read out the length and angle degrees of freedom. As interferometer configurations become more complex, and new sensing schemes are introduced, it is important to ensure that methods are available to optimize the system parameters to allow the experimental realization to match the theoretical design. In an experimental test, on a suitable model apparatus, we show that currently available numerical modelling tools allow the development and implementation of reliable methods of setting the key system parameters. Here we present an example technique showing how these parameters can be optimized and compare the numerical model with the experimental reality. The effects of mode-matching and misalignment on the sensing signals and on the process of optimizing them are also considered

[en] Diffraction gratings have been considered as input couplers for Fabry-Perot cavities in future gravitational wave detectors. We experimentally demonstrate the use of a triple-suspended, diffractively coupled cavity and examine conventional Pound-Drever-Hall length sensing and control techniques to maintain the required operating condition. Utilizing the diffractively coupled Fabry-Perot cavity, we investigate the effects associated with translational grating motion and observe a unique 1/f slope in the magnitude of the frequency response when monitoring the forward-reflected error signal.

[en] GEO 600 uses two 8 m triangular ring cavities as a modecleaner system for the stabilization of the laser. To isolate the cavities with respect to the seismic noise the optical components are suspended as double pendulums. The resonances of these pendulums are damped by a local-control loop via magnet-coil actuators acting on the intermediate masses. The suspension scheme and the measured key data (i.e. finesse, linewidth, visibility, throughput and in-lock durations of the cavities, as well as the isolation performance and the resulting frequency stability) of the modecleaner system will be given in this paper. Furthermore an overview of the GEO 600 interferometer suspension will be given

[en] 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 the interferometer 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 a final 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 m Hz^-1/2 at 10 Hz at each of the test masses

[en] The Glasgow group is involved in the construction of the GEO600 interferometer as well as in R and D activity on technology for advanced gravitational wave detectors. GEO600 will be the first GW detector using quasi-monolithic silica suspensions in order to decrease thermal noise significantly with respect to steel wire suspensions. The results concerning GEO600 suspension mounting and performance will be shown in the first section. Section 2 is devoted to the present results from the direct measurement of thermal noise in mirrors mounted in the 10 m interferometer in Glasgow which has a sensitivity limit of 4 x 10^-19 m Hz^-1/2 above 1 kHz. Section 3 presents results on the measurements of coating losses. R and D activity has been carried out to understand better how thermal noise in the suspensions affects the detector sensitivity, and in section 4 a discussion on the non-linear thermoelastic effect is presented