Experimental demonstration of a suspended diffractively coupled optical cavity

All-reflective optical systems are under consideration for future gravitational wave detector topologies. One approach in proposed designs is to use diffraction gratings as input couplers for Fabry–Perot cavities. We present an experimental demonstration of a fully suspended diffractively coupled cavity and investigate the use of conventional Pound–Drever–Hall length sensing and control techniques to maintain the required operating condition

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

Search for Gravitational Waves from Intermediate Mass Binary Black Holes

We present the results of a weakly modeled burst search for gravitational waves from mergers of non-spinning intermediate mass black holes (IMBH) in the total mass range 100--450 solar masses and with the component mass ratios between 1:1 and 4:1. The search was conducted on data collected by the LIGO and Virgo detectors between November of 2005 and October of 2007. No plausible signals were observed by the search which constrains the astrophysical rates of the IMBH mergers as a function of the component masses. In the most efficiently detected bin centered on 88+88 solar masses, for non-spinning sources, the rate density upper limit is 0.13 per Mpc^3 per Myr at the 90% confidence level.Comment: 13 pages, 4 figures: data for plots and archived public version at https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=62326, see also the public announcement at http://www.ligo.org/science/Publication-S5IMBH

Novel sensing and control schemes for a three-mirror coupled cavity

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

Modeling of multistage pendulums: Triple pendulum suspension for GEO 600

Extensive dynamic six degree-of-freedom modeling of a multiple pendulum suspension has been developed for application to the design and analysis of the planned suspensions for the main mirrors in GEO 600, the German/United Kingdom gravitational wave detector. Two models were developed independently, and results from both were compared with the experiment to verify their applicability. The models have been applied to investigate the optimization of parameters for achieving the desired modal behavior. In addition, the levels of cross coupling between degrees of freedom due to mechanical misalignments have been investigated and shown to be within acceptable limits

Techniques in the optimization of length sensing and control systems for a three-mirror coupled cavity

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

GEO 600 triple pendulum suspension system: Seismic isolation and control

In this article we describe aspects of the suspension system for each of the main optics (test masses) in the GEO 600 interferometric gravitational wave detector currently under construction in Germany. In particular we describe the triple pendulum, which is a key aspect of the overall system, discuss the details of the mechanical design, present transfer functions of the isolation performance, and display examples of impulse responses for the local control used to damp the low frequency modes of the triple pendulum. This local control is achieved using co-located sensing and feedback at the highest mass of the triple pendulum, providing a significant attenuation of local control noise to the fused silica test mass, the lowest mass of the triple pendulum. In order to enhance the vertical isolation, the triple pendulum will incorporate two stages of cantilever springs, and fused silica fibers will be used in the lowest pendulum stage in order to minimize thermal noise from the pendulum modes. It is expected that the thermal noise associated with the internal modes of the fused silica test mass (mass similar to 6 kg) will set the sensitivity limit for GEO 600 from 50 to similar to 200 Hz. The measured performance from individual stages of the prototype suspension system indicates that a seismic noise level which is a factor similar to 3 lower than this thermal noise level at 50 Hz can be achieved

Performance of the Glasgow 10 m prototype gravitational wave detector operating at lambda=1064 nm

We report on measurements performed with the Glasgow prototype gravitational wave detector which has recently been converted to operate with a Nd:YAG laser at 1064 nm. The differential displacement amplitude spectral density above a few hundred Hz is similar to 5 x 10(-19) m/root Hz which is consistent with the limitations imposed by shot noise in this frequency range. We also present sensitivity spectra over a lower frequency regime to highlight the impact of improved suspension designs

Experimental demonstration of a suspended, diffractively coupled Fabry‚ Perot cavity

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