Principles of calibrating the dual-recycled GEO 600

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Abstract
GEO 600 is a dual-recycled interferometric gravitational wave detector which uses advanced optical techniques to achieve its design sensitivity. The peak response of GEO is tunable over a range of frequencies within the detection band of the instrument. The accurate calibration of the detector is an essential step in preparing the data for distribution to the various analysis groups in the community as well as allowing for the extraction of science results. Calibrating the dual-recycled interferometer requires the optical response of the instrument to be determined and corrected for on time scales associated with any fluctuations in the response. Due to the optical readout scheme used in GEO 600, any gravitational wave signals seen by the detector are split between two output signals. This means that, in order to recover a calibrated strain signal that has an optimal signal-to-noise ratio, the two outputs need to be combined pre- or postcalibration. This article presents a detailed method for calibrating the dual-recycled GEO configuration. The method builds upon the calibration scheme developed to calibrate the power-recycled GEO configuration. The main focus of this article is on the new aspects of the calibration process: on-line measurement, parameterization, and correction of the optical response of the instrument in an on-line, time-domain calibration pipeline. Some discussion is also given regarding the combination of the two main detector output signals in producing an optimal calibrated strain signal.

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