LIGO Document G0900519-v1

Accurate measurement of the time delay in the response of the LIGO gravitational wave detectors

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It is important to have a quantitative understanding of the time delay in the response of interferometric gravitational wave detectors when performing timing sensitive data analyses such as coincidence searches between several detectors or long signal integrations for continuous wave searches. We present a method to precisely measure the time delay using photon radiation pressure. An intensity modulated laser beam illuminating on a mirror applies a modulated radiation pressure force on the mirror. By comparing the phase between the interferometer's output and the intensity monitor of the actuation laser, we can measure the time delay of the interferometer's response with a good precision. The independence of the photon actuator from the control loops of the interferometer, which is used for the standard LIGO calibration, makes it an independent verification of the timing error in the system. We achieved a 1 micro second relative accuracy and a 10 micro second systematic error of the time delay by a measurement performed with the 4km interferometer at the LIGO Hanford observatory.
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