DC-readout of a signal-recycled gravitational wave detector

TL;DR

This paper discusses the implementation and advantages of DC-readout in gravitational wave detectors, highlighting its potential to overcome phase noise issues associated with traditional heterodyne methods.

Contribution

It reports the first measurements of DC-readout in a signal-recycled gravitational wave detector and compares its performance with heterodyne readout.

Findings

DC-readout was successfully implemented on GEO 600.

Performance improvements over heterodyne readout were observed.

Implications for future gravitational wave detector designs are discussed.

Abstract

All first-generation large-scale gravitational wave detectors are operated at the dark fringe and use a heterodyne readout employing radio frequency (RF) modulation-demodulation techniques. However, the experience in the currently running interferometers reveals several problems connected with a heterodyne readout, of which phase noise of the RF modulation is the most serious one. A homodyne detection scheme (DC-readout), using the highly stabilized and filtered carrier light as local oscillator for the readout, is considered to be a favourable alternative. Recently a DC-readout scheme was implemented on the GEO 600 detector. We describe the results of first measurements and give a comparison of the performance achieved with homodyne and heterodyne readout. The implications of the combined use of DC-readout and signal-recycling are considered.