DC-readout of a signal-recycled gravitational wave detector
  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. 
S.Hild, H.Grote, J.Degallaix, S.Chelkowski, K.Danzmann, A. Freise,   M.Hewitson, J.Hough, H.Lueck, M.Prijatelj, K.A.Strain, ...