The sensitivity of GEO 600 depends on the bandwidth which in its turn depends on the signal recycling factor. The signal recycling of GEO 600 thus provides the opportunity to change the spectral characteristics of the detector response especially those due to the shot noise limitations. By choosing low or high mirror reflectivites of the signal recycling mirror we can selcect a wideband or a narrowband behaviour, respectively. The center frequency of the maximum sensitivity can be tuned to the desired frequency by shifting the signal recycling mirror thus changing the resonance frequency of the signal recycling cavity.
This diagram shows the development of the amplitude spectral densities of GEO600 during the first Science Runs (S1 to S3).