(ed.). Gravitational waves (IOP, 2001)(422s).

The physics of interferometers 33Figure 3.2. TAMA300 sensitivity as a function of frequency. The vertical axis is the1σ noise level, measured in strain per root Hz. To get a limit on the gravitational waveamplitude h, one must multiply the height of the curve by the square root of the bandwidthof the signal. This takes into account the fact that the noise power at different frequenciesis independent, so the power is proportional to bandwidth. The noise amplitude is thereforeproportional to the square root of the bandwidth.TAMA300 [4] (Japan) is located in Tokyo, and its arm length is 300 m.It began taking data without power recycling in 1999, but its sensitivity is notyet near 10 −21 . Following improvements, especially power recycling, it shouldget to within a factor of ten of this goal. However, it is not planned as anobserving instrument: it is a prototype for a kilometre-scale interferometer inJapan, currently called JGWO. By 2005 this may be operating, possibly withcryogenically cooled mirrors.GEO600 [5] (Germany and Britain) is located near Hannover (Germany).Its arm length is 600 m and the target date for first good data is now the end of2001. Unlike TAMA, GEO600 is designed as a leading-edge-technologydetector,where high-performance suspensions and optical tricks like signal recycling canbe developed and applied. Although it has a short baseline, it will have a similarsensitivity to the larger LIGO and VIRGO detectors at first. At a later stage, LIGOand VIRGO will incorporate the advanced methods developed in GEO, and at thatpoint they will advance in sensitivity, leaving GEO behind.As we can see from figure 3.3 the sensitivity of GEO600 depends on itsbandwidth, which in its turn depends on the signal recycling factor. GEO600 canchange its observing bandwidth in response to observing goals. By choosing lowor high reflectivity for the signal recycling mirror, scientists can make GEO600wide-band or narrow-band, respectively. The centre frequency of the observingband (in the right-hand panel of figure 3.3 it is ∼600 Hz) can be tuned to anydesired frequency by shifting the position of the signal recycling mirror, thus