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

34 Gravitational-wave detectorsNoise amplitude spectral density10 −2110 −2210 −23SeismicNoise10 −20 Frequency (Hz)TotalThermal NoiseUncertainty PrincipleShot Noise(broad band)10 2 10 3Noise amplitude spectral density10 −2110 −2210 −23SeismicNoise10 −20 Frequency (Hz)TotalShot Noise(narrow bandat 600 Hz)Thermal NoiseUncertainty Principle10 2 10 3Figure 3.3. GEO600 noise curves. As for the TAMA curve, these are calibrated in strainper root Hz. The figure on the left-hand side shows GEO’s wideband configuration; thaton the right-hand side shows a possible narrowband operating mode.changing the resonance frequency of the signal recycling cavity. This featurecould be useful when interferometers work with bars or when performing widebandsurveys.LIGO [6] (USA) is building two detectors of arm length 4 km. One islocated in Hanford WA and the other in Livingstone LA. The target date forobserving is mid-2002. The two detectors are placed so that their antenna patternsoverlap as much as possible and yet they are far enough apart that there will be ameasurable time delay in most coincident bursts of gravitational radiation. Thisdelay will give some directional information. The Hanford detector also containsa half-length interferometer to assist in coincidence searches. The two LIGOdetectors are the best placed for doing cross-correlation for a random backgroundof gravitational waves. LIGO’s expected initial noise curve is shown in figure 3.4.These detectors have been constructed to have a long lifetime. With such longarms they can benefit from upgrades in laser power and mirror quality. LIGO hasdefined an upgrade goal called LIGO II, which it hopes to reach by 2007, whichwill observe at 10 −22 or better over a bandwidth from 10 Hz up to 1 kHz.VIRGO [7] (Italy and France) is building a 3 km detector near Pisa. Itstarget date for good data is 2003. Its expected initial noise curve is shown infigure 3.4. Like LIGO, it can eventually be pushed to much higher sensitivitieswith more powerful lasers and other optical enhancements. VIRGO specializesin sophisticated suspensions, and the control of vibrational noise. Its goal is toobserve at the lowest possible frequencies from the ground, at least partly to beable to examine as many pulsars and other neutron stars as possible.3.3 The physics of resonant mass detectorsThe principle of operation of bar detectors is to use the gravitational tidal forceof the wave to stretch a massive cylinder along its axis, and then to measure theelastic vibrations of the cylinder.