Prototype detectors using laser interferometry have been constructed by various research groups around the world – at the Max-Planck-Institut für Quantenoptik in Garching , at the University of Glasgow , at California Institute of Technology , at the Massachusets Institute of Technology , at the Institute of Space and Astronautical Science in Tokyo  and at the astronomical observatory in Tokyo . These detectors have arm lengths varying from 10 m to 100 m and have or had either multibeam delay lines or resonant Fabry–Pérot cavities in their arms. The 10 m detector at Glasgow is shown in Figure 8.
Several years ago the sensitivities of some of these detectors reached a level – better than 10–18 for millisecond pulses – such that the technology could be considered sufficiently mature to propose the construction of detectors of much longer baseline which should be capable of reaching the performance required to have a real possibility of detecting gravitational waves. Thus an international network of gravitational wave detectors is now under construction.
The American LIGO project comprises the building of two detector systems with arms of 4 km length, one in Hanford, Washington State, and one in Livingston, Louisiana. One half length, 2 km, interferometer is also being built inside the same evacuated enclosure at Hanford. Construction at both sites is proceeding well and should allow prelimimary coincident operation to be carried out in 2001. A birds-eye view of the Hanford site showing the central building and the directions of the two arms is shown in Figure 9.
The French/Italian VIRGO detector of 3 km arm length at Cascina near Pisa is also well into construction with the interferometry in the central part being ready for testing in 2000 and final operation expected for late 2002. As mentioned earlier it is designed to have better performance, down to 10 Hz than the other detectors as shown in Figure 10, where the level of contributing noise sources and some possible signal levels are shown.
The TAMA 300 detector, which has arms of length 300 m, is at a relatively advanced stage of construction at the Tokyo Astronomical Observatory. This detector is being built mainly underground; initial operation of the interferometer has been achieved in 1999 and power recycling is now being implemented. All the systems mentioned above are designed to use resonant cavities in the arms of the detectors and use standard wire sling techniques for suspending the test masses. The German/British detector, GEO 600, is somewhat different. It makes use of a four pass delay line system with advanced optical signal enhancement techniques, utilises very low loss fused silica suspensions for the test masses, and should have a sensitivity at frequencies above a few hundred Hz comparable to the first phases of VIRGO and LIGO when they are in initial operation. Construction is advancing well and initial operation of the GEO 600 detector is expected to commence in 2001. During the following years we can expect some very interesting coincidence searches for gravitational waves, at a sensitivity level of approximately 10–21 for pulses of several milliseconds duration. This level of sensitivity is expected to be improved on when the longer LIGO and VIRGO detectors are upgraded. Indeed plans for an upgraded LIGO, LIGO 2, are already on the drawing board. Plans are to use 30 kg sapphire test masses for this detector, suspended by fused silica fibers or ribbons, along with an improved seismic isolation system, increased laser power, of the order of 100 W, and signal recycling . The contribution of the different noise source to the expected interferometer sensitivity as contained in , is shown in Figure 11.
It should be noted that very recent work by Braginsky and colleagues in Moscow  is suggesting that a form of mechanical loss known as thermoelastic damping  is important in bulk crystalline materials such as as sapphire, and may be represented by a noise line somewhat higher than the thermal noise in the above figure. This is currently under investigation (at the beginning of 2000).
Living Rev. Relativity 3, (2000), 3
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