Prior to the start of the 21st century there existed several prototype laser interferometric detectors, constructed by various research groups around the world – at the Max-Planck-Institüt für Quantenoptik in Garching , at the University of Glasgow , at the California Institute of Technology , at the Massachusetts Institute of Technology , at the Institute of Space and Astronautical Science in Tokyo  and at the astronomical observatory in Tokyo . These detectors had arm lengths varying from 10 m to 100 m and had either multibeam delay lines or resonant Fabry–Pérot cavities in their arms. The 10 m detector that used to exist at Glasgow is shown in Figure 12.
The sensitivities of some of these detectors reached a level – better than 10–18 for millisecond bursts – such that the technology could be considered sufficiently mature to propose the construction of detectors of much longer baseline that would be capable of reaching the performance required to have a real possibility of detecting gravitational waves. An international network of such long baseline gravitational wave detectors has now been constructed and commissioned, and science-quality data from these has been produced and analysed since 2002 (see Section 6.1 and Section 6.2 for a review of recent science data runs and results).
The American LIGO project  comprises two detector systems with arms of 4 km length, one in Hanford, Washington, and one in Livingston, Louisiana (also known as the LIGO Hanford Observatory 4k [LHO 4k] and LIGO Livingston Observatory 4k [LLO 4k], or H1 and L1, respectively). One half length, 2 km, interferometer was also contained inside the same evacuated enclosure at Hanford (also known as the LHO 2k, or H2). The design goal of the 4 km interferometers was to have a peak strain sensitivity between 100 – 200 Hz of 3 × 10–23 Hz–1/2  (see Figure 15), which was achieved during the fifth science run (Section 6.1). A birds-eye view of the Hanford site showing the central building and the directions of the two arms is shown in Figure 13. In October 2010 the LIGO detectors shut down and decommissioning began in preparation for the installation of a more sensitive instrument known as Advanced LIGO (see Section 6.3.1).
The French/Italian Virgo project  comprises a single 3 km arm-length detector at Cascina near Pisa. As mentioned earlier, it is designed to have better performance than the other detectors, down to 10 Hz.
The TAMA300 detector , which has arms of length 300 m, at the Tokyo Astronomical Observatory was the first of the “beyond-prototype” detectors to become operational. This detector is built mainly underground and partly has the aim of adding to the gravitational-wave detector network for sensitivity to events within the local group of galaxies, but is primarily a test bed for developing techniques for future larger-scale detectors. Initial operation of the interferometer was achieved in 1999 and power recycling was implemented for data taking in 2003 .
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, GEO600 , built near Hannover, Germany, 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, despite its smaller size, was designed to have a sensitivity at frequencies above a few hundred Hz comparable to the first phases of Virgo and LIGO during their initial operation. It uses both power recycling (Section 5.1) and tunable signal recycling (Section 5.2), often referred to together as dual recycling.
To gain the most out of the detectors as a true network, data sharing and joint analyses are required. In the summer of 2001 the LIGO and GEO600 teams signed a Memorandum of Understanding (MoU), under the auspices of the LIGO Scientific Collaboration (LSC) , allowing complete data sharing between the two groups. Part of this agreement has been to ensure that both LIGO and GEO600 have taken data in coincidence (see below). Coincident data taking, and joint analysis, has also occurred between the TAMA300 project and the LSC detectors. The Virgo collaboration also signed an MoU with the LSC, which has allowed data sharing since May 2006.
The operation and commissioning of these detectors is a continually-evolving process, and the current state of this review only covers developments until late-2010. For the most up-to-date information on detectors readers are advised to consult the proceedings of the Amaldi meetings, GWDAW/GWPAW (Gravitational Wave Data Analysis Workshops), and GWADW (Gravitational Wave Advanced Detectors Workshops) – see  for a list of past conferences.
For the first and second generations of detector, much effort has gone into estimating the expected number of sources that might be observable given their design sensitivities. In particular, for what are thought to be the strongest sources: the coalescence of neutron-star binaries or black holes (see Section 6.2.2 for current rates as constrained by observations). These estimates, based on observation and simulation, are summarised in Table 5 of  and the realistic rates suggest initial detectors would expect to see 0.02, 0.004 and 0.007 events per year for neutron-star–binary, black-hole–neutron-star, and black-hole–binary systems, respectively (it should be noted that there is a range of possible rates consistent with current observations and models)1. Second generation detectors (see Section 6.3.1), which can observe approximately 1000 time more volume than the initial detectors might, expect to see 40, 10, and 20 per year for the same sources. With such rates a great deal of astrophysics could be possible (see  for examples).
Living Rev. Relativity 14, (2011), 5
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