Advanced LIGO (aLIGO) [166, 214, 66] and Advanced Virgo (AdvVirgo) [62, 67] are the second generation detectors. They are planned to have a sensitivity increase over the levels of the initial detectors by a factor of 10 – 15 times. These increased sensitivity levels would expand the volume of space observed by the detectors by 1000 times meaning that there is a realistic detection rate of neutron-star–binary coalescences of around 40 yr–1 [3, 205]. The technological issues required to reach these sensitivities, such as choice of test mass and mirror coating materials, suspension design, interferometric layout, control and readout, would need a separate review article to themselves, but we shall very briefly summarise them here.
Advanced LIGO will consist of three 4 km detectors in the current LIGO vacuum system; two at the Hanford site3 and one at Livingston. It will apply some of the technologies from the GEO600 interferometer, such as the use of a signal recycling mirror at the output port and monolithic silica suspensions for the test masses, rather than the current steel wire slings. Larger test masses will be used with an increase from 11 to 40 kg, although the masses will still be made from fused silica. The mirror coating is likely to consist of multiple alternating layers of silica and tantala, with the tantala layers doped with titania to reduce the coating thermal noise . The seismic isolation systems will be replaced with improved versions offering a seismic cut-off frequency of 10 Hz as opposed to the current cut-off of 40 Hz. As stated for Enhanced LIGO (in Section 6.1.2), the laser power will be greater than for initial LIGO and a DC readout scheme will be used. Initial/Enhanced LIGO was shut down to begin the installation of these upgrades on 20 October 2010. The design strain amplitude sensitivity curve for aLIGO (and AdvVirgo and LCGT) is shown in Figure 18.
AdvVirgo will apply similar upgrades to those for aLIGO and over a similar timescale (for details see  and ). Plans are to add a signal recycling mirror, monolithic suspensions, increased laser power to 200 W, improved coatings, and to potentially use non-Gaussian beams (see, e.g., ), although this option is unlikely. The seismic isolation system will not be changed. Virgo will shut down to begin these upgrades in July 2011.
The Large-scale Cryogenic Gravitational-Wave Telescope (LCGT) [234, 247, 207] is a planned Japanese detector to be sited underground in the Kamioka mine. The LGCT will consist of a detector with 3 km arms, using sapphire mirrors and sapphire suspensions. Initially it will operate at room temperature, but will later be cooled to cryogenic temperatures. This detector is planned to have similar sensitivities to aLIGO and AdvVirgo, with a reach for binary coalescences of about 200 Mpc with SNR of 10. There currently exists a technology demonstrator called the Cryogenic Laser Interferometer Observatory (CLIO) [324, 164], which has a 100 m baseline and is also sited in the Kamioka mine. This is to demonstrate the very stable conditions (i.e., low levels of seismic noise) existing in the mine and also the cryogenically-cooled sapphire mirrors suspended from aluminium wires. In experiments with CLIO at room temperature (i.e. 300 K), using a metallic glass called Bolfur for its wire suspensions, it has already been used to produce an astrophysics result by looking for gravitational waves from the Vela pulsar , giving a 99.4% confidence upper limit of = 5.3 × 10–20. Tests with the cryogenic system activated and using aluminium suspensions allowed two mirrors to be cooled to 14 K.
Having a network of comparably-sensitive detectors spread widely across the globe is vital to gain the fullest astrophysical insight into transient sources. Position reconstruction for sources relies on triangulating the location based on time-of-flight delays observed between detectors. Therefore, having long baselines, and different planes between as many detectors as possible, gives the best positional reconstruction – in  it is shown that for the 2 US aLIGO sites sky localisation will be on the order of 1000 square degrees, whereas this can be brought down to a few square degrees with the inclusion of more sites and detectors. Observation with multiple detectors also provides the best way to give confidence that a signal is a real gravitational wave rather than the accidental coincidence of background noise. Finally, multiple, differently-oriented, detectors will increase the ability to reconstruct a transient sources waveform and polarisation.
Currently design studies are under way for a third-generation gravitational-wave observatory called the Einstein Telescope (ET) . This is a European Commission funded study with working groups looking into various aspects of the design including the site location and characteristics (e.g. underground), suspensions technologies; detector topology and geometry (e.g. an equilateral triangle configuration); and astrophysical aims. The preliminary plan is to aim for an observatory, which improves upon the second-generation detectors by an order of magnitude over a broad band. There are many technological challenges to be faced in attempting to make this a reality and research is currently under way into a variety of these issues.
Investigations into the interferometric configuration have already been studied (see [145, 174, 178]), with suggestions including a triple interferometer system made up from an equilateral triangle, an underground location, and potentially a xylophone configuration (two independent detectors covering different frequency ranges, i.e., ultimately giving six detectors in total, although constructed over a period of years). Three potential sensitivity curves are plotted in Figure 19 for different configurations of detectors.
Living Rev. Relativity 14, (2011), 5
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