A figure of merit for the sensitivity of a detector is to calculate its horizon distance. This is the maximum range out to which it could see the coalescence of two neutron stars that are optimally oriented and located (i.e., with the orbital plane perpendicular to the line-of-sight, and with this plane parallel to the detector plane, so that the antenna response is at its maximum) at a signal-to-noise ratio of 8 . The horizon distance can be converted to a range that is an average over all sky locations and source orientations (i.e. not the best case scenario) by dividing it by 2.26 ) – we shall use this angle averaged range throughout the rest of this review.
The first interferometric detector to start regular data taking with sufficient sensitivity and stability to enable it to potentially detect gravitational waves from the the galactic centre was TAMA300 . Over the period between August 1999 to January 2004 TAMA had nine data-taking periods (denominated DT1–9) over which time its typical strain noise sensitivity, in its most sensitive frequency band improved from 3 × 10–19 Hz–1/2 to 1.5 × 10–21 Hz–1/2 . TAMA300 operated in coincidence with the LIGO and GEO600 detectors for two of the science data-taking periods. More recently focus has shifted to the Cryogenic Laser Interferometer Observatory (CLIO) prototype detector [324, 164], designed to test technologies for a future second-generation Japanese detector called the Large-scale Cryogenic Gravitational-Wave Telescope (LCGT) (see Section 6.3.1).
The first LIGO detector to achieve lock (meaning having the interferometer stably held on a dark fringe of the interference pattern, with light resonating throughout the cavity) was H2 in late 2000. By early 2002 all three detectors had achieved lock and have since undergone many periods of commissioning and science data taking. Over the period from mid-2001 to mid-2002 the commissioning process improved the detectors’ peak sensitivities by several orders of magnitude, with L1 going from 10–17 – 10–20 Hz–1/2 at 150 Hz. In summer 2002 it was decided that the detectors were at a sensitivity, and had a good enough lock stability, to allow a science data-taking run. This was potentially sensitive to local galactic burst events. From 23 August to 9 September 2002 the three LIGO detectors, along with GEO600 (and, for some time, TAMA300), undertook their first coincident science run, denoted S1 (see  for the state of the LIGO and GEO600 detectors at the time of S1). At this time the most sensitive detector was L1 with a peak sensitivity at around 300 Hz of 2 – 3 × 10–21 Hz–1/2. The best strain amplitude sensitivity curve for S1 (and the subsequent LIGO science runs) can be seen in Figure 15. The amount of time over the run that the detectors were said to be in science mode, i.e., stable and with the interferometer locked, called their duty cycle, or duty factor, was 42% for L1, 58% for H1 and 73% for H2. For the most sensitive detector, L1, the inspiral range was typically 0.08 Mpc.
For the second science run (S2), from 14 February to 14 April 2003, the noise floor was considerably improved over S1 by several upgrades including: improving and stabilising the optical levers used to measure the mirror orientation to reduce the low frequency ( 50 Hz) noise; replacing the coil drivers that are used as actuators to control the position and orientation of suspended mirrors, to improve the mid-frequency ( 50 – 200 Hz) noise floor; and increasing the laser power in the interferometer to reduce shot noise and improve the high frequency ( 200 Hz) sensitivity (see Section IIA of  for a more thorough description of the detector improvements made for S2). These changes improved the sensitivities by about an order of magnitude across the frequency band with a best strain, for L1, of 3 × 10–22 Hz–1/2 between 200 – 300 Hz. The duty factor during S2 was 74% for H1, 58% for H2 and 38% for L1, with a triple coincidence time when all three detectors were in lock of 22% of the run. The average inspiral ranges during the run were approximately 0.9, 0.4 and 0.3 Mpc for L1, H1 and H2 respectively. This run was also operated in coincidence with the TAMA300 DT8 run.
For the the third science run (S3), from 31 October 2003 to 9 January 2004, the detectors were again improved, with the majority of sensitivity increase in the mid-frequency range. This run was also operated partially in coincidence with GEO600. The best sensitivity, which was for H1, was 5 × 10–23 Hz–1/2 between 100 – 200 Hz. The duty factors were 69% for H1, 63% for H2 and only 22% for L1, with a 16% triple coincidence time. L1’s poor duty factor was due to large levels of anthropogenic seismic noise near the site during the day.
The fourth science run (S4), from 22 February to 23 March 2005, saw less-drastic improvements in detector sensitivity across a wide frequency band, but did make large improvements for frequencies 70 Hz. Between S3 and S4 a better seismic isolation system, which actively measured and countered for ground motion, was installed in L1, greatly reducing the amount of time it was thrown out of lock. For H1 the laser power was able to be increased to its full design power of 10 W . The duty factors were 80% for H1, 81% for H2 and 74% for L1, with a 56% triple coincidence time. The most sensitive detector, H1, had an inspiral range of 7.1 Mpc.
By mid-to-late 2005 the detectors had equaled their design sensitivities over most of the frequency band and were also maintaining good stability and high duty factors. It was decided to perform a long science run with the aim of collecting one year’s worth of triple coincident data, with an angle-averaged inspiral range of equal to, or greater than, 10 Mpc for L1 and H1, and 5 Mpc or better for H2. This run, S5, spanned from 4 November 2005 (L1 started slightly later on 14 November) until 1 October 2007, and the performance of the detectors during it is summarised in . One year of triple coincidence was achieved on 21 September 2007, with a total triple coincidence duty factor of 52.5% for the whole run. The average insprial range over S5 was 15 Mpc for H1 and L1, and 8 Mpc for H2.
After the end of S5 the LIGO H2 detector and GEO600 were kept operational while possible in an evening and weekend mode called Astrowatch. This observing mode continued until early 2009, after which H2 was retired. During this time commissioning of some upgrades to the 4 km LIGO detectors took place for the sixth and final initial LIGO science run (S6) – some of which are summarised in . The aim of these upgrades, called Enhanced LIGO , was to try and increase sensitivity by a factor of two. Enhanced LIGO involved the direct implementation of technologies and techniques designed for the later upgrade to Advanced LIGO (see Section 6.3.1) such as, most notably, higher-powered lasers, a DC readout scheme (see Section 5.4), the addition of output mode cleaners and the movement of some hardware into the vacuum system. The lasers, supplied by the Albert Einstein Institute and manufactured by Laser Zentrum Hannover, give a maximum power of 30 W, which is around 3 times the initial LIGO power. The upgrade to higher power required that several of the optical components needed to be replaced. These upgrades were only carried out on the 4 km H1 and L1 detectors due to the H2 detector being left in Astrowatch mode during the commissioning period. The upgrades were able to produce 1.5 – 2 times sensitivity increases at frequencies above 200 Hz, but generally at lower frequencies various sources of noise meant sensitivity increases were not possible. S6 took place from July 2009 until 20 October 2010, at which point decommissioning started for the full upgrade to Advanced LIGO. Typically the detectors ran with laser power at 10 W during the day (at higher power the detector was less stable and the higher level of anthropogenic noise during the day meant that achieving and maintaining lock required lower power) and 20 W at night, leading to inspiral ranges from 10 – 20 Mpc.
GEO600 achieved first lock as a power-recycled Michelson (with no signal recycling) in late 2001. Commissioning over the following year, detailed in , included increases in the laser power, installation of monolithic suspensions for the end test masses (although not for the beam splitter and inboard mirrors), rearrangement of the optical bench to reduce scattered light and implementation of an automatic alignment system. For the S1 run, carried out in coincidence with LIGO (and, in part, TAMA300), the detector was kept in this configuration (see  for the status of the detector during S1). It had a very high duty factor of 98%, although its strain sensitivity was 2 orders of magnitude lower than the LIGO instruments. The auto-alignment system in GEO600 has since meant that it has been able to operate for long periods without manual intervention to regain lock, as has been the case for initial LIGO.
Following S1 the signal recycling mirror was installed and in late 2003 the first lock of the fully dual-recycled system was achieved (see [289, 318, 163] for information on the commissioning of GEO600 as a dual-recycled detector). Other upgrades included the installation of the final mirrors, suspended as triple pendulums, and with monolithic final stages. Once installed it was found that there was a radius of curvature mismatch with one of the mirrors, which had to be compensated for by carefully heating the mirror. Due to this commissioning effort GEO600 did not participate in the S2 run. Very soon after the implementation of dual-recycling GEO600 took part in the S3 run. This occurred over two time intervals from 5 – 11 November 2003, dubbed S3I, and from 30 December 2003 to 13 January 2004, dubbed S3II. During S3I GEO600 operated with the signal-recycling cavity tuned to 1.3 kHz, and had a 95% duty factor, but was then taken off-line for more commissioning work. In the period between S3I and II various sources of noise and lock loss were diagnosed and mitigated, including noise from a servo in the signal recycling cavity and electronic noise on a photo-diode . This lead to improved sensitivity by up to an order of magnitude at some frequencies (see Figure 16). For S3II the signal recycling cavity was tuned to 1 kHz and, due to the upgrades, had an increased duty factor of 99%. GEO600 operated during the whole of S4 (22 February to 24 March 2004), in coincidence with LIGO, with a 97% duty factor. It used the same optical configuration as S3, but had sensitivity improvements from a few times to up to an order of magnitude over the S3 values .
The main changes to the detector after S4 were to shift the resonance condition of the signal recycling cavity to a lower frequency, 350 Hz, allowing better sensitivity in the few hundred Hz regime, and increasing the circulating laser power, with an input power of 10 W. The pre-S5 peak sensitivity was 4 × 10–22 Hz–1/2 at around 400 Hz, with an inspiral range of 0.6 Mpc . GEO600 did not join S5 at the start of the LIGO run, but from 21 January 2006 was in a night-and-weekend data-taking mode whilst noise hunting studies and commissioning were conducted. For S5 the signal recycling cavity was re-tuned up to 550 Hz. It went into full-time data taking from 1 May to 16 October 2006, with an instrumental duty factor of 94%. The average peak sensitivity during S5 was better than 3 × 10–22 Hz–1/2 (see  for a summary of GEO600 during S5). After this it was deemed more valuable for GEO600 to continue more noise hunting and commissioning work, to give as good a sensitivity as possible for when the LIGO detectors went offline for upgrading. However, it did continue operating in night-and-weekend mode.
GEO600 continued operating in Astrowatch mode between November 2007 and July 2009 after which upgrades began. The plans for the GEO600 detector are to continue to use it as a test-bed for more novel interferometric techniques whilst focusing on increasing in sensitivity at higher frequencies (greater than a few hundred Hz). This project is called GEO-HF . The upgrading towards GEO-HF has been taking place since Summer 2009 . The main upgrades started during 2009 were to change the read-out scheme from an RF read-out to a DC read-out system  (also see Section 5.4), install an output mode cleaner, place the read-out system in vacuum, injecting squeezed light [302, 114] into the output port, and finally increasing the input laser power to 35 W. Running the interferometer with squeezed light will be the first demonstration of a full-scale gravitational-wave detector operating beyond the standard quantum limit. GEO-HF participated in S6 in an overnight and weekend mode, alongside a commissioning schedule, and is continuing in this mode following the end of S6.
In summer 2002 Virgo completed the commissioning of the central area interferometer, consisting of a power-recycled Michelson interferometer, but without the 3 km Fabry–Pérot arm cavities. Over the next couple of years various steps were made towards commissioning the full-size interferometer. In early 2004 first lock with the 3 km arms was achieved, but without power-recycling, and by the end of 2004 lock with power recycling was achieved. During summer 2005 the commissioning runs provided order-of-magnitude sensitivity improvements, with a peak sensitivity of 6 × 10–22 Hz–1/2 at 300 Hz, and an inspiral range of over 1 Mpc. In late 2005 several major upgrades brought Virgo to its final configuration. See [58, 59, 60, 61] for more detailed information on the commissioning of the detector.
Virgo joined coincident observations with the LIGO and GEO600 S5 run with 10 weekend science runs (WSRs) starting in late 2006 until March 2007. Over this time improvements were made mainly in the mid-to-low frequency regime ( 300 Hz). Full-time data taking, under the title of Virgo Science run 1 (VSR1), began on 18 May 2007 and ended with the end of S5 on 1 October 2007. During VSR1, the science-mode duty factor was 81% and by the end of the run maximum neutron-star–binary inspiral range was frequently up to about 4.5 Mpc. The best sensitivity curves for WSR1, WSR10 and VSR1 can be seen in Figure 17.
At the same time as commissioning for Enhanced LIGO was taking place there was also a similar effort to upgrade the Virgo detector, called Virgo+. The main upgrade was to the lasers to increase their power from 10 to 25 W at the input mode cleaner, with upgrades also to the thermal compensation system on the mirrors, the control electronics, mode cleaners and injection optics [64, 141]. Virgo+ started taking data with Enhanced LIGO for Virgo Science Run 2 (VSR2) and sensitivities of Virgo+ close to the initial Virgo design sensitivity were reached. VSR2 finished on 8 January 2010 to allow for further commissioning and noise hunting. This was followed by VSR3, which began on 11 August 2010 and ran until 20 October 2010. Further Virgo+ runs are expected during 2011. Following these the upgrades to Advanced Virgo will begin.
Living Rev. Relativity 14, (2011), 5
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