2.2 The physics of gravitational waves from compact binary coalescences
3.1.1 Subsystems of the LIGO interferometers
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noise by using signals from the onsite seismometers to control movement of the vacuum chambers for the end test masses” [66]. Hanford has been using a piezoelectric pre-isolation (PEPI) system since S2, but will be upgraded to HEPI for Advanced LIGO [66].
102 103
Frequency (Hz)
10−2310−22 10−21 10−20 10−19
Strain amplitude sp ectral densit y (1 / √ Hz)
H1L1
Figure 3: Representative strain amplitude sensitivity of the LIGO detectors during S6.
0 100 200 300 400
Time (days) since the start of S6
4 6 8 10 12 14 16 18 20 22
Inspiraldetectionrange(Mpc)
H1 L1
Figure 4: The inspiral detection range of the LIGO detectors throughout S6 to an optimally oriented and located binary neutron star merger. The rapid improvements between epochs can be attributed to hardware and control changes implemented during commissioning periods.
Figure 3.4: The range (See Equation (2.25) to which the LIGO detectors are sensitive to a binary neutron star inspiral signal, shown to illustrate the changing sensitivity as various hardware or software upgrades are made throughout the course of the run [6].
A very important part of the detectors’ proper function areservos, also known as control loops. These stabilize the laser amplitude and frequency at the pre-stabilized laser table (PSL), damp the pendulum motion of the suspended optics, control the lengths of various cavities and the angular positions of the optics, and more. For example, the lengths of the two Fabry-Perot cavities in the arms and the power-recycling cavity are kept at an integer number of wavelengths so that new light that enters interferes constructively with the light already resonant in the cavities. There is also a servo that controls the Michelson phase so that the anti-symmetric port stays at the dark fringe [62]. The detector strain signal is derived from the sensing and actuation signals of the differential arm motion control loop — see Section 3.3 below.
Virgo detectors operate in a similar fashion; see Reference [63].
• IO: The input optics subsystem shares an optical table with the PSL subsystem. This subsystem’s components (see Figure 3.5) are responsible for additional mode matching and controlling the power and frequency of the laser that enters the interferometer. It uses an electro-optic modulator to phase modulate the beam to produce radio frequency (RF) sidebands, which are sent into the interferometer and to the length and alignment control subsystems [7]. The input mode cleaner is used to further stabilize the laser frequency, and further define the transverse mode of the beam before entering the main interferometer. Moreover, the Faraday isolator prevents light from back-scattering into the PSL subsystem. The mode matching telescope widens the diameter of the laser while further stabilizing the frequency and isolating the TEM00mode that will be resonant between the ITMs and ETMs [7].
• COC: The core optics components subsystem consist of the two input test masses, the two end test masses, the beam splitter, and the recycling mirror [69]. The optics are made from fused silica and have specialized reflective and anti-reflective coatings applied [70].
• COS: The core optics support subsystem generates optical pick-off beams from each of the core optics and takes them outside of the vacuum so they can be used by the LSC and ASC subsystems [71].
• SUS: The suspensions subsystem controls the position of the suspended optics (input test masses, end test masses, and mode cleaner optics). These optics are suspended via a single wire that loops around the barrel of the mirror. The optics have four magnets glued to them. These magnets are used in conjunction with optical sensor and electro-magnetic (OSEM) actuators to adjust the angular and horizontal positions of the mirrors. Once the interferometer is in lock, SUS is only used to damp pendulum motion of the optics; length control is left to the LSC subsystem, and angular control is left to the ASC subsystem [8].
• LSC: The length-sensing and control subsystem receives length-sensing information from the photo- diodes and sends them to the actuators (OSEMs) on the suspensions, which adjust the longitudinal distances between the input and end test masses (ITMs and ETMs) such that the fundamental mode fulfills the required interference conditions. The common arm (CARM) signal is fed back to the ETMs at low frequency and to the frequency stabilization servo at high frequencies. Servo filters process these signals to keep stable feedback control of the loop [72]. This is the subsystem that measures and controls DARM (the error signal that is converted into a GW signal; see the following section). This subsystem operates at 16348 samples/second [8].
• ASC: The alignment-sensing and control subsystem has two main parts: 1) the initial alignment- sensing (IAS) of theopticsto configure them such that lock is possible by interfacing with the COC, COS, SUS, SEI, and IOO subsystems; 2) ASC of the cavitiesvia wavefront sensors and OSEM ac- tuation [73]. This second part (sometimes referred to as angular-sensing and control) tracks and fixes the pitch and yaw of 8 mirrors (beam splitter, ETMX, ETMY, ITMX, ITMY, two mode matching
telescopes, and recycling mirror) such that there is maximal power buildup in the Fabry-Perot cavi- ties. Wavefront sensors, quadrant photodiodes, and a camera are used to examine the laser light and its sidebands. See Figure 3.6 for the locations of these components. This information is fed into a control loop that controls the mirrors’ positions via the OSEMs. Wavefront sensors are quadrant photodiodes equipped with RF electronics; they use the Pound-Drever-Hall method to produce error signals for the control loop. Each wavefront sensor produces two channels — the in-phase and quad- phase demodulation of the input beam with the RF sidebands. The ASC subsystem operates at 2048 samples/second [8].
• PEM: The physical environmental monitors subsystem is composed of numerous seismometers, mag- netometers, accelerometers, weather stations, mains voltage monitors, temperature sensors, and an AOR radio receiver distributed throughout each LIGO site. See Figure 3.7 for the locations of these monitors [74]. Most of these sensors are passively recording information about the state of the envi- ronment, and are used later to assess data quality. The seismometer information, however, is used in the seismic isolation subsystem.
• SEI: The seismic isolation subsystem uses information from the network of seismometers to actively subtract seismic noise from the tables holding the optics. Figure 3.8 shows a seismic isolation config- uration. The configuration includes four passive isolation stacks for each core optic, but these are not controlled by a servo. In S6, the active isolation comes from the sensor and actuator (hydraulic external pre-isolator for L1 and piezoelectric pre-isolator for H1). The control loop in L1 began using Weiner feed-forward filtering in the middle of S6 [9].
• OMC: The output mode cleaner subsystem was added during S6 to support the new DC readout plan.
This subsystem removes any spurious higher-order modes that have arisen while the laser is in the interferometer arms. The OMC also removes the RF sidebands, as they are no longer necessary (and, in fact, add extra noise) for DC readout using homodyne detection. The subsystem includes several optics for beam alignment and purification, as well as photodiodes for readout; see Figure 3.9 for the locations of these components. One of the optics is outfitted with a piezoelectric actuator (for fast position correction) and another with a thermal actuator (for slow position correction). The OMC has its own vacuum and seismic isolation system, consisting of two active pendula mounted on an active isolator [10].
• TCS: The thermal compensation system, upgraded during S6, corrects for under- or over-heating of the ITMs so that their effective radius of curvature is close to the design value (otherwise the light coming back through the ITMs will not be reflected by the recycling mirror, leading to a loss of laser power).
The subsystem includes optics, a camera for each ITM, an optical imaging system, a servo, and a CO2
laser for applying the heat [52].
• CDS: The control and data system provides the closed loop control of the instruments’ servos. It is responsible for monitoring and control of the vacuum system, providing diagnostics to monitor interferometer performance, collaborate with the PSL, ASC, SUS, LSC, and SEI subsystems. It is also responsible for bringing the interferometer into “lock” [69].
• DAQ: The data acquisition subsystem records both digital and analog information from all the subsys- tems’ various sensors. The amount of data can be up to 5 Mbytes per second during S6 (this number will increase by an order of magnitude for Advanced LIGO) [69].
These subsystems are responsible for getting the detector into lock (i.e., the fundamental mode of the laser is resonant in the Fabry-Perot cavities, and the mirror positions are stable), keeping the detector under length and alignment control at its design configuration, and recording information from the various components of the detector. Each time-varying piece of information is recorded in a data channel. Data channels are described in the following subsection.
Figure 3.5: An illustration of the input optics subsystem for LIGO during S6 (enhanced LIGO). The electro- optic modulator produces the RF sidebands that are used by other subsystems; this is the last component that is outside the vacuum. The mode cleaner suppresses laser light that is not in the TEM00mode, provides frequency stabilization, and passively reduces beam jitter above 10 Hz. The Faraday isolator prevents back- propagation of the laser and provides access to the symmetric port beam for length and alignment-sensing [7].