Reference Design Baseline Definition

The LIGO Scientific Collaboration, through its Working Groups, has worked with the LIGO Laboratory to identify a reference design for the Advanced LIGO detector upgrade. The reference design represents a dramatic improvement over the initial complement of LIGO instruments. The reference design is planned to lead to a quantum noise limited interferometer array with considerably increased bandwidth and sensitivity.

The basic optical configuration is a power-recycled and signal-recycled Michelson interferometer with Fabry-Perot "transducers" in the arms; see Figure 1. Using the initial LIGO design as a point of departure, Advanced LIGO requires the addition of a signal-recycling mirror at the output "dark" port, and changes in the RF modulation and control systems. This additional mirror allows the gravitational wave induced sidebands to be stored in the arm cavities or extracted (depending upon the state of "resonance" of the signal recycling cavity), and allows one to tailor the interferometer response according to the character of a source (or specific frequency in the case of a fixed-frequency source). For wideband tuning, "quantum noise" dominates the instrument noise sensitivity at most frequencies (see Figure 2). Additional details may be found in Interferometer Sensing and Controls Subsystem (ISC) section.

Figure 1 Schematic of an Advanced LIGO interferometer, with representative mirror reflectivities optimized for neutron star binary inspiral detection. Several new features compared to initial LIGO are shown: more massive, sapphire test masses; 20´ higher input laser power; signal recycling; active correction of thermal lensing; an output mode cleaner. (ETM = end test mass; ITM = input test mass; PRM = power recycling mirror; SRM = signal recycling mirror; BS = 50/50 beam splitter; PD = photodetector; MOD = phase modulation). Mode-matching and beam-coupling telescopes not shown.

The laser power is increased from 10 W to 100-200 W, chosen to be optimal for the desired interferometer response, given the quantum limits and limits due to available optical materials. The resulting circulating power in the arms is roughly 0.5 MW, in comparison with the initial LIGO value of ~10 kW. The Nd:YAG pre-stabilized laser design resembles that of initial LIGO, but with the addition of a more powerful output stage; see Prestabilized Laser Subsystem (PSL) section. The conditioning of the laser light also follows initial LIGO closely, with a ring-cavity mode cleaner and reflective mode-matching telescope, although changes to the modulators and isolators must be made to accommodate the increase in power; see Input Optics Subsystem (IO).

Whereas initial LIGO uses 25-cm diameter, 11-kg, fused-silica test masses, the test mass optics for Advanced LIGO are larger in diameter (~32 cm) to reduce thermal noise contributions and more massive (~-40 kg) to keep the radiation pressure noise to a level comparable to the suspension thermal noise. Two materials are under study: sapphire and fused silica, and both can be configured to lead to a satisfactory LIGO upgrade. The baseline choice for the core optics substrate material is sapphire. Sapphire promises superior sensitivity for the measured material parameters, and full-size samples are now under characterization. The beamsplitter and other suspended optics, where thermal noise is less important, are made of fused silica. Polishing and coating are not required to be significantly better than the best results seen for initial LIGO; see Section 10, Core Optics Components (COC) section. Compensation of the thermal lensing in the test mass optics (due to absorption in the substrate and coatings) is added to handle the much-increased circulating power - of the order of 1 MW in the arm cavities; see Auxiliary Optics Subsystem (AOS).

The test mass is suspended by fused silica ribbons or tapered fibers attached with hydroxy-catalysis bonds, in contrast to the steel wire sling suspensions used in initial LIGO. Fused silica has much lower loss (higher Q) than steel, and the fiber geometry allows more of the energy of the pendulum to be stored in the earth’s gravitational field while maintaining the required strength, thereby reducing suspension thermal noise. The resulting suspension thermal noise is anticipated to be less than the radiation pressure noise and comparable to the Newtonian background (“gravity gradient noise“) at 10 Hz. The complete suspension has four pendulum stages, and is based on the suspension developed for the UK-German GEO-600 detector. The mechanical control system relies on a hierarchy of actuators distributed between the seismic and suspension systems to minimize required control authority on the test masses. The test mass magnetic actuators used in the initial LIGO suspensions are eliminated (to reduce thermal noise and direct magnetic field coupling from the permanent magnet attachments) in favor of electrostatic forces for locking the interferometer and photon pressure for the operational mode. The much smaller forces on the test masses reduce the likelihood of compromises in the thermal noise performance and the risk of non-Gaussian noise. Local sensors (electrostatic and occultation) and magnets/coils are used on the top suspension stage for damping, orientation, and control; see Suspension Subsystem (SUS).

The isolation system is built on the initial LIGO piers and support tubes but otherwise is a complete replacement, required to bring the seismic cutoff frequency from ~40 Hz (initial LIGO) to ~10 Hz. RMS motions (dominated by frequencies less than 10 Hz) are reduced by active servo techniques, and control inputs complement those in the suspensions in the gravitational-wave band. The attenuation offered by the combination of the suspension and seismic isolation system eliminates the seismic noise contribution to the performance of the instrument, and for the low-frequency operation of the interferometer, the Newtonian background noise dominates. See Seismic Isolation Subsystem (SEI).

Reference Design Parameters

The Advanced LIGO reference design is summarised in Table 1.

Reference Design Sensitivity Goal

The anticipated improvement in the performance of the reference design detector for wideband tuning is indicated in Figure 1 (equivalent strain noise as a function of frequency). This instrument is designed to deliver an improvement over initial LIGO in the rms noise and limiting sensitivity by a factor of more than 10 over a very broad frequency band. This translates into an increase of event rate by more than 1000 for extragalactic sources, so that several hours of operation will exceed, in physics reach, the integrated observations of the 1-year initial-LIGO Science Run. These Advanced LIGO interferometers will also have a greater frequency range with both a reduced lower cutoff (10 Hz vs. 40 Hz) and a better high frequency performance (~8 times greater in frequency for comparable sensitivity). Finally, they will have the capability for a reshaping of the noise curve. This allows e.g., narrowbanding with much enhanced sensitivity near some chosen frequency as shown in Figure 2.

Figure 2 Noise Anatomy of Advanced LIGO. This model of the noise performance is based on our current requirements set, and represents the principal contributors of the noise and the least-squares sum of those components expressed as an equivalent gravitational wave strain.

At the initial LIGO sensitivity, it is plausible but not probable that gravitational waves will be detected. With Advanced LIGO it is probable to detect waves from a variety of sources and extract rich information from them. Specifically (cf., Figure 3), Advanced LIGO is capable of the following science:

• Inspiraling neutron star (NS) and black hole (BH) binaries: 1.4 M NS+NS binaries will be detectable to a distance of 300 Mpc (estimated event rate ~2/yr to 3/day); 1.4 MNS+10 MBH, detectable to 650 Mpc (estimated ~1/yr to 4/day); 10 M BH+BH, detectable to redshift z=0.4 (estimated ~1/mo to 30/day - if black holes form in completely symmetric events, then none will be seen, but this possibility is actually not supported by current astronomical observations). The inspiral waves will reveal the bodies’ masses and spins and will enable precision tests of general relativity at far higher post-Newtonian order than is possible today [6 orders higher in (orbital speed) /(speed of light).] New relativistic effects will be seen, e.g., radiation reaction due to tails of waves and perhaps even tails of tails.
• Tidal disruption of a NS by a BH: When the NS in a NS+BH binary nears its black-hole companion, it can be torn apart by the hole’s spacetime curvature. The disruption waves should carry information about the NS structure and equation of state. Extracting this information will require three interferometers: two operating in wideband mode to measure the inspiral waves and deduce from them the BH and NS masses and spins, and one with noise curve optimized for the high-frequency (~300 to ~1000 Hz) disruption waves. This 3-interferometer configuration can also seek NS equation-of-state information by measuring the influence of tidal coupling on the wave spectrum from inspiraling NS+NS binaries.
• BH+BH mergers and ringdowns: When rapidly spinning BH’s collide, they should trigger large-amplitude, nonlinear oscillations of curved spacetime around their merging horizons. Little is known about the dynamics of spacetime under these extreme circumstances; we can learn about it by comparing LIGO’s observations of the emitted waves with supercomputer simulations. Advanced LIGO can detect the merger waves from BH binaries with total mass as great as 2000 M, to cosmological redshifts as large as z=2.
• Supernovae: Empirical evidence suggests that neutron stars in type II supernovae receive kicks of magnitude as large as ~1000 km/s. These violent recoils imply the supernova’s collapsing-core trigger may be strongly asymmetric, emitting waves that might be detectable out to the Virgo cluster of galaxies (event rate a few/yr) and perhaps beyond. Even when the collapse is spherical and emits no waves, the collapsed core (proto-neutron star) is predicted to be unstable to convective overturn. The gravitational waves from this convection may be detectable throughout our Galaxy and its orbiting companions, the Magellanic Clouds. By cross correlating the gravitational waves with neutrinos from just one such (very rare) event, we could learn much about the proto-neutron star’s convecting core.
• Gamma-ray bursts: The triggers of gamma ray bursts are thought to be the collapse of massive stellar cores (hypernovae) and/or the merger of NS+NS or NS+BH binaries, all of which emit strong gravitational waves. The next generation of orbiting gamma-ray telescopes will be operational in the time frame of Advanced LIGO, providing astrophysical triggers for LIGO’s searches. With the aid of these triggers, and with predicted enhancements of the gravitational waves along the burst’s beaming direction (toward earth), estimates suggest coincident detections of a few per year. Any such detection would reveal the nature of the gamma-burst trigger. The third interferometer, with noise curve reshaped for better sensitivity at high frequencies, may enable observations of the trigger’s dynamics.
• Spinning neutron stars: The narrowband tunability of the third interferometer will be exploited to search with high sensitivity at high frequencies for gravitational radiation arising from spinning NS’s: known pulsars and Low-Mass X-Ray Binaries (LMXB’s), and unknown pulsars. If (as is plausible) a NS’s accretion torque, in an LMXB, is counterbalanced by its gravitational radiation-reaction torque, then its wave strength is predictable from the observed X-ray flux, and about 10 known LMXB’s would be detectable by Advanced LIGO with narrow-banding (the dots near the minimum of the narrow-band curve) but only one (Sco X-1, the star in Figure 3) without narrow-banding. These LMXB’s may serve as "calibration sources" for LIGO. A NS’s crustal shear or internal magnetic field is predicted to be able to support non-axisymmetric ellipticities as large as e~10^-6 or even 10^-5. A narrowbanded interferometer could detect a known millisecond pulsar with e as small as 2x10^-8(1000Hz/f)²;(r/10kpc), where f is the wave frequency (most likely twice the spin frequency) and r is the distance. In an all-sky, all-frequency search the sensitivity would be degraded by a factor of a few to ~15.
• Stochastic Waves: The sensitivity improvement of Advanced LIGO, coupled with the decrease in lower frequency cutoff, means that an observational measurement of the stochastic gravitational wave background can be performed with a sensitivity after 1 year of observation of WGW~5x10^-9 (WGW is the ratio of the stochastic gravity-wave energy density contained in a bandwidth Df = f to the total energy density required to close the universe; a flat spectrum is assumed). The sources of such background in the LIGO band are all highly speculative and could be weaker than 5x10^-9 if they exist at all, but also might be stronger and detectable. Some examples can be given: cosmic strings and other topological defects in the structure of spacetime, first-order phase transitions in the states of quantum fields at temperature ~109 K in the very early universe, Goldstone modes of scalar fields that arise in supersymmetric and string theories, coherent excitations of our 3+1 dimensional universe, regarded as a brane in a higher dimensional universe, and the birth of the universe as described by string-motivated "pre-big-bang" cosmology.
• The Unexpected: We are very ignorant of the gravitational universe, and it seems quite probable that Advanced LIGO’s observations will bring some significant surprises.

Figure 3 The estimated signal strengths hs(f) from various sources (thin lines, filled circles and star) compared with the noise h(f) (heavy lines) of three interferometers: initial LIGO, Advanced LIGO in a wideband (WB) mode, and Advanced LIGO narrowbanded (NB) at 600 Hz. See text for explanations of sources. The signal strength hs(f) is defined in such a way that, wherever a signal point or curve lies above the interferometer's noise curve, the signal, coming from a random direction on the sky and with a random orientation, is detectable with a false alarm probability of less than one per cent using currently understood data analysis algorithms.

Reference Design Options and Selection

The Advanced LIGO reference design has as its baseline that all three LIGO interferometers will be upgraded as described. It assumes, furthermore, that the upgrades will produce identical interferometers, though they may be run with different detailed parameters such as output laser power and different signal tuning and signal-recycling mirror transmission. The principal options for the reference design are described below.

Number of Upgraded Interferometers

The upgrade could be restricted to a single interferometer at each LIGO site. The Hanford 2-kilometer interferometer could be retained in its present configuration or decommissioned. However, in the discovery phase of LIGO observations, prior to confirmed observation of gravitational waves, the third interferometer may provide additional confidence and an increase of the volume of the universe that LIGO can see by as much as 50%; in the phase after initial detections, an additional interferometer could be tuned and used in combination with the other LIGO instruments and with other networked detectors to significant astrophysical advantage. If the upgrade of the third interferometer is dropped from the scope of the Advanced LIGO project, it will reduce the costs and required resources.

2-Kilometer Interferometer Upgraded but Not Converted to 4 Kilometer Length

This option could be employed if it is felt that a half-size gravitational wave signal is useful in separating genuine signals and that retaining this feature outweighs the advantages of increasing sensitivity that accompanies an increase in arm length. At this time, the improved sensitivity of the longer interferometer is compelling, and we choose to increase the arm cavity length in the reference design. If extending the arm cavity is dropped from the scope of this upgrade, the costs and resources required will be modestly reduced from those required in the baseline design.

Simultaneous Implementation of the Upgrade

Our baseline plan calls for a staged implementation of the upgrade, in which the Livingston instrument installation is started first, with the installation at Hanford to follow by 8 months. This distributes both fabrication and installation demands over a reasonable period. An alternative would be to engage in a simultaneous installation at the two observatories. This would stress the manpower and the facilities, and would require some duplication of installation equipment. It would potentially reduce the duration during which the pair of LIGO observatories is "off-line." Simultaneous implementation may increase the costs, resources and schedule required to complete the Advanced LIGO upgrade.

Test Mass Substrate Material

Sapphire is selected as the substrate material in this reference design. It offers significant advantages in reducing thermal noise and in control of thermal distortions on the optics. It requires greater development and carries greater risk than fused silica in crystal growth, cost, optical performance, polishing and coating. Our program will carry fused silica as a fallback option, with some impact on the detector sensitivity, with a well-defined date for confirmation of sapphire or adoption of fused silica for the baseline. If sapphire is dropped from the baseline reference design, the costs, schedule and resources required for Advanced LIGO will likely be unchanged.

Future Incremental Upgrades to Advanced LIGO

The Reference Design balances technical challenges and improved performance. The stability of the design through the intensive R&D effort to date has demonstrated its robustness. The design is, however, flexible and can accommodate all foreseeable improvements to this type of detector; a room-temperature, transmissive-optic, Fabry-Perot Michelson. Some examples that have been explored are as follows:

• Quantum non-demolition techniques: The baseline sensing system experiences the stored light as an "optical spring" which helps to reduce the quantum noise below the naïve limit. Modifications to the interferometer’s input and/or output port fields may allow further reduction of quantum noise.
• Newtonian background cancellation: The changes in mass distribution near the test masses (primarily due to seismic noise) appear as a low-frequency noise limit. Monitoring this motion with an array of seismometers may allow a regression or cancellation to observe at lower frequencies.
• Non-Gaussian laser light profiles: The thermal motion of the mirror surface, especially for the thermoelastic noise which dominates in the case of sapphire, has a smaller net effect for larger light beams. Introduction of slightly non-spherical end test masses would lead to non-Hermite-Gaussian modes with a larger waist that could reduce this noise source, giving better sensitivity at intermediate frequencies.
• Variable reflectivity signal recycling mirror: The tunability of the interferometer response is limited with a fixed transmission signal-recycling mirror. Forming a low-finesse output-coupling cavity from a substrate coated on both sides could allow a thermally tuned output coupler, giving a broader range of instrument response functions.

These options may be proposed after observation with the present baseline design for Advanced LIGO. Some may be able to be incorporated into the design shortly after, or coincident with, the commissioning of the baseline. For example, the variable reflectivity signal-recycling mirror has been proposed as an Advanced LIGO enhancement from the Australian consortium ACIGA.