Advanced LIGO subsystems
are the organizational units of the overall project. Follow the links below to view the mission and progress of each subsystem.

Auxiliary Optics Core Optics
Data Acquisition Data and
Input Optics

A Comprehensive Overview of Advanced LIGO


Context and Summary

Gravitational waves offer a remarkable opportunity to see the universe from a new perspective, providing access to astrophysical insights that are available in no other way. The Advanced LIGO detector upgrade, completed in March 2015, enabled the first detections of gravitational waves in September 2015. The instruments are still undergoing commissioning, but will uttimately be more than ten times more sensitive, and over a much broader frequency band, than initial LIGO, seeing a volume of space more than a thousand times greater than initial LIGO, and extending the range of compact masses that can be observed at a fixed signal strength by a factor of four or more.

This proposal to build Advanced LIGO has grown out of the LIGO Scientific Collaboration and has broad support both nationally and internationally from that community.

A closely coordinated community R&D program, exploring the instrument science and building and testing prototype subsystem elements, helped bring the design to fruition. The LIGO laboratory led and coordinated the fabrication and construction of the instruments, with the continued strong participation of the community. The joint United Kingdom/German GEO Project made substantial contributions to this construction project. The UK participants provided the suspension subsystem, including suspension assemblies, their controls, and installation and commissioning. The German participants undertook  the design and fabrication of the pre-stabilized laser subsystem. The GEO Project is a full partner in Advanced LIGO, participating at all levels in the effort.

Australian groups are also made capital contributions to Advanced LIGO. A consortium of Australian National University and the University of Adelaide profided  Hartmann phase sensors, a pre-lock length stabilization system, and specialized beam-pointing equipment to Advanced LIGO. ANU and Adelaide are full partners in Advanced LIGO.


The LIGO Mission

From its outset, LIGO has been approved by the National Science Foundation to directly observe gravitational waves from cosmic sources, and to open the field of gravitational wave astronomy. The program and mission of the LIGO Laboratory is to:

LIGO is envisioned as a new capability contained in a set of facilities and not as a single experiment. The LIGO construction project provided the facilities that support the scientific instrumentation, and the initial set of laser interferometers to be used in the first LIGO scientific observation periods. The facilities include the buildings and vacuum systems at the two observatory sites. The two observatories are located at Hanford, Washington and Livingston, Louisiana. The performance requirements on the LIGO facilities were intended to accommodate the initial interferometers and future interferometer upgrades and replacements, and possible additional interferometers with complementary capabilities. The requirements on the LIGO facilities were intended to permit future interferometers to reach levels of sensitivity approaching the ultimate limits of ground-based interferometers, limited by the practical constratints on a 4km large facility at a specific site. Advanced LIGO represents the second generation of instruments to be installed in the LIGO infrastructure, and will take the science of gravitational radiation from the discovery mode to a mode of regular astrophysical observation.

LIGO Detector Scientific Goals

The scientific program for LIGO is both to test relativistic gravitation and to open the field of gravitational wave astrophysics. More precise tests of General Relativity (and competing theories) will be made. LIGO will enable the establishment of a brand new field of astronomy, using a completely new information carrier: the gravitational field.

Once fully commissioned, the Advanced LIGO detectors will be able to see inspialing binaries made up of two 1.4 M neutron stars to a distance of 300 Mpc, some 15x further than the initial LIGO, and giving an event rate some 3000x greater. Neurron star - black hole (BH) binaries will be visible to 650 Mpc; and coalescing BH+BH systems will be visible cosmological distance, to z=0.4. The existence of gravitational waves is a crucial prediction of the General Theory of Relativity. That theory makes a number of unambiguous predictionas about the character of gravitational radiation. These can be verified by observations with LIGO. These inclde probes of strong-field gravity associated with black holes, high-order post-Newtonian effects in inspiraling binaries, the spin character of the radiation field, and the wave propagation speed.

Since many prospective gravitational wave sources have no corresponding electromagnetic signature (e.g., black hole interactions), there are good reasons to believe that the gravitational-wave sky will be substantially different from the electromagnetic one. Maping the gravitational-wave sky will profide an understanding of the universe in a way tht electromagnetic observations cannot. As a new field of astrophysics it is quite likely that gravitational wave observations will uncover new clases of sources not anticipated in our current thinking.


Detector Design Fundamentals

The effect of a propagating gravitational wave is to deform space in a quadrupolar form. The effect alternately elongates space in one direction while compressing space in an orthogonal direction and vice versa, with the frequency of the gravitational wave. A Michelson interferometer operating between freely suspended masses is ideally suited to detect these antisymmetric distortions of space induced by the gravitational waves; the strains are converted into changes in light intensity and consequently to electrical signals via photodetectors.

Limitations to the sensitivity come from two sources: extraneous forces on the test masses, and a limited ability to sense the response of the masses to the gravitational wave strain. The thermally excited motion of the test mass and the suspension is a fundamental limitation, intrinsic to the way in which the measurement is performed; this influence is managed through the selection of low-mechanical-loss materials and designs which capitalize on them. Seismic motion causes forces on the mirrors due to the direct coupling through the isolation and suspension system, a technical noise source which is minimized through design; and due to the time-varying mass distribution near the mass (the Newtonian background).

Sensing limitations arise most fundamentally due to the statistical nature of the laser light used in the interferometry, and the momentum transferred to the test masses by the photons (linking the sensing and stochastic noise limitations to sensitivity). Technical noise sources that limit the ability to sense include frequency noise and intensity fluctuations in the laser light. Scattered light, which adds random phase fluctuations to the light, can also mask gravitational signals. In the limit, valid for LIGO, that the instrument is short compared with the gravitational wavelength, longer arms give larger signals. In contrast, most competing noise sources remain constant with length; this motivates the 4km baseline of the Observatories. More generally, the scientific capability of LIGO is defined within the limits imposed by the physical settings of the interferometers and by the facility design, by the design of the initial detectors and ultimately by future interferometers designed to progressively exploit the facility capabilities.

Although the rates for gravitational wave sources have large uncertainty, an improvement in strain sensitivity linearly improves the distance searched for detectable sources. This increases the detection rate by the cube of the sensitivity improvement.

The Observatories

LIGO Facility Scientific Capability

The LIGO facility design envisaged a progression of increasingly sensitive interferometers capable of extending the physics reach of the observatories. In the design of the observatories, LIGO incorporated critical design features into its facilities in order to optimize LIGO's ultimate performance capabilities. These features include a building foundation and infrastructure which provides a clean, quiet environment for the instruments; a 4km long "L" ultra-high vacuum beam tube system that brings scattered light and index fluctuations due to residual gas to a negligible level; and a system of large vacuum chambers and pumping subsystems capable of providing a flexible envelope for a wide range of detector designs, and delivering a vacuum quality that complements the beam tube subsystem. Advanced LIGO requires no changes in this infrastructure to meet its scientific goals.

The LIGO Observatories

LIGO Hanford Observatory (LHO), located on the U.S. Department of Energy Hanford site in eastern Washington, comprises 5 major experimental halls for the interferometer spread over 5 miles. 1.2-m diameter ultrahigh vacuum tubing connects these halls. Three support buildings house laboratories, offices, and an amphitheater, and two additional buildings are associated with maintenance and operations. Approximately 90,000 square feet of this space is under tight environmental control to minimize contamination of sensitive equipment. The physical plant has been designed to provide a low vibration environment similar to the surrounding undeveloped shrub-steppe environment.

Figure 1:  LIGO Hanford Observatory (LHO) in aerial view. The 4-km interferometer arms are shown with the 5 main buildings along the orthogonal arm layout

Figure 2:  LIGO Livingston Observatory (LLO) corner region in aerial view.

The LIGO Livingston Observatory, located in pine forests between Baton Rouge and New Orleans, Louisiana, is the site of a single 4-km laser interferometer gravitational wave detector. The beam tube dimensions are identical to those at LHO. The instrument design, and sensitivity, is the same as LHO.

Initial LIGO

The NSF Cooperative Agreement of May 1992 initiated LIGO Construction and Construction Related Research and Development. The Project schedule and cost estimates were reviewed by the NSF during September 1994 and presented to the National Science Board in November 1994. The LIGO construction effort was completed, on cost and close to schedule. 'First lock' of the initial LIGO instruments was acheived in 2000. All of the instruments met the sensitivity goal fo rinitial LIGO of an RMS stranin sensitivity of 10-21 in a 100 Hz band in 2005. An integrated year of data was taken an analyzed for gravitational-wave signals; none were found, but in the process both challenging upper limits and astrophysically interesting non-detectins were made.


LIGO Scientific Collaboration

A fundamental goal of LIGO has been to become a true national facility available to the scientific community. In order to accomplish this, LIGO has broadened the participation to include the community of scientists interested in participating in the LIGO research program by creating the LIGO Scientific Collaboration (LSC). There are now some 650 members from 59 institutions in 11 countries. The LSC consists of both LIGO Laboratory scientists and those from collaborating groups. The LSC is organized so as to provide "equal scientific opportunity" to all members whether they are from within LIGO Laboratory or the broader LSC. It is growing steadily and will remain open to new members over the coming years. The international partners are involved in all aspects of the LIGO research program.

The full LSC collaboration meets twice yearly in an extended meeting, and various working groups meet more frequently. The LSC has produced White Papers that outline the plans for technical development of LIGO and for science data analysis. A publication policy and a conference committee are active, as well as the other functions necessary to make it a "full service" organization. the LSC works closely with the Virgo Collaboration, and the data from the LIGO detectors is combined with that of the Virgo detector (located near Pisa, Italy) with researchers from both collaborations sharing the analysis effort.

The Advanced LIGO design, both in basic conception and in the detailed R&D, is very much a product of the LSC (with a strong LIGO Laboratory element). The technical working groups have been and continue to be central to the advancement of the design, and this proposal is made with the strong support of the many participating institutions in the LSC.

LIGO has been organized such that the search for astrophysical signals and interpretations will be performed through the LSC. Preparation tasks for the runs are organized within the LSC, LSC members participate in the data taking runs, and the analysis of the data is coordinated through the LSC proposal driven process. LIGO is available to all interested researchers through participation in the LSC, an open organization. To join, a research group defines a research program with the LIGO Laboratory through the creation of a Memorandum of Understanding (MOU) and relevant attachments. The group then presents its program to the LSC. When the group is accepted into the LSC it becomes a full scientific partner in LIGO.


Overview of Advanced LIGO

The sensitivity goals for the Advanced LIGO detector systems are chosen to enable the advance from plausible detection to likely detection and rich observational studies of sources. These sensitivity goals require an instrument limited only by fundamental noise sources over a very wide frequency range. To achieve this sensitivity, almost every aspect of the interferometer must be revised from the initial LIGO design. The system briefly described below is the reference concept that is the basis for structuring the R&D program and the detailed studies of system tradeoffs performed as R&D results define the feasible parameters. A more complete description of the proposed detector, organized by subsystem, is found in the Advanced LIGO Reference Design. While still preliminary and subject to change, the curves for the strain sensitivity for various modes of operation can be found at Advanced LIGO anticipated sensitivity curves.

The basic optical configuration is a power-recycled and signal-recycled Michelson interferometer with Fabry-Perot "transducers" in the arms. Using the initial LIGO design as a point of departure, this requires the addition of a signal-recycling mirror at the output "dark" port, and changes in the interferometer readout and control systems. This additional mirror allows the gravitational wave induced sidebands to be stored or extracted (depending upon the state of "resonance" of the signal recycling cavity), and leads to a tailoring of the interferometer response according to the character of a source (or specific frequency in the case of a fixed-frequency source). The upgrade includes the three LIGO interferometers, allowing e.g., one interferometer at Hanford and the interferometer at Livingston to be tuned to be broadband, and the second interferometer at Hanford to be used as a higher-frequency narrowband detector.

To improve the quantum-limited sensitivity, the laser power is increased from the initial LIGO value of 10 W to ~200 W. The conditioning of the laser light follows initial LIGO closely, with a ring-cavity mode cleaner and reflective mode-matching telescope.

Whereas initial LIGO uses 25-cm, 11-kg, fused-silica test masses, the fused silica test mass optics for Advanced LIGO are larger in diameter (~34 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. 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 power - of the order of 1 MW in the arm cavities. The test mass is suspended by fused silica fibers, in contrast to the steel wire sling suspensions used in initial LIGO. The resulting suspension thermal noise is anticipated to be less than the radiation pressure noise (in broad-band observation mode) and to be comparable to the Newtonian background ("gravity gradient" noise) at 10 Hz. The complete suspension has four pendulum stages, contributing to the seismic isolation and providing multiple points for actuation.

The seismic 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 (for initial LIGO) to 10 Hz. RMS motions (frequencies less than 10 Hz) are reduced by active servo techniques. The result is to render the seismic noise negligible at all observing frequencies. Through the combination of the seismic isolation and suspension systems, the required control forces on the test masses will be reduced by many orders of magnitude in comparison with initial LIGO, reducing also the probability of non-Gaussian noise in the test mass.

The overall performance of Advanced LIGO is dominated at most frequencies by the quantum noise of sensing the position of the test masses, with a contribution at mid-frequencies from the internal thermal noise of the test masses. This design, with modest enhancements after it enters scientific use, should take this interferometer architecture to its technical endpoint; it is as sensitive as one can make an interferometer based on familiar technology: a Fabry-Perot Michelson configuration with external optical readout using room temperature transmissive optics. Further advances will come from R&D that is just beginning, such as the exploration of cryogenic optics and suspensions, purely reflective optics, and a change in the readout to one which fully exploits our understanding of the quantum nature of the measurement (e.g., quantum non-demolition speed meters). These later developments will be timely for instruments to be developed in the second decade of this century.





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