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

An Overview of Detector Upgrades

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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