LIGO: The Laser Interferometer Gravitational-Wave Observatory

TL;DR

The paper provides a comprehensive overview of LIGO’s design, operation, and scientific potential for gravitational-wave detection. It details the Michelson-Fabry-Perot-Mylarized interferometer architecture, the laser and optics chain, vibration isolation, sensing and control schemes, calibration, and environmental monitoring, together with a thorough noise-budget analysis that identifies displacement and sensing noise as primary limits. It summarizes the data-analysis infrastructure and the four main GW search categories—compact binary coalescences, bursts, continuous waves, and stochastic backgrounds—reporting upper limits from the early S5 data and horizon distances for key sources, while highlighting the network's capabilities for vetoing artifacts and combining data across detectors. Finally, the paper discusses near-term upgrades (Enhanced LIGO) and the more ambitious Advanced LIGO plan to achieve at least an order-of-magnitude improvement in sensitivity, promising to turn gravitational-wave detection into a robust observational science with rich astrophysical returns.

Abstract

The goal of the Laser Interferometric Gravitational-Wave Observatory (LIGO) is to detect and study gravitational waves of astrophysical origin. Direct detection of gravitational waves holds the promise of testing general relativity in the strong-field regime, of providing a new probe of exotic objects such as black hole and neutron stars, and of uncovering unanticipated new astrophysics. LIGO, a joint Caltech-MIT project supported by the National Science Foundation, operates three multi-kilometer interferometers at two widely separated sites in the United States. These detectors are the result of decades of worldwide technology development, design, construction, and commissioning. They are now operating at their design sensitivity, and are sensitive to gravitational wave strains smaller than 1 part in 1E21. With this unprecedented sensitivity, the data are being analyzed to detect or place limits on gravitational waves from a variety of potential astrophysical sources.

Figures (14)

• Figure 1: A gravitational wave traveling perpendicular to the plane of the diagram is characterized by a strain amplitude $h$. The wave distorts a ring of test particles into an ellipse, elongated in one direction in one half-cycle of the wave, and elongated in the orthogonal direction in the next half-cycle. This oscillating distortion can be measured with a Michelson interferometer oriented as shown. The length oscillations modulate the phase shifts accrued by the light in each arm, which are in turn observed as light intensity modulations at the photodetector (green semi-circle). This depicts one of the linear polarization modes of the GW.
• Figure 2: Aerial photograph of the LIGO observatories at Hanford, Washington (top) and Livingston, Louisiana (bottom). The lasers and optics are contained in the white and blue buildings. From the large corner building, evacuated beam tubes extend at right angles for 4 km in each direction (the full length of only one of the arms is seen in each photo); the tubes are covered by the arched, concrete enclosures seen here.
• Figure 3: Optical and sensing configuration of the LIGO 4 km interferometers (the laser power numbers here are generic; specific power levels are given in Table 1). The IO block includes laser frequency and amplitude stabilization, and electro-optic phase modulators. The power recycling cavity is formed between the PRM and the two ITMs, and contains the BS. The inset photo shows an input test mass mirror in its pendulum suspension. The near face has a highly reflective coating for the infrared laser light, but transmits visible light. Through it one can see mirror actuators arranged in a square pattern near the mirror perimeter.
• Figure 4: Schematic layout of the frequency stabilization servo. The laser is locked to a fixed-length reference cavity through an AOM. The AOM frequency is generated by a Voltage Controlled Oscillator (VCO) driven by the MC, which is in turn driven by the common mode arm length signal from the REF port. The laser frequency is actuated by a combination of a Pockels Cell (PC), piezo actuator, and thermal control.
• Figure 5: Antenna response pattern for a LIGO gravitational wave detector, in the long-wavelength approximation. The interferometer beamsplitter is located at the center of each pattern, and the thick black lines indicate the orientation of the interferometer arms. The distance from a point of the plot surface to the center of the pattern is a measure of the gravitational wave sensitivity in this direction. The pattern on the left is for $+$ polarization, the middle pattern is for $\times$ polarization, and the right-most one is for unpolarized waves.