Following the initial LIGO scientific observation period, planned for 2003 through 2006, LIGO detector systems will require an upgrade to significantly improve the detection sensitivity. Such staged improvements were a central part of the original LIGO design and program plan.

LIGO consists of conventional facilities and the interferometric detectors. The LIGO facilities (sites, buildings and building systems, masonry slabs, beam tubes and vacuum equipment) have been specified, designed and constructed to accommodate future advanced LIGO detectors. The initial LIGO detectors were designed with technologies available at the initiation of the construction project. This was done with the expectation that they would be replaced with improved systems capable of ultimately performing to the limits defined by the facilities.

In parallel with its support of the initial LIGO construction, the National Science Foundation (NSF) initiated support of a program of research and development focused on identifying the technical foundations of future LIGO detectors. At the same time, the LIGO Laboratory worked with the interested scientific community to create the LIGO Scientific Collaboration (LSC) that advocates and executes the scientific program with LIGO.

The LSC, which includes the scientific staff of the LIGO Laboratory, has worked to define the scientific objectives of upgrades to LIGO. It has developed a reference design and an enhanced R&D program plan. This development has led to this proposal for construction of the Advanced LIGO upgrade following the initial LIGO scientific observing period.

In this Advanced LIGO Project Book, the definition and conceptual program plan for construction of Advanced LIGO are described. It is intended that this Project Book will be developed further and formally maintained as a working baseline definition document for Advanced LIGO.