  The Advanced LIGO gravitational wave detectors are second generation
instruments designed and built for the two LIGO observatories in Hanford, WA
and Livingston, LA. The two instruments are identical in design, and are
specialized versions of a Michelson interferometer with 4 km long arms. As in
initial LIGO, Fabry-Perot cavities are used in the arms to increase the
interaction time with a gravitational wave, and power recycling is used to
increase the effective laser power. Signal recycling has been added in Advanced
LIGO to improve the frequency response. In the most sensitive frequency region
around 100 Hz, the design strain sensitivity is a factor of 10 better than
initial LIGO. In addition, the low frequency end of the sensitivity band is
moved from 40 Hz down to 10 Hz. All interferometer components have been
replaced with improved technologies to achieve this sensitivity gain. Much
better seismic isolation and test mass suspensions are responsible for the
gains at lower frequencies. Higher laser power, larger test masses and improved
mirror coatings lead to the improved sensitivity at mid- and high- frequencies.
Data collecting runs with these new instruments are planned to begin in
mid-2015.
