LIGO interferometry

Interferometric gravitational wave antennas are based on enhanced Michelson interferometers. The sensitivity goal of the Laser Interferometer Gravitational-wave Observatory (LIGO) is 10^-21 rms, integrated over a 100 Hz bandwidth centered at 150 Hz. The use of Fabry-Perot cavities is essential for reaching the required sensitivity, but sets challenges for the control system which must maintain the cavities near resonance. We present the major design features of the LIGO length and frequency sensing and control system which will hold the differential length to within 5×10^-14 m of the operating point. We also highlight the restrictions imposed by couplings of noise into the gravitational wave readout signal and the required immunity against them.