Interferometric gravitational-wave antennas are based on Michelson interferometers whose sensitivity to small differential length changes has been enhanced by the addition of multiple coupled optical resonators. The use of optical cavities is essential for reaching the required sensitivity but sets challenges for the control system, which must maintain the cavities near resonance. The goal for the strain sensitivity of the Laser Interferometer Gravitational-Wave Observatory (LIGO) is 10^-21 rms, integrated over a 100-Hz bandwidth centered at 150 Hz. 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 x10^-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.
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