ALIGNMENT OF AN INTERFEROMETRIC GRAVITATIONAL-WAVE DETECTOR

Interferometric gravitational wave detectors are designed to detect sm all perturbations in the relative lengths of their kilometer-scale arm s that are induced by passing gravitational radiation. An analysis of the effects of imperfect optical alignment on the strain sensitivity o f such an interferometer shows that to achieve maximum strain sensitiv ity at the Laser Interferometer Gravitational Wave Observatory require s that the angular orientations of the optics be within 10(-8) rad rms of the optical axis, and the beam must be kept centered on the mirror s within 1 mm. In addition, fluctuations in the input laser beam direc tion must be less than 1.5 x 10(-14) rad/root Hz in angle and less tha n 2.8 x 10(-10) m/root Hz in transverse displacement for frequencies f > 150 Hz in order that they not produce spurious noise in the gravita tional wave readout channel. We show that seismic disturbances limit t he use of local reference frames for angular alignment at a level appr oximately an order of magnitude worse than required. A wave-front sens ing scheme that uses the input laser beam as the reference axis is pre sented that successfully discriminates among all angular degrees of fr eedom and permits the implementation of a closed-loop servo control to suppress the environmentally driven angular fluctuations sufficiently . (C) 1998 Optical Society of America.