Absolute-length determination of a long-baseline Fabry-Perot cavity by means of resonating modulation sidebands

A new method has been demonstrated for absolute-length measurements of a lo ng-baseline Fabry-Perot cavity by use of phase-modulated light. This method is based on determination of a free spectral range (FSR) of the cavity fro m the frequency difference between a carrier and phase-modulation sidebands , both of which resonate in the cavity. Sensitive response of the Fabry-Per ot cavity near resonant frequencies ensures accurate determination of the F SR and thus of the absolute length of the cavity. This method was applied t o a 300-m Fabry-Perot cavity of the TAMA gravitational wave detector that i s being developed at the National Astronomical Observatory, Tokyo. With a m odulation frequency of similar to 12 MHz, we successfully determined the ab solute cavity length with resolution of 1 mu m (3 X 10(-9) in strain) and o bserved local ground strain variations of 6 x 10(-8). (C) 1999 Optical Soci ety of America OCIS codes: 120.2230, 120.3180, 120.3940, 120.5060.