Quantum-limited optical phase detection at the 10-10-rad level

Interferometric detection of gravitational waves at a level of astrophysical interest is expected to require measurement of optical phase differences of less than or equal to 10(-10) rad. A fundamental limit to the phase sensing is the statistics of photon detection-Poisson statistics for light in a coherent state. We have built a laboratory-scale interferometer to achieve and investigate the phase detection sensitivity required for the Laser Interferometer Gravitational-Wave Observatory. With 70 W of circulating power, we have obtained a phase sensitivity of 1.28 x 10(-10) rad/root Hz at frequencies above 600 Hz, limited by quantum noise. Below 600 Hz, excess noise above the quantum limit is seen, and we present our investigations into the sources of this excess. Compared with the results of previous such experiments, the phase sensitivity over the full 100-Hz-10-kHz band of interest has been improved by factors of up to 100, with a factor-of-2.5 improvement in the quantum-limited level. (C) 2002 Optical Society of America.