Optical Pathlength Fluctuations in an Interferometer Due to Residual Gas

Abstract

Planning is now underway for large gravitational wave antennae to be constructed in the next 5 years. These antennae will use ultra-precise laser interferometers to monitor gravitationally-induced motions of test mass over multi-kilometer baselines with a precision of 10⁻¹⁷m or better. With such a high precision, these interferometers are sensitive to a number of noise sources which are normally ignored in other applications of optical interferometry. This letter discusses one such noise source, the fluctuations in optical pathlength (equivalent to fluctuations in the index of refraction of the residual gas) due to the statistical variation of the number of atoms in the laser beam. Since the cost of the vacuum system is the major expense associated with the large antennae, it is important to estimate the vacuum requirements as accurately as possible.