The Polarization Sagnac Interferometer For Gravitational Wave Detection

Future advanced interferometric gravitational wave detectors will be limited by thermal distortions induced by high circulating power. An all-reflective configuration based on the Sagnac interferometer, presented here, is well suited to operation with high circulating power. A polarization scheme is presented that allows the interferometer to be used in a reciprocal configuration, so that static imperfections and thermally induced distortions of the beamsplitter and optics have a minimal effect on the interference contrast. The necessary low-frequency response of the interferometer requires delay-lines in the arms. To deal with the noise introduced by scattered light in the delay lines, a laser frequency sweep is presented that frequency shifts the scattered light so that it does not produce noise in the measurement band. The control requirements and alignment tolerances are calculated for the components of the detector and they are compared with the levels necessary for an alternative interferometer configuration, the Fabry-Perot Michelson, to highlight the advantages and disadvantages of the polarization Sagnac interferometer. The all-reflective delay-line polarization Sagnac interferometer design is demonstrated on a 10 m prototype interferometer with suspended optics that incorporates the laser frequency sweep to provide a shot-noise-limited phase sensitivity of Δφ=10^-9 rad Hz^-1/2 at frequencies as low as 200 Hz. Scaling this prototype to several kilometers with kilowatts of circulating power requires several technical improvements in high-power solid-state lasers, second harmonic generation, and the fabrication of large mirrors, which are likely to be made in the next 10 years.