Results from the Stanford 10 m Sagnac interferometer

The design of a 10 m all-reflective prototype Sagnac interferometer with suspended optics is described and the experimental results are presented. It uses a polarization scheme to allow detection of the dark fringe on the symmetric port of the beamsplitter for optimal 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 frequency shifts the scattered light so that it does not produce noise near zero frequency. This results in a shot-noise-limited phase sensitivity of delta_phi = 1.6 x 10^-9 rad Hz^-1/2 at frequencies as low as 200 Hz. Scaling this prototype to several kilometres 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.