Sagnac interferometer for gravitational-wave detection.
We have investigated a zero-area Sagnac interferometer as a broad band gravitational-wave detector. Frequency response measurements of a laboratory-scale interferometer are in excellent agreement with theory. The measured contrast ratio, 0.996, is insensitive to induced birefringence, laser-frequency variation, arm imbalance, and dc mirror displacement. A near shot-noise-limited phase sensitivity of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\mathrm{rad}/\sqrt{\mathrm{Hz}}$ was measured at the interferometer's maximum sensitivity frequency, 90.9 MHz.