Polarization Sagnac interferometer with postmodulation for gravitational-wave detection

We describe a polarization Sagnac interferometer with an in-loop half-wave plate that allows signal detection at the reciprocal port of the beam split ter while maintaining the ability to detect the signal at a dark fringe. Po stmodulation and balanced heterodyne detection are used to recover the sign al. This topology is simple to control because of its common-path character istics and its collinear signal and local oscillator. The robustness of thi s scheme to amplitude and frequency fluctuations of the laser is demonstrat ed. Intraloop birefringence in this interferometer acts as a loss, reducing the power on the detector. The magnitude of this loss is discussed and exp erimentally verified. (C) 1999 Optical Society of America.