Polarization Sagnac interferometer with a common-path local oscillator forheterodyne detection

We describe a free-space common-path polarization Sagnac interferometer. Th e polarization Sagnac interferometer is used in a symmetric fashion with de tection on the dark fringe of the interference to avoid photodetector satur ation when high-powered illumination is used. By modulating the carrier fie ld that is reflected from the interferometer, we generate a local oscillato r for heterodyne detection of the signal field. We calculate the effect of optical element misalignments and imperfect polarization on the shot-noise- limited sensitivity of the interferometer. The predictions are experimental ly verified with a 2-m arm-length tabletop polarization Sagnac interferomet er. (C) 1999 Optical Society of America [S0740-3224(99)02609-0].