The Polarization Sagnac Interferometer as a Candidate Configuration for an Advanced Detector

We present a delay-line polarization Sagnac (1) interferometer that utilizes the increased laser power which will likely be available to future advanced detectors (2) to greatly simplify many laser and interferometer control requirement. We describe how the signal sidebands and the local oscillator are made common path by using a polarizing beamsplitter and controlling the circulating light's polarization state. The common path nature of the interfering beams simplify alignment, reduce the interferometer control effort, and lower the laser amplitude and frequency stability requirements by 6 orders of magnitude over a frontally modulated Michelson interferometer. We present experimental verifications of the robustness of this configuration, which was implemented on a 2m tabletop interferometer The advantage of common mode noise rejection of the Sagnac interferometer due to the symmetry of the counter-propagating beam's paths is extended by utilizing the beamsplitter in a symmetric fashion. Detection of the dark fringe of interference on the symmetric port of the beamsplitter is achieved in a polarization Sagnac interferometer by using a polarization beamsplitter to split the input light into two polarization components which counter-propagate around the interferometer loop. An in-loop polarization-changing element, such as a half wave plate oriented at 45° with respect to the polarizing beamsplitter axis, swaps the polarization states of the counterpropagating beams. The reflected polarization is then transmitted upon reencountering the beamsplitter, and the transmitted polarization is then reflected. The polarization state of the output is effected by any relative phase delay between the two polarization components inside the interferometer loop. In this way the measurement of differential phase is transformed into a measurement of polarization ellipticity. The polarization state of the output beam is given by the Jones vector representing transmission through the entire optical system