title: Heterodyne interferometer with subatomic periodic nonlinearity.
abstract: A new, to our knowledge, heterodyne interferometer for differential displacement measurements is presented. It is, in principle, free of periodic nonlinearity. A pair of spatially separated light beams with different frequencies is produced by two acousto-optic modulators, avoiding the main source of periodic nonlinearity in traditional heterodyne interferometers that are based on a Zeeman split laser. In addition, laser beams of the same frequency are used in the measurement and the reference arms, giving the interferometer theoretically perfect immunity from common-mode displacement. We experimentally demonstrated a residual level of periodic nonlinearity of less than 20 pm in amplitude. The remaining periodic error is attributed to unbalanced ghost reflections that drift slowly with time.
authors.affiliation: Physics Laboratory, National Institute of Standards and Technology, Building 221, Room A141, Gaithersburg, Maryland 20899, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Wu
authors.forename: J
authors.initials: J
authors.lastname: Lawall
authors.forename: R D
authors.initials: RD
authors.lastname: Deslattes