A Heterodyne Interferometer With Subatomic Periodic Nonlinearity
C M. Wu, John R. Lawall, R Deslattes
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 acoustic-optic-modulators, avoiding the main source of periodic nonlinearity in traditional heterodyne interferometers. In addition, laser beams of the same frequency are used in the measurement and reference arms, giving the interferometer theoretically perfect immunity from common-mode displacement. We have experimentally demonstrated a residual level of periodic nonlinearity of less then 20 pm in amplitude. The remaining periodic error is attributed to unbalanced ghost reflections, that drift slowly with time.