Heterodyne interferometer with subatomic periodic nonlinearity

A new, to our knowledge, heterodyne interferometer for differential displac ement measurements is presented. It is, in principle, fi-ee of periodic non linearity. A pair of spatially separated light beams with different frequen cies is produced by two acousto-optic modulators, avoiding the main source of periodic nonlinearity in. traditional heterodyne interferometers that ar e based on a Zeeman split laser. In addition, laser beams of the same frequ ency are used in the measurement and the reference arms, giving the interfe rometer theoretically perfect immunity from common-mode displacement. We ex perimentally demonstrated a residual level of periodic nonlinearity of less than 20 pm in amplitude. The remaining periodic error is attributed to unb alanced ghost reflections that drift slowly with time. (C) 1999 Optical Soc iety of America.