Acousto-optic displacement-measuring interferometer: a new heterodyne interferometer with Ångstrom-level periodic error

A concept for a new displacement-measuring interferometer has been developed. The motivation behind this work is minimization of the periodic error caused by unwanted leakage between the two frequencies in commercially available heterodyne systems. Typically, the two frequencies are carried on a single beam and separated by polarization-dependent optics. Imperfect optics, non-ideal laser heads and mechanical misalignment may cause each frequency to be leaked into both the reference and the measurement paths, ideally consisting of a single light frequency. The new polarization-independent interferometer described here uses an acousto-optic modulator as the beam splitter and eliminates the frequency leakage by spatially separating the two light beams. Bench-top experiments have shown a reduction in periodic error to 0.18 nm (1.8 Å). The device description, as well as suggestions for simple architecture implementations, are presented.

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