A differential interferometric heterodyne encoder with 30 picometer periodic nonlinearity and sub-nanometer stability

A differential interferometric heterodyne encoder with spatially separated input beams was developed to minimize periodic nonlinearities resulting from polarization mixing. The laser beams with different frequencies were delivered by two polarization-maintaining fibers to the encoder head. Under laboratory conditions this encoder demonstrated a system stability of 38 pm (standard deviation) and 100 pm over 30 s and 1 h respectively. In a comparison measurement with a differential heterodyne interferometer, this encoder showed periodic nonlinearities of less than 30pm without any additional correction.