Differential type of phase-locked laser diode interferometer free from external disturbance

A phase-locked laser diode interferometer with differential detection to eliminate external disturbance is proposed. In this interferometer, the measurements are implemented at two different points at the same time. The surface profile that contains the disturbance is obtained at the scanned measuring point, and the disturbance is obtained at the fixed measuring point. The exact profile is obtained by subtracting the latter from the former. The limitations and characteristics are examined theoretically. The analytical results agree well with the experimental results. The repeated measurement accuracy is estimated to be ~5 nm in this interferometer.