Dual mode phase measurement of heterodyne optical interferometry

We present a new digital phase measuring scheme for optical heterodyne interferometry, which provides high measurable velocity up to 6 m/s with a fine displacement resolution of 0.1 nanometer. The main idea is combining two distinctive digital phase measuring techniques with complementing characteristics to each other; one is the Doppler shift frequency counting with 20 MHz beat frequency for high velocity measurement and the other is the synchronous phase demodulation with 2.0 kHz beat frequency for extremely fine displacement resolution. The two techniques are operated in switching mode in accordance with the object speed in a synchronized way. Experimental results prove that the proposed dual mode phase measuring scheme is realized with a set of relatively simple electronic circuits for beat frequency shifting, heterodyne phase detection, and low-pass filtering.