Dual mode phase measurement for optical heterodyne interferometry

We present a new digital phase measuring scheme for optical heterodyne interferometry, which provides high measurable velocity up to 6 m s -1 with a fine displacement resolution of 0.1 nm. The main idea is combining two distinctive digital phase measuring techniques with mutually complementary characteristics; one is counting the Doppler shift frequency with a 20 MHz beat frequency for high velocity measurement and the other is synchronous phase demodulation with a 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.