Two-way frequency-conversion phase measurement for high-speed and high-resolution heterodyne interferometry

Abstract

He-Ne heterodyne laser interferometers are widely used as standard tools for displacement metrology to produce high-resolution measurements. However, such systems use specially designed complex and costly high-speed electronics. In this paper, we present a new unsophisticated method of phase measurement that achieves a resolution of 0.15 nm using simple electronics for a target speed of 2.4 ms(-1). The method adopts a frequency-conversion technique to lower the original beat frequency to 100 kHz by mixing it with a stable reference signal generated from a phase-locked loop. At the same time, to avoid the accompanying unwanted decrease in the maximum measurable speed caused by the lowered beat frequency, a two-way frequency conversion and a special fringe-counting method are combined to perform the required phase unwrapping, using widely available programmable digital gates. This allows velocity measurements up to the limit achievable using the original beat frequency.