Simple digital phase measuring algorithm for low noise heterodyne interferometry
We present a digital algorithm for measuring the phase difference between two sinusoidal signals that combines the modified fringe counting method with two sample zero crossing to enable sequential signal processing. This technique can be applied to a phase meter for measuring dynamic phase differences with high resolution, particularly for heterodyne interferometry. The floor noise obtained from a demonstration with an electrical apparatus is $5\times10^ 8} \mathrm{rad/\sqrt{Hz} at frequencies above approximately 0.1 Hz. In addition, by applying this method to a commercial heterodyne interferometer, the floor noise level is confirmed to be $7\times10^ 14} \mathrm{m/\sqrt{Hz} from 4 kHz to 1 MHz. We also confirm the validity of the algorithm by comparing its results with those from a standard homodyne interferometer for measuring shock motion peak acceleration greater than 5000 m/s^2 and a 10 mm stroke.