Patent ID: 7224463

Claim:
A method, comprising: obtaining two quadrature fringe signals from an interferometer which causes two signals in two signal paths to interfere with each other; continually sampling the two quadrature fringe signals to obtain digital data samples from the two quadrature fringe signals at a data sampling rate faster than an expected fringe rate; while continuing the sampling of the two quadrature fringe signals, using a plurality of the digital data samples that are collected over a sampling period to perform a linear least square fitting to establish coefficients for an ellipse traced by the two quadrature fringe signals as a phase difference between the two signal paths changes, wherein the linear least square fitting uses a nonparametric equation for the ellipse and applies a normalization to one selected coefficient of the ellipse to compute the coefficients of the ellipse; and using digital data samples that are collected over a subsequent sampling period to update the coefficients of the ellipse to dynamically account for a change in the two signal paths in the interferometer.