Patent ID: 6950192

Claim:
A method comprising: providing an interference signal S(t) from two beams directed along different paths, wherein the signal S(t) is indicative of changes in an optical path difference n{tilde over (L)}(t) between the different paths, where n is an average refractive index along the different paths, {tilde over (L)}(t) is a total physical path difference between the different paths, and t is time; providing one or more coefficients representative of one or more errors that cause the signal S(t) to deviate from an ideal expression of the form A 1 cos(ω R t+φ(t)+ζ 1 ), where A 1 and ζ 1 are constants, ω R is an angular frequency difference between the two beams, and φ(t)=n/k{tilde over (L)}(t), with k=2π/λ and λ equal to a wavelength for the beams; calculating a function of values of the signal S(t); and reducing the deviation of S(t) from the ideal expression using an error signal S ψ (t) generated from the coefficients and one or more error basis functions derived at least in part from the function of values of the signal S(t).