The invention features a method including: (i) providing an interference signal S(t) from two beams derived from a common source and 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; (ii) providing coefficients representative of one or more errors that cause the signal S(t) to deviate from an ideal expression of the form A₁ cos(_Rt+(t)+₁), where A₁ and ₁ are constants, _R is an angular frequency difference between the two beams, and (t)=nk{tilde over (L)}(t), with k=2/ and equal to a wavelength for the beams; (iii) calculating a quadrature signal {tilde over (S)}(t) based on the signal S(t); and (iv) reducing the deviation of S(t) from the ideal expression using an error signal S(t) generated from the coefficients and error basis functions derived from the signals S(t) and {tilde over (S)}(t).