Patent ID: 7576868

Claim:
A method comprising: directing a first portion of a beam including a first frequency component along a first path; frequency shifting a second portion of the beam to generate a shifted beam that includes a second frequency component different from the first frequency component and one or more spurious frequency components different from the first frequency component, wherein frequency shifting the second beam portion comprises modulating the phase of the second beam portion to generate the shifted beam, and the phase modulation comprises Serrodyne modulation; directing at least a portion of the shifted beam along a second path different from the first path; measuring an interference signal S(t) from interference between the beam portions directed along the different paths, wherein the signal S(t) is indicative of changes in an optical path difference n{tilde over (L)}(t) between the paths, where n is an average refractive index along the paths, {tilde over (L)}(t) is a total physical path difference between the paths, and t is time; and providing an error signal to reduce errors in an estimate of {tilde over (L)}(t) that are caused by at least one of the spurious frequency components of the shifted beam, the error signal being derived at least in part based on the signal S(t).