Patent ID: 6947148

Claim:
Interferometric apparatus for measurig a change in distance to an object, said interferometric apparatus comprising: means for providing optical paths for making interferometric measurements comprising first and second measurement legs, said first and second measurement legs structured and arranged such that at least one of them has a variable physical length to the object, the optical path length difference between said first and second measurement legs varying in accordance with the difference between the respective physical lengths of their associated optical paths, at least one of said measurement legs including an optical element that introduces undesirable phase shifts in beams interacting with it; source means for generating at least one light beam; means for introducing predetermined portions of said at least one light beam into said first and second measurement legs as first and second measurement beams that travel along respective ones of said first and second measurement legs over their associated optical paths, at least one of said measurement beams experiencing said undesirable phase shifts as a result of interacting with said optical element, means for compensating for the presence of said undesirable phase shifts in said measurement beams so that said measurement beams emerge from said measurement legs as exit beams containing compensated information about the respective optical path lengths through said first and second measurement legs; and means for combining said exit beams to produce mixed optical signals containing information corresponding to compensated phase differences between each of said exit beams from corresponding ones of said associated optical paths of said first and second measurement legs.