Patent ID: 7616322
Filing Date: 2009-11-10
Classification: G01B,G03F

Abstract:
1. A method comprising: providing an interference signal S(t) from interference between 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; and providing an error signal to reduce errors in an estimate of {tilde over (L)}(t), the error signal being derived at least in part based on one or more collective properties of a distribution of multi-dimensional values, at least one of the multi-dimensional values in the distribution being generated from a plurality of samples of the signal S(t); wherein one of the paths is associated with a position of a reference object and the other path is associated with a position of a moveable measurement object; wherein the position of the moveable object is controlled by a servo system having a bandwidth and f wherein the servo system controls the position of the measurement object based on the signal S(t) and the error signal.