Patent ID: 8233154
Filing Date: 2012-07-31
Classification: G01V,G02B

Abstract:
1. A method for precisely measuring changes in the displacement of a first object relative to a second object, along a selected direction to be referred to as the x-direction; wherein the first object is rigidly affixed to an optical grating whose lines extend in a direction, to be referred to as the y-direction, that is different from the x-direction; wherein the second object is rigidly affixed to a measurement grating that is parallel to the optical grating, and whose lines also extend in the y-direction; said method comprising the steps of: (a) illuminating the optical grating with two mutually coherent or nearly coherent beams of light; such that zero order light from the first beam and plus or minus first order diffracted light from the second beam intersect at a non-zero fixed angle relative to one another; so that diffracted light from the first beam overlaps and interferes with diffracted light from the second beam to generate interference fringes extending in the y-direction, wherein the positions of the interference fringes in the x-direction correlate with the positions of the lines in the optical grating in the x-direction; and wherein the intensities of the two beams are such that the intensity of the interfering diffracted light from the first beam is approximately the same as the intensity of the interfering diffracted light from the second beam; whereby the interference fringes effectively form an image of the optical grating with a large depth of focus along the direction of the interference fringes; (b) observing any changes in the position of the interference fringes relative to the measurement grating, as a measure of any changes in the position of the first object relative to the second object along the x-direction; whereby: (c) the observed position of the interference fringes relative to the measurement grating is highly sensitive to small translations of the two objects relative to one another along the x-direction, but is relatively insensitive to small translations along the y-direction.