Systems and methods for cyclic error correction in a heterodyne interferometer

Abstract

Generally, in accordance with the various illustrative embodiments disclosed herein, a heterodyne optical interferometer incorporates error correction elements to correct a cyclic error that may be present in an interferometric measurement. The cyclic error can be caused by various factors such as an imperfect polarization relationship between two wavelength components, deficiencies in optical propagation paths (such as light leakage), imperfect optical coatings, and/or imperfect components. The cyclic error, which typically manifests itself as erroneous displacement information characterized by a low velocity sinusoidal frequency component, can be reduced or eliminated by using birefringent optical elements and other optical elements to alter certain characteristics of one or both wavelength components and reduce light leakage components in one or more light propagation paths in the heterodyne optical interferometer.

What is claimed is: 
     
         1 . A heterodyne interferometer comprising:
 a polarizing optical beam splitter configured to receive a composite light beam comprising a first wavelength component having a first polarization and a second wavelength component having a second polarization, the polarizing optical beam splitter further configured to cooperate with a movable target reflector to produce a measurement beam;   a measurement system configured to receive the measurement beam and determine a displacement characteristic of the movable target reflector; and   one or more light attenuating elements inserted in one or more light propagation paths in the heterodyne interferometer to attenuate one or more light leakage components that contribute to a cyclic error in the displacement characteristic determined by the measurement system.   
     
     
         2 . The heterodyne interferometer of  claim 1 , wherein the cyclic error is characterized at least in part, by a low velocity component present in an interferometric measurement executed by the measurement system for determining the displacement characteristic of the movable target reflector. 
     
     
         3 . The heterodyne interferometer of  claim 2 , wherein:
 the polarizing optical beam splitter includes a polarizing beam splitting coating configured to direct the first wavelength component along a first light propagation path towards the movable target reflector and to direct the second wavelength component along a second light propagation path that is orthogonal to the first light propagation path; and   the one or more light leakage components comprise at least one of a portion of the second wavelength component propagating along the first light propagation path or a portion of the first wavelength component propagating along the second light propagation path.   
     
     
         4 . The heterodyne interferometer of  claim 2 , wherein:
 the polarizing optical beam splitter includes a polarizing beam splitting coating configured to direct the first wavelength component along a first light propagation path towards the movable target reflector and to direct the second wavelength component along a second light propagation path that is orthogonal to the first light propagation path;   the composite light beam is propagated over a third light propagation path between a laser and the polarizing optical beam splitter;   the measurement beam is propagated over a fourth light propagation path between the polarizing optical beam splitter and the measurement system; and   the one or more light attenuating elements are inserted in one or more of the first light propagation path, the second light propagation path, the third light propagation path, or the fourth light propagation path.   
     
     
         5 . The heterodyne interferometer of  claim 4 , wherein the one or more light attenuating elements comprise a diattenuator arranged between a first birefringent optical element and a second birefringent optical element for attenuating at least one of the first wavelength component or the second wavelength component of the composite light beam propagating over the third light propagation path. 
     
     
         6 . The heterodyne interferometer of  claim 1 , wherein the one or more light attenuating elements comprises one or more of a birefringent optical element, a diattenuator, a combiner, an optical filter, an optical coating, a Faraday rotator, or a polarizer. 
     
     
         7 . The heterodyne interferometer of  claim 1 , wherein the one or more light attenuating elements comprises a compound optical element that is an integrated assembly of at least two light attenuating elements. 
     
     
         8 . The heterodyne interferometer of  claim 7 , wherein the at least two light attenuating elements comprise a first light attenuating element that is adjustable with a first degree of resolution and a second light attenuating element that is adjustable with a second degree of resolution. 
     
     
         9 . A method comprising:
 receiving in a polarizing optical beam splitter of a heterodyne interferometer, a composite light beam comprising a first wavelength component having a first polarization and a second wavelength component having a second polarization;   utilizing the polarizing optical beam splitter to produce a measurement beam that is indicative of a displacement characteristic of a movable target reflector;   receiving the measurement beam in a measurement system;   executing an interferometric measurement in the measurement system for determining the displacement characteristic of the movable target reflector;   detecting a cyclic error in the interferometric measurement; and   attenuating at least one of the first wavelength component or the second wavelength component present in one or more light propagation paths in the heterodyne interferometer for reducing the cyclic error in the interferometric measurement.   
     
     
         10 . The method of  claim 9 , wherein the cyclic error is characterized at least in part, by a low velocity component present in the interferometric measurement. 
     
     
         11 . The method of  claim 10 , wherein attenuating the at least one of the first wavelength component or the second wavelength component comprises one of inserting one or more light attenuating elements in the one or more light propagation paths or adjusting one or more light attenuating elements located in the one or more light propagation paths. 
     
     
         12 . The method of  claim 11 , wherein the one or more light attenuating elements comprises one or more of a birefringent optical element, a diattenuator, a combiner, an optical filter, an optical coating, a Faraday rotator, or a polarizer. 
     
     
         13 . The method of  claim 12 , wherein the one or more light attenuating elements comprises a compound optical element that is an integrated assembly of at least two light attenuating elements. 
     
     
         14 . The method of  claim 10 , wherein attenuating the at least one of the first wavelength component or the second wavelength component comprises modifying at least one of a polarization characteristic or an amplitude of the at least one of the first wavelength component or the second wavelength component. 
     
     
         15 . A method comprising:
 utilizing a composite light beam to execute an interferometric measurement in a heterodyne interferometer, the composite light beam comprising a first wavelength component having a first polarization and a second wavelength component having a second polarization;   detecting a cyclic error in the interferometric measurement; and   attenuating at least one of the first wavelength component or the second wavelength component present in one or more light propagation paths in the heterodyne interferometer for reducing the cyclic error in the interferometric measurement.   
     
     
         16 . The method of  claim 15 , wherein attenuating at least one of the first wavelength component or the second wavelength component is directed at reducing light leakage components present in the one or more light propagation paths in the heterodyne interferometer. 
     
     
         17 . The method of  claim 16 , wherein the interferometric measurement is directed at determining a displacement characteristic of a movable target reflector, and wherein utilizing the composite light beam to execute the interferometric measurement comprises:
 utilizing a polarizing optical beam splitter to receive the composite light beam and direct the first wavelength component of the composite light beam along a first light propagation path towards the movable target reflector and direct the second wavelength component of the composite light beam along a second light propagation path that is orthogonal to the first light propagation path.   
     
     
         18 . The method of  claim 17 , wherein a portion of the second wavelength component propagating along the first light propagation path is a first light leakage component and a portion of the first wavelength component propagating along the second light propagation path is a second light leakage component. 
     
     
         19 . The method of  claim 18 , wherein attenuating at least one of the first wavelength component or the second wavelength component comprises modifying at least one of a polarization characteristic or an amplitude of at least one of the first light leakage component or the second light leakage component. 
     
     
         20 . The method of  claim 18 , wherein attenuating at least one of the first wavelength component or the second wavelength component comprises an optical filtering of the at least one of the first wavelength component or the second wavelength component.