Interferometric cyclic error compensation

A method for reducing cyclic error contributions in an interferometry measurement, the method comprising:

• directing two beams derived from a common source along different paths in an interferometer, wherein the two beams have orthogonal polarizations and frequencies that differ by a heterodyne frequency;
• producing a first output beam derived from a portion of each of the two beams having a first common polarization;
• producing a second output beam derived from a portion of each of the two beams having a second common polarization substantially orthogonal to the first common polarization;
• generating first and second signals derived from intensity measurements of the first and second output beams, respectively;
• calculating a superheterodyne signal corresponding to a product of the first and second signals to substantially eliminate at least some first-order cyclic errors present in the first and second signals;
• and
• extracting the phase of the superheterodyne signal to provide information related to the different paths in the interferometer.

An apparatus comprising:

• an interferometer configured to direct two beams derived from a common source along different paths and produce a first output beam derived from a first portion of each of the two beams and a second output beam derived from a second portion of each of the two beams;
• first and second detectors positioned to measure an intensity of the first and second output beams, respectively;
• and
• an electronic processor coupled to the first and second detectors, wherein during operation the electronic processor calculates a product of a first signal derived from the intensity of the first output beam and a second signal derived from the intensity of the second output beam.