In heterodyne interferometry, a laser source generates a beam that includes two polarizations separated slightly in frequency (a “heterodyne beam”). The interference between the two polarizations in an interferometer provides a highly accurate measurement of the position of a measurement reflector relative to a reference reflector. By coupling the measurement reflector to a measurement object, the relative position of the measurement object can be determine with a similar degree of precision. Unfortunately, the generation of the heterodyne beam may produce heat and mechanical vibrations that can detrimentally affect the performance of the interferometer. The laser source may be placed at some distance from the interferometer in order to avoid these effects, but this in turn requires the use of some technique, such as a fiber optic connection, to transmit the laser beam from the laser source to the interferometer. Unfortunately, polarization mixing and birefringent optical effects, which disproportionately affect one polarization, when the heterodyne beam is transmitted to the interferometer can reduce the fidelity of the heterodyne interferometry measurement. Furthermore, such effects may vary based on temperature, mechanical vibrations, and other surrounding conditions, thus making it difficult to predict these effects in advance or to compensate for them.
USPTO Backgrounds
USPTO Backgrounds