Heterodyne interferometric system with subnanometer accuracy for measurement of straightness

A generalized laser interferometer system based on three design principles, i.e., heterodyne frequency, prevention of mixing, and perfect symmetry, is described. These design principles give rise to an interferometer in a highly stable system with no periodic nonlinearity. A novel straightness sensor, consisting of a straightness prism and a straightness reflector, is incorporated into the generalized system to form a straightness interferometer. A Hewlett-Packard commercial linear interferometer was used to validate the interferometer's parameters. Based on the present design, the interferometer has a gain of 0.348, a periodic nonlinearity of less than 40 pm, and a displacement noise of 12 pm/mean square root of Hz at a bandwidth of 7.8 kHz. This system is useful for precision straightness measurements.