| A lightweight and electrically passive interferometer is demanded by a number of applications for precision metrology such as machine tool metrology and on-machine feedback. To satisfy these requirements, a fiber-optic coupled heterodyne displacement measuring Michelson interferometer is developed. In this interferometer, the laser light is coupled into the interferometer with a polarization-maintaining (PM) fiber to separate the application environment of the interferometer from the laser head. The primary limitation of accuracy encountered with this fiber-optic coupled heterodyne interferometer is the optical mixing arising from the imperfection of fiber-optic coupling system. The optical mixing results in the nonlinearity (first order and second order errors) in the displacement measurement of the interferometer. Thermal translation is developed to measure the nonlinearity. A theoretical model using Jones calculus is developed and corresponding experiments are carried out with good agreement. A strategy of combined optical alignment and signal processing is determined to reduce nonlinearity below the level of 2 nm. |
