Nonlinearity of a double-path interferometer qualified with a non-constant moving speed.

A new double-path heterodyne interferometer with spatially separated input beams was developed to minimize the influence of periodic errors. The fully fiber-coupled optics exhibit balanced beam paths which reduce the influence of variations of the ambient conditions. The use of corner cube reflectors enables the interferometer to tolerate larger angular motions of the measuring mirror, compared with common plane mirror interferometers. The nonlinearities were characterized in a measurement with a non-constant moving speed by means of amplitude spectra for spline-interpolated data. The experimental results indicated that the amplitude of the periodic error was kept within 0.63 nm and caused by multi-reflections.