Determining the residual nonlinearity of a high-precision heterodyne interferometer

The comparison of two methods for determining the residual nonlinearity of the dual-frequency interferometer that has resolution of the order of a nanometer is presented. The first method estimates the maximum residual nonlinearity by determining the visibility of the mea- surement signal. This method has the advantages of easy operation and high resistance to environmental influence. The other method is high- precision calibration with another interferometer. To verify these two methods, a compact differential dual-frequency interferometer (DDFI) was set up and calibrated with a Fabry-Perot interferometer of China National Institute of Metrology (CNIM). The residual nonlinearity was es- timated as 1.4 nm using the first method and 2 nm using the second. These two results fit well in spite of environmental influence.