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 nan ometer is presented. The first method estimates the maximum residual nonlin earity by determining the visibility of the measurement signal. This method has the advantages of easy operation and high resistance to environmental influence. The other method is high-precision calibration with another inte rferometer. To verify these two methods, a compact differential dual-freque ncy interferometer (DDFI) was set up and calibrated with a Fabry-Perot inte rferometer of China National Institute of Metrology (CNIM). The residual no nlinearity was estimated as 1.4 nm using the first method and 2 nm using th e second. These two results fit well in spite of environmental influence. ( C) 1999 Society of Photo-Optical Instrumentation Engineers.