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 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 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 estimated as 1.4 nm using the first method and 2 nm using the second. These two results fit well in spite of environmental influence. # 1999 Society of Photo-Optical Instrumentation Engineers. S0091-3286(99)01208-8 Subject terms: displacement interferometry; heterodyne interferometer; nonlinearity; high-precision calibration. Paper 980323 received Aug. 24, 1998; revised manuscript received Jan. 26, 1999; accepted for publication Feb. 3, 1999.
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