Accuracy Improvement by Nonlinearity Reduction in Two-Frequency Laser Heterodyne Interferometer

A new method of nonlinearity reduction for accuracy improvement in the displacement measurement system based on the heterodyne Doppler-interferometry method is presented. A two-frequency He-Ne laser at 633 nm central wavelength and 152 MHz free spectral range is used as a source. The electrical induction and unwanted electromagnetic leakage between the reference and measurement paths produce nonlinearity that it is considerably decreased in the presented system. In the designed system, the signal to leakage ratio for moving targets in the range of -20 m/s to +20 m/s is increased to 55 dB and sub-nanometer accuracy of the displacement measurement is obtained.