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Laser interferometer systems are known for their high resolution, and especially for their high range/resolution ratio. In dimensional metrology laboratories, laser interferometers are popular workhorses for the calibration of displacements. The uncertainty is usually limited to about 10 nm due to polarization-and frequency mixing. For demanding applications however nanometer uncertainty is desired. We adapted a commercially available heterodyne laser interferometer by feeding the measurement signal into a fast lock-in amplifier and use the laser interferometer reference signal as a reference. By measuring both the in-phase and quadrature component an uncorrected phase can be directly measured. By recording both components while the phase changes between 0 and 2π a typical ellipse is recorded from which the first and second harmonics of periodic deviations can be derived. These can be corrected independent of their origin. Measurements using a Babinet-Soleil compensator show that this method can reduce significant non-linearities (16 nm top-bottom) to a standard deviation down to about 0.01 nm. This is confirmed in the evaluation of some standard optical-set-ups. With this method, also optical set-ups can be analysed to predict the non-linearities when a non-compensated standard interferometer is used.

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