Subject of this contribution is a laser interferometer for absolute distance measurement (ADI) in a range of 1...50m. The 2σ measurement uncertainty is 7 microns + 1ppm. The frequency of the beat note generated by an extended-cavity diode laser and a stabilised helium-neon laser is changed by a certain amount, and the optical path difference is then calculated from the observed phase shift of the superheterodyne interferometer signals. A straight-line fit algorithm is theoretically proved to make the calculation of the distance ratio insensitive against statistical and systematic phase errors. Experiments at the 50m length comparator of the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany) show that the systematic measurement errors are smaller than the statistical errors.
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