SI-traceable absolute distance measurement over more than 800 meters with sub-nanometer interferometry by two-color inline refractivity compensation

Abstract

In this work, we demonstrate two-color inline refractivity compensation in a heterodyne synthetic wavelength interferometer for a measurement of absolute distances over several hundred meters with sub-millimeter accuracy. Two frequency-doubled Nd:YAG lasers with a coherence length of more than 1 km are used as light sources. Direct SI traceability is achieved by controlling the lasers' frequency difference in the radio frequency regime. The resulting synthetic wavelengths at 532 nm and 1064 nm are used for the absolute distance measurement and dispersion-based inline refractive index compensation. A standard deviation of 50 μm is achieved for distances up to 864 m. This performance corresponds to a standard deviation of the observable, the difference of the four optical wavelengths, on a sub-nanometer level. Comparison against white light interferometry confirms sub-millimeter accuracy over this distance. Temporally resolved data over 864 m provide quantitative insights into the influence of chromatic beam paths.