Dual frequency sweeping interferometry with range-invariant accuracy for absolute distance metrology

Coherent absolute distance interferometry is one of the most interesting techniques for length metrology. In frequency sweeping interferometry (FSI), measurements are made without ambiguity, by using a synthetic wavelengths resulting from a frequency sweep. FSI-based sensors are simple devices and fulfill an important role on any demanding space mission metrological chain. Their parameterization flexibility allows either technological or application-related tradeoffs to be performed. Accuracy is mainly dependent on the capability to measure the synthetic wavelength, using a Fabry-Perot interferometer (FP) that counts resonances as the frequency sweeps, and on the number of detected synthetic fringes. For large ranges, the number of fringes dominates performances, leading to a linear decrease of the accuracy with range. By increasing the size of the interferometer reference arm, and by measuring both the distance and the reference arm independently, it is possible to ensure high accuracy for small distance measurements, even for much larger range. In the context of the ESA PROBA3 mission (coronagraph and demonstration of metrology for free-flying formation), we are prototyping a FSI sensor composed of a mode-hop free frequency sweep external cavity diode laser, a high finesse FP (to measure accurately the frequency sweep range) and a dual measurement system to enable the measurements at 150 m with an accuracy at the tens of micrometer level. Its uncertainty budget, interferometers design and preliminary experimental results are detailed in this paper.