Dual frequency sweeping interferometry for absolute distance metrology at long ranges: implementation and performance

Abstract

Frequency sweeping interferometry (FSI) is a technique where absolute distance measurements are made without ambiguity, by using synthetic wavelengths resulting from a frequency sweep. In FSI, the measurement uncertainty increases with the distance, as consequence of the propagation of the uncertainty in the synthetic wavelength measurement. For long ranges, this component of the uncertainty budget is one of the major drawbacks of the technique. To overcome this problem, we introduced the concept of the dual FSI mode, where the measurement process for longer ranges is reduced to the close range case, by limiting the Optical Path Diference in the interferometer. This was achieved by increasing the reference arm with a long reference fiber, and using a second ancillary interferometer to calibrate continuously the fiber length and compensate temperature variations. In the context of the ESA PROBA3 space mission (coronagraph and demonstration of metrology for free-flying formation), we implemented a FSI sensor composed of a mode-hop free frequency sweep external cavity diode laser, a high finesse Fabry-Perot interferometer (to measure accurately the frequency sweep range) and a dual measurement system. This dual FSI concept, presented in San Diego in 2008, was now implemented and fully tested in view of the PROBA3 mission. Accuracies smaller than 32 μm for a measurement range from 51 m to 61 m were achieved using a reference fiber with 71 m, maintaining the reduced complexity inherent to FSI technique, a mandatory condition for space applications. Implementation issues and performance results are also discussed in this paper.