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
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.