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Sub-ppm absolute distance measurements using an optical frequency comb generated by a conventional dual-drive Mach-Zehnder modulator

A simple technique to generate an optical frequency comb, based on a conventional dual-drive Mach-Zehnder intensity modulator, has been used as optical source for a high accuracy distance measurement in an interferometric set-up. The modulator has been driven by a direct-digital synthesizer that is able to deliver a pure ramp in frequency between 13 GHz and 14 GHz. We have obtained about 15 modes, corresponding to a spectral span of 200 GHz. This optical signal, launched in a Michelson interferometric set-up, allowed performing absolute distance measurement by sweeping the radio-frequency of the direct digital synthesizer. Measurements have been compared to a standard, which was a mode-locked femtosecond laser along with a counting interferometer. Absolute distance measurements over a range of 1 to 24 meters gave an accuracy of about ± 10 microns, with a repeatability of ± 5 microns, corresponding to a sub-ppm absolute distance measurement.