Superheterodyne configuration for two-wavelength interferometry applied to absolute distance measurement

• Published in: Applied optics. Optical technology and biomedical optics Vol. 49; no. 4; p. 714
• Main Authors: Le Floch, Sébastien, Salvadé, Yves, Droz, Nathalie, Mitouassiwou, Rostand, Favre, Patrick
• Format: Journal Article
• Language: English
• Published: United States 01.02.2010
• ISSN: 2155-3165
• DOI: 10.1364/AO.49.000714

We present a new superheterodyne technique for long-distance measurements by two-wavelength interferometry (TWI). While conventional systems use two acousto-optic modulators to generate two different heterodyne frequencies, here the two frequencies result from synchronized sweeps of optical and radio frequencies. A distributed feedback laser source is injected in an intensity modulator that is driven at the half-wave voltage mode. A radio-frequency signal is applied to this intensity modulator to generate two optical sidebands around the optical carrier. This applied radio frequency consists of a digital ramp between 13 and 15 GHz, with 1 ms duration and with an accuracy of better than 1 ppm. Simultaneously, the laser source is frequency modulated by a current modulation that is synchronized on the radio-frequency ramp as well as on a triangle waveform. These two frequency-swept optical signals at the output of the modulator illuminate a Michelson interferometer and create two distinct distance-dependent heterodyne frequencies on the photodetector. The superheterodyne signal is then detected and bandpass filtered to retrieve the absolute distance measurement. Experiments between 1 and 15 m confirm the validity of this new concept, leading to a distance accuracy of +/- 50 microm for a 1 ms acquisition time.