Calibration of the Fabry-Pérot free spectral range using a tunable laser in a Michelson interferometer

Coherent interferometric absolute distance metrology enables different techniques for length metrology. Measurements are made without ambiguity, by using either one or several synthetic wavelengths resulting from the beating of two or more wavelengths or, in the case of frequency sweeping interferometry (FSI), from a frequency sweep. These techniques require accurate measurements of the frequency variation, a task which is usually performed with a Fabry-Pérot (FP) interferometer. Sensor performances are highly dependent on the FP free spectral range (FSR), which must be accurately known. We describe a method to measure the FSR of an FP using a tunable laser in a Michelson interferometer, using both absolute and relative distance measurements. The optical setup is based on a frequency-sweeping sensor and a controlled translation table. An uncertainty of 45 ppm in the measurement of a 1-GHz FSR is obtained. The method is indicated for the calibration of the FP in the frequency-sweep-range-measuring subsystem of a frequency sweeping sensor absolute distance sensor as it does not require any addition hardware, configuring a self-calibration approach.