Virtual-interferometer approach to frequency-scanning nonlinearity calibration via harmonic extraction from a...

Frequency-scanning nonlinearity is a primary factor limiting the ranging accuracy of frequency-scanning interferometry (FSI). Here, we propose, for the first time, to our knowledge, an efficient nonlinearity compensation approach based on high-order harmonic extraction (HHE) from a Fabry-Pérot (F-P) etalon. By employing generalized demodulation (GD), high-order harmonic components carrying frequency-scanning nonlinearity information are extracted from the F-P time-domain pulse signal, enabling the construction of a "virtual interferometer" with a stable and selectable optical path difference. Using the extracted harmonic signal as a reference, the measurement interference signal is resampled at equidistant optical-frequency intervals to compensate for frequency-scanning nonlinearity. Since the F-P etalon is an intrinsic optical frequency calibration element in FSI systems, the proposed method eliminates the need for long-fiber auxiliary interferometers, thereby reducing system complexity and improving environmental robustness. Experimental results show that the frequency-scanning nonlinearity is reduced to 5.97 × 10^-9. In an 18 m absolute ranging experiment, a mean ranging error of 65.65 µm is achieved, with a relative precision better than 4.8 × 10^-6 over an absolute distance of 18 m.