Spurious jumps in wavelength tracking of fiber-optic Fabry-Perot interferometric sensors

Wavelength tracking is a commonly used method for demodulating fiber-optic Fabry-Perot interferometric sensors due to its high resolution and straightforward implementation. We report the observation of random spurious jumps in a commonly used wavelength-tracking method based on curve fitting. These jumps were unrelated to the phase ambiguity of the spectral fringes and led to measurement errors. We analyzed the origin of the spurious jumps through Monte Carlo simulations where the fringe valley positions were obtained using polynomial curve fittings. The simulation results show that the spurious jumps arose mainly from the systematic errors of the curve-fitting function for modeling the sensor spectrum and manifested themselves by the changes in the pixel set for curve fitting. The centroid method also suffered from the spurious jumps. We proposed a modified correlation demodulation method free of the spurious jumps. In this method, the information of the measurand was obtained through the correlation between the measured sensor spectral frames and a sufficiently large number of calibrated frames of the sensor over the measurement range. The simulation and experimental results show that the modified correlation method was free of the spurious jumps encountered in the regular wavelength tracking. The resolution of the method was also studied and compared with the curve-fitting method.