OPD Demodulation Methods
5.4.1 OPD Demodulation Based on Spectrum Interrogation and Fringe Counting
After obtaining the spectrum of FFP sensors using above-mentioned spectrum interrogation methods, the next mission is how to demodulate the OPDs of FFP sensors from their spectra. A simple method is called fringe counting [16-19]. For a certain fringe m as shown in Figure 5.11, since the peak wavelength corresponds to the minimum value of the cosine curve, we can get
For the fringe m — q, we can get
Therefore, the OPD of the FFP cavity is derived as
Demodulation resolution using this method strongly depends on the peak or pit bandwidth and fringe number. Generally, in order to achieve high resolution, an FFP sensor with narrow peak or pit bandwidth and more fringes are highly desired.
To further improve the performance of demodulation, a combination of one-peak and two-peak tracking methods was proposed, which presented a nanometer-scale demodulation stability . A two-step method including fringe counting and phase estimation was also proposed to improve demodulation performance . A spectrum fitting algorithm was also demonstrated by Han et al. to achieve both high-resolution, absolute measurement of the cavity length, and a large dynamic measurement range simultaneously, but errors might be introduced by various imperfections of the sensor .