Four-Wavelength Passive Demodulation Technique for the Measurement of Static/Dynamic Composite Signals

A four-wavelength passive demodulation technique for the measurement of static/dynamic composite signals is proposed and experimentally demonstrated. Those previously reported passive demodulation methods for extrinsic Fabry–Perot interferometrics (EFPIs) will collapse if the cavity length changes significantly during signal demodulation. However, the proposed technique can demodulate the measurand even if the cavity length changes significantly. The phase difference between different interferometric signals is first calculated by using four interferometric signals. The phase compensation of these interferometric signals is then implemented to eliminate the influence of cavity length variations on the demodulation. The change in the cavity length can then be recovered. Dynamic signals, static signals, and static/dynamic composite signals were interrogated by the demodulation technique with relative errors of $\sim $ 0.1%. An EFPI with a cavity length ranging from 32.53 to $1219.29~\mu \text{m}$ is detected by the same demodulator. The fast measurement of a cavity length change with an amplitude of $\sim $ 80 $\mu \text{m}$ is achieved. The technique is applicable to different types of signals applied on EFPIs with an unsteady cavity length.