Quasi-distributed fiber sensitization sensing technique based on FMCW interferometry and the virtual Vernier effect

A quasi-distributed fiber-optic sensitized sensing technique based on Fabry Perot (FP) microcavities is proposed by integrating frequency-modulated continuous-wave (FMCW) interferometry with the Vernier effect. An open-cavity structured FP sensor (FP-Sen) was fabricated for seawater salinity detection. Precise localization and interferometric spectral demodulation of multiple FP-Sens were achieved by compensating for the nonlinear tuning errors of the laser source using a digital signal processing (DSP) algorithm. The virtual interferometric spectrum (VIS) of multiple virtual sensors (V-Sens) was constructed according to the interferometric formula and superimposed with the interferometric spectrum of the FP-Sen to generate the optical Vernier effect. Sensitized salinity measurements were then realized by monitoring the drift of the Vernier envelope spectrum. Experimentally, a quasi-distributed FMCW system with three cascaded FP-Sens was employed to obtain high spatial resolution fiber-optic link information and to verify the feasibility of multipoint salinity sensitization. The sensitivity of the FP-Sen reached 2.235 nm/‰ within the salinity range of 25-30‰, achieving a 9-times sensitivity enhancement compared to a single sensor. Additionally, stability analysis using violin plots demonstrated that the fabricated FP-Sens exhibited good stability. The proposed technique offers clear advantages in practical applications requiring a large number of sensitized sensing points.