Wide Wavelength Range Demodulation Method of Cascaded FBGs Based on LPFG Spectral Control

To perceive spatial principal strain in frozen environments, the 6-axis strain sensor was proposed in previous research. When monitoring ice instability, it is necessary to demodulate the cascaded 6 FBGs of the sensor over a wide wavelength range. The edge filtering demodulation method based on dual-peak resonance LPFG spectral control is proposed, which achieves accurate demodulation of the center wavelength of cascaded 6 FBGs over a large wavelength range (each demodulation range is 15 nm, and the total range is 90 nm). We have etched a dual-peak resonance LPFG using CO2 laser and designed a demodulation system based on piezoelectric ceramics (PZT). By precisely controlling the bending degree of LPFG through PZT, the spectrum of the LPFG can be regularly changed. The linear mathematical model of the demodulation system has been established, which theoretically verifies the feasibility of the proposed demodulation method. To solve the nonlinear error caused by the linearity poor at the edge of the demodulation interval, the BP-neural network demodulation model was established. The average error is smaller, which can reach 0.014 nm, with an MSE of 0.0005. This work can provide useful technical support for the widespread application of FBG multi-dimensional strain sensors.