A novel Bragg grating sensor interrogation system utilizing a scanning filter, a Mach-Zehnder interferometer and a 3×3 coupler

This work describes a new technique for fibre Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning Fabry-Perot (SFP) bandpass filter used to wavelength-multiplex multiple gratings in a single fibre, and an unbalanced Mach-Zehnder fibre interferometer made with a 3×3 coupler to detect strain-induced wavelength shifts. A passive technique for interferometer drift compensation using non-sensing FBGs is included in the system. A prototype complete system interrogates four gratings in a single fibre at a Nyquist sampling rate up to 10 kHz, with a noise floor measured near 4 n Hz^-1/2 above 0.1 Hz. The inclusion of the interferometer drift compensation technique is shown to make quasi-static measurements feasible.