Twin-grating fiber optic sensor demodulation

We present a demodulation algorithm for sensors consisting of two Bragg gratings separated by a short distance. This sensor works as an unbalanced interferometer sensitive to temperature or axial strain variations. The sensor demodulation procedure here presented is based on the evaluation of the phase of the interference pattern produced by light reflected from both gratings and on the determination of the Bragg wavelength shift. This information can be extracted from the phase distribution of the optical spectrum image in the Fourier domain. The demodulation method here presented has been thoroughly analyzed theoretically and tested experimentally. Experimental tests were performed with temperature sensors. The proposed demodulation procedure provides high accuracy and absolute measurements.