Precise measurement technique of long period fiber grating sensors using Fourier transform method

A precise measurement technique of long period of fiber grating sensor (LPG) is demonstrated by employing a cascaded chirp LPG (C-CLPG) as a sensing device and by adopting a Fourier transform method for an interrogation technique. In this approach, transmittance of the C-CLPG exhibits periodic channeled spectrum and its strain- and/or temperature-dependent wavelength shift is analyzed by obtaining a cross-spectrum function between the original and the shifted channeled spectra, in which the phase term of the cross-spectrum corresponds to the wavelength shift. Since the phase component is determined by using all the channeled spectrum over a wider bandwidth of the C-CLPG, more precise measurements of the strain and temperature are expected than that of the peak search method in that the wavelength shift is determined by the shift of one of peaks of the channeled spectrum. In the experiment, the highly precise strain and temperature measurements are presented by comparing with the results of the peak search method.