High-sensitivity sensor design based on cascaded long-period fiber grating with film coating

A cascaded long-period fiber grating (CLPFG) sensor with film coating is presented. Two long-period fiber gratings (LPFGs) are cascaded to form a Mach-Zehnder interferometer. The optical transmission spectrum and the sensing characteristics for ambient refractive index (RI) measurement of the CLPFG sensor are analyzed in a form of transfer matrix based on rigorous coupled-mode theory. The results indicate that it is highly sensitive to the film RI and surrounding RI, so it can be used as a gas sensor or a solution sensor. Data simulation shows that the resolution of the RI of the films is predicted to be 10(-7). Further, a cascaded chirped LPFG is introduced to study its sensing performance. The influence of the chirp coefficient and the grating structural parameters on the transmission spectrum of the cascaded chirped LPFG is analyzed. Data simulation shows that the resolution of the RI of the films is predicted to be 10(-9). In contrast to the conventional measurement method based on interrogating the wavelength change, intensity detection of this cascaded chirped LPFG sensor means that no optical spectrum analyzer is required in the measuring system, which is favorable for practical applications, especially for in situ environmental motoring. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)