Novel high sensitivity sensor design based on cascaded long-period fiber gratings

A cascaded long-period fiber grating (CLPFG) sensor with film coating is presented in this paper. Two LPFGs are cascaded to form a Mach-Zehnder interferometer. The optical transmission spectrum and the sensing characteristics for ambient refractive index measurement of the cascaded LPFG 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 refractive index and surrounding refractive index, so it can be used as gas sensor or solution sensor. In addition, the influence of the film optical parameters on the sensitivity is analyzed. By using optimization method, the optimal film optical parameters and the grating structure parameters are obtained. Data simulation shows that the resolution of the refractive index of the films is predicted to be 10(-7). Further, the 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 cascaded chirped LPFG is analyzed. Data simulation shows that the resolution of the refractive index of the films is predicted to be 10(-9). In contrast to conventional measurement method based on interrogating the wavelength change, the 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.