Characteristics analysis of cascaded long-period gratings with nm-thick film coating

ABSTRACT Cascaded long period grating with film coating is sensitive to the thickness and refractive index of the film material and the surrounding refractive index, so it can be used in the applications such as gas sensing, liquid concentration sensing and bio-sensing. In this paper, the characteristic of cascaded long period grating coating with higher refractive index nanostructured film overlay is investigated theoretically by using coupled-mode theory and transfer matrix method. The responses of interference fringe peak wavelength to the thickness and refractive index of the film and the surrounding refractive index are numerical calculated. The theoretical simulation results show that the minima wavelength of the interference fringes within the long period gratings attenua tion bands are sensitive to the thickness and refractive index of the film coating and the refractive index of the surroundin g environment. By choosing these parameters optimally, the sensitivity of the peak wavelength shift of the interference fringes can be enhanced. The theoretical analysis can provide guidance to the application of the nanostructured film coated cascaded long period grating sensor. Keywords: cascaded long period grating, refractive index, film overlay, coating, optical fiber sensor