title: Refractive index sensitivity of nano-film coated long-period fiber gratings.
abstract: We demonstrate the fabrication of long-period fiber gratings (LPFGs) coated with high index nano-film using the atomic layer deposition (ALD) technology. Higher index sensitivity can be achieved in the transition region of the coated LPFGs. For the LPFG coated by nano-film with a thickness of 100 nm, the high index sensitivity of 3000 nm/RIU and the expanded index sensitive range are obtained. The grating contrast of the over-coupled LPFGs and conventional LPFGs are measured and the over-coupled gratings are found to have a higher contrast in the transition region. The cladding modes transition is observed experimentally with increasing surrounding index using an infrared camera. The theoretical model of the hybrid modes in four-layer cylindrical waveguide is proposed for numerical simulation. The experimental results are well consistent with theoretical analysis.
authors.forename: Fang
authors.initials: F
authors.lastname: Zou
authors.forename: Yunqi
authors.initials: Y
authors.lastname: Liu
authors.forename: Chuanlu
authors.initials: C
authors.lastname: Deng
authors.forename: Yanhua
authors.initials: Y
authors.lastname: Dong
authors.forename: Shan
authors.initials: S
authors.lastname: Zhu
authors.forename: Tingyun
authors.initials: T
authors.lastname: Wang