Cladding mode reorganization in high-refractive-index-coated long-period gratings: effects on the refractive-index sensitivity

Abstract

We theoretically and experimentally investigated the reorganization of cladding modes in high-refractive-index- (HRI-) coated long-period gratings (LPGs), focusing on the influence of refractive-index sensitivity. When azimuthally symmetric nanoscale HRI coatings are deposited along LPG devices, a significant modification of the distribution of cladding modes occurs, depending on the layers' features (refractive index and thickness) and on the external refractive index. In particular, if these parameters are properly chosen, a transition between cladding modes and overlay modes occurs. Numerical and experimental effects of the mode transition on the sensitivity of the surrounding refractive index are described.

Micro-structured fiber Bragg gratings. Part II: Towards advanced photonic devices

“…On the other side an extension of this model to a cylindrical four-layer waveguide has been adopted to investigate the effects of a HRI coating on the cladding modes in a standard fiber [12,13]. The same approach can be efficiently used in case of HRI coating cast on a thinned optical fiber.…”

“…The effects of HRI coatings on standard fibers with LPGs have been widely documented [12,13,17]. In that case, the HRI layer affects only the cladding modes since the core mode is insensitive to the surrounding medium.…”

“…In particular, the cladding mode with the highest effective refractive index becomes guided within the overlay region and a reorganization of the remaining cladding modes takes place, in order to recover the original field distribution. This transition mechanism induces a significant improvement on the sensitivity of the attenuation bands on the overlay features and on the SRI [12,13]. Here, the attention is focused on the investigation of nano-sized HRI coatings on cladding stripped FBGs.…”

“…The presence of HRI coating on the defect region of a MSFBG induces strong changes on the guided modes leading to the possibility to improve the sensitivity of the final device according to previous works [12,13].…”

Optical Fibre Long-Period Gratings Functionalised with Nano-Assembled Thin Films: Approaches to Chemical Sensing

“…It has been shown previously experimentally and theoretically [27,28] that the effective indices of the cladding modes, and thus the central wavelengths of the core-cladding mode coupling bands of LPGs, show a high sensitivity to the optical thickness of high refractive index coatings when the coating's optical thickness is such that one of the low order cladding modes is phase matched to a mode of the waveguide formed by the coating. This is termed the mode transition region, in which the cladding modes are reorganized, with each mode taking on the characteristics (effective index and electric field profile) of its adjacent lower order cladding mode [29].…”