Long period grating in multicore optical fiber: an ultra-sensitive vector bending sensor for low curvatures

Abstract

Long period grating was UV inscribed into a multicore fiber consisting of 120 single mode cores. The multicore fiber that hosts the grating was fusion spliced into a single mode fiber at both ends. The splice creates a taper transition between the two types of fiber that produces a nonadiabatic mode evolution; this results in the illumination of all the modes in the multicore fiber. The spectral characteristics of this fiber device as a function of curvature were investigated. The device yielded a significant spectral sensitivity as high as $1.23\mathrm{nm}/{\mathrm{m}}^{-1}$ and $3.57\mathrm{dB}/{\mathrm{m}}^{-1}$ to the ultra-low curvature values from 0 to $1{\mathrm{m}}^{-1}$. This fiber device can also distinguish the orientation of curvature experienced by the fiber as the long period grating attenuation bands producing either a blue or red wavelength shift. The finite element method (FEM) model was used to investigate the modal behavior in multicore fiber and to predict the phase-matching curves of the long period grating inscribed into multicore fiber.