Spectral characteristics and bend response of Bragg gratings inscribed in all-solid bandgap fibers

In this paper, we investigate the spectral characteristics and bend response of fiber Bragg gratings (FBGs) in all-solid photonic bandgap fibers (PBGFs). We inscribe FBGs within the secondary bandgap by ultraviolet (UV) side illumination and observe the couplings to backward core mode, guided LP01 and LP11 supermodes and radiative LP02 supermodes. The mechanisms of these resonant couplings in the FBG are described in detail. We demonstrate that only those supermodes with certain phase relationships and symmetric mode field profiles are responsible for the supermode resonances. When the fiber grating is bent, the guided supermode resonances become chirped as a result of the strain gradient over the fiber cross section. Meanwhile, the core resonance is enhanced, due to more energy of the core mode distributed in the cladding rods. The bend response is direction dependant owing to the nonuniform UV-induced average index raises and index modulation over the high-index rod lattice.