Numerical Analyses of the Polarization Dependent Cladding Mode Coupling in Localized Fiber Bragg Gratings

The polarization dependence of cladding mode coupling in fiber Bragg grating (FBG) plays a significant role in the excitation of surface plasmon resonances (SPR) for sensing applications. In this work, we numerically analyze the polarization dependent cladding mode coupling enabled by localized fiber Bragg gratings (LFBGs), focusing on the properties that generate from the modal structures and the position of the localized refractive index modifications (RIMs). Our analyses reveal that, both centric and eccentric LFBGs allow strong coupling to cladding modes. In centric LFBGs, coupling to EH/HE
_1,m
modes dominates, whereas in eccentric LFBG, additional coupling to high azimuthal order cladding modes is permitted. Strong polarization dependence is found in eccentric LFBG: for certain cladding modes, coupling from one polarization can be suppressed due to the cancellation of negative and positive mode coupling. More interestingly, we found that cladding modes with even and odd azimuthal orders response differently to input polarizations, which accounts for the unique polarization dependence (i.e., neighboring resonances exhibit opposite polarization preference in the transmission spectra) in eccentric LFBG. Our numerical results can be verified by the good agreement with existed experimental reports.