Tuning the resonance of polarization-degenerate LP cladding mode in excessively tilted long period fiber grating for highly sensitive refractive index sensing.

This work presents a sensitive refractive index sensor based on the dual resonance of an excessively tilted long period fiber grating (Ex-TLPFG). The Ex-TLPFG is tuned to couple the guided core mode with only the polarization-degenerate cladding mode LP, which consists of TE/TM and HE vector modes. It is found that the p-polarized LP1,lp mode exhibits a higher sensitivity than that of the s-polarized LP1,ls mode. An optimized sensitivity as high as 12182.9  nm/RIU is achieved for the p-polarized LP1,2p mode at the low refractive index region by tuning the initial resonance condition. The sensing performance is also evaluated through the power measurement method for a single resonance band. It is demonstrated that the improved sensitivity in this work for diameter-reduced Ex-TLPFG is much higher than that for the conventional LPFG based devices, which makes this sensing platform very attractive for a variety of index sensing applications.