New understanding of “mode transition” of coated long period fiber grating

Abstract

Based on the concept of effective refractive index (ERI) and the power density distribution of cladding mode, we reinvestigate the phenomenon of “mode transition” of coated long-period fiber grating (LPFG), and gives a new physical image for “mode transition”. By modifying the ERI range of cladding mode, we point out that the step-like growth of ERI is the self-property of each cladding mode, it has nothing to do with “mode transition”. Thus, there is no transition process between high and low order modes, and we rename the “mode transition region” to “mode barrier region”. Furthermore, the power density distribution of cladding mode shows some lower-order cladding modes will guided by the overlay in mode barrier region. Then, we present the distribution of sensitivity to SRI in mode barrier region, which provides a correct theoretical guidance for high-sensitivity coated LPFG sensors design. Finally, we propose two new characteristics of mode barrier region. First, a new kind of dual-peak resonance based on mode barrier region can be used for high sensitivity sensing and it expand the application scope of dual-peak resonance, meanwhile, it can avoid cross-sensitivity. Second, the coupling coefficient of even-order cladding mode will increase in mode barrier region, which solves the problem that the too small coupling coefficient of even-order cladding mode. Furthermore, the coupling coefficient of every cladding mode has higher response to overlay thickness, surrounding refractive index (SRI) and some other parameters in mode barrier region, so the sensitivity can be greatly improve for an intensity-modulated LPFG sensor.