Refractive Index Sensing Based on the Reflectivity of the Backward Cladding-Core Mode Coupling in a Concatenated Fiber Bragg Grating and a Long Period Grating

This study demonstrates the coupling between the backward cladding mode and the backward core mode in a fiber Bragg grating (FBG) cascaded with a long period grating (LPG) to create induced wavelength channels for detecting refractive index variation. Cladding-to-core mode coupling is based on the phase matching condition in which the wave vector of the LPG is equal to the difference between the backward core mode and the backward cladding mode wave-vector in FBG. Moreover, by means of varying the refractive index of the surrounding cladding layer region from an FBG end to the other LPG end, the reflectivity of both the induced wavelength channels and the Bragg wavelength channel is therefore changed due to different mode couplings. This property of index sensing of concatenating an FBG and an LPG by monitoring the reflection power may be exploited for chemical sensing and environmental monitoring applications.