Figure 2:

Schematic illustrations of the three different OFGs used for RI sensing along with their respective coupling mechanism sketched with arrows (green: counterpropagating core mode; blue: counter-/co-propagating cladding modes): etched FBG (A), tilted FBG (B), and LPG (C).

Given the coupling between the fundamental core mode and cladding modes, the light intensity (yellow cylinder) decreases after the interaction with the grating. In the case of both EFBG and TFBG, an RI change will induce a shift of the resonant attenuation bands toward longer wavelengths; conversely, in an LPG, an RI change will induce a shift of the resonant attenuation bands toward shorter wavelengths.

© De Gruyter