Femtosecond laser exposure is used to fabricate a fiber Bragg grating (FBG) over a fiber core cladding on a short section of photon-sensitive thin-core fiber (TCF) using a side-illumination technique. The cladding modes are excited in the fusion region due to the core mismatch between the leading-in single-mode fiber and the TCF, and one resonant cladding mode and core mode are then transmitted downstream as separate wavelengths by the FBG.
Furthermore, the transmission intensities for the two resonant dips could be controlled by changing the focal-line position of laser beam. Because the cladding modes have unique field shapes, they react to perturbations both inside and outside the fiber, which makes them good candidates for measuring fiber bending and ambient refractive index variation.