Fig. 1. Schematic diagram of experiment setup

Fig. 2. Schematic illustration of the refractive index in FBG

Fig. 3. (a) Spectra of weak FBG with dose of UV exposure; (b) UV-induced loss α varies with UV-induced refractive index change δn

Fig. 4. Refractive index change and loss increasing during FBG inscription. (a) δnmod, δneff and δnmod/δneffvaries with dose of UV exposure; (b) loss coefficient of FBG varies with its coupling coefficient

Fig. 5. (a) δnmod/δneff and (b) α/κ varies with the distance between the phase mask and the fiber core

Fig. 6. (a) δnmod/δneff and (b) α/κ vary with the distance between the phase mask and the fiber core, using the two different phase masks