Fig. 1. Properties of single-mode fiber. (a) Refractive indexes of different modes; (b) phase matching curves

Fig. 2. Transmission spectra versus grating parameter. (a) Grating period; (b) modulation depth of refractive index; (c) number of periods

Fig. 3. Grating fabrication system. (a) Schematic of LPFG fabrication system; (b) microscopic structure of LPFG

Fig. 4. Experimental results. (a) Transmission spectrum versus grating period (P=7.2 W, N=40, T=1000 ms); (b) resonant wavelength versus grating period; (c) transmission spectrum versus scanning cycle (Λ=750 μm, N=30, T=1000 ms); (d) resonant wavelength versus scanning cycle; (e) transmission spectrum versus number of cycles (Λ=750 μm, P=7.2 W, T=1000 ms)

Fig. 5. Setup and test results of LPFG temperature characteristic testing. (a) Testing setup; (b) LPFG transmission spectrum versus temperature; (c) central wavelength of loss speak in transmission spectrum versus temperature