Fig. 1. Measured spectrum of CTFBGs fabricated for the following experiments: (a) CTFBG I and (b) CTFBG II.

Fig. 2. Experimental configuration for the suppression of SRS in a tandem pumping fiber amplifier. HR, highly reflective FBG; OC, output coupler FBG; LD, laser diode; YDF-20/400, Yb-doped 20/400 fiber; YDF-30/250, Yb-doped 30/250 fiber; CLS, cladding light stripper.

Fig. 3. Changing spectra of output as the pump power increases (a) without and (b) with a CTFBG inserted, and comparison of (c) spectra at pump power 3490 W, (d) signal ratio, and (e) output power versus pump power with and without CTFBG I together with the beam quality and profile of the output.

Fig. 4. Changing spectra of the output as the pump power increases with (a) CTFBG I and (b) CTFBGs I and II inserted. Comparison of (c) spectra at pump power 4790 W, (d) signal ratio, and (e) output power versus pump power with CTFBG I or both CTFBGs together with the beam quality and profile of the output.