Fig. 1. Absorption spectra with cross-sectional image of fiber shown in inset

Fig. 2. Amplification testing system. (a) Forward pump; (b) backward pump; (c) bidirectional pump

Fig. 3. Gain performance of different BEDFs in forward pump amplification testing system. (a) Gain spectra; (b) noise figure (NF) spectra

Fig. 4. Gain performance of BEDF-4 under different pump conditions. (a) Gain spectra; (b) NF spectra

Fig. 5. Gain performance of BEDF-2 under bidirectional pumping. (a) Gain spectra; (b) NF spectra