Fig. 1. Experimental setup of the single-pass (solid, without C1 and C2) and double-pass (dashed) BDFA.

Fig. 2. Experimental setup of the BDFL.

Fig. 3. (a) Absorption spectrum (the inset shows the cross section) and (b) unsaturable loss of the BPSF.

Fig. 4. The optical amplification characteristics of the single-pass fiber amplifier for a 200 m BPSF.

Fig. 5. The amplification characteristics of the 200 m BPSF in the single-pass and double-pass amplifiers with the signal power of −30 dBm and the pump one of 785 mW at 1240 nm.

Fig. 6. The variations of the gain and the NF under different (a) pump and (b) signal powers in the double-pass amplification setup.

Fig. 7. (a) The dependence of the laser slope efficiency on the fiber length for different reflectivities of the OC FBGs. (b) The relation between the output power and the pump power (the inset shows the laser spectrum measured at ∼1313 nm).