Fig. 1. Energy-level diagram of Er3+ ion^[16]. Pump signal absorption and cooperative upconversion are indicated by black solid arrows and red dashed arrows, repectively. Stimulated absorption and emission at the signal wavelength are shown by blue dashed arrows.

Fig. 2. Absorption and amplification region creation in an EDF.

Fig. 3. A typical forward pump EDFA.

Fig. 4. EDFA gain with various doping radii.

Fig. 5. EDFA gain spectra with various doping radii.

Fig. 6. EDFA gain performance as a function of NA with various pump powers.