Fig. 1. Energy level diagram of Er³⁺ and Yb³⁺ ions.

Fig. 2. Numerical simulation results of various pump schemes: (a) different pump wavelengths; (b) different pump directions.

Fig. 3. Numerical simulation results of threshold power to: (a) effective length; (b) effective diameter.

Fig. 4. Schematic of the experimental setup. Pre-Amp, pre-amplifier; ISO, optical isolator; MFA, mode field adapter; CLS, cladding light stripper; LD, laser diode; CL, collimating lens; WP, wedge prism; FC, fiber collimator.

Fig. 5. Spectra of backward light at different output powers.

Fig. 6. Backward optical powers at different cooled plate temperatures.

Fig. 7. Backward optical powers at different passive fiber lengths.

Fig. 8. (a) Output power; (b) optical spectrum of signal light for the seed, pre-amplifier, and main amplifier, respectively; (c) output spectra of signal light and ASE.