Fig. 1. Energy level diagram of Er³⁺/Yb³⁺ co-doped fiber

Fig. 2. Refractive index profile of optical fiber preform

Fig. 3. Change curves of absorption coefficient and background loss. (a) Cladding absorption coefficient and background loss; (b) core absorption coefficient

Fig. 4. ASE test structure chart

Fig. 5. Curves of laser output power of 1.55 μm with pump power

Fig. 6. Laser performance test experimental setup

Fig. 7. Output power of laser with wavelength of 1550 nm varies with pump power and output spectrum when output power is maximum