Fig. 1. Gain performance of Er³⁺/Yb³⁺ co-doped phosphate planar waveguide amplifier^[31]. (a) Internal gain under different 405 nm LED pump power densities; (b) output signal intensity with 275 nm LED pump light on and off

Fig. 2. Structure and gain performance of erbium-doped Al₂O₃ slot waveguide amplifier^[34]. (a) SEM image of the cross-section of silicon nitride slot waveguide; (b) SEM image of the cross-section of the waveguide after depositing Al₂O₃; (c) signal enhancement of hybrid slot waveguides with lengths of 250, 700, and 1200 μm; calculated (real point) and simulated (solid line) optical gains as functions of pump optical power for hybrid slot waveguide with length of (d) 250 μm, (e) 700 μm, and (f) 1200 μm

Fig. 3. Structure and gain performance of Si₃N₄∶Er waveguide amplifier^[35]. (a) Waveguide amplifier excited by 1480 nm pump light; (b) key manufacturing processes, optical transverse electrical mode diagrams, and SEM images; (c) measured and simulated waveguide gain; (d) output power, with inset of output spectrum

Fig. 4. Schematic diagrams of on-chip LNOI∶Er waveguide amplifier. The insets show pump wavelength and signal wavelength, and mode images around the input and output port of the waveguide^[36]

Fig. 5. Schematic diagrams of erbium-doped lithium niobate optical waveguide amplifier^[37]. (a) Crystal structure of lithium niobate and the positions of Er³⁺ in the unit cell; (b) schematic diagrams of waveguide amplifier and signal amplification principle

Fig. 6. Schematic diagrams of experimental setup for measuring gain of erbium chloride silicate^[38]. (a) Schematic diagram of gain measurement; (b) schematic diagram of coupling between tapered fiber and both ends of nanowire; (c) top view microscope image of the coupled system

Fig. 7. Structure of erbium silicate planar waveguide amplifier. The insets show the calculated fundamental transverse electrical mode distribution of the waveguide. (a) Strip waveguide^[39]; (b) slot waveguide^[40]; (c) hybrid waveguide^[41]

Fig. 8. Structure and gain performance of hybrid optical waveguide amplifier based on lithium-doped erbium ytterbium silicate^[42-43]. (a) Schematic diagram of device structure; (b) output signal spectra of waveguide under different pumping powers, with inset of corresponding gains for different pump powers