Fig. 1. Types of single-mode fiber low-loss full-band (O—U band) fiber amplifiers

Fig. 2. Discontinuous band characteristics of parallel structure C+L band erbium-doped fiber amplifier^[16]

Fig. 3. Structure of C+L band hybrid optical amplifier^[24]

Fig. 4. L-band optical fiber amplifier^[31]. (a) Three-pass amplifier configuration; (b) gain spectra of different signal optical powers

Fig. 5. Gain and NF of bismuth/erbium co-doped fibers (illustration: ESA cross-section)^[32]

Fig. 6. Gain characteristics of different EDFAs^[33]. (a) Erbium/ytterbium/phosphorus co-doped fibers with different doping concentrations; (b) erbium/cerium co-doped optical fibers with different doping concentrations

Fig. 7. L-band optical fiber amplifier^[35]. (a) 980 nm dual-pump dual-pass amplification configuration; (b) gain and NF characteristics

Fig. 8. Gain characteristics of PEDF1 and PEDF2^[36]

Fig. 9. Gain and NF characteristics with 1210 or 1240 nm pumps combined with 1267 nm pump^[37]

Fig. 10. Gain and NF characteristics of 152 m BPSF in single-pass and dual-pass amplification configurations^[38]

Fig. 11. Gain characteristics of bismuth-phosphorus co-doped silica optical fiber^[42]

Fig. 12. Gain spectral characteristics of different signal optical powers^[43]

Fig. 13. Gain and NF characteristics of BDFs at different wavelengths under two amplification systems^[46]

Fig. 14. Gain characteristics of BDFA^[47]. (a) Signal power is -10 dBm and -23 dBm (inset shows the fluorescence characteristics of BDSF); (b) different signal powers at 1420 nm

Fig. 15. Gain and NF characteristics of 220 m BPSF^[49]

Fig. 16. Gain (solid line) and NF (dashed line) spectra at pump wavelengths of 793 nm and 808 nm [pump power of single pump is 14 W; pump power of dual pump is 14 W (793 nm) +4 W (808 nm)] ^[51]. (a) Backward pump; (b) forward pump

Fig. 17. Gain and NF characteristics of amplifiers with different BPSFs under different input signal powers^[53]. (a) 117 m BPSF-1; (b) 195 m BPSF-2

Fig. 18. Gain characteristics of fiber amplifiers with different BDFs under 1240 nm and 1310 nm pumps^[54]. (a) BDF-1; (b) BDF-2

Fig. 19. Fluorescence and gain characteristics of BDFs^[55]. (a) Fluorescence spectra under 1180 nm or 1275 nm pump; (b) gain characteristics of two-level BDFA under different input signal powers

Fig. 20. Gain characteristics of BDF under different fiber amplification systems^[56]. (a) 1275 nm single-pump system at -23 dBm input power; (b) comparison of 1275 nm and 1310 nm single-pump systems with dual-pump systems respectively

Fig. 21. Gain characteristics of BDFA^[57]. (a) Gain and NF characteristics at different wavelengths; (b) gain characteristics at 1700 nm varying with input pump power

Fig. 22. Gain and NF characteristics of BDF^[59]

Fig. 23. Gain and NF characteristics of S-band thulium-doped fluoride fiber amplifiers^[63]

Fig. 24. U-band fiber amplifier^[66]. (a) Schematic diagram of system structure; (b) gain and NF curves; (c) output signal spectra after amplification

Fig. 25. Comparison of gain characteristics of the first-stage and second-stage amplifiers of TDFA^[67]