The exponential growth in data transmission capacity during the information era has led to rapid developments in large models, high computing power, large clusters, and ultra-high rate, ultra-large capacity all-optical network communication, challenging traditional optical transmission windows. The transmission capacity of current optical fiber communication systems has encountered a “bottleneck”, making bandwidth expansion one of the most effective methods to increase communication capacity. The fiber amplifier serves a vital function in this context. Consequently, research on ultra-wideband fiber amplifiers enables broader communication bands for transmission applications.

The amplification characteristics of fiber amplifiers are fundamentally linked to the performance of their doped optical fibers. Erbium-doped fibers (EDFs) maintain an essential position in wideband amplification within the C+L band, attributed to the luminescence characteristics of erbium ions. Amplification in other communication bands necessitates alternative doping elements. Bismuth-doped fibers (BDFs), a recent research focus, demonstrate versatility in forming various luminescent active centers through element combinations, exhibiting broadband luminescence characteristics within 1150?1700 nm, enabling effective amplification across O, E, S, L and U bands. Thulium-doped fibers (TDFs) utilize thulium ions’ wide gain spectrum characteristics in the 1450?2100 nm range, encompassing S, L and U bands.Recent years have witnessed substantial advancements in fiber amplifiers primarily based on EDFs, BDFs, or TDFs for achieving ultra-wideband amplification. Erbium-doped fiber amplifiers (EDFAs) continue to expand into the C+L band and L⁺ band regions. Bismuth-doped fiber amplifiers (BDFAs) have achieved notable progress in multiple-band amplification, leveraging bismuth ions’ broadband luminescence characteristics, particularly in O+E, E+S, and O+E+S bands. Thulium-doped fiber amplifiers (TDFAs) have emerged as crucial components for expanding amplification in S and U bands. Thus, analyzing these three primary types of doped fiber amplifiers holds substantial importance for achieving ultra-wideband amplification.

Researchers have explored various EDFA configurations utilizing different optical components to enhance performance (Fig. 3). In 2019, Almukhtar et al. from the University of Malaya achieved approximately 14 dB flat gain between 1535 and 1605 nm through a two-stage dual-pass system. In 2023, Zhai et al. from the University of Southampton developed a dual-pump dual-pass amplifier system, achieving L-band gains exceeding 20 dB, representing a significant advancement in L-band fiber amplifiers (Fig. 7). In 2024, Liu et al. from Shanghai University fabricated PbS/Er co-doped fibers using atomic layer deposition (ALD) and modified chemical vapor deposition (MCVD), extending the bandwidth with gains above 20 dB to 1627 nm for the first time (Fig. 8).

The BDFA employs bismuth active centers associated with phosphorus (BACs-P) in BDF for effective O-band amplification. In 2019, Thipparapu et al. from the University of Southampton constructed a dual-pump dual-pass optical fiber amplification system, achieving peak gains approaching 40 dB (Fig. 10). In 2023, Mikhailov et al. from the United States demonstrated gains exceeding 20 dB between 1255 and 1355 nm. In 2024, Chen et al. from Shanghai University implemented a 1240 nm dual-pump dual-pass amplification system, achieving maximum gains of 0.5 dB/m (Fig. 13). BDFAs achieve E-band amplification utilizing bismuth active centers associated with silicon (BACs-Si) in BDF. In 2024, Liu et al. from Huazhong University of Science and Technology achieved wideband amplification between 1390 and 1510 nm using only 16 m optical fiber length, with unit length gains reaching 4.06 dB/m in the dual-pass amplification system (Fig. 11).

The BDFA can utilize both BACs-P and BACs-Si in BDF for high-performance O+E band amplification. In 2024, Zhai et al. from the University of Southampton demonstrated high-gain and wideband amplification across O, E, and S bands, achieving gains exceeding 20 dB between 1335 and 1473 nm at -10 dBm signal input power (Fig. 17). That same year, Yang et al. from Shanghai University fabricated BDF using ALD combined with MCVD technology, achieving gain intensities exceeding 15 dB between 1280 and 1495 nm (Fig. 18). Regarding L-band and L+U-band amplification, Liu et al. from Huazhong University of Science and Technology fabricated high-germanium-bismuth-doped fibers in 2024, achieving amplification between 1595 and 1670 nm using a 1550 nm pump, with gains reaching 26.3 dB at 1670 nm (Fig. 22).

The S-band TDFA primarily utilizes fluoride and tellurium glass fibers with relatively low phonon energy as gain media. In 2006, Aozasa et al. conducted a systematic investigation of thulium ion doping concentration’s influence on S-band TDFA gain characteristics. With a thulium ion doping concentration of 6×10^-3, they achieved a gain of 22 dB within the wavelength range of 1477?1507 nm (Fig. 23). In the U-band, Chen et al. (2014) employed thulium-germanium co-doped silica fibers as the gain medium, incorporating fiber gratings to optimize the amplifier system structure, and extended the gain bandwidth of the U-band fiber amplifier to 1628 nm (Fig. 24).

Considering future market opportunities and development trajectories, new band optical fiber amplifiers demonstrate significant potential in data center optical interconnection applications, enabling the high-speed interconnection of AI computing power networks. EDFAs have achieved significant advances in the C+L band and L⁺ band, delivering gains exceeding 22 dB throughout the L band, and successfully extending the bandwidth with gains above 20 dB to 1627 nm. BDFAs have achieved amplification with gain intensities exceeding 15 dB within the wavelength range of 1280?1495 nm, while the gain per unit fiber length in the E+S band has reached 4.06 dB/m. TDFAs have demonstrated amplification with full-band gains greater than 25 dB in the S and U bands. Nevertheless, opportunities remain for optimizing doped ion concentrations in optical fibers, and enhancement of preparation processes is required to improve amplification system performance. Future developments integrating the mutual regulation mechanisms of erbium ions, bismuth ions, thulium ions and other ions, while focusing on optimization of preparation processes and amplification system parameters, will facilitate enhanced fiber amplifier performance during amplification using active doped fiber. The advancement of full-band ultra-wideband fiber amplifiers presents promising future prospects.