Tremendous advances in high-speed communications technologies have enabled modern services, including live 4K video streaming, real-time remote surgery, artificial intelligence (AI), the Internet of Things (IoT), virtual reality (VR), cloud storage, and social media. The further development of these digital platforms will inevitably require increased data transmission rate, which significantly exceed the current capabilities of existing high-speed communications systems. To meet the ever-growing data traffic, it is necessary to develop and implement new advanced solutions. Multi-wavelength transmission technology is considered one of the most promising approaches, which can potentially increase a bandwidth of transmission data over optical fiber systems by utilizing an extended range of wavelengths (from O- to U-band), where the optical loss of conventional single-mode fiber is below 0.2-0.3 dB/km. However, the success of this approach depends on the development of new amplification technologies, as traditional optical amplifiers based on fibers doped with rare earth ions, especially Er³⁺ ions, are inherently incapable of providing effective amplification beyond the C+L telecom bands. This has spurred research into promising amplification media, which began more than 20 years ago.

Bismuth (Bi)-doped fibers (BDFs) are a unique active medium suitable for optical amplifiers and lasers operating in a spectral range of 1.15-1.78 μm. The progress achieved in the development of BDFs and optical devices based on them gives hope that multi-band technologies capable of operating over the entire available spectral range can be successfully implemented in the near future. This is confirmed by the presence of commercially available devices developed by a number of telecom companies, as well as the start of implementation of bismuth-doped fiber amplifiers (BDFAs) for O-, E-, and S-band data transmission over optical communication systems. However, the progress achieved in the development of BDFA and BDF lasers was due not only to the solution of applied problems, but also to a deeper understanding of the fundamental principles of formation of bismuth active centers (BACs) and their physical nature. This review presents the main achievements in terms of optical characteristics of Bi-doped materials (crystals, ceramics, bulk glasses and optical fibers) and devices developed using these materials. This highlights that the structure and chemical composition of the glass matrix strongly influence the resulting optical properties of these media. Some fabrication strategies such as the modulation of topological order, coordination engineering, smart confined doping, and direct cluster control, and novel approaches for performance analysis (for example, "hidden potential") of BDFs are emphasized and discussed. The peculiar properties of bismuth active centers (BACs), in particular, optical anisotropy and "dark precursors", characterizing their structure and possible process leading to their formation are considered. Also, this review evaluates novel designs of BDFs, especially, heterogeneous glass-core fibers, which can be used for solution of the practical problems. For instance, such designs can be useful for developing a broadband flattop optical amplifier with adopted characteristics. In addition to bismuth-doped materials, this review includes the mainstream results in BDFAs for advanced optical technologies, summarizing the obtained results over two decades. Despite the significant progress the prospects for commercial production of BDFs remain uncertain that primarily due to difficulties in the reproducibility of Bi-doped fiber parameters and the high level of unsaturable loss in highly Bi-concentrated fibers. Addressing these challenges is essential to advancing commercialization and ensuring rapid deployment of this technology.