Significance At present, gain fibers of lasers and amplifiers used in optical communication systems are more common in rare earth ion-doped glass fibers. However, the inherent f-f transition of rare earth ion leads to the narrow transmission bandwidth which cannot meet the increasing demand for network data traffic transmission. Bi-activated optical glasses and fibers can exhibit broadband NIR luminescence in a spectral region of 1 000-1 800 nm spanning the whole low-loss optical communication window, which possesses unique advantages over traditional rare-earth ions and transition metal ions doped glasses or glass-ceramics. Moreover, Bi-doped glass fibers have achieved laser output and optical signal amplification in the range of 1 150-1 550 nm and 1 600-1 800 nm. This fully shows that Bi-doped glass fiber is expected to solve the problem of insufficient data transmission capacity, and becomes a gain material for the next generation of fiber lasers and amplifiers. Progress The research progress of Bi-doped glass and fiber can be illustrated by the discussion of the luminescence mechanism, the performance improvement of Bi-doped glass, the exploration of optical fiber preparation methods, and the application progress of Bi-doped fiber. Bi has the electronic configuration of (Xe) 4f145d106s26p3, where the outer 6s and 6p electrons have the significant interaction with the host glass, thereby showing host dependent absorption and emission properties and exhibiting a number of oxidation states such as +1, +2, +3 and +5. Thus, there are a number of hypotheses regarding the origin of NIR luminescence centers in Bi glasses: Bi clusters, BiO, Bi5+, Bi+ and some other low valence states of Bi ions including metallic Bi, point defects, and Bi dimers. At present, it is generally accepted that the NIR luminescence of Bi comes from low-valence Bi ions such as Bi+ and Bi0 (Fig.1 and Fig.2). Because Bi related NIR photoluminescence is quite sensitive to the local chemical environment, broadband NIR luminescence can be achieved in a variety of matrix glasses. However, low efficiency and narrow bandwidth (emission bandwidth is difficult to cover the communication C- and L-band with important applications) are the main problems of Bi-doped glass. Thus, diverse strategies were proposed to improve the optical performance of Bi-doped glass, such as modifying glass structure, constructing local reduction environment, employing high-energy radiation, co-doping multiple ions and inducing multiple Bi emission centers (Fig.4-8). The efficient luminescence of glass is critical to the gain characteristics of subsequent fibers. Similarly, the preparation method of the optical fiber is also very important to obtain a high-performance optical fiber. Thus, various fiber preparation methods, such as MCVD (Modified Chemical Vapor Deposition), molten core, and rod-in-tube method, were explored for the preparation of Bi-doped fibers with different needs (Fig.9-11). The Bi fiber prepared by MVCD method shows the characteristics of high purity and low loss, and is the most commonly used method at present. By co-doping Bi and different modified ions (Al, P, and Ge) in the core glass, laser output and optical signal amplification in different spectral regions (1160-1775 nm) can be achieved and expanded (Fig.12 and Tab.1). The wavelength of Bi fiber lasers and amplifiers can cover the 1160-1775 nm region, which not only includes the area covered by rare earth ions-activated fiber lasers, but also makes up for the gaps in other communication bands of today's fiber lasers. Conclusions and Prospects Over the years, significant results have been achieved in theory, preparation method, performance optimization and practical application for Bi-doped glass and optical fiber, which have laid the foundation for the development of new, broadband, high-efficiency and tunable lasers and amplifiers, and are also very in line with the development needs of large-capacity and high-speed optical communications in the future. In addition, there are many other challenges, one of which is figuring out the active state in the Bi-doped glass and optical fiber that causes NIR emission. Many hypotheses were reported based on experimental facts, but none confirmed all the properties in the Bi-doped fibers. By understanding the active state of the Bi that contributes to NIR emission, fiber manufacturing conditions can be optimized to develop highly efficient fibers for lasers and amplifiers. This requires a great deal of attention, and once solved, it will revolutionize the next generation of Bi-doped fiber lasers and amplifiers. © 2023 Chinese Society of Astronautics. All rights reserved.
