The mid-infrared (MIR) spectral band, spanning the 2.5‒5.0 µm wavelength range, is included within the atmospheric transmission window and features medium molecular absorption and concentrated thermal radiation energy. These unique characteristics render MIR lasers crucial for applications in atmospheric remote sensing, medical diagnostics and therapy, gas sensing, precision detection, and material processing. Among the various laser technologies, fiber lasers are particularly favored because of their excellent beam quality, compact structure, effective heat dissipation, and high optical-to-optical conversion efficiency. With advancements in fiber laser technology, the output wavelength range has gradually expanded into the MIR band. Fluoride glasses, which are known for their low phonon energy, broad MIR transmittance, and high solubility for rare-earth ions, have been identified as ideal gain media for MIR lasers. Among the various types of fluoride glass, fluoroaluminate glass stands out because of its moisture resistance and mechanical strength, which significantly promote its application in high-power laser systems. This study traces the evolution of the composition, structure, MIR emission characteristics, and fiber fabrication techniques of fluoroaluminate glasses since the 1950s, focusing on the latest representative research findings on MIR fluoroaluminate fiber lasers. Hence, this study provides a strong basis for the future research on MIR fluoroaluminate fiber lasers and the applications of the technology.

The development of fluoroaluminate glass, with its unique properties (including its low refractive index, high glass transition temperature, and enhanced mechanical strength), has attracted significant interest since it was patented by Sun in 1949. The glass-forming regions of the AlF₃-RF₂, AlF₃-RF₂-YF₃ (AYF), and AlF₃-YF₃-PbF₂ (AYP) systems have been extensively studied, leading to improved stability and reduced crystallization tendencies, as shown in Figs. 1‒4. The structure of fluoroaluminate glass, which is primarily composed of [AlF₆]^3- octahedra, has been investigated using techniques such as X-ray diffraction, neutron diffraction, Raman spectroscopy, and molecular dynamics simulation, revealing insights into its network structure. Furthermore, the MIR luminescent properties of fluoroaluminate glass doped with rare-earth ions have been investigated, with significant achievements in realizing emissions at 2.7 μm, 3.5 μm, and beyond, thereby demonstrating the potential applications of the material in the advancement of laser technology.

There have been several innovations in the techniques used for the fabrication of fluoroaluminate glass fibers. The development of AlF₃-BaF₂-CaF₂-YF₃-SrF₂-MgF₂-ZrF₄-NaF(AZF) glass fibers with low losses is a particularly significant milestone. The subsequent successful fabrication of glass fibers from both A1F₃-BaF₂-YF₃-PbF₂-MgF₂ (ABYPM) and AYF glass systems underscores the progressive refinement of fluoroaluminate fiber technology.

Fluoroaluminate glass fibers with higher mechanical strength and stronger resistance to moisture can theoretically facilitate the output and stability of high-power MIR lasers. Continuous research on fluoroaluminate fibers doped with rare-earth ions, such as Er³⁺, Ho³⁺, and Dy³⁺, has resulted in the active development of MIR fiber laser technology. In 1990, Yanagita et al. reported a 2.715 μm laser that employed Er³⁺-doped AZF glass fiber with a maximum output power of 2.1 mW. Over nearly 25 years of development, the outputs of MIR fluoroaluminate fiber lasers have been scaled up to approximately 10 W, as shown in Table 3. In 2018, Jia et al. used a 1120 nm fiber laser to pump Ho³⁺-doped ABYPM glass fiber, achieving laser output at 2.868 μm. The maximum unsaturated output power was 57 mW with a slope efficiency of 5.1% (Figs. 7 and 8). In the following two years, He et al., Wang et al., and Zhang et al. optimized the doping of rare-earth ions, using co-doped Ho³⁺/Pr³⁺ to reduce the population of the laser lower energy levels, and achieved higher power and higher efficiency MIR laser output at approximately 3 μm. Significantly, Liu et al. achieved MIR laser emission with a Watt level of approximately 2.87 μm in 2021 (Figs. 12‒14). In 2022, Xu et al. first reported the use of 800 nm femtosecond laser line-by-line direct writing technology to write fiber Bragg gratings into co-doped Ho³⁺/Pr³⁺ AZF glass fibers (Figs. 16 and 17). The scheme to enhance the output performance of approximately 3 μm lasers via cascaded laser output has also been successfully implemented. In 2024, Liu et al. used a 1150 nm Raman fiber laser to pump Ho³⁺-doped AYF glass fiber and achieved cascaded laser output at approximately 3 μm and approximately 2 μm under normal atmospheric conditions at room temperature and humidity of 40%. The maximum unsaturated output power was 11.6 W (5.82 W @~3 μm and 5.76 W@~2 μm) with a slope efficiency of 29% (Fig. 15). This comprehensive overview of the evolution of fluoroaluminate glass and its pivotal role in the advancement of MIR laser technology, highlights the significant strides made in this field, and establishes a robust foundation for future innovations.

Conclusions and Prospects In this study, we review the progress in the research on fluoroaluminate glass and optical fibers in the development of MIR lasers and analyze the significance of the material with regard to the advancement of laser technology. Recent studies have demonstrated that MIR fluoroaluminate fiber lasers achieve remarkable output levels (approaching approximately 10 W) when a 1150 nm fiber laser is employed as the pump source. In the future, by developing new types of aluminum fluoroaluminate glass systems, exploring the preparation of double-cladding fluoroaluminate fibers, researching fiber grating engraving technology, and studying heterojunction fiber fusion technology, an all-fiber-structure MIR fluoroaluminate fiber laser can be developed. Such lasers are expected to deliver higher power outputs, improved efficiency, and extended wavelengths in the MIR spectrum. These advancements aim to satisfy the growing demands of practical applications and are expected to drive new breakthroughs in both the scientific research on MIR lasers and the industrial applications of the technology.