The mid-infrared band of 2?5 μm, which contains the atmospheric transmission window and the fingerprint absorption peaks of many molecules, holds vast potential applications in areas such as biomedicine, precision machining, and mid-infrared photoelectrical countermeasures. Although high-power mid-infrared lasers can be generated using methods such as nonlinear frequency conversion, the existing devices are often bulky and structurally complex, making them unsuitable for scenarios with complex optical paths or spatial constraints. Optical fibers, as an excellent and flexible light guide medium, can realize miniaturized and lightweight systems and have been widely studied. The materials mainly used for optical fibers include silicate glass, tellurite glass, fluoride glass, and sulfide glass. Among them, silicate glass fibers have an infrared cut-off edge at approximately 2.5 μm, making them unable to transmit lasers in the mid-infrared range. Tellurite glass fibers exhibit significant intrinsic absorption in the wavelength range greater than 4 μm, leading to rapid increases in loss and an inability to meet low-loss transmission in the 2?5 μm full spectrum. Sulfide fibers have a broader infrared transmission range; nevertheless, the absorption bands caused by S—H and Se—H bonds are challenging to eliminate, resulting in higher average loss in the 2?5 μm wavelength range. Currently, the main choice for low-loss transmission across the entire 2?5 μm range is fluoroindate fiber (InF₃-based fiber). This study introduces the design and preparation of InF₃-based fibers suitable for high-power mid-infrared laser transmission.

The early investigation by the research team on the crystallization behavior of fluoroindate glass indicated that in the molten state of multi-component InF₃-based glass, high field strength fluoride cations compete for the nearby fluoride ions, leading to phase separation. The formation of phases reduces the activation energy for non-uniform nucleation in the melt, prompting spontaneous crystallization during the glass cooling stage. Based on this, the research team utilized inorganic glass engineering software for simulation analysis. Combining the team own accumulated experience in fluoroindate glass formulations, the composition of InF₃-based optical fiber core/cladding glass is designed. Using the designed glass composition, a precursor for fluoroindate glass is prepared through the melt-quenching method. Combined with the team developed physical-chemical dehydroxylation technique, water is efficiently removed from the glass. Finally, high-power transmission InF₃-based optical fibers are drawn by employing a “short heating zone” specialized optical fiber drawing process.

Figure 1 displays the mid-infrared transmittance spectrum of a 10 mm thick InF₃-based glass precursor, which reveals no significant absorption near the 2.8 μm wavelength, indicating effective elimination of hydroxyl groups in the glass. Through differential thermal analysis (DTA), the transition temperature (T_g) and crystallization temperature (T_c) of the glass are investigated (Fig.2). The glass transition temperatures for the core and cladding glasses are 295 ℃ and 299 ℃, respectively. The glass transition temperatures for the core glass and cladding glass are close, which is favorable for the fiber drawing process. The crystallization temperatures for the core and cladding glasses are 384 ℃ and 381 ℃, respectively. Using the formula ?T=T_c-T_g, the thermal stability parameters (?T) for the core and cladding glasses are calculated to be 89 ℃ and 82 ℃, which indicates that both glasses have good thermal stability, making them suitable for subsequent fabrication of fibers. The optical fiber preform rod is heated to the vicinity of the glass transition temperature. Once the preform rod forms a molten tip, it is drawn and elongated into an optical fiber under the influence of gravity. The dimensions of the fiber are illustrated in Fig.3, with a core diameter of 200 μm and a cladding diameter of 260 μm. The average loss of the optical fiber is ≤0.22 dB/m @ 3?5 μm [Fig.3(b)]. In practical applications, optical fibers may encounter sharp and pointed objects, potentially leading to damage during use. Armoring the optical fiber to create a fiber optic cable can effectively address this issue, significantly enhancing the safety of optical fiber use (Fig.4). The transmission results of optical fibers and cables are shown in Fig.5. Pulsed laser with a wavelength of 3.7?4.8 μm is used as the target light source. The transmission of pulsed laser at the 10 W level has been realized in a laboratory environment through spatial coupling.

The Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences has successfully produced high-power transmitting energy InF₃-based optical fibers, demonstrating initial capabilities for independent production of mid-infrared fluoroindate optical fibers. The fiber products with independent intellectual property rights are initially realized the indigenization substitution. The manufactured cables have achieved pulsed laser output at the 10 W level in the 3.7?4.8 μm wavelength range, showcasing excellent mid-infrared laser energy transmission performance. With the ongoing optimization and adjustments to fiber composition and manufacturing processes, the optical and mechanical properties of the fibers will further improve, providing robust support for the development of high-end infrared optical systems in our country.