The ISO 20473:2007(e) standard established by the International Organization for Standardization defines the mid-IR band as the specific spectral range spanning 3–50 µm in wavelength^[1]. However, practical applications demonstrate that the recognized boundaries of the mid-IR spectrum remain inconsistent across different technical fields. In laser technology, the 2–25 µm wavelength range generally constitutes the mid-IR spectrum. This spectral range is characterized by three principal features, specifically atmospheric transmission windows, the molecular fingerprint region, and the thermal radiation band.

The atmospheric medium, consisting of gaseous molecules with minor aerosol and particulate constituents, exhibits intricate light-matter interactions. Photon propagation through atmospheric pathways is governed by refraction, scattering, and absorption. Molecular nitrogen and oxygen demonstrate negligible absorption beyond 0.3 µm in the atmospheric transmission spectrum. The predominant attenuation mechanisms arise from strong absorption features of water vapor (H2O:2.7, 3.2, 6.3 µm), carbon dioxide (CO2:2.7, 4.3 µm), and ozone (O3:4.8, 9.6, 14.2 µm). Comparative spectral analysis reveals that while maximum atmospheric transmittance in the visible region reaches approximately 65%, specific mid-IR windows—particularly 2.1–2.4 µm, 3.3–4.2 µm, and 8.2–11.7 µm—exhibit enhanced transmission exceeding 65%, with the 3.4–4 µm spectral band achieving exceptional transmittance up to 90%^[2,3]. These transmission characteristics substantiate the strategic advantage of mid-IR radiation for long-distance atmospheric laser communication systems.

Different molecules exhibit varying absorption strengths for photons at different wavelengths, and their characteristic absorption spectra regions are known as “molecular fingerprint regions”. By employing spectroscopic identification techniques, the types of molecules present can be detected and distinguished based on the known absorption lines of specific molecules. The mid-IR region contains the absorption lines of many important molecules. In addition to the major atmospheric constituents mentioned earlier, gases such as carbon monoxide (CO:4.6 µm), nitric oxide (NO:1.8, 2.7, 5.2, 5.4 µm), nitrous oxide (N2O:2.8, 3.9, 4.1, 4.5, 7.8 µm), and nitrogen dioxide (NO2:3.5, 6.0 µm), as well as solids, liquids, and gases containing C-H bonds (e.g., methane CH4:3.3, 6.5, 7.6 µm), can all be detected using mid-IR light sources^[4]. In particular, with the rapid development of organic chemistry and biochemistry in the 21st century, the detection of organic substances has become increasingly important.

The Planck blackbody radiation experiment demonstrates that the mid-IR region also falls within the range where thermal radiation energy from objects is concentrated. According to Wien’s displacement law, as the absolute temperature of a blackbody increases from 50 to 5800 K, both the total radiated energy and the peak wavelength shift progressively toward shorter wavelengths. When the blackbody temperature reaches approximately 200 K, it begins to emit photons with a wavelength of around 5 µm. Although the peak of radiation moves toward the near-infrared or even visible region as the temperature rises, high-energy mid-IR photon emission remains present throughout. As a result, the mid-IR spectral region plays a crucial role in thermal source detection.

Based on the above three key characteristics, the mid-IR spectral region has attracted widespread attention in areas such as fundamental spectroscopy^[5–9], perception-based imaging^[10–14], medical diagnostics and treatment^[15–17], aerospace^[18–21], and security applications^[22–26].

Mid-IR lasers can be generated by multiple methods, including gas lasers, quantum cascade lasers (QCLs), solid-state lasers, and optical parametric amplifiers (OPAs). Compared with those methods, fiber lasers possess comprehensive advantages such as superior beam quality, high conversion efficiency, compact size, exceptional reliability, and efficient thermal management, positioning them as the most promising candidate for next-generation high-power mid-IR laser generation.

It can be seen from Fig. 1 that, as the gain medium for mid-IR lasers, fluoride fibers, particularly those based on fluorozirconate, fluoroaluminate, and fluoroindate glass, offer several key advantages that make them highly suitable for mid-IR photonics. These fibers exhibit a wide transmission window and low phonon energy, enabling efficient mid-IR light propagation and reducing non-radiative losses, which are critical factors for laser and amplifier applications. Additionally, their excellent rare-earth solubility and structural flexibility allow for tailored doping strategies and customized fiber designs. The good chemical compatibility and high fiber producibility further support scalable fabrication. As a result, fluoride fibers have found broad applications in mid-IR lasers, optical sensing, fiber amplifiers, laser delivery systems, nonlinear optics, and even visible laser generation, demonstrating their versatility and importance in advancing next-generation optical technologies.

Beyond their inherent material advantages, fluoride fibers have wide optical applications due to their ultra-low theoretical transmission loss, estimated at ∼10−3dB/km—three orders of magnitude lower than that of conventional silica fibers. However, impurity-induced absorption and scattering result in the measured loss far exceeding theoretical values. The dominant impurities responsible for practical losses include transition metal ions, hydroxyl (OH−) groups, and microcrystallites formed during fabrication. Transition metal ions introduce electronic absorption bands in the visible-NIR range, while OH− groups exhibit strong vibrational absorption near 2.87 µm. Microcrystallites and phase separation further elevate scattering losses. Advanced purification techniques, such as reactive atmosphere processing and zone refining, have reduced OH− content to <10⁻⁶ and transition metal concentrations to 10⁻⁹ levels in state-of-the-art fibers, enabling losses as low as 0.003 dB/m in laboratory demonstrations. Overcoming these limitations by material purification and advanced fabrication techniques could revolutionize long-haul optical communication, enabling signal transmission over 10,000 km without the need for amplification or regeneration. This prospect is up to breakthroughs in producing fluoride glass fibers that approach their theoretical loss limits, which is also a challenge that continues to drive research in glass chemistry and photonic engineering.

Current research prioritizes the development of fluoride glass compositions resistant to devitrification and moisture degradation. A major obstacle lies in the lack of reliable methods to predict the stability of novel glass formulations, necessitating empirical trial-and-error approaches. To address this, laboratories employ rigorous compositional screening and advanced processing techniques to suppress nucleation and crystallite formation during fiber fabrication. By meticulously controlling impurities and structural defects, researchers aim to achieve scattering losses low enough to realize the theoretical ultra-low attenuation potential of fluoride fibers in the mid-IR spectrum.

This review demonstrates the potential of fluoride glass fibers in mid-IR photonics, with a focus on their material superiority, fabrication techniques, and applications. This review begins with the intrinsic material properties of fluoride glass, particularly their extended mid-IR transparency windows, while addressing persistent challenges in thermal stability and chemical durability. A systematic evaluation of advanced fabrication methodologies follows, including innovative preform and fiber drawing techniques essential for high-performance mid-IR waveguides. This review subsequently explores mid-IR fiber lasers, with special emphasis on rare-earth ion doping lasing, nonlinear effects governing supercontinuum generation, and performance of advanced laser systems. Practical applications are assessed through case studies spanning environmental gas sensing, minimally invasive biomedical diagnostics, and advanced defense systems, demonstrating how fluoride fiber innovations are redefining operational capabilities in these domains. Finally, the work identifies unresolved challenges and presents the proposed research directions to enable next-generation mid-IR photonic systems. By synthesizing cross-disciplinary advances in materials science, photonics engineering, and applied physics, this review aims to establish a foundational framework for researchers advancing the frontiers of mid-IR technology through fluoride fiber innovations.

Fluoride glass, a class of amorphous materials composed of heavy metal fluorides, including fluorozirconate, fluoroaluminate, and fluoroindate glass, have emerged as promising candidates for advanced mid-IR photonic devices. With ultra-broad optical transparency from the ultraviolet to the mid-IR and low phonon energy, fluoride glass address the shortcomings of oxide glass in infrared optical applications.

The evolution of fluoride glass technology has been characterized by three distinct developmental phases, each marked by fundamental breakthroughs in material design. During the foundational phase, systematic compositional adjustments coupled with structural characterization studies enabled the development of fluorozirconate, fluoroaluminate, and fluoroindate glass systems exhibiting enhanced functional attributes. The second phase witnessed strategic lanthanide doping within the glass matrix, where engineered pumping schemes generated tunable fluorescence emissions spanning discrete spectral regions. These photonic characteristics subsequently drove targeted compositional refinement, particularly in dopant-host interactions and energy transfer mechanisms. Building on these advances, the current phase focuses on translating strongly luminescent rare-earth-doped glass into functional optical fibers serving as critical components in mid-IR photonic systems, particularly for laser and amplifier systems.

This section systematically introduces recent advancements in fluoride glass-forming systems, structures, and mid-IR luminescent properties. Furthermore, it also summarizes the physical properties and chemical durability of fluoride glass.

The most established and widely used fluoride glass are the fluorozirconate glass. Within this glass family, ZBLAN with the composition of 53ZrF4−20BaF2−4LaF3−3AlF3−20NaF (in mole fraction), has been the most widely used. It exhibits high stability against crystallization, enabling low-loss fiber fabrication. By contrast, no fibers have been reported for divalent-fluoride-based glass, a fact that is ascribed to their low crystallization stability. Recently, fluoroindate glass have drawn increasing interest due to their extended transmission compared with ZBLAN glass. Meanwhile, fluoroaluminate glass have gained comparable interest for mid-IR applications due to their high glass transition temperature (Tg) and good chemical stability.

Although ZrF4 does not exist in a vitreous form, it exhibits the highest glass-forming ability among fluoride systems. It can be used for preparing glass with binary combinations such as ZrF4−BaF2 or ZrF4−ThF4^[27,28]. However, ternary combinations are needed to obtain samples thick enough for physical and optical characterization. In 1974, Poulain et al.^[29] prepared the first fluorozirconate glass in the ZrF4−BaF2−NaF ternary system, and its glass formation region is shown in Fig. 2(a)^[30]. Nonetheless, it is possible to obtain these glass when the cooling rate is fast enough. Additionally, the introduction of LaF3 into binary systems has been shown to improve the glass-forming ability^[31,32]. Other ternary fluorozirconate glass systems have been proposed and studied, such as ZrF4−BaF2−NdF3^[33], ZrF4−BaF2−ThF4^[34], and ZrF4−BaF2−GdF3^[35,36]. More stable glass can be synthesized in the ZrF4−BaF2−LaF3 systems^[37,38].

Early investigations showed that zirconium could be replaced by hafnium^[39]. However, the development of HfF4-based glass was hampered by the limited purity and the high price of hafnium compounds. In general, a direct Hf/Zr substitution may be carried out in most cases and does not significantly change the physical properties, except for density and refractive index, which are slightly decreased. Glass compositions allowing slower cooling rates may be found in ZrF4−BaF2−LaF3−NaF quaternary systems^[40]. And the most efficient stabilizing agent is AlF3. The stabilizing effect of AlF3 led to the development of several quaternary glass, incorporating 4% (mole fraction) AlF3. The glass formation regions of ZrF4−BaF2−LaF3−AlF3 and ZrF4−BaF2−NaF−AlF3 are shown in Figs. 2(b) and 2(c). When NaF is added to these glass, the glass-forming ability is further improved, and the standard ZBLAN glass was derived from these studies^[40]. The demonstration that adding a few percent of AlF3 improves glass-forming ability resulted in the development of the standard compositions that are now commonly used^[41], the most common of which is ZBLAN glass^[40].

Due to the strong tendency of fluoride glass to devitrify, it is often necessary to introduce multiple components into fluoride glass to enhance their stability^[43]. Therefore, research on the effects of adding or replacing components on the performance of these glass is of great importance. Qiu et al. explained the impact of divalent metal fluorides on the stability of fluorozirconate glass systems using the mixed cation effect^[44]. They found that introducing MgF2 and CaF2 to replace BaF2 in the glass decreases the thermal stability, whereas replacing BaF2 with SrF2 increases the thermal stability. This phenomenon can be explained from the perspective of the glass structure: since the ionic radii of Ba and Sr are similar, their mutual replacement does not significantly affect the glass structure. The primary effect is the interaction between different cations, leading to the “mixed cation effect”. In contrast, the ionic radii of Mg and Ca differ considerably from Ba, and the replacement would cause a greater impact on the glass structure, preventing the occurrence of the mixed cation effect. Furthermore, Smektala and Metachi investigated the effects of YF3, PbF2, and HfF4 on ZBLAN glass, and their study indicated that YF3 strongly stabilizes the glass formation, whereas the introduction of PbF2 and HfF4 reduces the glass’s stability^[45].

In 1981, Almeida et al.^[46] studied the structure of fluorozirconate glass and analyzed the 50Zr−50BaF2 fluorozirconate glass using Raman spectroscopy. They proposed that each Zr ion is coordinated by six F ions, forming a [ZrF6] octahedron, which establishes a glass network with a zigzag chain structure. The Ba ions act as crosslinking agents between these chains. However, in 1985, Inoue et al. measured the radial distribution functions of 2BaF2−3ZrF4 and BaF2−3ZrF4 glass systems and compared them with calculated functions for different coordination numbers. They concluded that the most appropriate coordination was Zr coordinated with eight F ions. Kawamoto et al.^[47] used Raman spectroscopy, X-ray scattering, and molecular dynamics to study BaF2−ZrF4, PbF2−ZrF4, and SrF2−ZrF4 glass. They found that the glass and the corresponding crystalline materials had similar Raman spectra, suggesting that Zr is coordinated with eight F ions.

Research on the mid-IR luminescent properties of fluorozirconate glass has been primarily conducted in ZBLAN and ZBYA systems.

In 2011, Tian et al.^[48] reported intense 2.7 µm emission from Er3-doped fluorozirconate glass. The emission characteristics and energy transfer processes under 980 nm laser diode excitation were systematically investigated. In 2013, Huang et al.^[49] studied the intensive fluorescence of 2.7 µm in highly Er3+ doped ZBYA (ZrF4−BaF2−AlF3−YF3) glass and suggested that Er3+ doped ZBYA glass has potential application in 2.7 µm lasers. They also investigated the energy transfer mechanism of Er3+/Ho3+, Er3+/Pr3+, and Er3+/Tm3+ co-doped ZBYA glass to improve the 2.7 µm emission^[50,51]. In 2017, Liao et al.^[52] reported that the mid-IR emission of ∼2.7μm was realized in Er3+/Tm3+ co-doped ZBLAN glass by 793 nm (or 808 nm) and 980 nm laser excitation. In 2021, Gan et al.^[53] obtained fluorescence emission of 2.72 µm by pumping Er3+-doped ZBLAN glass with 808 and 980 nm lasers. In 2020, Zhao et al.^[54] realized 3.9 µm mid-IR emission in Ho3-doped ZBYA glass, with the results demonstrating the potential viability of this material as an effective 3.9 µm mid-IR laser source.

In 2010, Gomes et al.^[55] employed an optical parametric oscillator (OPO) tunable laser to pump 4% (mole fraction) Dy3+-doped ZBLAN glass substrate, achieving a fluorescence emission peak at 2.88 µm. In 2020, through co-doping Dy3+/Tm3+ ions in ZBYA glass, the calculated emission cross-section value at 2.9 µm reached 3.79×10−21cm2. Experimental findings indicate that fluoride glass shows promising potential as an excellent candidate material for 2.9 µm mid-IR lasers^[56]. In 2022, Xu et al.^[57] reported achieving mid-IR emission at ∼2.9µm in Dy3+/Tm3+ co-doped ZBYA glass using an 808 nm laser diode as the pumping source. In 2013, Gomes et al.^[58] achieved fluorescence emission at 3.69 µm in Yb3+/Pr3+ co-doped ZBLAN glass under 974 nm laser pumping. Their study determined that optimal mid-IR fluorescence emission occurred with 1% (mole fraction) Pr3+ and 5% (mole fraction) Yb3+.

In summary, fluorozirconate glass, particularly ZBLAN, remain the cornerstone of mid-IR fiber technology due to their low phonon energy and exceptional rare-earth ion solubility. However, their susceptibility to devitrification and moisture-driven degradation poses significant challenges for long-term reliability. These limitations underscore the need for advanced stabilization techniques, such as AlF3 doping or hybrid compositions, which will be explored in the following sub-sections on fluoroaluminate and fluoroindate systems.

Research on fluoroaluminate glass originated in 1949 when Sun^[59] disclosed a quaternary AlF3−PbF2−SrF2−MgF2 system, with AlF3 as the glass-forming host. Later in 1976, Nozhkin et al.^[60] identified the mineral usovite (Ba2CaMgAl2F14) in Russia, redirecting research focus toward AlF3−RF2 (R=Ba, Ca, Sr, Mg) systems.

Fluoroaluminate glass initially garnered limited attention due to its great crystallization tendency. Early binary AlF3-based systems faced challenges from the high melting point of AlF3(∼1290°C), necessitating rapid quenching (e.g., liquid nitrogen cooling, roller quenching) to produce millimeter-scale glass^[61–63]. This limitation drove the shift toward ternary and multicomponent systems. In 1979, Videau et al.^[64] identified the AlF3−BaF2−CaF2 ternary system with enhanced crystallization resistance, though rapid cooling remained essential for glass formation.

The AlF3−RF2−YF3 (AYF) system became pivotal in fluoroaluminate glass research. Kanamori et al.^[65] systematically mapped the glass-forming region of the AlF3−BaF2−CaF2−YF3 system in 1981. By 1983, they extended this work to the AlF3−SrF2−MgF2−YF3 quaternary system, synthesizing stable glass via conventional melt-quenching^[66], thereby establishing foundational design principles, demonstrated in Fig. 3(a).

In 1985, Hu et al.^[67] enhanced stability by integrating multiple alkaline earth metals into the AlF3−BaF2−CaF2−SrF2−MgF2−YF3 system, as can be observed in Fig. 3(b). Subsequent advances included doping 40AlF3−10BaF2−30CaF2−10SrF2−10MgF2 (mole fraction) with 10%–15% (mole fraction) YF3/YbF3 to improve glass-forming ability^[68]. Seddon et al.^[69] later confirmed LiF additions minimally affected glass formation. X-ray diffraction guided refinement by 1993 yielded a stable six-component composition, 37AlF3−12BaF2−15CaF2−9SrF2−12MgF2−15YF3 (mole fraction)^[70]. Further innovations incorporated LaF3^[71] and modifiers like TeO2^[72], Ba(PO3)2^[73], and CaCl2^[74], all boosting glass-forming capacity.

In 1987, Izumitani et al.^[75] at HOYA Corporation developed AlF3−BaF2−CaF2−YF3−SrF2−MgF2−ZrF4−NaF (AZF) and AlF3−BaF2−CaF2−YF3−SrF2−MgF2−BeF2 (ABF) glass, achieving superior glass-forming capacity. Iqbal et al.^[76] enhanced AZF thermal stability in 1995, while Zhang et al.^[77] substituted NaF with LiF in 2021 for further optimization. Wang et al.^[78] explored the AlF3−YF3−PbF2 (AYP) system in 1991, synthesizing AlF3−BaF2−YF3−PbF2−MgF2 (ABYPM) glass and mapping its glass-forming region, depicted in Fig. 3(c). Substituting PbF2 with MgF2, CaF2, and BaF2, or YF3 with YbF3 improved thermal stability and crystallization resistance. In 2022, Zhang et al.^[79] discovered that the introduction of ZnF2 can lower the phonon energy of AlF3−BaF2−YF3−SrF2 glass while maintaining good thermal stabilities. In 2024, Mao et al.^[80] Further studied the synthesis of fluoride glass ceramics with this component through melt-quenching technology, followed by heat treatment, to controllably form ZnF2 nanocrystals in a fluoroaluminate glass matrix doped with Er3+.

Consistent with the confusion effects^[81], elevated compositional complexity effectively inhibits crystallization. The evolution from binary to multicomponent fluoroaluminate glass has substantially enhanced thermal stability, primarily attributed to the formation of low-melting-point eutectic phases. Furthermore, component diversification reduces critical cooling rates, thereby improving the system’s resistance to devitrification and mechanical robustness.

The structural investigation of fluoroaluminate glass originated with the 40AlF3−20BaF2−40CaF2 composition. In 1979, Videau et al.^[64] identified a dual coordination network comprising dominant [AlF6]3− octahedra and minor [AlF4]− tetrahedra through XRD and Raman spectroscopy. This interpretation was challenged in 1986 by Kawamoto et al.^[82], who compared the glass’s Raman spectrum with crystalline Na3AlF6 (exclusively [AlF6]3− octahedra), concluding that the glass structure solely consists of interconnected [AlF6]3− units. They attributed the tetrahedral signatures reported by Videau’s group to either rapid melt quenching artifacts or inherent metastability.

Further validation emerged in 1988 when Yasui et al.^[62] employed nuclear magnetic resonance (NMR) spectroscopy to confirm [AlF6]3− octahedra as the fundamental structural unit. Concurrently, Nanba et al.^[83] conducted a multimodal analysis combining XRD, neutron diffraction (ND), and molecular dynamics (MD) simulations. Their results revealed a three-dimensional network of branched [AlF6]3− chains, with isolated octahedra being exceptionally rare. Notably, discontinuous [CaFn] polyhedral chains were identified as integral components of the structural framework.

The quenching methodology significantly influences the structural configuration of fluoroaluminate glass. In 1992, Nanba et al.^[84] demonstrated this phenomenon through comparative studies of twin-roller quenched and copper-plate quenched glass. XRD and neutron diffraction analyses revealed shorter F-F peak distances in copper-plate quenched glass versus twin-roller quenched counterparts, indicative of reduced interatomic spacing. This structural compaction likely arises from corner- or edge-sharing [AlF6]3− octahedra forming branched chain networks. In contrast, twin-roller quenched glass exhibited numerous isolated [AlF6]3− units, potentially attributable to faster cooling rates inhibiting structural reorganization.

Complementary studies by Chen et al.^[85] investigated composition-structure relationships in AYF systems via Raman spectroscopy. At an equimolar AlF3:YF3 ratio, the glass network comprised coexisting [AlF6]3− octahedra and [AlF4]− tetrahedra. Progressive reduction of AlF3 content diminished tetrahedral populations, with [YFn] polyhedra emerging as dominant species at lower AlF3 ratios, completely replacing [AlF4]− units.

Poulain et al.^[86] further elucidated cation roles, proposing that large-radius cations act as network modifiers disrupting structural periodicity, whereas smaller cations fail to integrate into the glass matrix^[82,87].

Investigations of mid-IR luminescent properties in fluorozirconate glass have predominantly focused on AYF, ABYPM, and AZF glass systems.

In 2014, Huang et al.^[88] synthesized AYF glass with high thermal and chemical stability, systematically investigating the 2.7 µm fluorescence emission process in Er3+-doped glass. Their study revealed that compared to ZBLAN glass, Er3+-doped AYF glass exhibited a higher fluorescence branching ratio (20%) and emission cross-section (9.4×10−21cm2) in the ∼2.7μm band, which significantly enhanced 2.7 µm fluorescence emission. In 2021, Zhang et al.^[89] fabricated Er3+-doped ABYPM glass and achieved efficient 3.5 µm fluorescence emission under 638 nm laser diode pumping, further demonstrating the mid-IR luminescent potential of fluoroaluminate glass.

In 2015, Zhou et al.^[90] first realized ∼4μm emission from Ho3+-doped AYF glass through 900 nm Ti: sapphire laser pumping, providing novel insights into mid-IR laser materials for longer wavelengths. In 2021, Liu et al.^[91] obtained intense 2.4 µm emission observed in Ho3+-doped AZF glass under pumping of a 638 nm laser diode. In 2014, Zhou et al.^[92] reported ∼2.85μm mid-IR emission in Yb3+/Ho3+ co-doped AYF glass under 980 nm laser diode excitation. Additionally, the effectiveness of co-doping sensitizer ions (e.g., Tm3+, Er3+, Nd3+, and Yb3+) with Ho3+ to achieve efficient mid-IR fluorescence has been verified. In 2021, Zhang et al.^[93] achieved ∼3.9μm fluorescence emission in Ho3+/Nd3+ co-doped ABYPM glass using 808 nm laser diode excitation. Through analysis of energy transfer mechanisms, they identified that a Ho3+:Nd3+ molar ratio of 2:1 yielded the most efficient ∼3.9μm emission.

In 2016, Zhou et al.^[94] investigated the mid-IR luminescence properties of heavily Dy3+-doped AYF glass under 808 nm laser diode pumping. The emission intensity at 2.86 µm progressively increased with Dy3+ concentration up to 10% (mole fraction) without observable fluorescence quenching, attributed to the high dispersion of a Dy3+-doped glass network and minimal cross-relaxation probability between Dy3+ ions.

In 2021, Zhang et al.^[77] achieved broadband fluorescence emission spanning 2.6–4.1 µm in AZF glass through Yb3+-sensitized Pr3+ under 976 nm laser pumping. Their study determined that optimal mid-IR fluorescence emission occurred with 0.3% (mole fraction) Pr3+ and 1% (mole fraction) Yb3+. This work first elucidated the mid-IR fluorescence mechanism in Pr3+/Yb3+ co-doped AZF glass, offering new possibilities for realizing long-wavelength mid-IR fiber lasers.

The development of fluoroaluminate glass represents a strategic shift toward thermally stable and chemically durable mid-IR materials. With glass transition temperatures exceeding 350°C, these systems address the thermal shortcomings of ZBLAN. However, their higher phonon energy restricts luminescence efficiency, necessitating innovative sensitization schemes. The subsequent discussion on fluoroindate glass introduces an alternative approach, leveraging extended infrared transparency and reduced phonon energy for longer-wavelength applications.

In 1983, Videau first studied fluoride glass based on the InF3−BaF2−YF3 system, but it did not attract significant attention at that time. Subsequently, fluoroindate glass has been paid more and more attention by researchers due to the development of optical fiber, fiber lasers, and fiber amplifiers. Extensive research on fluoroindate glass began in the early 1990s. The study of ternary and multicomponent fluoride glass using InF3 as the main component has gradually revealed the characteristics and advantages of fluoroindate glass, which has resulted in rapid development of fluoroindate glass; however, there are few studies on their structure. It is generally considered that the structure of the fluoroindate glass is similar to that of fluoroaluminate glass, and it is also composed of [InF6] octahedral units. GaF3 is commonly added to fluoroindate glass to improve the stability of the glass and form the InF3−GaF3-based glass, resulting in the so-called BIG glass^[95]. BIG glass is almost as stable as ZBLAN glass. In recent years, a series of relatively stable glass systems such as InF3−BaF2−ZnF2−SrF2 and PbF2−InF3−GaF3 have been developed^[96,97]. Investigators have shown a significant interest in glass with a high content of InF3.

Messaddeq et al.^[98] studied the glass formation of the InF3−SrF2−BaF2−ZnF2−X (X=PbF2, CdF2, NaF, CaF2) system. In these systems, the larger glass-forming region is the glass system containing CaF2 and PbF2, which can be prepared in some cases with a thickness of 15 mm. Bulk samples can be prepared by adding a small amount of GdF3 into the glass, and the most stable glass contains GdF3 less than 4%. Soufiane et al.^[99] added GdF3, NaF, and GaF3 to the In-Zn-Ba-Sr fluoride glass to increase the thermal stability.

Boutarfaia et al. studied the glass formation in the InF3−SrF2−BaF2−YF3 quaternary system; the optimal concentration of YF3 is 5–10%. It was determined for the first time that in a fluoride system, YF3 may possess a glass-forming ability in addition to stabilization. Later, Boutarfaia et al. studied the formation and crystallization kinetics of multicomponent In/Ga-based glass. They used a base glass with a composition of 30InF3−10YF3−25BaF2−5SrF2−10NaF−10ZnF2−10GaF3. According to the “interference principle,” LiF, KF, and CsF were introduced into the glass to replace NaF^[100].

Indium fluoride has the high-temperature cubic polymorph of the ReO3 type at normal pressure^[101]. As the temperature decreases, it passes into the trigonal polymorph of the VF3 type, space group R3c. The melting temperature of InF3 is 1320°C^[102,103]. Indium fluoride is characterized by its high vapor pressure^[104]; it vaporizes strongly upon heating above 900°C, undergoes easy hydrolysis upon slight heating, and absorbs moisture when stored in air. Its hydrate InF3·3H2O can hardly be dehydrated because its hydrolysis with water of crystallization occurs even at room temperature^[105]: InF3·3H2O→InOHF2+HF+2H2O.

The ionic radius of In3+ is substantially smaller than those of lanthanide cations, closer to the radius of the scandium ion, and significantly larger than the radii of gallium and aluminum ions^[106].

A morphotropic transition occurs on going from InF3 to trifluorides of heavy rare-earth elements (Lu-Er, Y). Indium fluoride is partially dissolved in high- and low-temperature RF3 polymorphs corresponding to the structural types a−YF3 and b−YF3, respectively^[107]. Being a strong Lewis acid, indium fluoride forms numerous compounds with fluorides of divalent metals^[95,108].

In recent years, research on the mid-IR luminescent properties of fluoroindate glass has attracted widespread attention from researchers.

In 2021, Wang et al. fabricated Er3+-doped fluoroindate glass, observing strong ∼3.5μm fluorescence emission, and further analyzed its spectral characteristics^[109]. In the same year, He et al.^[110] achieved ∼3.3μm fluorescence emission in Er3+-doped fluoroindate fibers, highlighting their potential as gain media for mid-IR fiber lasers.

In 2016, Gomes et al.^[111] systematically investigated the ∼3.9μm fluorescence emission process in Ho3+-doped fluoroindate glass, analyzing its fundamental spectral properties. They proposed that Ho3+-doped fluoroindate glass fibers could serve as gain fibers for ∼3.9μm mid-IR fiber lasers. However, the ∼3.9μm radiative transition (I55→I56) of Ho3+ ions is a self-terminating process, as the lower energy level I56 exhibits a longer lifetime (4.8 ms) compared to the upper level I55 (13 µs)^[111], making population inversion challenging. To address this, co-doping with additional rare-earth ions has been introduced to reduce the lower-level lifetime and facilitate population inversion. Consequently, Ho3+/Tm3+^[112], Ho3+/Eu3+^[113], and Ho3+/Tb3+^[114] co-doped fluoroindate glass have been developed, demonstrating enhanced ∼3.9μm fluorescence emission and offering new insights for rare-earth ion co-doping strategies in gain fibers. Due to limitations in Ho3+ energy-level configurations, which preclude the use of conventional commercial laser pumps, Wang et al. designed and fabricated Ho3+/Nd3+ co-doped fluoroindate glass, achieving ∼3.9μm fluorescence emission under pumping by a standard 808 nm laser diode^[115].

In 2018, Majewski et al.^[116] synthesized Dy3+-doped fluoroindate glass, achieving fluorescence emission centered at 4.3 µm. Their work further confirmed the suppressed multiphonon relaxation rates in rare-earth-doped fluoroindate glass systems, reinforcing their suitability for mid-IR photonic applications.

Subsequently, He et al.^[117] developed Pr3+/Yb3+ co-doped fluoroindate glass, reporting broadband fluorescence emission spanning 2.7–4.2 µm, thereby laying the groundwork for Pr3+-doped fibers in mid-IR laser applications.

Fluoroindate glass, exemplified by BIG compositions, extend the operational wavelength range of fluoride fibers beyond 5 µm while maintaining moderate thermal stability. Their low phonon energy and flexible coordination geometry enable efficient rare-earth doping for emissions up to 4.5 µm. Nevertheless, challenges such as InF3 volatility during melting and susceptibility to surface crystallization must be overcome to realize their full potential. The following section transitions to fabrication techniques, where advances in preform design and drawing processes aim to mitigate these material limitations.

Central to their technological prominence are the exceptional optical properties of fluoride glass, which arise from their unique structural and compositional attributes.

Fluoride glass achieves continuous transparency across ultraviolet, visible, and mid-IR regimes, a critical enabler for multispectral photonic applications. The transmission spectra of several fluoride glass are depicted in Fig. 4.

As expected, the multiphonon absorption edge shifts to longer wavelengths following the sequence of fluoroaluminate, fluorozirconate, and fluoroindate glass. Table 1 shows the maximum phonon energy of fluoride glass.

The refractive index of fluoride glass is intrinsically linked to their composition and structural network. Fluorozirconate glass, such as ZBLAN, typically exhibit nD of ZBLAN glass that is close to 1.5, attributed to the high polarizability of heavy metal ions and the relatively open fluorozirconate network. In contrast, fluoroaluminate glass demonstrates lower refractive indices (1.4–1.5 at nD) due to the stronger Al-F bonds and compact network structure, which reduce electronic polarizability. Fluoroindate glass, such as InF3−GaF3−BaF2 systems, bridges this gap with an intermediate index, leveraging the moderate polarizability of In3+ ions and the flexible coordination geometry of In-F bonds.

The compositional dependence of refractive indices in these glass is further modulated by dopants and network modifiers. For instance, substituting ZrF4 with heavier HfF4 in fluorozirconate glass elevates the refractive index by ∼2% owing to increased electron density. Conversely, incorporating alkali fluorides (e.g., LiF) into fluoroaluminate glass reduces indices by weakening the network connectivity. In fluoroindate glass, the addition of PbF2 enhances polarizability, pushing indices toward 1.60, a critical threshold for mid-IR lens design. These tailored refractive properties, coupled with ultra-low optical dispersion, position fluoride glass as indispensable materials for achromatic lenses, low-loss waveguides, and nonlinear photonic devices.

Fluoride glass, classified as “soft glass” due to its viscoelastic behavior and “low-melting glass” owing to reduced vitrification temperatures, exhibits distinct thermal phase transitions. Each glass-forming system demonstrates multiple characteristic thermal parameters, typically determined via differential scanning calorimetry (DSC). The Tg marks the onset of structural relaxation from a rigid amorphous state to a supercooled liquid. Heating beyond Tg induces an unstable thermal regime, triggering crystallization in most fluoride compositions. Crystallization kinetics are defined by two critical temperatures: the onset temperature (Tx) and the peak crystallization temperature (Tc), corresponding to the initiation and maximum rate of devitrification, respectively. Furthermore, the ΔT=Tx−Tg criterion proposed by Dietzel^[118] provides a quantitative measure to assess the crystallization resistance of fluoride glass during fiber drawing processes. Fluorozirconate and fluoroindate glass generally exhibit Tg in the range of 250°C–300°C, whereas fluoroaluminate-based systems demonstrate markedly higher Tg values exceeding 350°C. Table 2 summarizes the nominal compositions and thermal stability parameters of representative fluoride glass.

The thermo-mechanical properties of glass are important considerations in various applications, mainly including fracture toughness, thermal conductivity, thermal expansion, and elastic modulus. These parameters are shown in Table 3.

The fabrication of fluoride glass involves distinct challenges for each system. ZBLAN’s narrow working temperature window necessitates rapid quenching to avoid crystallization, while fluoroaluminate glass require high-temperature melting in inert atmospheres to prevent AlF3 decomposition. Fluoroindate glass face InF3 volatility, which complicates stoichiometric control. Recent advances in reactive atmosphere processing have reduced InF3 loss to <5%, enabling repeatable production of low-loss fibers.

The initial development of fluoride glass was motivated by their exceptional resistance to fluorinating agents such as F2, HF, ClF3, and SF6 under controlled environments. However, under ambient atmospheric conditions where hydrolysis and oxidation dominate, this corrosion resistance becomes less relevant. Consequently, contemporary research prioritizes assessing their aqueous stability, with particular emphasis on hydrolysis mechanisms at glass-water interfaces.

In fluorozirconate glass, the ZBLAN glass fiber has long been the most popular gain medium in mid-IR fiber lasers. However, the poor chemical and thermal stability of ZBLAN glass limits its further applications^[130]. In 1984, fluorozirconate glass with the composition ZrF4−BaF2−YF3−AlF3 (ZBYA) was first proposed^[131]. Since then, ZBYA glass has been extensively studied due to its superior thermal and chemical stability compared to ZBLAN glass^[132,133]. Xu et al.^[126] compared the chemical stability of ZBYA glass and ZBLAN glass through water immersion tests on both types of zirconium fluoride glass. The initial transmission spectra and weight of the samples were recorded, followed by immersing the two glass samples in deionized water to ensure complete surface contact with water. The sample weights were measured, and transmission spectra were recorded at 12, 24, and 36 h (after drying at 100°C for 12 h). The experiments were conducted at room temperature. As shown in Fig. 5, both types of glass exhibited a certain degree of weight loss after water immersion, which is mainly due to the exchange between F− and OH− ions and the high solubility of ZrF4 in water. Nevertheless, the weight loss percentage of ZBYA glass was lower than that of ZBLAN glass, as the presence of monovalent alkali metal Na+ ions accelerates the water corrosion rate of ZBLAN glass^[134]. These results indicate that ZBYA glass undergoes a slower hydrolysis reaction and possesses higher chemical stability compared to ZBLAN glass, suggesting that ZBYA glass fibers may have a longer operational lifespan in humid environments.

To assess the chemical stability of fluoroaluminate glass (ABYPM, ABCYSMLZ, ABCYPM, and AYF), water immersion tests were conducted. Initial transmission spectra and weights were measured before submerging samples in deionized water (24 h, RT). After oven-drying (100°C, 12 h), weight loss and spectral changes were quantified. As shown in Fig. 6, AYF glass demonstrated the highest water resistance with only 1.8% mass loss and minimal spectral variation^[128].

Figure 6 reveals that surface-bound water induces broadband transmission attenuation in fluoride glass. Comparative analysis of fluoroaluminate glass systems (AYF vs. ABYPM/ABCYSMLZ/ABCYPM) and fluorozirconate glass counterparts shows AYF exhibits optimal moisture stability. Notably, ZBLAN and ZBYA glass display 28%–32% transmittance reduction in visible (400–700 nm) and mid-IR (3–5 µm) regions.

For fluoroindate glass, as shown in Fig. 7, fluoroindate bulk core glass^[129] exhibits a moderate reduction in optical transparency following deionized water immersion, attributed to surface-mediated F−/OH− ion exchange and subsequent hydration layer formation. Comparative mass loss analyses reveal that fluoroindate glass demonstrates lower hydrolytic degradation than fluorozirconate counterparts, as confirmed by gravimetric testing^[135]. This enhanced aqueous corrosion resistance stems from the glass network’s covalent bond strengthening and reduced hydroxyl group penetration kinetics.

As quantified in Table 4, among the three types of fluoride glass, fluoroaluminate glass has the best chemical stability. AYF spectral variation demonstrates superior environmental durability, suggesting strong application potential in moisture-sensitive photonic systems^[128].

Table 5 summarizes the defining characteristics of fluorozirconate, fluoroaluminate, and fluoroindate glass. While fluorozirconate glass remain the workhorse for mid-IR fiber lasers due to their low phonon energy and high rare-earth solubility, their susceptibility to crystallization and moisture limits long-term reliability. Fluoroaluminate systems address these shortcomings with superior thermal stability and chemical resistance, albeit at the cost of higher phonon energy that restricts mid-IR luminescence efficiency. Fluoroindate glass bridge this gap, offering extended infrared transparency and moderate thermal properties, though their practical adoption requires advancements in mitigating InF3 volatility during melting.

Fluoride glass have driven their adoption in mid-IR sensing, biomedical imaging, and high-power laser systems, leveraging their ability to host rare-earth ions with high quantum efficiency. Recent innovations in nanostructured fluoride glass-ceramics further extend their utility to optoelectronic devices and radiation detection. Despite these advancements, challenges such as hygroscopic degradation, mechanical brittleness, and scalable fabrication persist, prompting research into advanced synthesis techniques like containerless melting and chemical vapor deposition. Future endeavors aim to integrate fluoride glass with quantum photonic architectures and sustainable manufacturing paradigms, signaling a paradigm shift in optical material design.

Currently, three companies worldwide—Le Verre Fluoré from France, Fiberlabs from Japan, and Thorlabs from the United States—produce commercial undoped ZBLAN, fluoroaluminate, and fluoroindate fibers, as well as rare-earth-doped ZBLAN and fluoroindate fibers. The background loss of rare-earth-doped fluoride fibers ranges from 0.2 to 0.7 dB/m, while the minimum loss of undoped ZBLAN fibers is approximately 0.01–0.02 dB/m. Commercial fluoroindate fibers exhibit a loss of 0.2 dB/m, with detailed fiber parameters listed in Table 6.

These performance metrics underscore the critical relationship between material composition and optical performance in fluoride fibers. To fully realize the low-loss potential, precise control over fabrication processes becomes significant. The following section systematically examines the manufacturing technologies, beginning with the preparation of preforms—the fundamental determinant of fiber quality. Particular emphasis will be placed on innovative approaches addressing crystallization suppression and defect minimization during fiber drawing, key challenges that directly impact the attenuation characteristics discussed. This technical analysis establishes the foundation for understanding recent advancements in fluoride fiber fabrication methodologies.

Preform fabrication technology is the foundation for low-loss optical fiber manufacturing. Fluoride preform fabrication methods, including interfacial-gel polymerization^[136], direct melting^[137], and mechanical extrusion^[138,139], established the groundwork for fiber production. As for silica fibers, chemical vapor deposition (CVD) has become the dominant approach, including Corning’s outside vapor deposition (OVD)^[140], AT&T Bell Labs’ modified CVD (MCVD)^[141], NTT’s vapor axial deposition (VAD)^[142], and Philips’ plasma CVD (PCVD)^[143,144]. While CVD-derived methods remain indispensable for silica fiber, their applicability to fluoride glass fibers is limited due to the absence of compatible vapor deposition techniques, which is ascribed to the complex composition of fluoride glass. The fabrication of fluoride preforms generally employs melting-casting techniques, which require overcoming material-specific challenges like crystallization susceptibility and thermal instability. This section will give several methods for fluoride preform preparation.

In 1981, Mitachi et al.^[145] proposed fluoride glass preform fabrication by the hot-jointing method, firstly putting a precision-polished core rod into a preheated semi-cylindrical mold and then pouring the cladding melt, followed by annealing to consolidate the structure. While achieving functional preforms, interfacial defects persisted due to melt surface tension and incomplete bonding (Fig. 8).

Hot-jointing represents an early milestone in fluoride preform fabrication, enabling core-clad structures through melt adhesion. However, interfacial defects and incomplete bonding limit its applicability to low-loss fiber production. This drawback motivated the development of built-in casting methods, which prioritize dimensional precision and contamination control, as detailed in the next sub-section.

Preforms are fabricated using a built-in casting method involving sequential infusion of core and cladding melts into thermally conditioned molds, followed by annealing treatment. Due to the inherent brittleness of fluoride glass systems, achieving high-quality core-cladding interfaces demands rigorously controlled extended annealing processes with precise thermal management. A critical challenge arises from mold surface imperfections, which can introduce contaminants such as crystallites or bubbles at the interface, compromising both optical performance and mechanical strength. To address this, modified techniques prioritize enhanced interfacial integrity through optimized mold designs to minimize surface defects and contamination suppression strategies to eliminate impurities during the infusion process. These refinements collectively improve structural homogeneity and reduce interfacial stress concentrations in the final preform.

Mitachi et al.^[146] first demonstrated the traditional built-in casting method by pouring cladding melt into a preheated brass mold near the Tg, followed by mold inversion to drain the excess melt and form a hollow cladding tube. The subsequent core melt injection and annealing process leverages natural surface tension to ensure smooth interfaces and minimize contamination risks. In 1986, Kanamori et al.^[147] refined the traditional built-in casting method, achieving a breakthrough with optical fibers exhibiting transmission loss below 1 dB/km. Critical challenges persist in precisely regulating the central hole dimensions (heavily dependent on temperature-mediated material flow dynamics) and optimizing drainage timing to prevent hole overexpansion. Furthermore, variations in glass compositions demand extensive empirical adjustments to mitigate defects such as bubble formation and surface irregularities, requiring meticulous trial-and-error optimization [Fig. 9(a)].

Tran et al.^[148] introduced rotational casting in 1982, employing centrifugal forces by mold rotation above 3000 r/min to distribute cladding melt uniformly, forming concentric fluoride glass tubes with controlled inner diameters. This method enables adjustable core-cladding ratios and repeatable preform production. The process also permits direct in-mold cladding melting to streamline fabrication, though maintaining rotational stability is critical to minimize interface defects caused by turbulent flow or uneven cooling [Fig. 9(b)]. Lu et al.^[149] validated the technique in 1987, achieving fiber transmission loss of 1 dB/km.

Ohishi et al.^[150] developed the jacketing method as an improved variant of the built-in casting technique to produce single-mode fluoride glass fibers with smaller core diameters. The process begins by fabricating a core-cladding preform using conventional built-in casting, after which a hollow jacket tube is formed by inverted pouring of cladding melt into a larger preheated mold. During fiber drawing, the preform is inserted into the jacket tube within a dry nitrogen environment to suppress moisture-induced interfacial reactions. A cladding-to-core diameter ratio greater than 5:1 is critical for ensuring optical confinement, while deviations such as insufficient cladding thickness or defects at the jacket-cladding interface can introduce extrinsic scattering losses [Fig. 9(c)].

Sakaguchi and Takahashi^[151] introduced the modified built-in casting method to mitigate core-cladding tapering and enhance dimensional precision. This method involves sequentially pouring cladding and core melts into a preheated cylindrical mold, which is then transferred to a perforated substrate before solidification initiates. Gravitational drainage of unsolidified melts into the central bore creates a concentric structure, with minimized tapering achieved through precise timing and alignment during the transfer phase. The technique improves structural uniformity compared to traditional approaches but requires meticulous coordination of thermal and rheological conditions to prevent flow irregularities or misalignment-induced defects [Fig. 9(d)].

The lifting method employs vertical mold displacement to create core-cladding structures through controlled drainage of molten materials. In this process, a preheated funnel-shaped cylindrical mold is partially filled with cladding melt, followed by rapid injection of core melt. As the mold is vertically extracted, unsolidified cladding material drains outward while the core melt flows inward to occupy the central cavity. Premature mold extraction risks excessive core melt infiltration and oversizing of the core, whereas delayed extraction can cause partial cladding solidification and structural collapse. Successful execution hinges on precise synchronization of melt viscosity, gravitational flow dynamics, and extraction timing to ensure dimensional uniformity. Compared to static casting methods, this technique offers superior control over core-cladding geometry but requires iterative parameter calibration to address interfacial defects such as irregularities or adhesion issues. Key challenges include balancing the interplay between material flow behavior and thermal conditions to maintain structural integrity while minimizing geometric deviations [Fig. 9(e)].

Suction casting, developed by Ohishi et al.^[152], creates core-cladding structures by leveraging thermal contraction to drive melt redistribution. The process involves filling a cylindrical mold—equipped with a bottom accumulator—with cladding melt, followed by core melt infusion during partial solidification. Cooling-induced shrinkage generates suction forces that draw the core material into the central cavity, allowing adjustable core-cladding ratios. However, this method demands mold customization for varying preform sizes and precise thermal management to synchronize shrinkage rates across materials. Key challenges include interfacial defects caused by uneven cooling gradients and residual stresses arising from differential contraction between core and cladding materials [Fig. 9(f)].

Built-in casting techniques, including rotational and suction casting, significantly improved preform homogeneity by leveraging controlled melt flow and drainage. These methods reduced scattering losses to <1dB/km in experimental fibers but remain limited by mold-induced crystallites. The subsequent discussion on extrusion and rod-in-tube methods explores alternative strategies to bypass these challenges through mechanical shaping and assembly.

The rod-in-tube technique fabricates fiber preforms by assembling precision-machined core rods and cladding tubes cast from glass melts^[153–158]. Core and cladding components are individually produced by controlled melt casting and annealing, followed by mechanical processing to ensure dimensional compatibility. Successful assembly requires tight diameter matching to eliminate interfacial gaps.

While this method offers exceptional geometric flexibility for diverse fiber architectures due to simplified equipment and independent core-cladding ratio control, its implementation with fluoride glass faces significant hurdles. Fluoride glass brittleness increases fracture risks during machining, reducing production yields compared to silica-based systems. Surface contamination from polishing residues and environmental exposure is amplified by fluoride materials’ moisture sensitivity, with prolonged air exposure degrading interfacial quality through hydration-induced optical losses. Additionally, excessive surface roughness promotes defects like microbubbles and scattering centers, while incomplete gap elimination during drawing traps residual air pockets. Strict process controls, particularly in drilling, polishing, and assembly, are essential to mitigate these issues and maintain dimensional precision [Fig. 10(a)].

The extrusion technique, developed by Roeder et al. in 1970^[138,159], fabricates fiber preforms by pressing softened glass components through molds under controlled viscosity conditions. Core and cladding glass blocks are prepared and heated to their deformation temperature in a dry atmosphere within a press cylinder, enabling plastic deformation under pressure^[160–165].

This method offers distinct advantages for fluoride glass systems, including reduced processing temperatures that reduce crystallization risks and preserve material stability. Its versatility allows fabrication of diverse core-cladding geometries through mold customization while maintaining high production efficiency. The low-temperature, high-pressure environment minimizes thermal degradation, preventing transmission loss escalation during preform formation [Fig. 10(b)]. Miura et al.^[166] demonstrated the technique’s potential in 1991, achieving fluoride fibers with 0.9 dB/m loss at 2.94 µm, a milestone attributed to precise viscosity management and interfacial homogeneity. Bei et al.^[167] demonstrated that precise temperature control during extrusion completely suppresses surface crystallization, even when operating close to the glass transition temperature.

Fiber drawing is performed using a computer-controlled tower integrating feeding mechanisms, heating furnaces, gas management systems, diameter measurement units, coating-curing modules, tension sensors, capstan-driven drawing systems, and winding spools (Fig. 11). The feeding mechanism ensures precise delivery of preforms into the heating furnace to minimize diameter fluctuations, as irregular feed rates can induce uncoated fiber thickness variations exceeding ±1%. The heating device, typically a resistive furnace, generates temperature gradients peaking at the drawing zone. Fluoride glass systems require thermal stability within ±0.2°C due to their rapid viscosity-temperature dependence; deviations beyond 1°C risk diameter irregularities, surface pitting, or crystallization. Convection heating improves stability for moisture-sensitive glass, reducing crystallization risks by 30%–50% compared to radiative methods. Meanwhile, the gas environment is maintained via dry inert gas purging (nitrogen/argon at 5–10 L/min) to prevent fluoride glass hydration while avoiding turbulence-induced fiber vibration. For tube-rod assemblies, pressure control within ±50Pa prevents core-cladding separation or deformation. Diameter measurement employs laser-based gauges with 10 nm resolution to monitor cladding dimensions, enabling real-time adjustments to feed and draw speeds, while secondary sensors ensure coated fiber thickness uniformity within 2%–5%.

The coating system applies UV-curable polyurethane acrylate, widely used in telecom for its thermal expansion compatibility with silica (±5% mismatch tolerance). Multi-layer coatings enhance mechanical durability by distributing stress, reducing microbending losses by 40%–60%. Tension control relies on in-line meters to maintain forces between 50–200 g. Excess tension (>250g) triggers temperature adjustments to prevent fiber breakage, while low tension (<30g) indicates viscosity instability requiring speed modulation.

The drawing and winding systems use synchronized capstan pairs to regulate draw speeds (0.1–2 m/s), determining final fiber diameters through mass balance principles. Take-up spools auto-adjust rotational speeds via tension feedback, limiting winding-induced strain to <0.1%. Fluoride glass fabrication demands stringent avoidance of crystallization and moisture exposure. Tube-rod methods require precise negative pressure control to eliminate core-cladding gaps, while coating adhesion depends on thermal expansion matching to mitigate microbending losses. Challenges such as residual air pockets, surface contamination, and machining-induced fractures necessitate rigorous process controls across drilling, polishing, and assembly stages to ensure dimensional and interfacial integrity.

Fiber fabrication mainly includes two methods: preform fiber drawing and crucible fiber drawing, each with distinct operational frameworks and material compatibilities^[168].

This method draws fibers directly from preforms mounted on a feeding mechanism. The geometry of the resulting fiber, such as its cladding and core dimensions, is determined by the initial preform’s cladding-to-core diameter ratio^[169]. Single-step drawing typically accommodates large diameter ratios, whereas achieving smaller ratios usually necessitates intermediate steps. For instance, preforms with reduced cladding-core ratios may require secondary polishing and the insertion of additional cladding tubes prior to the final drawing stage. While this multi-stage approach is feasible for mechanically robust glass like silicates and phosphates, its application to fluoride glass remains limited due to their susceptibility to crystallization during prolonged or repeated thermal processing.

The crucible fiber drawing method fabricates optical fibers by directly melting glass raw materials (blocks, cullet, or powder) within a precisely controlled crucible. Enhanced feeding mechanisms, heating systems, and gas management regulate softening conditions to achieve high-quality fiber fabrication. This approach eliminates material-intensive preprocessing steps (e.g., cutting, polishing) required in preform-based methods, significantly improving production efficiency and reducing costs. Crucible techniques enable continuous fabrication of ultra-long fibers without splicing limitations and accommodate diverse refractive index profiles, from step-index to graded-index designs, with adjustable core dimensions^[170,171]. For crystallization-prone fluoride glass, precise thermal regulation of the drawing nozzle is critical to avoid prolonged exposure to crystal nucleation temperatures. The double-crucible variant enhances mechanical performance through tunable core-cladding dimensions, surpassing traditional rod-in-tube preforms in structural consistency.

Recent advancements in fluoride fiber drawing, exemplified by Thorlabs, Inc., focus on refining ZBLAN (ZrF4−BaF2−LaF3−AlF3−NaF) fiber fabrication to mitigate crystallization and optical losses in space environment. Innovations include precision melt-casting of preforms under oxygen-free atmospheres and optimized thermal gradients during drawing, reducing defects like microbubbles and interfacial stress. Enhanced homogeneity via dopant distribution control and hybrid preform designs has achieved record-low losses (∼0.01dB/m) in the mid-IR spectrum. Improved mechanical durability through space-tailored polymer coatings addresses microcrack susceptibility. Microgravity experiments demonstrate suppressed crystallization, suggesting potential for space-based manufacturing. These advancements position fluoride fibers as critical for next-generation space communications, infrared sensing, and high-power laser delivery.

Mid-IR lasers refer to coherent light sources with wavelengths in the 3–50 µm spectral range, as defined by the ISO 20473:2007 standard. However, in laser technology, the 2–25 µm band is more commonly classified as the mid-IR region, with fiber lasers typically emitting in the 2.5–5 µm range^[186]. The unique wavelengths of mid-IR lasers are strongly absorbed by water molecules and biological tissues^[187], and their interaction with various chemical bonds induces thermal effects in materials^[188], enabling novel solutions for non-invasive medical diagnostics^[189], therapeutic treatments^[190], and material processing technologies such as cutting and welding^[191]. For example, selective photothermal effects can be utilized for precise tumor ablation in medical therapy as well as for high-precision industrial machining^[192,193]. Current mid-IR fiber laser technologies primarily include rare-earth-doped fiber lasers^[194], solid-state lasers^[195], quantum cascade lasers^[196], free-electron lasers^[197], gas lasers^[198], and optical parametric oscillators/amplifiers^[199]. Among them, mid-IR fiber lasers have the advantages of excellent beam quality, high efficiency, easy thermal management, and compactness.

Rare-earth (RE)-doped fiber lasers are high-efficiency light source systems consisting of a pump source, gain medium, and resonant cavity, which are shown in Fig. 13. The gain medium of these lasers is the rare-earth-doped optical fiber, where rare-earth ions absorb pump energy and transition from the ground state to an excited state. As pumping continues, the population inversion of excited ions is achieved, leading to stimulated emission. Within the resonant cavity, stimulated emission undergoes oscillation and amplification, ultimately generating high-quality laser output. Fiber lasers offer advantages such as high optical-to-optical conversion efficiency^[194,200] and near-ideal beam quality^[201], making them highly suitable for precision machining and metrology applications. Additionally, their superior heat dissipation capability stems from the fiber’s geometric structure and material properties, while low optical loss and high durability ensure excellent stability and long operational lifetimes^[202,203]. Furthermore, the flexibility of optical fibers facilitates compact and modular laser designs, enhancing system integration^[204]. However, challenges such as low pump coupling efficiency and limited emission wavelength range restricted by rare-earth ion energy transitions remain to be addressed in fiber laser design and applications^[205].

Compared to conventional silicate glass fibers, fluoride fibers exhibit lower phonon energy, enabling low-loss transmission of laser light beyond 2 µm, making them an ideal fiber host for mid-IR fiber lasers. Recent advancements in the stability of fluoride glass materials and fiber fabrication techniques have spurred extensive research on fluoride-glass-based mid-IR fiber lasers, resulting in continuous improvements in output power and wavelength coverage (Figs. 14 and 15). This section systematically investigates RE-doped fiber lasers operating in different wavelength bands, including 2.7–3.0 µm, 3.0–3.3 µm, ∼3.5μm, and ∼3.9μm bands.

Since the first achievement of 2.7 µm laser output in Er3+-doped ZBLAN fiber by Allain et al.^[219] in 1988, research on ∼3μm fiber lasers achieved several groundbreaking advancements in terms of power, efficiency, and wavelength coverage. In 1994, Frerichs^[220] first reported the generation of 2.8 µm laser emission from Er3+-doped ZBLAN fiber pumped by a 980 nm laser. The laser exhibited a threshold of less than 1 mW, an output power of 6 mW, and an efficiency of 9.3%. In 1999, Sumiyoshi et al.^[221] utilized a 1.15 µm wavelength Raman fiber laser to pump Ho3+-doped ZBLAN fiber, achieving high-efficiency, high-power cascaded laser output at room temperature. In the same year, Sandrock et al.^[222] reported the generation of 2.8 µm laser emission from an M-type Er3+-doped ZBLAN fiber pumped by a 970 nm laser. The fiber had an Er3+ doping concentration of 5% (mole fraction) and a ring-shaped gain medium cross-section. With a laser cavity length of 50 cm, the system produced 1 W of 2.8 µm laser output, achieving a slope efficiency of 25%. The laser wavelengths were ∼3 and ∼2μm, respectively, with a total output power of 3 W and an efficiency as high as 65%. This marked a milestone achievement in the field of mid-infrared fiber lasers, representing the first time that watt-level mid-infrared laser output was realized in ZBLAN glass fiber. This breakthrough not only demonstrated the immense potential of ZBLAN fiber in mid-infrared laser generation but also laid the groundwork for its practical applications. They employed the dual-wavelength laser beam as a laser scalpel to cut the soft tissue of a rabbit, validating its precision and efficient cutting capability.

In 2007, Zhu and Jain^[223] employed a 975 nm laser diode array of 100 W to pump double-cladding heavily Er3+-doped ZBLAN fiber, as shown in Fig. 16(a), achieving laser output at a wavelength of 2.78 µm. The output power exceeded 9 W, with a slope efficiency of 21.3%, marking the successful application of high-power pumping technology. In 2010, Tokita et al.^[224] reported a tunable Er3+-doped ZBLAN fiber laser pumped by a 975 nm laser with a stable output power of 10 W. The gain fiber was cooled by continuous nitrogen gas flow to facilitate convective heat dissipation. The laser exhibited a tunable wavelength range from 2.71 to 2.88 µm and operated continuously at a maximum power of 11 W for 1 h, with a power fluctuation of only 0.13%. In 2011, Faucher et al.^[225] reported an all-fiber Er3+-doped ZBLAN laser operating at 3 µm, which achieved a slope efficiency surpassing the theoretical Stokes efficiency limit. This laser emitted at a wavelength of 2.825 µm and reached a maximum output power of 20.6 W. In 2015, Fortin et al.^[226] achieved 30.5 W of 2938 nm laser output through an all-fiber structure, elevating the power of mid-infrared fiber lasers to the tens of watts level. In 2017, Aydin et al.^[227] enhanced the slope efficiency of a 2.8 µm laser to 50% through cascaded emission of a 1.6 µm laser, surpassing the Stokes limit efficiency and representing a significant breakthrough in the efficiency optimization of mid-infrared lasers. The experimental setup of the cascade laser is shown in Fig. 16(b).

In 2018, Laval University^[228] demonstrated a 41.6 W laser output at 2824 nm from an Er3+-doped ZBLAN fiber laser employing a bidirectional pumping scheme. The experimental setup is illustrated in Fig. 16(c). The laser cavity was pumped bidirectionally at 980 nm to effectively distribute the thermal load within the active fiber. A slope efficiency of 22.9% was achieved, and the long-term stability of the laser was significantly improved by employing end-cap fusion splicing at the fiber output and annealing of fiber gratings. In 2019, Osaka University^[229] achieved a 35 W of 2838 nm laser output by directly inscribing a fiber Bragg grating (FBG) into Er3+-doped ZBLAN fiber using 515 nm femtosecond pulses and the plane-by-plane inscription technique. This demonstrated the unique advantages of femtosecond laser inscription technology in mid-infrared lasers. In the same year, Harbin Engineering University^[230] realized a 2.866 µm laser output in Ho3+/Pr3+ co-doped AlF3-based glass fiber, as shown in Fig. 17(a), achieving an output power of 173 mW and a slope efficiency of 10.4%. This result validates for the first time the potential of fluoroaluminate glass fibers as an effective gain medium for mid-infrared lasers. In 2021, Liu et al.^[231] utilized a Ho3+/Pr3+ co-doped AlF3-based glass fiber, for the first time at room temperature, to achieve 1.13 W of continuous-wave laser output at 3 µm (M2=1.2) and a long-time operating stability of 45 min without any additional packaging or active thermal management. They also demonstrated tunable laser output within the range of 2.842–2.938 µm, as shown in Fig. 17(b). In the same year, Newburgh et al.^[232] reported research on a quasi-continuous-wave Er3+: ZBLAN fiber laser, achieving an output power of approximately 70 W with a slope efficiency of 29%, setting a record for the efficiency of non-cascaded Er: ZBLAN fiber lasers.

In 2021, researchers at the Harbin Engineering University^[233] gained an achievement by successfully inscribing fiber Bragg gratings (FBGs) with various parameters into AlF3-based fibers for the first time at mid-infrared (MIR) wavelengths around 2.86 µm, as shown in Fig. 17(c). The team fabricated a second-order FBG with 99% reflectivity at one end of a 16.5 cm gain fiber. When pumped by a 1150 nm laser, this fiber laser system demonstrated exceptional performance, delivering over 1 W of output power at 2863.9 nm with 17.7% overall laser efficiency and maintaining a narrow full width at half-maximum (FWHM) bandwidth of 0.46 nm. This pioneering work represents a significant advancement in MIR fiber laser technology, combining high-power output with precise spectral control in a compact fiber-based system. In 2022, Xu et al.^[126] achieved, for the first time, 2.9 µm laser emission using a self-developed Ho3+/Pr3+ co-doped ZBYA fiber, with an output power of 2.16 W and a slope efficiency of 24%, demonstrating the potential of ZBYA fiber in mid-infrared lasers. In the same year, Zhang et al.^[234] innovatively employed a 1.72 µm wavelength laser to pump Er3+-doped fiber, achieving laser emission at 2.785 µm with a slope efficiency of 30.9%. By leveraging the excited-state absorption (ESA) mechanism, they significantly enhanced laser efficiency, offering a novel approach to pumping schemes for mid-infrared lasers. Wang et al.^[235] at the Harbin Engineering University achieved the first watt-level ∼2.9μm laser emission from a Ho3+/Pr3+ co-doped fluoroindate glass fiber. The experimental setup employed a 1150 nm Raman laser to pump a 27 cm long fiber, which generated a maximum output power of 1.35 W at 2864 nm wavelength with an impressive slope efficiency of 21.14%.

In 2023, Zhang et al.^[236] achieved a 33.8 W mid-infrared 2.8 µm laser output by employing high-efficiency thermal management technology in Er3+-doped fluoride fiber, as shown in Fig. 18(a), advanced mid-infrared fiber end-cap fabrication techniques, and high-power pump laser coupling technology. Using a high-power 976 nm semiconductor laser to single-end pump an 8 m long fluoride gain fiber, they set a new record for the highest power level in a single-end-pumped mid-infrared fiber laser, achieving an optical-to-optical conversion efficiency of 26.4%.

In 2024, Cheng et al.^[237] proposed a novel dual-wavelength pumping (DWP) method at wavelengths of 1642 nm and 1976 nm to effectively excite ions to an intermediate energy level. Compared to the traditional single-wavelength pumping system at 1150 nm, the 1642 nm laser contributes to the generation of a stronger self-circulation system, which is crucial for maintaining the population inversion for the 2.86 µm transition. This dual-wavelength pumping system significantly improved the generation efficiency of the 2.86 µm laser, achieving a slope efficiency of 53.7% in experiments. In 2024, Harbin Engineering University^[238] conducted systematic experimental investigations on Ho3+/Pr3+ co-doped glass and fibers with concentration ratios ranging from 1:0.01 to 1:1. Comprehensive measurements of fluorescence decay lifetimes for Ho3+:I56 and I57 energy levels in AlF3 glass samples were conducted. The results elucidated the dominant energy transfer processes that influence the efficiency of the 3 µm laser. In the same year, they^[128] successfully developed a cascaded laser system using a Ho3+-doped single-clad AYF fiber as the gain medium, pumped by a 1150 nm laser source. The system achieved outstanding dual-wavelength laser operation, delivering a maximum total output power of 11.6 W and a slope efficiency of 29%. The laser emitted simultaneously at two characteristic wavelengths of approximately 2 µm and 3 µm. The ∼3μm laser exhibited excellent stability, maintaining its maximum output power over 50 min.

In 2025, Tianjin University^[239] introduced an innovative all-fiber 2.8 µm laser configuration, as illustrated in Fig. 18(b), which effectively eliminates the requirement for conventional high-reflector components. This novel approach significantly simplifies the design of mid-infrared erbium-doped fiber lasers while enhancing both design flexibility and application adaptability.

Table 7 provides a summary of the literature on CW mid-IR fluoride fiber lasers operating in the 2.7–3.0 μm wavelength range. The 2.7–3.0 µm Er3+-doped ZBLAN fiber lasers have achieved remarkable progress, with power levels exceeding 40 W and slope efficiencies approaching 50%. However, thermal lensing and photodarkening at high pump densities necessitate active cooling strategies. The following sub-section on 3.0–3.3 µm lasers shifts focus to Dy3+-doped systems, where in-band pumping and cascaded energy transfer enable efficient longer-wavelength operation.

In 2016, Majewski et al.^[240] employed an Er3+-doped ZBLAN fiber laser to achieve a 2.8 µm output by directly exciting the upper laser level of the H613/2→H15/26 transition of the Dy3+ ion; a 3.04 µm laser was produced with a record slope efficiency of 51%. This configuration resulted in a slope efficiency of 51% at an emission wavelength of 3.04 µm. By increasing the fiber length, they successfully generated emission at 3.26 µm, achieving a slope efficiency of 32%. The schematic diagram of the Dy3+-doped ZBLAN fiber laser structure is shown in Fig. 19(a).

In 2018, Woodward et al.^[241] achieved an output power of 1.06 W at a wavelength of 3.15 µm by in-band pumping of Dy3+: ZBLAN fiber at 2.825 µm, with a slope efficiency as high as 73%, setting a record for the highest slope efficiency of mid-infrared fiber lasers at that time. This also marked the first fiber laser to achieve watt-level output in the 3.0–3.3 µm wavelength range. In 2019, Fortin et al.^[242] reported an all-fiber Dy3+-doped fiber laser, achieving a maximum output power of 10.1 W at a wavelength of 3.24 µm, which is the highest output power record for fiber lasers in this wavelength range, as shown in Fig. 19(b). Using femtosecond laser-written fiber Bragg gratings (FBGs), they realized an all-fiber design, significantly enhancing the system’s stability and practicality. In the same year, Majewski et al.^[243] reported an 800 nm direct diode-pumped Dy3+/Tm3+ co-doped ZBLAN fiber laser, achieving an output power of 12 mW at a wavelength of 3.23 µm. Although the slope efficiency was relatively low (0.3%), the study found that the energy transfer upconversion (ETU) process may have potential advantages for future 4 µm laser systems.

In 2020, Amin et al.^[244] reported two new near-infrared pump wavelengths (0.8 and 0.9 µm), addressing the harmful excited-state absorption (ESA) issues associated with longer pump wavelengths (e.g., 1.1, 1.3, and 1.7 µm) used in the past. These pump wavelengths exhibited Stokes efficiency limits of 70% and 79%, respectively, demonstrating higher energy conversion potential and providing new pumping schemes for the design of future mid-infrared fiber lasers. In 2021, Wang et al.^[245] achieved a maximum output power of 260 mW at a wavelength of 3.27 µm using a 3.6 m long 0.25% (mole fraction) Dy³⁺/4% (mole fraction) Er³⁺ co-doped ZBLAN fiber, with a slope efficiency of 5.73%. They also demonstrated lasers operating at 3.23 and 3.35 µm wavelengths using 2 and 9 m gain fibers, respectively, albeit with lower slope efficiencies. In 2023, Ososkov et al.^[246] achieved an output power of 0.36 W at a wavelength of 3.05 µm through in-band pumping of Dy3+-doped fluoroindate fiber at 2.83 µm, with a slope efficiency as high as 82%, approaching 90% of the Stokes efficiency limit. They also, for the first time, inscribed high-reflectivity fiber Bragg gratings (FBGs) in -based glass fiber, enabling stable narrow-linewidth output at 3.2 µm. This represents the highest performance record for InF3-based glass fiber lasers, laying the foundation for their application in high-power mid-infrared lasers.

In 2024, Zhao and Luo^[247] experimentally demonstrated efficient watt-level power output from an Er3+/Dy3+ co-doped fluoride fiber laser in the beyond-3 µm wavelength range, achieving a maximum output power of 3.03 W at ∼3.210μm with a slope efficiency of 19.1%, showcasing its potential for efficient, high-power operation. In the same year, Xu et al.^[248] achieved a groundbreaking milestone by demonstrating the first continuous-wave laser operation beyond 3 µm using their independently developed Ho3+/Pr3+ co-doped AlF3-based glass fiber. The experimental configuration featured an innovative hybrid resonant cavity that combined a high-reflection dichroic mirror (DM, with >99% reflectivity across 3–3.1 µm band) and a partial-reflection fiber Bragg grating (PR-FBG, R=80%). Under 1.15 µm laser pumping, the system delivered a maximum unsaturated output power of 1.014 W at 3.009 µm wavelength, achieving an optical conversion efficiency of 11.8% with a narrow spectral linewidth of 0.88 nm. A significant performance enhancement was observed after implementing a 200°C thermal annealing process for 30 min on the FBG, which boosted the output power at 3.036 µm from 348 to 1081 mW—a threefold improvement that clearly demonstrates the system’s exceptional optothermal stability. This work represents the first reported FBG-based Ho3+/Pr3+ co-doped AlF3 fiber laser operating in the 3–3.1 µm spectral region, offering an innovative technical solution for advancing high-performance mid-infrared fiber laser development. The achievement establishes new possibilities for applications in molecular spectroscopy, medical procedures, and infrared sensing systems while setting a benchmark for rare-earth-doped fluoride glass laser technology.

Dy3+-doped ZBLAN and fluoroindate fibers have unlocked the 3.0–3.3 µm regime through resonant pumping and energy transfer upconversion. While slope efficiencies >70% demonstrate the maturity of these systems, wavelength coverage remains constrained by multiphonon absorption. The exploration of 3.5 µm Er3+ lasers in the next sub-section highlights complementary approaches to span the mid-IR spectrum.

The development of mid-infrared 3.5 µm fiber lasers dates back to 1991, when Tobben et al.^[249] first reported a fiber laser operating at this wavelength. Using a 655 nm dye laser to pump an Er3+-doped ZrF4 fiber, they achieved an output power of 8.5 mW and a slope efficiency of 3% under liquid nitrogen cooling. In 2013, Henderson-Sapir et al.^[250] from the University of Adelaide proposed a dual-wavelength pumping (DWP) scheme using 985 and 1973 nm sources, as illustrated in Fig. 20(a). By establishing a “virtual ground state,” they addressed the ion population bottleneck, significantly enhancing the laser output performance. In 2014, the team^[251] further optimized the experimental setup and achieved a room-temperature output of 260 mW at 3.5 µm, with an optical-to-optical conversion efficiency of 16%. This represented an order-of-magnitude improvement in both output power and efficiency compared to previous demonstrations of similar lasers. Figure 20(a) illustrates the energy level diagram for an Er3+-doped ZBLAN fiber in which we explore 3.5 µm lasing from the F42/9→I2/94 levels. In 2016^[252], they utilized a dual-wavelength pumping scheme at 980 and 1973 nm to excite a 1.0% (mole fraction) Er3+: ZBLAN fiber, achieving a 1.45 W laser output at 3.47 µm with a slope efficiency of 27%. Additionally, they demonstrated, for the first time, tunable output spanning from 3.33 to 3.78 µm. In the same year, Fortin et al.^[253] from the Laval University developed the first all-fiber 3.44 µm laser. They replaced the high-reflectivity cavity mirror with a dichroic mirror deposited on the fiber end face and inscribed Bragg gratings directly in the fiber core to function as the partial-reflectivity cavity mirror. By butt-coupling silica fiber with fluoride fiber, they constructed an all-fiber system. The laser achieved an output power of 1.52 W with a slope efficiency of 19%. However, due to limitations in heterogeneous splicing technology, the system’s stability required further improvement. In 2017, Maes et al.^[254] from the Laval University directly inscribed HR-FBG (high-reflectivity fiber Bragg grating) and LR-FBG (low-reflectivity fiber Bragg grating) in the fiber core. By varying the reflectivity of the output LR-FBG, they measured the output characteristics and found that the optimal reflectivity for maximum output power was approximately 30%. As seen in Fig. 20(b), using a dual-wavelength pump at 976 and 1976 nm on a 5 m long Er3+:ZBLAN fiber with a doping concentration of 1.0% (mole fraction), they achieved an output power of 5.6 W, with an overall optical efficiency of 26.4%. Although the laser cavity still contained butt-coupled sections, further improvements in efficiency and stability were needed.

In the same year, Qin et al.^[255] reported a breakthrough in 3.5 µm erbium-doped fiber lasers. They employed a dual-wavelength pumping scheme by combining a commercially available 970 nm laser diode with a custom-built 1973 nm laser to pump a 2.5 m long Er3+: ZBLAN fiber doped at 1.5% (mole fraction). This configuration yielded a maximum output power of 0.85 W at 3.52 µm, with a slope efficiency of 25.14% relative to the 1973 nm pump power. Notably, by increasing the 1973 nm pump power, they were able to tune the laser wavelength from 3.52 to 3.68 µm. The experiment also produced a 3.68 µm laser output with a power of 0.62 W, which set a record at the time for the longest free-running wavelength achieved by an Er3+: ZBLAN laser without the use of tuning devices. In 2019, Maes et al.^[256] from the Laval University reported a significant advancement using 976 and 1976 nm pumps to excite a 2.5 m long, heavily doped Er3+: ZBLAN fiber with a doping concentration of 7.0% (mole fraction). This resulted in a 3.42 µm laser output of 3.4 W and a slope efficiency of 38.6%. Their results indicated that, compared to lightly doped fibers, heavily doped fibers require shorter lengths and lower 976 nm cladding pump power, providing a more efficient approach to high-power mid-IR laser generation. In the same year, Hunan University^[257] conducted experiments using a dual-end pumping scheme on a 2 m long Er3+:ZBLAN fiber laser with a doping concentration of 1% (mole fraction). With an output coupler reflectivity of 65%, they achieved a maximum output power of 1.72 W at 3.5 µm when the 1973 nm pump power reached 8.39 W. To some extent, compared to single-end pumping schemes, this dual-end pumping approach helps prevent fiber tip damage at medium pump power levels, thereby enabling further power scalability.

In 2021, Wang et al.^[258] introduced a novel dual-wavelength pumping method using 655 and 1981 nm. This approach enabled a high-power output of 1.72 W at 3.5 µm from an Er3+: ZBLAN fiber laser, achieving a slope efficiency of 31.5% relative to the 1981 nm pump power. Although the laser power was constrained by the available 1981 nm pump power, their results demonstrated that this pumping configuration could yield higher output power under equivalent pump conditions, offering a promising alternative for advancing 3.5 µm laser performance.

In 2022, leveraging advances in fiber splicing technology, Lemieux-Tanguay et al.^[259] from the Laval University achieved a 75% core-to-core transmission of 1976 nm light through heterogeneous splicing between silica and fluoride fibers. Through continuous optimization of the cavity design, they realized a 3.55 µm laser output with a power of 15 W, achieving an overall optical efficiency of 17.2% and a slope efficiency of 51.3% relative to the 1976 nm pump power. The experimental setup is illustrated in Fig. 20(c); the all-fiber resonator design provided exceptional stability at high output power, with peak-to-peak and root-mean-square deviations of 2.36% and 0.36%, respectively. This represents the highest output power achieved to date for an all-fiber 3.5 µm continuous-wave laser. In 2022, Zhang et al.^[260] utilized two independently developed 1973 nm pump sources to simultaneously pump a 4.8 m long Er3+: ZBLAN fiber with a doping concentration of 1% (mole fraction) from both ends. This dual-end pumping approach enabled them to achieve a maximum continuous-wave output power of 2.32 W at room temperature. The laser operated with an overall optical-to-optical conversion efficiency of 10.33%, emitting at a central wavelength of 3.54 µm. In the same year, Luo^[261] at the University of Electronic Science and Technology proposed a novel approach using a red laser diode (LD) to pump a double-clad Er3+/Dy3+ co-doped fluoride fiber. By directly exciting Er3+ to the high-energy-level F49/2 and leveraging energy transfer processes between Er3+ and Dy3+ as well as Dy3+ intra-band absorption, they achieved single-band laser output at 3.4 µm with a slope efficiency of 8.8% and a maximum power of 0.8 W using a 21% output coupler in a free-running Fabry-Pérot (F-P) cavity. With a 40% output coupler, they obtained dual-band laser output at 3.3 and 3.5 µm, achieving a slope efficiency of 10.7% and a maximum power of 0.95 W. Further power scaling was only limited by the available pump power. This work provides new opportunities for the miniaturization and integration of future commercial 3–5 µm lasers. In 2023, Zhang et al. from the Tianjin University^[262] optimized the pump wavelength using 1990 nm as the second pump source, which exhibits lower excited-state absorption (ESA2), combined with 976 nm in a dual-wavelength pumping scheme. They achieved a maximum output power of 7.2 W at 3.47 µm with a slope efficiency of 36%, representing the highest reported output power for a 3.5 µm band laser in a free-space optical setup.

In 2024, Xu et al.^[263] employed a dual-wavelength pumping scheme at 980 and 1150 nm, enabling simultaneous emission at 2.8 and 3.6 µm through excited-state absorption processes. For the first time, they demonstrated dual-wavelength laser operation at 2.8 and 3.6 µm using a self-developed Er3+-doped ZBYA fiber. Tianjin University^[264] further optimized the laser system, increasing the 3.5 µm laser output power to 16.4 W, as shown in Fig. 21. The system adopted a dual-end pumping architecture to reduce thermal load. The 1990 nm pump source was a self-developed, single-transverse-mode, thulium-doped fiber MOPA (master oscillator power amplifier) delivering a maximum output power of 56 W. The gain fiber was a fluoride fiber doped with 1% (mole fraction) Er3+ ions.

In the same year, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences (XIOPM)^[265], achieved a 3.5 µm all-fiber laser output with a power of 10.1 W and a slope efficiency of 41.7%. This breakthrough was based on self-made mid-IR fluoride fiber gratings and optimized heterogeneous fiber splicing and thermal management technologies. The system used a dual-wavelength pumping scheme, employing a 2×1 quartz fiber combiner to merge laser outputs at 1976 and 981 nm, which were then used to pump an Er3+-doped fluoride double-cladding fiber [(7% (mole fraction) Er3+ concentration, 2.5 m in length]. Low-loss splicing between quartz and fluoride fibers was achieved using an asymmetric heating technique, resulting in a high core-pumping efficiency (∼66%). A pair of fiber Bragg gratings with reflectivity of 98% at 3549 nm and 30% at 3549 nm were inscribed in the gain fiber via femtosecond laser direct writing, forming the resonant cavity. The entire optical system was mounted on an aluminum plate for thermal management, ensuring stable laser operation. Table 8 presents a summary of the literature on CW mid-IR ~3.5 μm fluoride fiber lasers.

Since J. Schneider first achieved ∼3.9μm fiber laser output in 1995 by pumping Ho3+-doped fluoride fiber with a 640 nm laser, significant progress has been made in ∼3.9μm fiber laser technology. Schneider’s experiment^[266] exhibited a slope efficiency of 1.5% and a maximum average output power of 1 mW. In 2018, Maes et al.^[218] from the Laval University extended the fiber transparency window to 5 µm using heavily Ho3+-doped InF3-based fiber. Utilizing energy transfer upconversion (ETU) and excited-state absorption (ESA) mechanisms, they achieved room-temperature 3.92 µm laser emission under 888 nm cladding pumping, with an output power of nearly 200 mW and a slope efficiency of approximately 10%.

In recent years, advancements in low-loss rare-earth-doped fluoride fibers, fiber Bragg grating (FBG) inscription, and heterogeneous fiber splicing technologies have further enhanced the performance of mid-IR fiber lasers. In 2024, Boilard^[267] demonstrated a monolithic fiber laser operating at 3920 nm with an output power of 1.7 W using Ho3+:InF3 fiber, as shown in Figs. 22(a)–22(d), further advancing the development of mid-IR fiber lasers toward higher power and efficiency.

In the same year, Lemieux-Tanguay et al.^[268] reported an all-fiber Er3+:ZrF4 dual-wavelength pumping (DWP) system, which achieved a stable continuous-wave laser emission of 2.0 W in the 3.8 µm band with a slope efficiency as high as 46.5%. The schematic of the 3.8 µm monolithic Er3+:ZrF4 DWP fiber laser is depicted in Fig. 22(e). The system also exhibited excellent stability at high power, with a root-mean-square deviation of only 0.58% over 2 h.

In 2025, Tianjin University^[269] demonstrated a highly efficient long-wavelength laser based on erbium-doped ZBLAN fiber, operating in the 3.5 µm band through the F49/2→I49/2 energy level transition. The research employed a high-transmission bandpass filter (BPF) as an intracavity wavelength-selecting element, successfully achieving specific laser output at approximately 3.78 µm. The laser achieved a maximum output power of 4.5 W at 3781.6 nm, with a slope efficiency of 17.1% relative to the absorbed 1990 nm pump power and a spectral linewidth of 0.17 nm. This achievement currently represents the highest reported output power for fiber lasers operating beyond 3.7 µm.

Supercontinuum (SC) generation occurs when intense optical pulses propagate through nonlinear media, with their temporal and spectral evolution governed by the interplay of dispersion effects and optical nonlinearities, resulting in dramatic spectral broadening, as illustrated in Fig. 23. The advent of photonic crystal fibers (PCFs) in 1996 revolutionized SC generation in silica-based systems. However, silica fibers are inherently limited to wavelengths below 2.5 µm due to strong material absorption in the mid-IR region. This restriction has spurred significant interest in soft-glass fibers—such as fluoride, tellurite, and chalcogenide (ChG) glass—which offer superior transparency across the mid-IR range (2–20 µm), making them ideal candidates for nonlinear media in mid-IR SC sources.

Fluoride fibers exhibit higher transmission losses beyond 2 µm but maintain significantly reduced losses below 1 dB/m in the 1–4 µm range. For instance, ZBLAN fibers demonstrate losses under 0.5 dB/m from visible to 4 µm, while InF3 fibers maintain similar performance from 2 µm to beyond 4.5 µm. Additionally, fluoride fibers possess a higher nonlinear coefficient compared to silica, enabling efficient spectral broadening. These characteristics enable SC sources to span from the visible to beyond 4 µm, surpassing the long-wavelength limitations imposed by silica’s intrinsic absorption. Fluoroindate fibers exhibit superior mid-IR supercontinuum generation compared to ZBLAN fibers due to their slightly higher nonlinear coefficient, extended anomalous dispersion range, and longer zero-dispersion wavelength, which collectively enhance nonlinear efficiency and enable broader spectral broadening in the mid-infrared region. This capability positions fluoride fibers as a superior platform for generating broadband mid-IR light for advanced photonic applications.

ZBLAN fiber is a leading nonlinear soft-glass medium for mid-IR applications, widely recognized for its extended long-wavelength transparency up to 4.5 µm. Its refractive index closely matches silica fibers, enabling low-loss fusion splicing and high-power mid-IR SC generation. However, its relatively low nonlinear coefficient (3.3×10−20m2/W) necessitates longer fiber lengths to accumulate sufficient nonlinear effects for broad spectral broadening. Additionally, ZBLAN’s deliquesce issue necessitates strict environmental control during handling and storage.

The first demonstration of SC generation in fluoride fibers was reported in 2006^[271], pumping by a 1550 nm mode-locked fiber laser in a 91 cm ZBLAN fiber. This system produced a 5 mW SC source spanning 1.8–3.4 µm. Despite its limited power and spectral width, this milestone spurred efforts to enhance SC performance. In 2009, Xia et al. from the University of Michigan^[272] employed a 1542 nm distributed feedback laser amplified via a three-stage Er/Yb co-doped system as the pump, and achieved a 10.5 W SC source spanning 0.8–4.0 µm from a 7 m ZBLAN fiber. Simulations suggested ZBLAN’s long-wavelength cutoff near 4.5 µm and a theoretical power tolerance of ∼40W, highlighting the influence of peak power on long-wavelength spectral components and the balance between nonlinear broadening and amplification.

In the field of power scaling and efficiency, in 2019, Yang from the National University of Defense Technology^[273,274] utilized a 1.9–2.6 µm Tm-doped amplifier-pumped SC source, achieving a 30-W-level SC output in ZBLAN fiber with a 1.90–3.35 µm range and 69% power efficiency. Despite progress, long-wave spectral components remained weak, and flatness was suboptimal. This study emphasized the critical role of high-peak-power seed sources and nonlinear accumulation in long fibers.

In order to enhance the flatness and stability, in 2020, the same group^[275] optimized seed pulse parameters and reduced splice loss to 0.26 dB at 2 µm, achieving a 20 W SC output with a 10 dB bandwidth extending beyond 4 µm using a 20 m ZBLAN fiber. This marked the first demonstration of >4μm spectral coverage at high power. Simultaneously, Xia et al. from the Ningbo University^[276] employed a Tm-doped fiber laser (TDFL) with spectral shaping to form a 1.5–2.3 µm seed source, realizing a 5.4 W SC output spanning 1.9–4.0 µm with excellent stability (0.03% power fluctuation over 2 h) and spectral flatness (<0.1dB RMS in the 2.1–3.5 µm range).

Recently, in 2023, Zhu et al.^[277] utilized a 2 µm MOPA system with a noise-like pulse seed source and two-stage Tm amplification to pump a 13.5 m ZBLAN fiber (13.5 µm core diameter), achieving a 33.1 W SC output spanning 1.90–3.68 µm with an efficiency of 75.06%, see Fig. 24. By exploiting soliton dynamics in the anomalous dispersion regime, they suppressed residual pump spikes and enhanced spectral flatness. Further adjustment of cavity dispersion enabled pulse-width modulation, optimizing both spectral width and output power.

Despite advancements, ZBLAN-based supercontinuum lasers face critical limitations including material constraints (low thermal stability and susceptibility to hydrolysis), spectral trade-offs (reduced power/flatness beyond 4 µm due to fiber loss), and low nonlinearity requiring long fibers. Future efforts should prioritize material innovation (e.g., modified glass for higher nonlinearity and resilience), hybrid architectures (dual-band/cascaded pumping to enhance nonlinearity while mitigating heat), and system robustness (hermetic coatings, adaptive thermal management) to advance high-power, broadband mid-IR sources for practical applications.

In conclusion, ZBLAN-based SC lasers have demonstrated remarkable progress in power, bandwidth, and stability. Addressing current material and architectural limitations will be key to unlocking their full potential for applications in spectroscopy, defense, and biomedical imaging, paving the way for next-generation mid-IR light sources.

Although the power and spectral width of SC lasers based on ZBLAN fibers have continued to improve, their long-wavelength spectral extension remains limited by the intrinsic loss characteristics of the fibers. In contrast, InF3-based fiber, with its superior long-wavelength transmission and improved physicochemical stability, has emerged as a critical medium for developing high-power mid-IR SC lasers (Table 9).

In 2016, Gauthier et al. from the Laval University^[278], constructed a 2.75 µm seed source (400 ps pulse width, 2 kHz repetition rate) generated by an optical parametric oscillator. This seed light was amplified in an Er3+-doped ZBLAN fiber and subsequently spliced to a low-loss nonlinear InF3-based fiber. They achieved an SC output spanning 2.4–5.4 µm, with spectral components beyond 3 µm accounting for 82% of the total power in a 15 m InF3-based fiber. However, due to CO2 molecular absorption near 4.2 µm, the spectral power density exhibited a sharp decline beyond this point.

In 2018, Théberge et al. from the Canadian Defence Research Centre^[279] utilized a 1.55 µm seed laser (50 ps pulse width) amplified by a multi-stage Yb/Er-doped fiber amplifier (EYDFA) and thulium-doped fiber amplifier (TDFA). This source generated a pre-broadened SC source covering 1.8–2.6 µm, which was then used to pump a 20 m InF3 fiber. They obtained an SC spectrum spanning 1–5 µm with an output power of 1 W, driven by nonlinear effects such as modulation instability, self-phase modulation, and Raman soliton self-frequency shifting. In 2020, Yang et al. from the National University of Defense Technology^[280] demonstrated a high-power mid-IR SC laser based on InF₃ fiber. As shown in Fig. 25(a), a 1550 nm nanosecond laser served as the seed source. The light was pre-amplified via an EYDFA, spectrally shifted using a single-mode fiber, and broadened through nonlinear effects in a TDFA, producing a flat 1.9–2.6 µm SC pump source. This was then injected into an 11 m InF3 fiber (7.5 µm core) by an asymmetric fusion splice (0.36 dB loss). The system achieved an 11.8 W average output with a spectral range of 1.9–4.9 µm and a 10 dB bandwidth of 2550 nm [Fig. 25(b)], marking a significant milestone in power scaling.

InF3-based fibers demonstrate distinct advantages for generating 3–5 µm SC lasers, yet they share challenges with ZBLAN fibers, including poor thermal stability and hydrolysis susceptibility, which undermine long-term reliability at high power. Additionally, the power proportion of SC components beyond 3 µm remains suboptimal. Future advancements may focus on optimizing pumping architectures, cascading configurations, and fiber dispersion engineering to enhance the efficiency and spectral power distribution of InF3-based SC systems.

Mid-IR fiber lasers have advanced significantly, driven by innovations in fluoride glass materials, fibers, pump sources, and system architectures. Current high-power systems, such as Er3+-doped ZBLAN fiber lasers, have achieved 41.6 W at 2.8 µm and 15 W at 3.55 µm using splice-free cavities, bidirectional pumping, and optimized fiber Bragg gratings (FBGs). Fluoroindate fibers have extended emission to 3.92 µm with a 1.7 W output, while SC sources based on ZBLAN and InF3 fibers have reached 33.1 W (1.9–4.02 µm) and 11.8 W (1.9–4.9 µm), respectively. However, challenges persist in thermal management, fiber tip degradation, and material limitations beyond 4 µm.

Future designs should prioritize material innovation with a wide infrared transmission window, high thermal and chemical stability, as well as high laser damage thresholds, to extend emission to 4–5 µm with improved rare-earth doping and reduced hydrogen contamination. All-fiber integration using low-loss silica-to-fluoride splices and monolithic FBG cavities will enhance reliability. Bidirectional pumping and advanced endcaps could mitigate thermal load and tip damage. For SC sources, large-core fibers and hybrid pumping schemes might boost power (>50W) and spectral coverage (1.9–5 µm). Addressing these challenges will enable transformative applications in defense, spectroscopy, and precision manufacturing.

The mid-IR spectral region’s significance in practical applications stems from its alignment with key molecular absorption features and atmospheric transmission properties. Biomedical and industrial applications predominantly utilize the 3–5 µm and 8–14 µm atmospheric windows, where mid-IR lasers achieve optimal tissue ablation efficiency and gas detection sensitivity. These sub-bands, though narrower than the ISO-defined 3–50 µm range, represent the most technologically and commercially viable regions for fiber laser deployment. For instance, the 3–5 µm window overlaps with fundamental vibrational modes of biomolecules and pollutants, while the 8–14 µm band corresponds to thermal radiation peaks for non-contact temperature sensing. As a result, mid-IR lasers have gained widespread application in recent years across various fields, including chemical sensing, environmental monitoring, biomedical applications, industrial processing, and military technology.

The mid-IR spectral range encompasses absorption wavelengths corresponding to the fundamental vibrational and rotational modes of numerous molecules. According to the principle of molecular vibrational absorption, when the frequency of mid-IR light matches the energy gap between molecular vibrational states, the molecule is excited to a higher energy level. The interaction between the molecule and the electromagnetic field induces changes in the molecular dipole moment, which manifest as characteristic absorption peaks in the infrared spectrum, commonly referred to as “molecular fingerprints”^[281]. By precisely measuring the wavelength and intensity of these absorption peaks, the chemical structure and composition of molecules can be revealed, enabling both qualitative and quantitative analysis of substances. Consequently, mid-IR light sources have extensive applications in chemical sensing and environmental monitoring.

As illustrated in Fig. 26, the absorption spectra of various gas molecules in the mid-IR region demonstrate that this spectral range covers the primary absorption lines of many important trace gases, liquids, and solids, including hydrogen cyanide (HCN), methane (CH4), nitrogen dioxide (NO2), formaldehyde (H2CO), carbon dioxide (CO2), nitrous oxide (N2O), carbon monoxide (CO), water molecules, and particulate matter (PM2.5)^[282]. By emitting laser beams of specific wavelengths into the atmosphere and comparing the collected echo signals of these molecules with the characteristic molecular fingerprint database, it is possible to accurately detect and analyze the types and concentrations of trace gases and particulate matter in the atmosphere, thereby facilitating the analysis and monitoring of air, soil, and water pollution^[283,284]. Numerous reports on the use of mid-IR lasers for the detection and sensing of special substances have been demonstrated^[285,286]. For instance, CH4, C2H6 (ethane), and NH3 (ammonia) are widely present in nature and commonly found in swamps, underground pipelines, and mines, where they pose risks due to their flammability, explosivity, and toxicity^[286–289]. By employing mid-IR fiber laser absorption spectroscopy, the concentrations of these gases can be monitored in real-time with high sensitivity, preventing explosion hazards and toxic gas leaks. This technology is particularly suitable for gas leakage detection in natural gas pipelines, mine safety monitoring, and underground gas storage facilities. Furthermore, CH4 and CO2 are the primary components of greenhouse gases. By tuning the operating wavelength of the laser, the molecular absorption line profiles can be precisely determined, enabling the effective detection of trace gas isotopic ratios^[290–292], and thereby achieving accurate monitoring of atmospheric greenhouse gas emissions^[293,294]. This application is essential for agriculture, landfill sites, oil and gas extraction, and industrial emissions control, contributing to effective pollution management and environmental protection.

The clinical application of lasers in medicine dates back to 1961 when a ruby laser was first used for retinal coagulation treatment in ophthalmology. Today, laser technology is widely utilized in biological research, with mid-IR lasers demonstrating significant potential in the biomedical field^[296,297]. In medical interventions, lasers primarily achieve precise tissue ablation through photothermal effects. This process is closely related to the absorption characteristics of water within biological tissues, which depend on the laser wavelength. When laser radiation interacts with biological tissues, the absorption of laser energy by water determines factors such as the penetration depth within the tissue, the extent of the damaged region, and the precision of surgical procedures^[298]. As shown in Fig. 27, the absorption spectrum of water molecules^[299] indicates that the absorption peak intensity in the 1.1–1.2 µm wavelength range is relatively low. Consequently, lasers operating in this spectral region can penetrate deeply into water-rich tissues, such as skin and other soft tissues, while precisely focusing on water-deficient tissues, such as fat, to achieve selective photothermal decomposition for therapeutic purposes^[300,301].

Furthermore, water molecules exhibit strong absorption in the 3 and 6 µm wavelength bands. Water absorption of mid-IR laser light at the 3 µm wavelength is 10,000 times stronger than that of near-infrared lasers at the 1 µm wavelength^[303]. When mid-IR laser radiation at this wavelength interacts with skin tissue, the energy is rapidly absorbed, causing a swift temperature increase in the superficial skin layers, which results in instantaneous tissue vaporization, separation, and precise ablation^[304]. Modern biological studies indicate that water constitutes a significant portion of soft tissues; thus, this property enables the application of mid-IR lasers in non-contact surgical procedures. In medical imaging and treatment, by precisely controlling laser parameters, mid-IR lasers can be utilized for high-resolution imaging of deep soft tissues, precision treatment, and drug delivery, while minimizing thermal damage to surrounding healthy tissues^[305,306]. In surgical applications, a focused laser beam enables strong absorption by water molecules in the tissue, leading to rapid evaporation, thereby allowing precise, non-contact tissue removal while simultaneously achieving rapid hemostasis. This approach offers advantages such as minimal incision depth and reduced recovery time, and such laser beams are referred to as “laser scalpels”^[307]. The replacement of traditional open surgery with this non-contact, minimally invasive laser technique has become an emerging trend in modern medicine^[308–311]. For instance, 3 µm laser systems have been demonstrated to precisely and effectively cut biological tissues, which shows the effects of 3 µm laser tissue cutting^[221]. Additionally, 3 µm laser systems are widely applied in dermatology, dentistry, orthopedics, and neurosurgery^[312–314]. In dermatology, 3 µm laser systems are used for removing skin lesions, including pigmentation disorders, scar revision, and laser resurfacing treatments. Based on the selective absorption of different skin pigments at specific laser wavelengths, a high-peak-pulse laser beam at the corresponding wavelength can be applied to the skin. Melanin absorbs the laser energy, undergoes rapid thermal expansion and fragmentation, and is subsequently phagocytosed and excreted, gradually eliminating scars and pigmentation spots. In ophthalmic surgery, 3 µm laser systems are used for corneal cutting and retinal repair, ensuring high precision and safety in delicate eye procedures. In dentistry, this laser wavelength is effective for minimally invasive treatments of enamel and dentin, reducing postoperative discomfort and inflammation. In orthopedic and neurosurgery, the high water absorption of mid-IR lasers enables precise bone tissue cutting and brain surgery, enhancing surgical safety and therapeutic outcomes. 3 µm laser systems are also employed in otologic (ear) surgery, further demonstrating their versatility in minimally invasive medical applications^[315].

Compared to solid-state lasers, quantum cascade lasers, and optical parametric oscillators, mid-IR fiber lasers offer superior beam quality, higher brightness, narrower linewidths, and compact structure. They also exhibit higher absorption efficiency, lower power consumption, enhanced processing precision, and extended operational lifespan, making them highly valuable in industrial applications, including safety monitoring, precision manufacturing, and materials processing.

In terms of safety monitoring, the mid-IR spectral region covers the “fingerprint absorption peaks” of numerous molecular species, such as carbon dioxide (CO2) at 2.3 and 3.8 µm, acetylene (C2H2) at 3.1 µm, methane (CH4) at 3.3 µm, nitrogen dioxide (NO2) at 3.5 µm, nitric oxide (NO) at 1.8 and 2.7 µm, and ammonia (NH3) at 3.4 µm^[316,317]. This characteristic makes mid-IR coherent light sources highly significant for industrial process control and gas safety detection. By employing mid-IR laser absorption spectroscopy, it is possible to achieve high-sensitivity, real-time, and non-contact detection of gas compositions, enabling rapid and precise detection and analysis of trace gases and atmospheric pollutants. This capability allows for the effective monitoring of industrial emissions, toxic gases, natural gas pipelines, oil field environments, and security-sensitive areas. For example, carbon monoxide (CO) is a toxic gas that is widely present in metallurgical plants, mines, automobile exhaust, and environments with incomplete combustion. Additionally, nitric oxide (NO), nitrous oxide (N2O), and hydrogen sulfide (H2S) are toxic gases commonly found in petrochemical industries, natural gas extraction, and wastewater treatment facilities. Utilizing mid-IR fiber laser sensors, it is possible to perform real-time CO concentration monitoring, preventing poisoning incidents and enhancing industrial safety. Furthermore, mid-IR lasers can be applied to monitor industrial emissions, vehicle exhaust, and flue gas from coal-fired power plants, thereby reducing pollutant emissions, protecting the environment, and ensuring industrial gas safety monitoring.

In precision manufacturing and materials processing, laser machining utilizes a controllable, clean, and highly concentrated laser beam to accurately heat, melt, or vaporize materials. Particularly when processing microscopic localized regions of materials, fiber laser processing technology surpasses conventional manufacturing techniques^[318,319]. Fiber lasers can operate in CW or pulsed modes. High-power CW fiber lasers are widely used in laser cutting, laser welding, and metal 3D printing^[320,321]. On the other hand, narrow-pulse-width fiber lasers can generate high peak power even at relatively low pulse energies. For instance, femtosecond lasers enable “cold processing” of materials, effectively minimizing the melt zone and heat-affected zone, making them suitable for selective material removal, precision drilling, and surface or internal micro-patterning^[322]. Compared to 1 µm lasers, mid-IR lasers offer higher processing efficiency and superior processing quality for polymeric materials. The fundamental stretching vibration of C–H bonds in polymer materials can resonate with 3.44 µm laser light, resulting in rapid and intense thermal effects^[323]. In 2018, Frayssinous et al. employed a 3.44 µm fiber laser to ablate high-density polyethylene (HDPE) and polypropylene. The processing efficiency achieved was three to five times higher than that of a 10.6 µm CO2 laser, with a lower ablation threshold and smoother, more evenly ablated regions^[324]. Other polymer materials, such as polyester resin, polylactic acid (PLA), nylon, and polycarbonate, also exhibit strong absorption in the 3 µm wavelength range. Robichaud employed a 3 µm short-pulse laser to process polyester resin and polymethyl methacrylate (PMMA), achieving high-quality machining and polymer optical fiber fusion splicing^[325]. The application of mid-IR lasers in polymer processing not only enhances processing quality and efficiency but also promotes the broader industrial adoption and technological advancement of polymer materials. Furthermore, as mid-IR laser processing and detection are non-contact methods, they prevent contamination and eliminate tool wear, which are common issues in traditional machining processes. Consequently, high-power-laser technology has now been extensively adopted in industrial applications^[326].

The high-power output and excellent beam quality of lasers are indispensable technologies in future high-tech war^[327]. For military and national security applications, the mid-IR spectral range (3–5 µm) encompasses the critical response region of military infrared-guided detectors, granting it significant strategic value in modern military countermeasure systems. Airborne and shipborne infrared countermeasure (IRCM) systems utilize high-power mid-IR laser beams to disrupt, deceive, or blind enemy missile seekers, causing them to deviate from their intended trajectory or lose guidance capability, thereby reducing the threat posed by advanced weapon systems^[328,329]. Additionally, mid-IR lasers exhibit excellent atmospheric transmission properties, allowing them to accurately locate and designate targets over long distances, thus providing precise targeting coordinates for laser-guided weapons. This capability is particularly crucial in nighttime operations or adverse weather conditions, where high-precision laser ranging can significantly enhance battlefield situational awareness^[330]. Furthermore, missiles and high-speed aircraft generate combustion byproducts such as CO2 and H2O, which exhibit distinct spectral absorption features in the mid-IR region. This unique spectral signature enables mid-IR laser detection systems to perform target identification and early warning. Using mid-IR spectral analysis technology, it is possible to remotely monitor the exhaust plume characteristics of airborne targets, facilitating precise identification and tracking of high-speed aerial threats, thus enhancing early threat detection and missile defense capabilities. In addition to playing a crucial role in infrared countermeasures, the long-wavelength characteristics of infrared lasers allow them to detect infrared radiation from the human body and other heat sources under low-light or nighttime conditions through thermal imaging technology. By analyzing the differences between objects based on the information from their emitted radiation and converting it into an electro-optical signal, these lasers are widely used in defense reconnaissance, aircraft carrier early warning, high-altitude surveillance, and precision strike targeting. They are often referred to as the “eyes” of modern high-tech weapons^[331,332]. Additionally, mid-IR laser-based LiDAR (light detection and ranging) systems can utilize return signals for terrain mapping and obstacle detection, and their high-precision measurement capabilities can provide critical data for tactical planning and security prevention^[333].

Comprehensively speaking, mid-IR laser technology provides new technical means and strategic solutions for modern military security applications. It not only enhances the anti-jamming capabilities of weapon systems but also serves as a technological foundation for long-range detection, precision guidance, and electro-optical countermeasures. This technology demonstrates broad application potential in air defense and missile interception, tactical strikes, and battlefield surveillance, playing a crucial role in the advancement of intelligent and information-based warfare systems. Its adoption is of significant importance in enhancing military operational efficiency and safeguarding national security.

Mid-IR lasers (2–5 µm) have drawn extensive attention in recent years due to their novel applications in medical diagnostics, gas sensing, materials processing, and national defense. This has created an urgent demand for efficient and low-loss mid-IR laser delivery media^[334–336]. Currently, traditional silica-based fibers are unsuitable for high-power, broad-bandwidth mid-IR laser transmission owing to their intrinsic IR absorption. Alternatively, fluoride fibers have shown great potential for mid-IR laser delivery due to their unique properties such as low phonon energy (<550cm−1), ultra-wide transmission window (0.3–7 µm), and tunable dispersion feature^[337,338]. In this part, three types of fluoride fibers, including fluorozirconate, fluoroaluminate, and fluoroindate fibers, are introduced, and their mid-IR laser delivery properties and optimization strategies are discussed to provide a theoretical and technological summary of the application of fluoride fibers in mid-IR laser delivery.

The field of fluoride glass was pioneered by the synthesis of fluorozirconate glass at the University of Rennes in 1974^[29]. In 1981, NTT Laboratories in Japan achieved 0.6 W power delivery at 2.7 µm using a GdF3−BaF2−ZrF4−AlF3 fluoride fiber, demonstrating the feasibility of mid-IR laser delivery^[339]. Subsequent research on fluoride fibers focused on developing ultra-low-loss communication fibers and demonstrated that the theoretical loss of ZBLAN fibers was as low as 0.001 dB/km, but practical losses were much higher due to fabrication limitations^[340]. In 1989, the Whitehurst team from the University of Manchester used ZBLAN fibers to transmit 2.94 µm (Er: YAG laser) light with 65% efficiency, 3.5 W average power, and a bend radius of 25 mm (300 µm core), highlighting their potential in laser surgery^[341]. Later, the Wuthrich team in Switzerland studied the damage thresholds of single-mode and multimode ZBLAN fibers during mid-IR laser delivery. They found that fiber end-face conditions critically influenced delivery power since contamination or poor cleaving might damage fiber ends. High-quality end faces enabled power intensity transmission up to 2.7MW/cm2 without damaging the fiber ends^[342]. In 1998, HOYA Corporation demonstrated that AlF3-based fluoride fibers surpassed ZBLAN fluorozirconate fibers in mid-IR laser delivery performance. The AlF₃ fibers achieved a higher laser damage threshold of 1.4kJ/cm2 compared to ZBLAN’s 1.0kJ/cm2, while also delivering over 8.7 W of power, more than double ZBLAN’s maximum of 4 W. These enhanced characteristics enabled the successful application of AlF3-based fibers in Er:YAG laser systems for dental surgery procedures^[343].

At the turn of the 21st century, François Séguin’s team in IR Photonics Inc. developed a double-clad structure and refined processes, enabling 2 m ZBLAN fibers to stably transmit Er, Cr: YSGG lasers (2.78 µm) with a peak power of 10.3MW/cm2 (1530J/cm2 fluence) for 23 h (1.14 million pulses) without aging^[344]. Nevertheless, when transmitting higher-power lasers, fiber damage tends to occur at the end faces due to thermal stress and crack propagation. Thus, low transmission loss, proper end-face treatment, and maintaining fiber integrity are essential for fluoride fibers to deliver high power.

In 2023, a research group in the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, utilized a “short-temperature-zone” drawing technique to suppress crystallization during fiber drawing. By optimizing parameters such as drawing temperature, feed rate, and pulling speed based on the temperature field, they produced fluoroindate fibers with a core/clad diameter of 200 µm/260 µm and an average theoretical loss of ≤0.22dB/m at 3–5 µm. Moreover, mechanical cutting of the fiber ends enabled a 10-W-level pulsed laser output at 3.7–4.8 µm, as shown in Fig. 28, laying the foundation for higher-power laser transmission and demonstrating the great potential of applying fluoride fibers to the laser industry and medical diagnostics^[345].

Over the past 50 years, fluoride fibers have undergone rapid development. Improvements in the fabrication process and optimizations in structural design have fostered the application of fluorozirconate fibers in clinical medicine (e.g., Er: YAG lasers) and laser material processing. Fluoroindate fibers, with lower phonon energy, demonstrated longer-wavelength (4–5 µm) laser transmission, while AlF3-based fibers, with better environmental durability, can be used in harsh conditions. Future work could focus on developing fluoride fibers with reduced fiber loss, enhanced damage threshold, and robustness for mid-IR laser delivery through optimizing fabrication methods and designs, further broadening the applications of fluoride fibers.

Mid-IR fiber laser technologies have undergone transformative advancements over the past two decades, exemplified by milestones such as a 41.6 W CW output at 2.8 µm in Er3+-doped ZBLAN fibers and 10-W-level outputs in Dy3+- and Er3+-doped systems at 3.24 and 3.5 µm. However, substantial gaps persist in power scalability and operational stability when compared to mature near-infrared laser systems. Addressing these limitations requires a concerted strategy integrating material innovation, photonic component engineering, and advanced thermal management. The following perspectives outline critical challenges and opportunities for advancing fluoride fiber technology and mid-IR laser systems. (1)Toward robust fluoride glass systems

Fluoride glass fibers face inherent material and fabrication limitations. Their inherent hygroscopic nature induces surface hydrolysis and hydroxyl contamination under ambient conditions, particularly evident in ZBLAN exposed to moisture^[135]. While fluoroaluminate glass enhanced aqueous resistance compared to conventional fluoride compositions, their durability still falls short of silica-based counterparts^[128]. Otherwise, lower tensile strength compared to silica renders fluoride fibers prone to surface defects during drawing, which act as crystallization nucleation sites, further compromising mechanical stability^[57,346]. Additionally, their low Tg and high thermal expansion coefficient exacerbate crystallization risks during fiber fabrication and operation. The fabrication process further demands precise thermal and atmospheric control to mitigate defects and scattering losses caused by low melt viscosity^[347]. Finally, the complex fabrication process requires stringent temperature and atmospheric control during preform casting or rotational molding to minimize crystallization and surface irregularities, while low-viscosity melts during drawing often introduce bubbles and defects that elevate scattering losses^[348]. Future research must prioritize hybrid glass systems that synergize broad mid-IR transparency, high rare-earth solubility, enhanced chemical resilience, and superior laser damage thresholds. Innovations in glass composition design and advanced fabrication techniques, such as precision rotational molding and optimized cooling protocols, will be pivotal for achieving these goals. (2)Developing mid-IR-specific photonic components

The reliance on free-space optics in current mid-IR systems introduces alignment sensitivity and insertion losses, hindering scalability. Emerging fiber-integrated solutions aim to overcome these limitations. High-reflectivity fiber Bragg gratings fabricated via femtosecond laser inscription enable precise spectral control with minimal parasitic losses. Concurrently, kW-grade end caps incorporating plasma-enhanced chemical vapor deposition coatings mitigate interfacial thermal lensing by suppressing thermal gradients. Ultra-low-loss combiners and broadband isolators based on magneto-optic fluoride fibers further enhance system robustness by minimizing signal degradation and back-reflections. These advancements collectively herald a transition toward compact, alignment-tolerant all-fiber architectures capable of sustaining high-power mid-IR operation across diverse applications, from precision surgery to remote sensing. (3)Resolution for thermal management bottlenecks

Conductive cooling alone proves insufficient for high-power mid-IR fiber lasers operating under extreme thermal loads. A hierarchical approach combining quantum-defect engineering, hybrid pumping, and cascaded wavelength conversion offers a comprehensive solution. Quantum-defect reduction via in-band pumping or multi-wavelength excitation directly lowers heat generation, while hybrid pumping schemes spatially distribute thermal loads across spectral bands. Cascaded energy transfer to longer wavelengths further dissipates residual heat through graded wavelength conversion. This multi-tiered framework not only addresses thermal bottlenecks but also enhances system scalability for applications demanding stringent power stability, such as directed energy systems and industrial material processing. (4)Addressing longstanding hygroscopic degradation in fluoride fibers

The hygroscopic nature of fluoride fibers remains a critical barrier to their practical deployment. To mitigate moisture-induced degradation, a multi-faceted strategy integrating surface engineering, compositional optimization, and environmental control is essential. Surface passivation techniques, such as atomic-layer-deposited coatings, reduce moisture permeability by over 90%, while fluoropolymer encapsulation extends operational lifetimes in humid environments. Hybrid glass systems incorporating stable covalent bonds at the surface further enhance moisture resistance. For scalable fabrication, containerless melting—though currently limited to small-scale batches—shows promise in suppressing heterogeneous nucleation, particularly in microgravity environments. Collaborative initiatives with commercial spaceflight providers could enable the production of meter-long, low-defect preforms. On Earth, hybrid crucible-levitation systems and hermetic packaging with integrated moisture getters offer near-term solutions. These approaches, combined with rigorous humidity control during storage and operation, are critical to unlocking the full potential of fluoride fibers in real-world applications.

The evolution of mid-IR fiber lasers hinges on interdisciplinary collaboration to bridge material science, photonics, and thermal engineering. Advances in fluoride glass chemistry, integrated mid-IR components, and intelligent thermal management will unlock unprecedented power scalability and reliability. Emerging applications in defense, environmental monitoring, and nonlinear optics will drive further innovation, positioning mid-IR fiber lasers as indispensable tools for next-generation photonic technologies. Realizing this potential demands sustained investment in fundamental research, industrial partnerships, and standardized testing protocols to accelerate technology maturation and commercialization. The integration of space-based manufacturing paradigms and terrestrial hybrid systems may further revolutionize fluoride fiber production, enabling breakthroughs in performance and accessibility.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 62225502 and 62090062), the CAS Youth Interdisciplinary Team (No. JCTD-2022-09), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2022244), the National Key R&D Program of China (Nos. 2020YFA0607602 and 2021YFB3500901), and the Shandong Provincial Natural Science Foundation of China (No. ZR2024MF033).