Space-based fabrication of ZBLAN optical fibers: A breakthrough in microgravity manufacturing

With the continuous growth of global communication demands, fiber-optic networks are undergoing rapid expansion, and the most widely used silica-based optical fibers are now approaching their intrinsic loss limits. Heavy-metal fluoride ZBLAN fibers theoretically exhibit losses in the mid-infrared band that can be approximately 20 times lower than those of silica fibers. However, this theoretical advantage has long been constrained by the inevitable crystallization problem during ground-based fabrication—ZBLAN glass possesses a critical cooling rate far higher than that of silica, rendering it highly susceptible to crystallization during the fiber drawing process on Earth, which leads to a significant increase in scattering loss. Although microgravity environments may reduce crystallization rates by suppressing buoyancy-driven convection, and theoretical models support this mechanism, the lack of space-qualified automated fiber drawing equipment has previously prevented the acquisition of fiber samples of sufficient length for systematic characterization. Therefore, validating the practical feasibility and performance enhancement effects of ZBLAN fiber drawing under microgravity conditions has become a key scientific issue in overcoming the bottleneck to realizing the ultra-low-loss potential of this material.

In a recent study published in Space: Science & Technology, an international research team developed a compact automated fiber drawing tower system and successfully drew approximately 12 km of ZBLAN fiber inside the Microgravity Science Glovebox aboard the International Space Station, marking the first-ever kilometer-scale fabrication of ZBLAN fiber in space. The study employed the same drawing equipment and process parameters as those used on the ground, fabricating both single-clad and step-index fiber structures. A systematic characterization of optical attenuation and mechanical properties was conducted by a consortium of five international institutions on the approximately 2.1 km sample exhibiting the optimal diameter uniformity. The results demonstrate that in the step-index fiber, the loss of the space-fabricated sample is significantly lower than that of its ground-based counterpart across the entire wavelength range; at 1.55 μm, the loss of the ground sample is 2–7 dB/m, whereas that of the space sample is reduced to 0.3–1.2 dB/m, and in the mid-infrared band, the loss of the space sample remains at 1–2 dB/m. The space sample also exhibits a 39% increase in Weibull modulus and an 80% improvement in ultimate tensile strength, indicating substantially enhanced mechanical reliability. This study provides the first experimental validation of the effectiveness of microgravity drawing in suppressing crystallization and improving the performance of ZBLAN fibers, opening a new technological pathway for the development of ultra-low-loss optical fibers and the fabrication of photonic materials in microgravity.

First, the motivation of this study lies in resolving the long-standing performance bottleneck of ZBLAN fibers. ZBLAN fluoride glass theoretically possesses far superior broad-spectrum transparency compared with silica fibers, particularly in the near- to mid-infrared bands, where its intrinsic loss can be as low as below 0.01 dB/km. However, during ground-based fiber drawing, gravity-driven buoyancy convection significantly accelerates crystal growth, causing the actual loss to exceed the theoretical value by several orders of magnitude. To circumvent this physical bottleneck, the research team leveraged the microgravity environment of the International Space Station, where buoyancy convection is largely suppressed and mass transport relies solely on molecular diffusion. They autonomously developed a compact automated fiber drawing tower and successfully drew approximately 12 km of ZBLAN fiber continuously aboard the ISS. This marks the first time that a sufficient length of space-fabricated fiber has been obtained for systematic investigation. Subsequently, a joint team comprising five international institutions selected the approximately 2.1 km sample exhibiting the best diameter uniformity from this batch, and conducted a comprehensive comparison with control samples drawn on the ground from the same preform batch. This rigorous experimental design ensures that the observed performance differences can be attributed solely to the change in gravitational environment, rather than to variations in processing or material batch.

Second, the comparative results reveal a clear and selective performance enhancement. In terms of optical attenuation, the single-clad structure (i.e., without a core–cladding interface) exhibits stable losses in the range of 0.08–0.25 dB/m regardless of whether the fiber was drawn in space or on the ground, with no appreciable difference between the two. In stark contrast, the step-index double-clad fibers display a markedly different behavior: the space-drawn samples exhibit significantly lower losses than their ground-based counterparts across the entire measured wavelength range. At the 1.55 μm communication window, the space fiber losses are 0.3–1.2 dB/m, whereas the ground fiber losses are as high as 2–7 dB/m; in the 2–4 μm mid-infrared region, the space samples maintain low losses of 1–2 dB/m, as shown in Fig. 1. Mechanical properties are also improved: the space-drawn step-index fiber exhibits a 39% increase in Weibull modulus and an 80% improvement in median ultimate tensile strength; even in the single-clad fibers, the ultimate tensile strength is enhanced by 25%, as illustrated in Fig. 2. These data provide the first experimental evidence from both optical and mechanical perspectives that microgravity drawing can simultaneously enhance the transmission efficiency and structural reliability of fibers, and that this advantage is particularly pronounced in complex structures featuring a core–cladding interface.

Finally, by analyzing the differences in preform fabrication processes, the paper elucidates the physical mechanism underlying the microgravity effect and offers a perspective on its industrial prospects. The step-index preform is fabricated using a rotational casting method, in which the high-index core glass is injected into a low-index cladding glass tube; this process involves slower cooling and reheating of the cladding, making it highly prone to the formation of micron-sized seeds at the core–cladding interface. In contrast, the single-clad preform is produced by rapid casting in a metallic mold, which cools quickly and generates virtually no seeds. During ground-based fiber drawing, gravity-driven convection accelerates mass transport, allowing these seeds to rapidly grow into microcrystallites, resulting in severe scattering losses. In the microgravity environment of space, convection is suppressed, impeding seed growth and thereby substantially reducing the excess loss caused by crystalline scattering. This perfectly explains why microgravity is ineffective for seed-free structures but yields significant improvements in step-index fibers containing seeds, as summarized in Table 1. However, the paper also clearly notes that while microgravity can effectively suppress crystalline scattering, it cannot eliminate the intrinsic absorption caused by transition metal impurities in the raw materials; achieving the theoretical loss limit will therefore still require further purification of feedstock materials. Nevertheless, this milestone work has demonstrated the scalability and practical value of space-based manufacturing, and points the way toward future autonomous space platforms for fabricating ultra-low-loss optical fibers and other difficult-to-process photonic materials, potentially propelling microgravity manufacturing from laboratory demonstration toward industrial-scale applications.