At the beginning of this month, Microsoft announced it had acquired Lumenisity® Limited, a company that is focused in the development of hollow core optical fibers. This piece of news is an excellent reason to talk about mid-infrared fibers, that is, fibers that transmit light in the mid-infrared (MIR), from 2 µm to 20 µm.
As it was mentioned in a previous post, silica is the most employed material in optical fibers, but silica fibers cannot be employed at wavelengths higher than 2 µm due to their high absorption in this wavelength range. Mid-infrared fibers include fibers that employ chalcogenide glasses, germanate fibers, fluoride fibers, polycrystalline fibers, sapphire fibers and hollow core fibers. In this post we are going to focus on fluoride fibers and hollow core fibers.
Fluoride fibers are fibers made of several fluoride glasses. Based on their composition, there are different types of fluoride fibers but two of the most common ones are ZBLAN fibers (ZrF4 – BaF2 – LaF3 – AlF3 – NaF) and AlF3-based fibers (AlF3 – BaF2 – SrF2 – CaF2 – MgF2 – YF3). The transmission window for ZBLAN fibers ranges from approximately 0.4 µm to 4 µm, while AlF3-based fibers only cover up to around 3.5 µm. The main applications of fluoride fibers are MIR spectroscopy, fiber-optic sensing, thermometry and imaging. Although it was initially thought they could be employed for optical communications, they are not adequate for this purpose.
The major disadvantages of fluoride fibers are their fragility and that they are hygroscopic (they absorb water from the medium), which limits their lifetime. Nevertheless, AlF3-based fibers are mechanically more robust and are less affected by moisture than ZBLAN fibers. It must also be considered that fluoride fibers are expensive as they are difficult to manufacture. This is due to the fact that there are no available gas precursors for fluoride fibers, so the manufacturing techniques employed in silica fibers cannot be used in this case.
Regarding hollow core fibers, they are fibers where the core is a hollow region (it is air instead of a solid material). This means the refractive index of the core is lower than that of the cladding, so light does not propagate following the total internal reflection mechanism, which requires the opposite condition. In conventional hollow core fibers, light propagation is explained by the existence of photonic bandgaps, which prevent the light guided by the hollow region from propagating into the cladding.
Hollow core fibers are generally considered to be a type of photonic crystal fibers, which are defined by a periodic arrangement of air holes in the section of the fiber. Their transmission window can extend from 2 µm up to 16 µm, although there are also hollow core fibers designed to work in the visible – near infrared region of the spectrum and even in the ultraviolet. Hollow core fibers are usually made of glass or sapphire, although other materials (plastic or metal), can also be employed.
One of the main applications of hollow core fibers is data transmission due to their low latency (time delay). The reason is that the speed of the guided light is usually close to the vacuum velocity of light. Other advantages of hollow core fibers for data transmission are their weak nonlinearities, which lower the costs and enhance the network quality, and the low propagation losses, which enable transmission over longer distances without repeaters. These features have been key in Microsoft’s decision of acquiring Lumenisity® Limited, the piece of news with which we have started this post. On the other hand, from a research point of view, hollow core fibers are interesting for gas sensing applications, as these structures favor the interaction between the guided light and the gases to be detected.
In conclusion, in this post we have described two of the main types of mid-infrared fibers: fluoride fibers and hollow-core fibers. Their characteristics, including their advantages and weaknesses, have been explained; as well as their applications, and it seems hollow core fibers are going to change the paradigm in data transmission in the next few years.
Written by J.J. Imas
 “Microsoft acquires Lumenisity®, an innovator in hollow core fiber (HCF) cable”, Girish Bablani
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 “Hollow-core photonic bandgap fibers: Technology and applications”, Francesco Poletti, Marco N. Petrovich et al., Nanophotonics, 2, 5-6, 12 2013
 Hollow-Core Photonic Crystal Fiber Gas Sensing, Ruowei Yu, Yuxing Chen et al., Sensors 2020, Vol. 20, Page 2996, 20, 10, 5 2020