Glass is a key material in optics and, in particular, in optical fibers, as they are usually made of silica (silicon dioxide, SiO2), the most abundant mineral found on the surface of the Earth, commonly found in the form of quartz and usually employed to manufacture glass, ceramics, abrasives and optical fibers. Regarding optical fibers, the cladding is typically made of fused silica (pure silica, without any other materials) while the core has a higher refractive index as it is made of silica doped with other materials, such as germanium. We will discuss about the optical fiber fabrication procedure in our next blog post. The difference in refractive index between core and cladding leads to the propagation of light through total internal reflection, enabling the use of optical fibers as waveguides. In this post, we are going to focus on the material that constitutes the optical fiber, glass, and its properties.
The first question that arises regarding glass is, to which phase of matter does it correspond? One may seem tempted to say that glass is solid as, for instance, glass have a fixed shape, a property we clearly associate to solids. However, solids are highly organized structures at atomic level, including crystals (the atoms are periodically arranged) and polycrystal structures (made of several “small” crystals). Glass, on the other hand, does not present this rigid order. Therefore, if glass is not a solid, it should be liquid, right? Glass is even able to “flow” (move their atoms), although very slowly. For example, this is supported by the fact that some panes in gothic cathedrals are thicker at the bottom that they are at top. Nevertheless, the answer is still not correct.
Glass is an amorphous solid, which can be considered an intermediate phase between solids and liquids. Glass has a more organized structure at atomic level than a liquid, but it lacks the long-range order that is characteristic of solids. The process of manufacturing a glass is the following. When a liquid at high temperature is cooled below its melting temperature (Tm) without solidifying, it becomes a supercooled liquid. If the material is cooled even further, below the glass-transition temperature (Tg), the movement of the atoms greatly reduces, and we now have glass.
There are liquids that can both crystallize and form a solid at Tm or become a supercooled liquid and then form a glass at Tg (see Figure 1). In these cases, in order to avoid solidification at Tm, fast cooling rates must be employed. On the other hand, Tg is not a temperature value, but a range of temperatures over which the glass transition (the process of the supercooled liquid becoming a glass) takes place. Furthermore, this range of temperatures is not fixed, but depends on the cooling rate, although it does not excessively vary.
Returning to optical fibers, silica is the most employed type of glass, predominantly for communications applications. Some of the properties of silica that justify its employment in optical fibers include the following: wide wavelength range with good optical transparency in the near infrared range (low absorption and scattering losses), high mechanical strength, chemical stability, and easy cleaving and fusion splicing.
Regarding silica fibers that are employed in the ultraviolet – visible region, better transmission (lower losses) is achieved in this region with a high concentration of hydroxyl groups (OH), while the opposite, a low OH concentration, is better when working in the near infrared range. This was also mentioned in one of our first blog posts. It is also worth mentioning that silica glass can be doped with different materials to increase (germanium dioxide, GeO2, alumina, Al2O3) or decrease (fluorine) the refractive index.
Nevertheless, there are also other non-silica glass fibers: phosphate glass fibers and fluoride glass (polymer fibers also exist, but we are talking about fibers that employ glass). Fluoride fibers, which are based on fluoride glasses, are particularly interesting because they possess high optical transparency in the mid-infrared spectrum, while silica fibers cannot be used in this range due to their high absorption (see Figure 2). Nevertheless, fluoride fibers are expensive as they are difficult to manufacture, and their use is challenging due to their fragility.
In conclusion, in this post we have explained the nature of glass as an amorphous solid and how it can be produced, as well as the main properties of silica fibers and the remaining glass fibers (fluoride fibers) that can be used. By the way, if the question still itches you, it has been demonstrated that the movement of the atoms in windowpanes is too slow to justify the fact they are thicker at the bottom. The real explanation, much less interesting, seems to be related to the manufacturing process employed in the Middle Ages.
Written by J.J. Imas
 Part I: The Truth about Glass. Clarus Glass boards
 Part II: The Truth about Glass. Clarus Glass boards
 Fact or Fiction?: Glass Is a (Supercooled) Liquid. Ciara Curtin. Scientific American.
 Glass transition. Wikipedia.
 Supercooled Liquids and Glasses, M. D. Ediger, C. A. Angell, and Sidney R. Nagel, J. Phys. Chem. 1996, 100, 31, 13200–13212
 Silica fibers. RP Photonics Encyclopedia.
 Fluoride fibers. RP Photonics Encyclopedia.