In this blog post, we’ll explore the principles behind optical fibers—which have made today’s ultra-high-speed optical communications possible—and “thermal migration,” the key technology used to manufacture them.
Today, we use the internet every single day without fail. The internet allows us to easily exchange information from all corners of the world, no matter where we are on the globe. As a result, people almost always use the internet when using computers to do homework, read the news, or play games. So, how is information from around the world delivered so quickly right to our fingertips? The answer lies in optical fiber. Optical fiber is a very thin, long cable made primarily from high-purity silicon dioxide (silica), which uses light as a signal to quickly transmit large volumes of data over long distances.
The concept of optical fiber began to develop in earnest in the mid-20th century. Although it had the major advantage of being able to transmit large amounts of data at much faster speeds than traditional metal cables, the loss of light inside the fiber was too significant at the time, making it difficult to use for telecommunications. So why is low light loss important? You’ve probably seen Namsan Tower from a distance. On a clear day, it’s visible from tens of kilometers away, but on rainy or foggy days, it’s hard to see. This is because light is absorbed or scattered by water droplets and dust particles in the air. Similarly, light passing through an optical fiber must suffer minimal loss to accurately deliver the signal to a recipient at a distant location. There are two main causes of light loss in optical fibers. The first is the absorption or scattering of light due to impurities and microscopic structural defects within the fiber. The material that makes up an optical fiber is composed of atoms, and these atoms and their bonding structures interact with light under specific conditions. Furthermore, if impurities or microscopic defects are present, the light may deviate from its original path or be partially absorbed, causing the signal to weaken. The second cause is light escaping from the interior of the fiber.
To solve the first problem, silica of very high purity—with virtually no impurities—must be used. In fact, optical fibers for telecommunications are manufactured to have an extremely high purity of 99.9999% or higher. In the fields of mechanical engineering and materials science, researchers have studied and applied various principles of heat and mass transfer—including a phenomenon known as “thermal electrophoresis”—in the process of manufacturing and controlling these high-purity materials. Since it is difficult to obtain silica of such high purity in its natural state, a purification process to remove impurities is absolutely necessary. “Thermal electrophoresis” refers to the phenomenon in which very small particles or molecules move when a temperature gradient exists. To understand this, let’s consider a kettle as an example. The water at the bottom of the kettle is heated first by the flame, while the water at the top remains relatively cool. Over time, the heated water becomes less dense and rises, while the cooler water sinks; this phenomenon is called “convection.” Separate from convection, fine particles or molecules can be subjected to forces of varying magnitudes and directions due to temperature differences, and as a result, the extent of their movement may vary depending on the type of particle. This phenomenon is called “thermoelectric migration.” When thermal drift occurs, different types of particles move along different trajectories; therefore, by controlling this process appropriately, it can be used to selectively separate or concentrate specific particles.
Advances in optical fiber manufacturing technology have made it possible to precisely control the composition and structure of materials, which has also effectively solved the problem of light escaping from the fiber. When a coin is placed in water, it appears to float at a shallower depth than it actually is; this is because light changes its speed and refracts as it passes through different materials. Generally, materials through which light travels slowly have a high refractive index, while those through which it travels quickly have a low refractive index. When light enters a material with a higher refractive index from one with a lower refractive index at an angle greater than a certain threshold, the phenomenon of “total internal reflection” occurs, in which the light is completely reflected and does not pass through the boundary. Optical fibers are designed with a higher refractive index in the core and a relatively lower refractive index in the cladding, ensuring that light travels along the interior of the fiber rather than escaping outward. Thanks to this structure, optical fibers can transmit light over long distances with very little loss and have become a core technology enabling today’s ultra-high-speed optical communications.
In the fields of mechanical engineering and materials science, researchers have studied various principles of heat and mass transfer—including thermal convection—to develop technologies for manufacturing high-purity materials and precisely controlling microstructures. These technological advancements have made it possible to increase the purity of optical fibers and significantly reduce light loss, while also allowing for the precise design of the refractive index within the fiber to maximize the total internal reflection effect. Optical fibers are a core technology that forms the foundation of today’s ultra-high-speed wired communications. The next time you use the internet at home or enjoy online games with friends, take a moment to reflect on the fact that behind the ultra-high-speed communications we take for granted lie various scientific principles—including “thermo-osmosis”—and cutting-edge manufacturing technologies.
