Optical fiber transmits light by utilising the phenomenon of internal reflection discussed earlier. It is primarily composed of two layers. At the centre is the core, where light propagates, made of a material with a high refractive index. Surrounding the core is the cladding layer made of a material with a lower refractive index than the core. Light entering the core undergoes internal reflection at the boundary with the cladding due to the difference in refractive index and the angle of incidence exceeding the critical angle, allowing it to be transmitted solely within the core.
While the refractive index of the core can vary depending on the material used, it typically ranges from approximately 1.46 to 1.5. On the other hand, the cladding has a slightly lower refractive index, usually between 1.44 and 1.46. Therefore, the core's refractive index is generally about 0.01 to 0.05 higher than that of the cladding. Despite this small difference in refractive index, the light can still undergo total internal reflection at the core-cladding boundary due to Snell's law, which relates the angle of incidence to the refractive indices.
Optical Fiber Materials
The most commonly used material for optical fiber production is pure silica glass (SiO₂). It has high transparency and low loss characteristics, making it suitable for long-distance communication. Doped silica glass is made by adding impurities to pure silica glass to adjust the refractive index or to amplify specific wavelengths of light. The impurities used and their characteristics are as follows.
Aluminium (Al₂O₃): Used to increase the refractive index of the core.
Germanium (GeO₂): Increases the refractive index of the core and is advantageous for high-speed data transmission.
Fluorides: Used to lower the refractive index of the cladding and improve the thermal resistance of the fiber.
Boron (B₂O₃): Used to lower the refractive index.
Phosphorus (P₂O₅): Used to adjust the refractive index and increase the strength of the fiber.
Additionally, polymer-based optical fibers are used in some cost-effective applications. These fibers are flexible and lightweight, but they can have higher signal loss compared to silica fibers, making them primarily suitable for short-distance communication or lighting. Recently, with the increasing application of optical sensors in automobiles, optical fibers are also being utilized. In cars, polymer optical fibers are more advantageous than glass-based fibers. This is because polymer optical fibers are lighter than silica-based ones, contributing to weight reduction, and they are flexible and easy to bend, allowing for easy installation in the complex shapes of vehicle interiors. Moreover, polymer optical fibers generally have lower production costs, providing cost-reduction benefits, and they are resistant to shocks and vibrations encountered during vehicle operation, ensuring stable performance in the automotive environment, thus enhancing durability and reliability. Electrically, they also have high resistance to electromagnetic interference (EMI), minimizing interference with the vehicle's electronic systems.
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