# Introduction to Photonics: Principles and the Most Recent Applications of Microstructures
## 4 Structure, Types, and Applications of Optical Fibers  
Optical fibers are flexible filaments made of very clear glass and can carry information in the form of light from one point to another. They are hair-thin structures formed through the formation of preforms, which are glass rods made into fine threads of glass and secured by plastic coatings. Various vapor deposition processes are employed by fiber manufacturers to draw the preform. The thread drawn from this preform is then usually wrapped into a cable configuration, which is then placed into an operative situation for years of dependable performance [26]. The two most important components of optical fibers are the core and the cladding. The \"core\", which is the axial part of the optical fiber, is made up of silica glass. The optical fiber core is that area of the fiber where light is transmitted. Sometimes, doping elements are used to modify the fiber refractive index, thereby changing the light velocity through the fiber. The \"cladding\", on the other hand, is the layer that surrounds the core completely. The cladding refractive index is less than that of the core. This enables the light inside the core to strike the core-cladding interface at a \"bouncing angle\", is confined inside the core by the total internal reflection, and keeps moving in the appropriate direction along the fiber length to a certain point. The cladding is usually surrounded by another layer known as \"coating,\" which normally is comprised of protective polymer films coated during the process of fiber drawing, before being in contact with any surface. Additional protective layers of \"Buffers\" are further applied on top of the polymer coatings as shown in Figure 3[27]. The mechanism of the modifications on the fiber surface can be characterized through the transmission spectrum measurement of the fibers. There are so many different possible configurations of fibers corresponding to different application purposes. The most important classification considers fibers as either single-mode fibers and multimode fibers. The concept of application-specific fibers was invented at Bell Laboratories in the mid-1990s, and this is followed by an introduction of fibers  
Figure 3: Structure of optical fiber. designed for network applications. These next designs that are used mainly for signal transmission in communications consist of 10-Gbps laser-optimized multimode fibers (OM3), Zero Water Peak Fiber (ZWPF), Non-Zero Dispersion Fibers (NZDF), and fibers that are specially designed for the marine application. Specially designed fibers, like erbium-doped fibers, and dispersion compensating fibers perform tasks that supplement the transmission fibers. The differences between the different transmission fibers are responsible for variations in the number and range of different wavelengths or pathways via which the light is received or transmitted; this is the distance at which a signal can travel without being amplified or regenerated, and the speed at which this signal can travel. The silica fibers are the common type of fibers that can transmit light with wavelengths below the mid-infrared range [28]. The silica as an optical waveguide is a strongly absorbing material for wavelengths above 2 um [29]. This is due to multiphoton absorption that causes vibrational resonance; however, there are different glass materials that can be used to fabricate the optical fibers in which these materials can transmit light at a longer wavelength [30]. The crystalline materials and hollow fiber waveguides are good candidates to perform these kinds of transmissions [31]. For instance, glasses such as the chalcogenides, which may have different compositions of sulfides, selenides or tellurides, have substantially lower vibration frequencies and therefore lower photon energy compared to silica [32]. This is due to the higher mass of chalcogenide ions compared to oxygen ions.