# Introduction to Photonics: Principles and the Most Recent Applications of Microstructures
## 4 Structure, Types, and Applications of Optical Fibers
Step-index multimode fibers were the first cords designed but are too slow regarding most applications because of the dispersion caused by the different path lengths of the various modes. Step-index fibers are barely used in modern telecommunications. Multimode fibers that are employed in communications possess the core size of 50 or 62.5 microns. The big core sizes allow the fibers to support many diagonal electromagnetic modes for a given polarization and frequency. Single-mode fibers enjoy lower fiber attenuation than multimode fibers and retain better reliability of each light pulse because they have no dispersion associated with multiple mode fibers. Hence, data can be transferred over a longer distance. Similar to multimode fibers, the earlier single-mode fibers were commonly characterized as step-index fibers (shown in Figure 5), which means the refractive index of the fiber cladding is a step below that of the core rather than graduated as in the case of graded-index fibers. Current single-mode fibers have grown into a more sophisticated design like depressed clad, matched clad, or other mysterious structures. Figure 4: Attenuation against wavelength transmission windows. The core size of single-mode fibers usually is nine microns. Because only one mode can propagate down the fiber length, the total internal reflection process does not occur; hence, the concept of numerical aperture becomes similar to those of multimode fibers. The numerical aperture of multimode fibers is usually larger than those of single-mode fibers. The most common lasers appropriate for applications over single-mode fiber include distributed feedback (DFB) and Fabry-Perot lasers. The attenuation of single-mode fibers is about 0.2 dB per km [43]. Optical fibers operate based on the principle of total internal reflection. Imagine rays of light striking a distinct boundary separating an optically less dense medium. A less dense medium is the one with a lower reflection index. At an appropriate incidence angle, these rays rather than passing through will be reflected fully. This phenomenon is referred to as the total internal reflection [44; 45]. Prisms in binoculars and camera viewfinders make use of total internal reflection. If the incidence angle is represented by the symbol (\(\alpha\)), and the angle of refraction as \(\beta\) (see Figure 6 at this boundary to the less-dense medium, (\(n_{G}>n_{A}\)) assuming air and glass are being considered), the condition \(\alpha<\beta\) holds. However, the angle \(\beta\) cannot be greater than \(90^{\circ}\). This is evident considering the [PERSON]'s law of refraction (Equation (1)):  
\[\frac{\sin\ \alpha}{\sin\ \beta}=\frac{n_{A}}{n_{G}} \tag{1}\]  
Bearing in mind that (\(\sin\ \beta\)) cannot be greater than one,  
\[\sin\ \alpha_{critical}=\frac{n_{A}}{n_{G}}<1 \tag{2}\]  
Figure 5: Multimode and single-mode fibers. For even greater angles of incidence, the rays of light are entirely reflected back into the denser medium almost without encountering any loss. It is the same principle that guides light around bends as well as inside optical fibers [46]. Optical fibers have applications for assisting us in various aspects of our lives--for example, in amplifiers [47], in telecommunications [48; 49], in medicine [50], in aerospace and aviation technology [51; 52], in engineering [53; 54], nanotechnology [55; 56], and in sensing applications. Optical fiber sensors have been studied for over 40 years. Several concepts have been suggested, and many methods have been established for various parameters as well as for various uses. Commercialization of optical fiber sensors has been carried out successfully. However, out of the many methods investigated, only a small amount of applications and methodologies have been commercialized successfully [57]. The optical fiber-based sensors possess many advantages over copper cables for their high sensitivity, small size, large bandwidth, lightweight quality, as well as immunity towards electromagnetic interferences [58; 59; 60].