# Introduction to Photonics: Principles and the Most Recent Applications of Microstructures
## 7 Waveguides and Applications  
A waveguide consists of a hollow, metal tube that is a unique form of transmission line. The technology of applying hollow pipes to streamline the movement of electromagnetic waves first appeared in 1897. The 1930s, following the development of the first microwave-producing equipment, necessitated the creation of a hollow waveguide for them. The success of these hollow waveguides motivated scientists to invent waveguides in the infrared region of the electromagnetic spectrum. These waveguides were initially used for medical purposes, but other areas of applications followed (Figure 12). [93]. Figure 11: Schematic representation of the principle of Fiber Bragg grating. It directs the waves in a similar way river banks head a tidal wave [93]. Nevertheless, since waveguides are regarded as single-conductor materials, there is a difference in the way electrical energy is propagated down a waveguide as compared to the way in which it is propagated through a two-conductor transmission system. Figure 13 shows the propagation of the TEM mode in the waveguide. From Table 1, it could be concluded that a major improvement in waveguides is the decrease in propagation losses. The waveguide dimensions become impossibly large for lower frequencies, while, when the rate is higher, the dimensions become impracticably smaller. Waveguides present numerous advantages when compared to their optical fiber counterparts. Waveguides can transmit wavelengths above 20 \(\upmu\)m. Their air core enables them to deliver high power lasers. Waveguides have relatively simple structures and they are cheap compared to the existing optical fibers. These qualities allow their use in applications where transmission of electromagnetic radiation requires a material of high mechanical, optical and thermal properties. Even though waveguides are associated with  
Figure 12: Rectangular and circular waveguides. Figure 13: Transverse electromagnetic (TEM) mode propagation of a waveguide. some setbacks such as losses upon bending as well as small numerical aperture, they appear to be the best option for use in sensors. Waveguides can be classified into two classes based on the principles on which they operate. These are attenuated total internal reflection and leaky-type waveguides. In attenuated total internal reflection-based waveguides, hollow core substances are present, which are enclosed by a wall that has a refractive index lower than that of the wavelength of the transmitted light. In the design of metallic waveguides, the inner wall is formed using a smooth metallic surface by depositing a metal film on the inner surface of plastic or a glass tube. Dielectric waveguides, on the other hand, are achieved by the formation of alternating high-low refractive index structure formed by the addition of multiple dielectric layers onto the metal surface. There has been a dramatic increase in the use of substrate integrated waveguides over the last decade due to their compact, low profile, and many other advantages over the conventional metallic waveguides. They resemble the conventional waveguides in their performances and can be made with printed circuit boards [94]. The need to explore waves at a millimeter scale for the subsequent generation of mobile communications has contributed significantly to the advancement of modern telecommunications components based on microwaves [95]. Substrate-integrated waveguides have found many potential applications outside the telecommunication industry. They are widely used in the automotive industry, as well as in biomedical devices for sensing applications. In terms of their dispersion and propagation characteristics, substrate integrated waveguides are similar to rectangular waveguides. Another important emerging area is opted fluidics, which combines the benefits of optics and microfluidics in order to achieve highly compact and highly functional materials.