# Introduction to Photonics: Principles and the Most Recent Applications of Microstructures
## 7 Waveguides and Applications
Specifically,  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{**Waveguide Name**} & \multicolumn{2}{c}{**Recommended Frequency**} & \multicolumn{2}{c}{**Cutoff Frequency**} & \multicolumn{2}{c}{**Cutoff Frequency**} & \multicolumn{2}{c}{**Inner Dimensions of Waveguide**} \\  & & \multicolumn{2}{c}{**Frequency (GHz)**} & \multicolumn{2}{c}{**Lowest Order**} & \multicolumn{2}{c}{**Next Mode**} & \multicolumn{2}{c}{**Opening**} \\ \hline
**EIA** & **RCSC** & **IEC** & **Mode (GHz)** & **(GHz)** & **A Inch** & **B Inch** \\ \hline - & WG9 & - & 2.20 to 3.30 & 1.686 & 3.372 & 3.5 & 1.75 \\ WR340 & WG9A & R26 & 2.20 to 3.30 & 1.736 & 3.471 & 3.4 & 1.7 \\ WR284  
fluidic elements are integrated into the photonics structure [96; 97]. A lot of optofluidic sensors are produced for healthcare and pharmaceutical researchers. Moreover, sensors are also available for biochemical analyses, environmental monitoring as well as biomedical researchers [98]. Integration of optical and fluidic structures can simply be achieved via optofluidic waveguides. Optofluidic waveguides are found to provide a very powerful foundation for building optofluidic sensors. The liquid core serves as the medium through which the light is guided with the help of highly reflective mirrors that are achieved by the sidewalls of the core. High sensitivity is usually achieved by taking advantage of flow or guidance of the light and the fluid through the same medium or channel. This direct interaction provides high sensitivity due to the small volume of the liquid as well as strong optical connections. Sensitivity can be achieved as low as the molecular level and this is the ultimate desire for any given analytical procedure. Just like other waveguide systems such as slot waveguides and photonic crystals, they have a high attractive capacity for optofluidic integrations with planar systems [99; 100]. These types of waveguides allow simultaneous confinement and propagation of light because of the interference of light that occurs at the claddings that are made up of alternating thin layers of low- and high-refractive index. The operating principle of these types of waveguides can be understood easier when one considers the 1D structure as presented below in Figure 14. Assuming that two layers of the cladding are considered, and their refractive indexes are respectively \(n_{1}\) and \(n_{2}\) (with \(n_{1}>n_{2}\)), propagation of light through the core is achieved by Fresnel reflections at the cladding interfaces. Repeated reflections at the interfaces cause the entrapped light to interfere. Interference cladding is designed to certify certain conditions to strengthen the intensity of the reflected light [101]. Presently, optical biosensors have emerged as the favorite choice to replace bulky laboratory instruments for such applications where a bulky quantity of samples is required to be simultaneously analyzed, like microplate array systems. Optical biosensors possess some desirable characteristics like small size, low-cost and being easier to use. These pleasant characteristics allow them to be used in online monitoring and sensing for the analysis of samples that are complex, and for real-time or online monitoring of experimental procedures [102]. Therefore, optical bio-sensing is now an active research field and commercialization of a number of platforms has already been realized. The application areas include environmental monitoring, clinical application, and food safety and control [103]. The most powerful and reliable biosensors are undoubtedly those based on fluorescence. Fluorescence intensity, decay time, and emission anisotropy are among the parameters that could be measured and used for sensing [104]. There is, therefore, a variety of options towards improving the performance of biosensors. Optical waveguides are dielectric configurations having two extreme wavelengths in the infrared  
Figure 14: 1D structure of narrow waveguide.