# Optoelectronic Instrumentation and Measurement Strategies for Optical Chemical (Bio)Sensing
## 2 Optical Devices
### Optical Fibers
Glass core fibers are used in the UV-visible and visible-NIR spectrum regions. They can be single-mode, multimode with a step index (such as polymer cladding silica (PCS) fibers) or gradient index, or fiber bundles made from such fibers [29]. Table 4 shows the main characteristics of optical fibers used for (bio)sensing applications. Multimode optical fibers are commonly used in FOCS. There are several schemes to connect optical fibers and sensing areas, as shown in Figure 4. The instrument shown in Figure 4a uses a single fiber or a fiber bundle. The instrument has a beam splitter to isolate the returning radiation and focus it on the photodetector. Figure 4b shows a bifurcated optical fiber. This scheme requires simple instrumentation and is highly efficient at collecting light. Figure 4c shows that the sensing element is deposited over the core fiber, acting as the cladding. The evanescent wave interacts with the sensing area and is attenuated according to the Lambert-Beer law. This schema requires a thin reagent layer and the response time is short. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Single-Mode Fiber** & **Graded-Index Multimode Fiber** & **Step-Index Multimode Fiber** \\ \hline \(\bullet\) & \(\varnothing_{core}\): 2–8 \(\upmu\)m & & \(\varnothing_{core}\): 50–200 \(\upmu\)m \\ \(\bullet\) & Requires laser & \(\bullet\) & Simpler & \(\bullet\) \\ \(\bullet\) & High bandwidth & \(\bullet\) & Laser or LED & \(\bullet\) \\ \(\bullet\) & Difficult to manipulate & \(\bullet\) & Shorter distances & \(\bullet\) \\ \(\bullet\) & Communications & \(\bullet\) & \(\bullet\) & Higher cost \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The main characteristics of optical fibers. Special optical fibers must be used for harsh environments [30]. The most common parameters are temperature and strain or stress sensing, although other parameters such as the pressure, magnetic field, voltage, and chemical species can also be measured. The coating of the optical fiber plays the key role in protecting the glass. If the coating survives, the glass will continue to perform. Special coatings such as polyimide, silicone, and high-temperature acrylate are available that are suitable for higher temperatures. In high-pressure ambient environments, some polyimide coatings may degrade and peel off from the glass, exposing the surface. A number of added functionalities can be obtained by replacing the polymer coating with metal. Unlike polymers, metals do not outgas in vacuums, do not ignite, and they lend themselves to mounting by soldering. Regarding harsh environments, metal-coated fibers also exhibit many attractive features. As with carbon-coated fibers, metal-coated fibers are protected against ingression of water and hydrogen. Furthermore, metal coatings provide unsurpassed heat resistance; fibers coated with aluminum or copper, for example, may operate at temperatures well above 400 \({}^{\circ}\)C. fiber Bragg grating temperature sensors have also been used nuclear environments due to the presence of ionizing radiation fields [31], as well as in harsh aerospace environment [32]; however, only in recent years has this technology matured sufficiently to allow real field applications.