# Optoelectronic Instrumentation and Measurement Strategies for Optical Chemical (Bio)Sensing
## 2 Optical Devices
### Optical Fibers  
The incorporation of optical fibers in the analytical system provides attractive characteristics, enhancing the advantages offered by optical methods of chemical measurement. The ability of fibers to transmit optical signals over long distances with little power loss makes direct measurement possible in places far from the instrumentation system. The flexibility of optical fibers allows chemical analyses to be performed in hard-to-reach places. Fiber optics provide a waveguide for radiation that interrogates the molecular recognition element of the optical sensor. Optical fibers, as shown in Figure 3, consist of a transparent dielectric cylinder (core) surrounded by a cladding of another dielectric material with a refractive index lower than that of the core. These two cylinders are protected by a coating. The propagation of light through the length of an optical fiber takes place through the phenomenon of total internal reflection. Any ray that enters through one end of the fiber into the acceptance cone will propagate through the core of the fiber. The maximum acceptance angle depends only on the refractive indices of the core (\(n_{1}\)) and the cover (\(n_{2}\)), via the following expression:  
\[sen\ \theta_{a}=\left(n_{1}^{2}-n_{2}^{2}\right)^{1/2} \tag{1}\]  
The light-gathering ability of the fiber is called the numerical aperture (\(NA\)) and corresponds to the sine of the acceptance angle:  
\[NA=sen\ \theta_{a} \tag{2}\]  
This value is of great importance for the coupling of light to optical fiber conductors. The greater the amount of light that the fiber can accept, the greater the transmission distance that can be reached, although if it is very large the system bandwidth degrades. Figure 3: Transmission of a ray of light inside an optical fiber. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Optical Filters**} & \multicolumn{2}{c}{**Monochromators**} \\ \hline \(\bullet\) & Central wavelength (CWL): 250–1500 nm & \(\bullet\) & Range: 180–240 μm interchangeable grid \\ \(\bullet\) & Bandwidth (FWHM): 5.0–10 nm & \(\bullet\) & Laboratory applications \\ \(\bullet\) & Fast and robust & \(\bullet\) & High resolution: 0.1 nm \\ \(\bullet\) & Low cost & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Wavelength selector devices used for optical chemical (bio)sensing. More details of the propagation of light in optical fibers can be found in [27; 28]. The transmission of light through a fiber depends on the wavelength of the radiation and the angle of reflection. Only certain combinations (modes) of wavelengths and reflection angles are allowed. Each mode corresponds to a single angle of reflection for each radiation wavelength. For a particular wavelength, the number of propagation modes decreases with the diameter of the fiber. Optical fibers with core diameters between 2 and 8 \(\upmu\)m allow a single propagation mode (single-mode fibers). Fibers with larger diameters, typically between 50 and 200 \(\upmu\)m, are multimode and allow simultaneous transmission of several wavelengths through the fiber; they are typically used over shorter distances. Multimode optical fibers are commonly used in FOCS because they are easier to connect and their tolerance for lower-precision components is higher due to the large size of their cores. The propagation of modes in an optical fiber depends on the profile of the refractive index of the nucleus. There are two types of multimode optical fibers:  
* Step-index multimode optical fibers, in which the refractive index is constant;
* Graded-index multimode optical fibers, in which the refractive index varies in parabolic shape from a maximum on the conductor axis to a minimum on the coating. The modal dispersion of this type of fiber is lower, although it is more expensive. The material used in fiber optic cables can be plastic, glass, or a combination of both glass for the core and plastic for the cover. Plastic fibers are used in the visible region of the spectrum, are lower in cost, and are easier to work with.