# Optoelectronic Instrumentation and Measurement Strategies for Optical Chemical (Bio)Sensing
## 2 Optical Devices
### Wavelength Selector  
In the design of optoelectronic systems, it is common to use a device that allows the selection of certain wavelengths. The most common devices used to perform this function are optical filters and monochromators. An optical filter located between the optical fiber and the emission light detector can greatly reduce the signal provided by the reflection of the excitation and the signal produced by light interference. The optical filters used in spectroscopy are generally interference-based. They are used to monitor light at certain wavelengths and are based on the arrangement of thin layers of dielectrics that produce interference between the wavefronts, resulting in very small bandwidths. The advancement of thin-film deposition technology has allowed for filters with a bandwidth of 5 nm (FWHM), central wavelengths (CWL) between 250 and 1500 nm, transmission \(>\)90% (at the peak of the central wavelength), and optical density \(>\) 4. These filters are available as components [25] or mounted directly on photodiodes [26].  
Other wavelength selection devices include monochromators, although their use is in laboratory instrumentation. They are based on the use of a diffraction network, which depending on the position, can diffract a polychromatic light source into its monochromatic components. In diode-matrix-based monochromators or CCDs, the spectrum is directed to a linear array of photodetectors. The size of the detector determines the spectral resolution. These monochromators are significantly cheaper, have no moving parts, and are very reliable. Other advantages are the simultaneous access to the entire spectrum and the integrated optoelectronic conversion; however, their resolution is lower compared to their diffraction network counterparts. Fiber optic instrumentation systems may also require the use of optical couplers or beam splitters to efficiently distinguish and separate incident and return radiation from traveling in a single fiber. Optical refraction and reflection components, such as lenses and mirrors, are also required in instruments to manipulate light and focus it more effectively on fiber optics. Table 3 summarizes the main characteristics of the wavelength selectors considered for optical chemical (bio)sensing.  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline  & **Photodiodes** & \multicolumn{2}{c}{**Avalanche Photodiodes**} & \multicolumn{2}{c}{**Photomultipliers**} \\ \hline \(\bullet\) & Range: UV–NIR & \(\bullet\) & Range: UV–NIR & \\ \(\bullet\) & Fast and robust & \(\bullet\) & High sensitivity and speed & \(\bullet\) & Large spectral range \\ \(\bullet\) & Low cost & \(\bullet\) & Reverse high voltage & \(\bullet\) & High voltage supply \\ \(\bullet\) & Relatively high noise & \(\bullet\) & Internal amplification & \(\bullet\) & Very high internal amplification \\ \(\bullet\) & Requires additional amplification & \(\bullet\) & Low light levels & \(\bullet\) & Low noise \\  & & & & & High cost \\  & & & & & Low mechanical stability [24] \\ \hline \hline \end{tabular}
\end{table}
Table 2: The main characteristics of light detectors used for optical sensing.