## 4 Fiber Sensing Technologies
### Optical Sensing for Gas Detection
#### 4.3.2 Gas Sensing System Design  
In the combustion process of coal waste piles, several gases are emitted; for emissions monitoring three were selected, namely ammonia (NH\({}_{3}\)), methane (CH\({}_{4}\)), and carbon dioxide (CO\({}_{2}\)). For the choice of the DFB lasers with emission wavelength in the near-infrared region, it is important to take into account the absorption lines intensity of these gases because in this region the intensity of the absorption lines is reduced compared to the infrared region. Thus, the DFB lasers of the experimental setup work at wavelengths of 1653.72 nm (for CH\({}_{4}\) detection), 1572.0 nm (for CO\({}_{2}\) detection), and 1512.21 nm (for NH\({}_{3}\) detection), where their respective gas absorption response is unique. The experimental setup implemented in the gas sensing units (NH\({}_{3}\), CO\({}_{2}\), and CH\({}_{4}\)) using the WMS is shown in Figure 4.  
The experimental setup for gas sensing included two types of sensing heads. One was based on commercially available flow cells (Wavelength References Inc., Corvallis, OR, USA--FC-H(80)-1/8-none-80 cm flowcell with no connectors and 1.8\" swagelok fittings for gas inlet/outlet) permitting free light propagation and interaction with the gas along an optical path length of \(\sim\)160 cm (the flow cell was operated in reflection), and another relying on microstructured optical fibers with gases diffusing in their hole. Through an optical switch, the system allows us to select the sensing head to be used. Figure 4: Schematic diagram of the system for gas sensing using WMS. Three reference gas cells were also used to fine tune the wavelength emission of the DFB lasers to the selected gas absorption line. These cells had a 100% concentration of each gas with a pressure of 0.263 atm at room temperature. The optical path length of the NH\({}_{3}\) and CH\({}_{4}\) cells was 5.5 cm, while it was 80 cm for the CO\({}_{2}\) cell. The control of the gas sensing system presented in Figure 4, as well as acquisition, processing, and data presentation, was performed using a LabVIEW@application (National Instruments, Oeiras, Portugal). The DFB laser diodes selected for each gas (NH\({}_{3}\), CO\({}_{2}\) and CH\({}_{4}\)) provided 7 mW at 1512.21 nm and 1572.0 nm and 10 mW at 1653.72 nm with a specified linewidth \(<\) 2 MHz. The fiber was spliced to a 99/1(%) directional coupler with the 99% arm sent to the optical switch. The 1% arm was used as the reference beam passing through the reference (NH\({}_{3}\), CO\({}_{2}\), and CH\({}_{4}\)) gas cell. Figure 5 shows the NH\({}_{3}\) gas absorption line for a concentration of 100% in the 1511.2-1513.6 nm region, a temperature range of 16-27 \({}^{\circ}\)C, and over the current range 27-103 mA. As a result, the NH\({}_{3}\) absorption line detected by the shift had a center wavelength of 1512.21 nm and a full width at half maximum (FWHM) of 59 pm. For the CO\({}_{2}\) and CH\({}_{4}\) absorption lines detected by the shift, the center wavelength was found to be 1572 nm and 1653.72 nm and the full width at half maximum (FWHM) of 40 pm and 41 pm, respectively. The laser wavelength and intensity was modulated by applying a sine wave signal with a frequency of 5 kHz. The initial injection DC current (laser bias) and temperature were set in the Laser Controller to 61.3 mA and 21.6 \({}^{\circ}\)C, respectively, in order to tune the emission wavelength DFB laser diode to the desired wavelength of the NH\({}_{3}\) absorption line (1512.21 nm). To tune the emission wavelength DFBs laser diodes to the desired wavelength of the CO\({}_{2}\) and CH\({}_{4}\) absorption lines (1572.0 nm and 1653.72 nm), the injection DC current and temperature were set in the Laser Controller to 97.8 mA and 65 mA and 33.24 \({}^{\circ}\)C and 21.73 \({}^{\circ}\)C, respectively. To lock the laser wavelength to the target gas absorption center line, the experimental setup had a feedback loop that consisted in part of the emitted light passing through the reference gas cell.