# In-Situ Testing of Methane Emissions from Landfills Using Laser Absorption Spectroscopy
## 2 Methodology
### Testing Apparatus  
Figure 2 shows the framework of the gas concentration testing system based on TDLAS. The system mainly consists of the laser transmitting unit, the operation control unit, and the signal receiving unit. First of all, the system program is used to input a triangular wave signal into the laser driver. The wavelength of the diode laser emission is scanned over the absorption peak of the target gas. According to the background temperature, the current, the drift coefficient of wavelength change with current, and the transmission coefficient of voltage change with current, the output wavelength range of the laser can be calculated. The output wavelength range should cover the absorption wavelength of the target gas. On this basis, a sinusoidal signal is added to modulate the wavelength signal. The tail fiber of a diode laser emits a laser, which passes through a collimator. The direct light passes through the target gas and scatters over a block at a certain distance. Then the scattered signal is input into the photoelectric detector with a lens. The signal captured by the detector is transmitted to the lock-in amplifier. Finally, the processed signal is transmitted to the LabVIEW program through the data acquisition card so as to realize the demodulation of the signal and the calculation of the gas concentration. The distance between the collimator and the block was 10 m, which meant 10 m long gas samples were measured.  
Figure 3 shows the structure and photographs of the self-designed testing apparatus. It mainly includes a laser emitter, photoelectric detector, focusing lens, collimator, controller, data acquisition card and power. The choice of the laser emitter depends on the absorption wavelength of the target gas (i.e., CH\({}_{4}\) in this study). Based on the HI  
Figure 2: The framework of the gas concentration testing system.  
TRAN database [28], the absorption spectrum of CH4 is least disturbed by other gases (e.g., N2, CO2, NH3, H2S) in the wavelength ranges of 1640-1770 nm 1320-1410 nm, and 1150-1168 nm. Among the three wavelength ranges, the absorption of CH4 at the wavelength range of 1640-1770 nm is greater than that of the other two wavelength ranges. Referring to the common laser emitter types, the absorption wavelength of CH4 in this experiment is chosen as 1653 nm. The DFB laser device (produced by Sichuan Tengguang Technology Co. Ltd, Mianyang, Sichuan) is used with a central output wavelength of 1653 nm and a line width of 2 MHz. The mode-hop free tuning range of the diode laser was over 50 GHz. The laser emitter is equipped with a TED200C temperature control device and an LDC205C current drive device (both produced by Thorlabs Company, Newton, NJ, USA). It has a limit operating current of 120 mA and an operating temperature range of \(-20\)\({}^{\circ}\)C\(\rightarrow\)\(+70\)\({}^{\circ}\)C. During the test, the signals with wavelengths of around 1653 nm are detected, and the PDA50B-EC photoelectric detector (produced by Thorlabs Company) is selected for this experiment. The detector is a germanium detector with an adjustable gain (0-70 dB), and the appropriate detector gain is chosen according to the strength of the signal. Using the focusing lens as the receiving unit of the system, the axes of the focusing lens and the output beam of the laser are controlled at the same height to facilitate the feedback of the optical signal to the photoelectric detector.