# A Dual-Gas Sensor Using Photoacoustic Spectroscopy Based on a Single Acoustic Resonator
## 3 Results  
### Calibration of CH\({}_{4}\) and NH\({}_{3}\)  
The PA instrument's response to different NH\({}_{3}\) and CH\({}_{4}\) concentrations was evaluated. CH\({}_{4}\) and NH\({}_{3}\) concentrations ranging from 20 to 500 ppm were generated by diluting a reference concentration of CH\({}_{4}\) (100 ppm) and NH\({}_{3}\) (500 ppm) with nitrogen for calibration. Amounts of 20-100 ppm CH\({}_{4}\) were used for the calibration of CH\({}_{4}\), and 100-500 ppm NH\({}_{3}\) was used for the calibration of NH\({}_{3}\).  
For CH\({}_{4}\) detection, the DFB laser was tuned to the center of the CH\({}_{4}\) absorption line at approximately 1653.8 nm (120 mA, 20 \({}^{\circ}\)C). A sawtooth signal (20 s in period, 800 mV in amplitude) was used to carry out the DFB wavelength scan. A 6.8-kilohertz sinusoidal signal (160 mV in amplitude) was added with the sawtooth signal to modulate the light amplitude for generating the PA signal. The value of 800 mV for the sawtooth signal was chosen to ensure that the scanning wavelengths of the two lasers covered the absorption peaks of both gases. A sinusoidal signal of 160 mV was chosen by investigating the dependence of the PA signals on the amplitude of the sinusoidal signal, as shown in Figure 5a. Figure 5a shows that the PA signal improved little with a modulation amplitude ranging from 160 to 250 mV, and that laser lifetime can benefit from the low modulation amplitude of 160 mV. Figure 5b shows the first harmonic photoacoustic signal of 20, 40, 60, 80 and 100 ppm CH\({}_{4}\) in nitrogen in the left region. Figure 6a shows the linear dependence of peak-peak PA signals on different CH\({}_{4}\) concentrations with a regression coefficient \(R^{2}\) = 0.99.  
Figure 4: (**a**) Simulated absorption lines of NH\({}_{3}\) and CH\({}_{4}\) around 1512 nm. (**b**) Simulated absorption lines of CH\({}_{4}\) around 1653 nm.  
Regarding the NH\({}_{3}\) measurement, the DFB laser was tuned to the center of the NH\({}_{3}\) absorption line at around 1511.7 nm (120 mA, 20 \({}^{\circ}\)C) with a sawtooth signal of 800 mV in amplitude and a modulation sine signal of 160 mV in amplitude. The first harmonic PA signal of NH\({}_{3}\) at 500 ppm was recorded in the right region in Figure 5b. A good linear correlation of the peak-peak PA signals on different NH\({}_{3}\) concentrations was obtained with \(R^{2}\) = 0.99, as shown in Figure 6b.  
Figure 7a shows the two spectra of CH\({}_{4}\) and NH\({}_{3}\), which indicate that NH\({}_{3}\) affected the measurement of CH\({}_{4}\) in the first half of the PA spectrum. In contrast, methane had no influence on the NH\({}_{3}\) signal. In order to eliminate this interference, the overlapped signal needed to be calibrated to obtain the average signal value of different NH\({}_{3}\) concentrations. As shown in Figure 7b, the PA signals brought in by NH\({}_{3}\) at different concentrations were plotted. The \(R\)-squared value obtained by linear fitting reached 0.997, which indicates that the interference signal is linearly related to the NH\({}_{3}\) concentration. Therefore, by first demodulating the concentration of NH\({}_{3}\), the interference signal that affects the CH\({}_{4}\) PA signal was obtained by calculation, and this part of the methane signal was subtracted in order to accurately retrieve the CH\({}_{4}\) concentration.  
Figure 5: (**a**) PA signals of different amplitudes of the sinusoidal signal. (**b**) First harmonic PA signals of CH\({}_{4}\) concentrations varying from 20 to 100 ppm in the left region and NH\({}_{3}\) concentrations varying from 100 to 500 ppm in the right region.  
Figure 6: Calibration of CH\({}_{4}\) and NH\({}_{3}\): (**a**) Response linearity of CH\({}_{4}\) at different concentrations. (**b**) Response linearity of NH\({}_{3}\) at different concentrations.