# A Dual-Gas Sensor Using Photoacoustic Spectroscopy Based on a Single Acoustic Resonator
## 2 Materials and Methods
### System Setup  
Figure 1 shows the schematic of our PAS instrument. The PAS instrument consists of two power-modulated light sources, an in-house-developed resonant PA cell to convert the light absorbed by trace gases to an acoustic signal, and a signal processing module. Two intensity-modulated DFB laser diodes emitting at around 1512 (SWLD-151220522-02, Sichuan Tengzhong Light Technology, Chengdu, China) and 1653 nm (SWLD-165317522-02, Sichuan Zhiguang Photonics Technology, Chengdu, China) were utilized as light sources for the simultaneous detection of ammonia and methane, respectively. The two lasers were fiber-coupled with SM fiber pigtail and mounted on butterfly clamp mountings (BF14-S, Sichuan Tengzhong Light Technology, Chengdu, China). Two fiber collimators (F280 APC, Thorlabs, Newton, MA, USA) with a focal length of 18.75 mm were used to reshape the laser beam for passing through the PA cell. The two laser beams were coupled together with a dichroic filter (3007010677, Union Optic, Wuhan, China). The DFB laser was powered by a laser controller (LDC202C, Thorlabs, Newton, MA, USA) with a current from 90 to 150 mA and a temperature of 22 \({}^{\circ}\)C. The wavelength of DFB laser 1 (around 1512 nm) was tuned from 1511.9 to 1512.3 nm when varying the current from 90 to 130 mA (center current: 110 mA; current scan amplitude: \(-20\) to 20 mA), and the wavelength of DFB laser 2 (around 1653 nm) was tuned from 1653.6 to 1654.3 nm when varying the current 110 to 150 mA (center current: 130 mA; current scan amplitude: \(-20\) to 20 mA), as shown in Figure 2a. The wavelength tuning of the DFB lasers with varying currents was measured by an optical spectrum analyzer (AQ6317, ANDO Electric, Tokyo, Japan). To ensure that the main absorption features were free from spectral interference, the scanning ranges were enlarged. The selected absorption lines of NH\({}_{3}\) and CH\({}_{4}\) at 1512.24 and 1653.72 nm were located at two sides of a period of the current scan, as shown in Figure 2. After the frequency scan of DFB 2 for the absorption line of CH\({}_{4}\) as shown in Figure 2b, the absorption line of NH\({}_{3}\) in Figure 2c was recorded. Therefore, through one simultaneous scan of the wavelengths of the two DFB lasers, the PAS signals of NH\({}_{3}\) and CH\({}_{4}\) were obtained (as shown in Figure 2d) without using any optical switch or time delay device. The driving current signal of the excitation laser was composed of a sawtooth wave signal and a high-frequency sine signal, which were added and programmed in a LabVIEW-controlled data acquisition (DAQ) card (USB 6211, Austin, NI, USA). The sawtooth wave signal was utilized to scan the wavelength for covering the absorption peak, while the sine signal was used to modulate the lasers for the lock-in amplifier. To simplify the system structure, only one DAQ card was used to control the DFB lasers. Without any optical switch or time delay device, two gases were detected simultaneously. The sampling rate of DAQ was 50 kHz. A longer integration time of the lock-in amplifier was required to enlarge the SNR of the PA signals. Therefore, the scan frequency was 0.05 Hz. The modulation frequency of the sine signal was identical to the resonant frequency of the PA cell for amplifying the acoustic signal. A reference square with the same frequency of the sine signal was also generated by the DAQ card for the lock-in amplifier (7270 DSP, AMETEK, Berwyn, IL, USA) to demodulate the PA signal. The modulation amplitude was 160 mV. Mass flow controllers (MFCs) were used to configure different concentrations of gases. The PA cell consisted of a cylindrical resonator and two buffer volumes. The PA cell was made of stainless steel 304, without coatings. The resonant cylinder was used as an acoustic resonator with a first longitudinal acoustic resonant frequency of 6800 Hz. The two buffer volumes at each extreme side of the PA cell were able to minimize the noise resulting from sample flow [32].