## 3 Results  
### Calibration  
Wavelength modulation spectroscopy (WMS) is one of the most effective techniques for eliminating the background noise for PAS measurement. To get the first harmonic (1f) signal, the resonant frequency of the resonator (\(f=6390\) Hz) determined the frequency of the sinusoidal modulation signal, which was simultaneously sent to the laser driver and the lock-in amplifier. An \(R\) value of the 1f PA signal, demodulated from the lock-in amplifier with an integrated time of 1 s, was acquired by the DAQ card (USB-6211, National instrument, Austin, TX, USA). Meanwhile, a sawtooth wave with the frequency of 50 mHz, representing a period of 20 s, was applied to slowly change the laser wavelength. The USB-6211 DAQ card has a maximum single channel sample rate of 250 kS/s. In the experiment, since all of the modulation signals were generated by the DAQ card, a high sample rate was required to ensure the signals would not be distorted. In addition, due to the relatively long sampling time, the sample rate should be limited in a certain range to avoid excessive sample points. Finally, the sample rate of 50 kS/s was set, which resulted in acquiring one million data points in one scanning period. We diluted the chloroform by mixing it with pure nitrogen through the MFCs. Different concentrations ranging from 63 to 625 ppm were generated by adjusting the flow rate of the two gas paths. In order to reduce the influence of airflow, the total flow rate was controlled to remain below 0.4 L/min. The 1f PA signals of different chloroform concentrations were integrated into one graph, as shown in Figure 4. The signal of pure nitrogen was regarded as the base signal. Figure 3: Wavelength-dependent absorption cross-section of gaseous chloroform and wavelength-tuning together with output power of the laser. (**a**) The absorption cross-section value of the chloroform gas around 1683 nm; the colored area was scanned by the laser. (**b**) The laser wavelength and the output power tuned by the injected current at the experiment temperature of 19.8 \({}^{\circ}\)C.  
As can be seen in Figure 3b, the wavelength shows a bad linear relationship with the injected current. During the experiment, the current increased linearly with time, which resulted in a nonlinear change of the wavelength. The wavelengths at corresponding time were given on the top \(X\)-axis. Obviously, four different peaks emerged in Figure 4, belonging to the 1f PA signal of the two adjacent absorption peaks of chloroform, separately. This indicates that the two absorption peaks of chloroform can clearly be distinguished, which reflects the capability of WMPAS in dealing with unconventional absorption lines. Generally, the intensity of the 1f signal was approximately proportional to the first derivative value of the gas absorption cross-section [17]. Due to the influence of adjacent absorption lines, the gas absorption cross-section was no longer the conventional Voigt type which is usually used to describe the broadening of the gas absorption line under normal pressure, and the amplitude of the 1f PA signal was affected accordingly. The intensity of the last peak (colored area) in Figure 4 was selected to retrieve chloroform concentration, as the peak amplitudes free-interference from other gas species (as discussed in Section 3.4) and the absorption shape at this frequency is close to the normal first harmonic curve. When the concentration of chloroform was 625 ppm, the peak value of the signal reached 3.75 mV, and the peak value decreased proportionally with the decrease in the gas concentration. Although improving the amplitude of the sine wave can enlarge the value of the peak signal, it will also broaden the 1f PA signal [33]. The large amplitude of the low-frequency sawtooth scanning wave led to an injected current variation range of 100 mA, while the maximum current modulation range of the laser was about 150 mA, which limited the amplitude range of the high-frequency sine wave.