# Design of a Laser Driver and Its Application in Gas Sensing
## 4 Results and Discussion
### Gas Sensing Experiments  
The experimental setup (see Figure 7) was verified by the results shown in Figures 14-16 for the measurements of CH\({}_{4}\), with the modulation index of lasing wavelength set to approximately 2.2 so that the largest possible amplitude of the second harmonic could be reached. Figure 14 gives an example of the recorded second harmonics. The output of the lock-in amplifier was continuously sampled by the data acquisition card. The program written in LabVIEW performed the averaging of the raw data. Through comprehensive consideration of both the signal-to-noise ratio and real-time requirement, the number of sampling points used for averaging was determined to be 10. The peak of the second harmonic obtained from the average using 10 samples is 3.05411 V. The wings of waveform are symmetrical and the DC offset is approximately zero, which is different from the traditional second harmonic detection. To visualize the variation of detection results with sample concentrations, we present the peaks of harmonic signals obtained by measuring all samples with their concentrations ranging from 400 ppm to 1%. According to the fitting, the peak of harmonic has a highly linear dependence on the sample concentration, as predicted in Section 3.2. In Figure 15, a regression coefficient of \(R=0.9976\) was derived from the linear fitting described by  
\[y =15.0207x, \tag{12}\]  
with \(y\) being the peak value of detector signal and \(x\) being the methane concentration. Figure 14: Waveform of second harmonic for 0.2% CH\({}_{4}\) buffered by N\({}_{2}\). Figure 13: Transient response of the current source when C5 equals 1 uF. Strictly speaking, the detection limit of a gas sensor should be measured under the condition that the signal is equal to the noise. However, in a limited number of measurements, it is difficult to achieve equal signal and noise intensity. Instead, the detection limit can be deduced from the experimental result of a high-concentration sample, such as the 400 ppm methane sample in our case. According to the wavelength-current relationship curve of the LD measured in advance by the spectrum analyzer, the wavenumber corresponding to the data point of the harmonic signal was obtained through conversion. As shown in Figure 16, the peak of the signal corresponding to the 400 ppm sample is 0.609 V, while the standard deviation of noise obtained by the nonlinear fitting based on the [PERSON]'s algorithm is 0.011 V, and the calculated SNR is 55.4 (0.609/0.011). Thus, when the signal is reduced to be equal to the noise, the corresponding concentration should be 7.2 ppm (400 ppm/55.4), which is the detection limit. Some CH\({}_{4}\) sensors that are applied in industry or reported in literature are summarized in Table 2. The PRMD series designed by [PERSON] et al. (2016) is an ultra-light handheld device that uses laser echoes for remote measurement in industry. [PERSON] et al. (2017) determined the optimum total pressure for the detection of CH\({}_{4}\) by experimental and theoretical approaches. The compact analyzer developed by [PERSON] et al. (2018) achieved a detection  
Figure 16: Measured data and its fitting of the harmonic for 400 ppm CH\({}_{4}\). Figure 15: Linear dependence of the harmonic on CH\({}_{4}\) mole fraction. limit as low as 1.4 ppb due to the selection of the interband cascade laser (ICL) and the fundamental \(v3\) band as the light source and target absorption line. It should be emphasized that the effective optical lengths of the above sensors are 10 m, 29.91 m and 54.6 m, respectively. According to the Beer-Lambert law, if all of the absorption lengths are converted to the same 15.4 cm, the theoretical detection limits obtained by [PERSON] et al. and [PERSON] et al. should become 32.5 ppm and 11.7 ppm, which are close to ours (7.2 ppm). Moreover, if the different absorption intensities of the selected spectral lines are taken into account, the gap between the detection limits obtained by us and [PERSON] et al. under the same conditions will be narrowed further.