## 5 Experiment
The peak amplitude difference increased with increasing visibility, and the average value was 0.0836 V. Thus, the laser detector based on the four-channel center-symmetric emission structure had a better anti-smoke interference ability. The amplitude growth rate of the target echo signal based on the visibility of 5 m is shown in \[\eta_{l}=\frac{p_{cN}}{p^{\prime}_{cN}}=\frac{\sum_{i=1}^{N_{photon}}\frac{n_{ photon}}{N_{photon}}\times\sigma_{sc}(r_{s1},r_{s2})\times\Delta v_{i}\Delta t}{ \sum_{i=1}^{N_{photon}}\frac{n_{photon}}{N_{photon}}\times\sigma_{sc}(r_{s1}, \cdots,r_{sn})\times\Delta v_{ij}\Delta t}<1 \tag{27}\]5 With the increase in visibility, the average amplitude growth rate was 180.2%. The amplitude of the target echo signal was indeed enhanced. \begin{table}
\begin{tabular}{l c c c c} \hline
**Smoke Visibility** & **8 m** & **12 m** & **15 m** & **Average** \\ \hline Amplitude growth rate & 74.2\% & 171.7\% & 294.6\% & 180.2\% \\ \hline \end{tabular}
\end{table} 
Table 2: Amplitude growth rate of target echo signals based on visibility of 5 m.  
Figure 21: Laser beat signals and their spectra at smoke visibility of 5 m: (**a**) The target was not set in the smoke scene, (**b**) the target was set in the smoke scene. Figure 23: Laser beat signals and their spectra at a smoke visibility of 12 m: (**a**) The target was not set in the smoke scene, (**b**) the target was set in the smoke scene. Figure 24: Laser beat signals and their spectra at a smoke visibility of 15 m: (**a**) The target was not set in the smoke scene, (**b**) the target was set in the smoke scene. Figure 22: Laser beat signals and their spectra at a smoke visibility of 8 m: (**a**) The target was not set in the smoke scene, (**b**) the target was set in the smoke scene. To further reflect the detection performance of the four-channel emission laser structure, the target distance was moved to a more distant 4.5 m position for the test, which corresponds to a change in beat frequency of  
\[f_{B}=\frac{4B}{cT_{m}}R=\frac{4\times 150\ \mathrm{MHz}}{3\times 10^{8}\ \mathrm{m/s}\times 0.5\ \mathrm{ms}}\times 4.5\ \mathrm{m}=18\ \mathrm{kHz} \tag{29}\]  
To evaluate the detection performance of the proposed structure, it is imperative to compare it with single-channel laser emission structure, which is currently the most widely adopted structure. For the same conditions of lower visibility, the beat signal spectrum under single and four-channel emission lasers is shown in Figures 25-27. In this case, single-channel emission selected one of the four channels individually. The individual peak amplitudes close to the 18 kHz position are labeled in Figure 27. Based on the results of Figure 25, the amplitude growth rates and ratios of the target echo signals are shown in Table 3. The SNR results are shown in Table 4.  
Figure 27: Laser beat signals and their spectra at smoke visibility of 6.2 m: (**a**) Four-channel emission laser, (**b**) single-channel emission laser. Figure 21:0 Laser beat signals and their spectra at a smoke visibility of 4.8 m: (**a**) Four-channel emission laser, (**b**) single-channel emission laser. Figure 21:1 Laser beat signals and their spectra at smoke visibility of 5.7 m: (**a**) Four-channel emission laser, (**b**) single-channel emission laser. Based on Table 3, with the increase in the number of optical channels, the overall power of the emission laser not only increased, but the ratio of the increase in the target echo signal amplitude also increased with the increase in visibility. Within the same range of visibility fluctuations as shown in Table 4, the SNRs of the beat signal spectrum were 36% and 88% for single and four channels, respectively. This implies a 147% enhancement in SNR resulting from the increase in the number of channels. The above conclusions clearly demonstrate the better target detection ability of the detector when employing this particular structure.