## 6 Conclusions  
According to the absorption spectra of methane and ethane, the infrared detection filters for these two gases were prepared, and the film structure was designed and optimized by using OptiLayer software. A G150-QCL laser interferometer is used to test the surface shape of the film and calculate its stress. By comparison, the stepped ion source is used to assist deposition, which not only provides the firmness of the film but also reduces the stress. Through the film test, the roughness of the Ge film is analyzed. The graphite bushing will increase the defects of the film. Changing the copper crucible can reduce the absorption and improve the bandpass transmittance. The average transmittance of the film is 90.5% in 3-3.5 um and 94. 24% in 6.5-8.2 um, and the cut-off depth is above OD3. The film has passed the environmental test. Compared with the latest infrared thermal imaging dual-band filter devices [21], the filter device in this experiment has a higher transmittance in the passband (about 2% higher at 3.31 um and about 5% higher at 6.71 and 7.67 um) and a deeper cut-off depth in the cut-off band (about one order of magnitude deeper). This enables the detection system to receive methane and ethane gases faster and more effectively block the influence of other gases.  
Although the prepared optical filter meets the requirements for use, there are certain differences between the experimental results and theoretical design of the infrared dual-band filter device for the methane and ethane gas detection system developed in this paper due to the cumulative error of film deposition and the display error of instrument equipment. The optical constants of the material have certain errors, and the energy loss caused by material absorption, scattering, and preparation process errors will be the focus of future research to improve experimental reproducibility. At the same time, dual-channel filter films have good practical applications and development prospects in fields such as optical remote sensing, the Internet of Things, nighttime driving, and security.  
Conceptualization, G.F. and X.F.; methodology, G.F. and X.F.; software, G.F.; validation, G.F., S.D., and B.W.; formal analysis, G.F.; investigation, Z.L. and Y.P.; resources, G.F. and S.D.; data curation, G.F. and Z.L.; writing--original draft preparation, G.F. and S.D.; writing--review and editing, G.F. and G.F.; visualization, G.F. and S.D.; supervision, G.F.; project administration, G.F. and S.D.; funding acquisition, G.F. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data used in this research are available from the corresponding author upon reasonable request. The data are not publicly available due to privacy.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Operation Process** & **Result** \\ \hline Adhesion test & Use 3 M adhesive tape to stick on the surface of the double bandpass filter, and then use LED spotlight to illuminate the surface of the lens after 3–5 times of vertical rapid tearing. Keep it in a constant temperature and humidity chamber with a humidity of 95%–100% and a temperature of (60 \(\pm\) 5) \(\times\) for 24 h, and then take it out. After the boiling pot reaches the set temperature of 80 degrees, the double bandpass filter is placed in the boiling pot for boiling experiment for 24 h, and the sample is checked after standing for 2 h at room temperature. & No demolding phenomenon \\ \hline Water boiling test & & \\ \hline \hline \end{tabular}
\end{table}
Table 9: Specific process table for environmental testing.  
**Conflicts of Interest:** Authors [PERSON], [PERSON] and [PERSON] were employed by the company Zhongshan Jilian Optoelectronic Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.