## 1 Introduction  
Methane and ethane are two common gases used in natural gas exploration and extraction, and simultaneous detection of both gases can provide more comprehensive information about underground gas reserves, helping to optimize drilling and improve extraction efficiency. However, when the concentration of methane gas is between 5% and 15% and the concentration of ethane gas is between 3% and 12.4%, an explosion will occur when exposed to open fire [1]. Meanwhile, methane, as a major greenhouse gas, has a more significant greenhouse effect than CO\({}_{2}\), seriously jeopardizing the Earth's ecological environment. Additionally, the chemical reaction between ethane and hydroxyl radicals will affect the oxidation ability of the atmosphere, and its reaction products are also one of the important sources of environmental pollution [2; 3]. Therefore, the real-time detection of methane and ethane is particularly important.  
Compared with other types of infrared gas detectors [4; 5; 6; 7], the uncooled focal plane detector has the advantages of small size, long service life, low power consumption, and low price. Figure 1[8] shows an infrared thermal imaging methane and ethane detection and identification system with an uncooled focal plane detector as the core. The dual bandpass filter device is located between the lens and the information processing system.  
[PERSON] et al. successfully developed an infrared methane detector for detecting methane gas concentration in mines; purely optical remote monitoring of low-level CH4 gas is realized for the first time by the method employing a 2 km long-distance, low-loss silica optical fiber link and a compact absorption cell in conjunction with a high radiant InGaAsP light-emitting diode (LED) at 1.33 \(\upmu\)m, and the minimum detection sensitivity of the system is 4% LEL [9]. [PERSON] et al. developed a methane concentration detector with a detection sensitivity of 2 \(\times\) 10\({}^{-5}\) using a 1.66 \(\upmu\)m tunable semiconductor laser light source [10]. In recent years, the detection of methane and ethane gas has been studied in many aspectsin China. In 2017, [PERSON] et al. [8] prepared a narrow-band filter film for single-channel detection of methane gas, with a transmittance of 93.87% at a single point of 3.31 um and a cut-off depth of OD2 at 1-3.2 um and 3.4-5 um; in 2019, [PERSON] et al. [11] used the method of DC pulse magnetron sputtering vacuum coating to prepare a filter film for detecting ethane with a transmittance of 72.09% at the central wavelength of 6.7 um and a cut-off depth of OD2 at 2.5-6.3 um and 7.1-10 um. In 2019, [PERSON] et al. [12] developed a dual-channel filter film for a methane gas detection system, which has an average transmittance of more than 80% at 3.16-3.46 um and 7.64-7.70 um and a cut-off depth of about OD2 at 3.66-7.42 um and 7.92-10.0 um. Judging from the results, dual-channel detection is more sensitive than single-channel detection.  
According to the data found, there are few reports on the filter film for detecting methane and ethane in the middle infrared and far infrared at the same time. In this paper, the fingerprint region of methane and ethane is studied, and a dual-channel filter is designed and prepared to detect methane and ethane simultaneously, which reduces the background noise and improves the detection sensitivity.