# Recent Developments in Modulation Spectroscopy for Methane Detection Based on Tunable Diode Laser
## 4 Application of Modulation Spectroscopy for Methane Detection
### Wavelength Modulation Spectroscopy Applied for Methane Detection
#### 4.1.3 Mid Infrared Methane Detection Systems Based on WMS  
Methane detection systems based on the mid-infrared laser have been researched for quite some time. However, traditional mid-infrared sensors have some drawbacks, such as the low power of the laser and the instability of a laser in single mode operation. QCL, as a convenient mid-infrared laser source, is widely employed for methane monitoring at room temperature. A small in situ sensor system was developed to monitor the concentration of CH\({}_{4}\) and N\({}_{2}\)O in real time [85]. The light source was a QCL operating at 7.83 \(\upmu\)m. The wavelength was scanned over the absorption lines of CH\({}_{4}\) and N\({}_{2}\)O at different operating temperatures, achieving simultaneous dual-species detection. The sampling volume of the multipass cell was only 225 mL, resulting in the compact size of the system. Moreover, an external cavity quantum cascade laser (EC-QCL), covering the absorption lines of four atmospheric greenhouse gases, was applied in a sensor system [86]. Additionally, gallium antimonite (GaSb)-based ICLs have been used for mid-infrared sensing [87; 88]. In addition, the commercial availability of ICLs was achieved in 2009. [PERSON] et al. [87] demonstrated a mid-infrared dual-gas detection system (CH\({}_{4}\) and C\({}_{2}\)H\({}_{6}\)) based on a single ICL [87]. However, the power consumption of \(\sim\)250 W was relatively high. The weight of the oil-free vacuum pump and pressure controller and readout in the system was heavy. To address the limitations, new portable sensor systems were developed [88; 89], in which competitive performances were revealed compared with other reported portable or handheld sensor devices. Mid-infrared sources based on DFG effects are also employed for methane detection. Commercial and off-the-shelf near-infrared lasers are purchased to act as the mixing sources. [PERSON] et al. [90] described a mid-infrared methane detection system using the difference frequency generation (DFG) process in a periodically poled lithium niobate (PPLN) crystal. The DFG system was used to implement TDLAS with WMS. The pump wavelength and the signal wavelength were provided by a fiber Bragg grating diode laser and a DFB diode laser, respectively. The fundamental absorption line of methane located around 3.4 \(\upmu\)m was addressed. Since then, several researchers have reported more absorption detection applications using DFG processes. A system utilizing a dual-wavelength amplifier for DFG process was firstly presented [91]. As shown in Figure 2, the dual-wavelength amplifier is used to amplify both of pump wavelength and signal wavelength, and simple 2f methane detection is carried out. However, the detectable concentration of methane at the same absorption line is less than achieved by [PERSON] et al. [90]. The minimum detectable methane concentration was at the level of 26 ppbv for an open-path interaction length of 8 m [92]. [PERSON] et al. [93] described a single-frequency CW difference-DFG source, which was tunable from 3.1 to 3.6 \(\upmu\)m. The output power of the source can reach tens of milliwatts. Therefore, the wideband-tunable mid-infrared source has the potential in the application of trace gas detection.  
Figure 2: The methane detection system based on a dual-wavelength amplifier. Reproduced with permission from [91], Optical Society of America, 2013.