# 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
#### 4.1.2 Near Infrared Methane Detection Systems Based on WMS  
Based on TDLs with WMS, researchers have successfully studied many high-performance methane detection systems in the near infrared region [71; 72; 78; 79]. Typically, a DFB laser is often used as a light source. A portable methane detection device was developed by using a DFB diode laser centered at 1.654 um[71]. The second harmonic wavelength modulation spectroscopy (2f-WMS) was adopted. Within the detection range of 0-10\({}^{6}\) ppm, the relative detection error was less than 7%. [PERSON] et al. [72] also designed a trace methane gas sensor. The frequency modulation technology was applied in WMS to move the bandwidth of detection from low frequency to high frequency, to reduce 1/f noise. [PERSON] et al. [78] described a portable CH\({}_{4}\) detection sensor. The temperature of the DFB laser was controlled by a software-based proportion-integration-differentiation (PID) algorithm. In addition, the measurement range was from 0 to 100%. Then, the absorption length was added from 0.2 m to 0.4 m, and an open gas sensing probe was set in an improved system [79]. In the lab group's previous reports, the two mid-infrared detection sensors had been developed [80; 81]. Though the near-infrared sensor has a longer effective path length of the gas cell, the mid-infrared sensor has a higher MDL of 5 ppm. This is due to the fact that the absorption line intensity at 3.31 um is stronger over two orders than that at 1.65 um. The wavelet-denoising (WD)-assisted wavelength modulation technique is successfully suppressing the noise in mid-infrared detection sensors. A sequential multipoint sensor applying 2f-WMS technique was firstly developed by [PERSON] [82]. The sensor did not use any multiplexing techniques to distribute the laser intensity among a multitude of gas cells.  
In some systems, the DFB laser is replaced with a vertical cavity laser (VCL) as a light source [83]. [PERSON] et al. [69] developed a portable natural gas leak detector based on a VCL. The detector could measure methane concentrations from ambient methane levels (1.8 ppm) to pure gas. In the detection process, the response time of the detector was 1-2 s, and the detection precision was below 1%.  
Due to the low cost of near-infrared laser and the development of optical communications networks, some instruments in near infrared have been expanded into the commercialized application. The LI-7700 Methane Analyzer (LI-COR Biosciences Lincoln, NE, USA) was developed for detecting methane by eddy covariance method, which had the advantages of light weight, open path, and low power requirement [84]. A tunable diode laser centered at 1.65 um and a Herriott cell with a 30 m effective path length were employed. The wavelength was modulated across the absorption band at the sub-MHz frequency. Pressure- and temperature-induced changes in line shape and population distribution, changing in laser power and mirror reflectivity as well, were compensated by using computational fitting algorithms. This ensured that measurements remained accurate over a wide range of pressure and temperature conditions. The Laser Gas(tm) iQ2 analyzer (NEO, AS) was the first all-in-one TDLAS analyzer to measure up to four gases (O\({}_{2}\), CO, CH\({}_{4}\), H\({}_{2}\)O) and temperature depending on configuration, which eliminated the need for multiple units for combustion analysis. Then the Laser Gas(tm) II Open Path (OP) Monitor (NEO, AS), a compact and high-performance gas analyzer, was created for long-distance monitoring in ambient air.  
Figure 1: The spectral output of the laser at a radio frequency: (**a**) unmodulated; (**b**) modulated with no absorption; (**c**) modulated with absorption. \(w_{c}\) and \(w_{c}\pm\Omega\) represent the carrier frequency and the side band frequencies, respectively; \(\Omega\) represents the frequency of the laser. Reproduced with permission from [77], the Institute of Electrical and Electronics Engineers, 2008.