# Recent Developments in Modulation Spectroscopy for Methane Detection Based on Tunable Diode Laser
## 4 Application of Modulation Spectroscopy for Methane Detection
### Frequency Modulation Spectroscopy Applied for Methane Detection
#### 4.2.2 Methane Detection Systems Based on FMS  
Generally, FMS can be applied for the situation where WMS is applied in reference [106]. Compared with that of WMS, processing electronics or detector of FMS should have a broader bandwidth in order to generate higher frequencies [107]. This is a crucial limitation of FMS. [PERSON] et al. [108] determined that the absorbance of detection using one-tone FMS in a 1-Hz bandwidth could be \(10^{-8}\) when the laser-induced shot noise of detector exceeded thermal noise. However, the power of many lead-salt diode lasers is not sufficient to produce shot-noise-limited spectroscopy. Frequency modulation of GaAlAs lasers was first demonstrated by [PERSON] et al. [109] in 1980. The method of FM-TDLAS was used by [PERSON] et al. [110] to measure CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\)O, and CO in air samples of a few cm\({}^{3}\). The lead-salt diode laser was tuned at a typical frequency of 1 kHz, and the rf signal of 100-195 MHz was superimposed on the laser current. Then, [PERSON] et al. [24] used a GaAlAs diode laser (DL) at 886 nm to obtain the sensitivities of three detection techniques in methane sensor in order to more clearly compare WMS and FMS with TTFMS. The laser was tuned on a third overtone methane transition for measuring the minimum detectable absorption. The modulation frequency was 1 kHz, 100 MHz, and \(390\pm 5\) MHz for WMS, FMS and TTFMS, and the minimum detectable absorptions were \(4.5\times 10^{-7}\), \(9.7\times 10^{-8}\) and \(6.4\times 10^{-8}\), respectively.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Title of Multipass** & **Components** & **Path Length** & **Detection Limit** & **Taken from Refs.** \\ \hline Dense-pattern & Two silver-coated & & & \\ multipass cell & concave spherical & 26.4 m & \(<\)79 ppb & [2] \\  & mirrors & & & \\ Confocal multipass & A confocal & & & \\ cell & configuration of six & 290 m & 1.2 ppb & [102] \\  & mirrors & & & \\ Multiple-reflection & Three mirrors & 140.3 m & 128 ppb & [103] \\  & Two twisted & & & \\ Multiple-pass & cylindrical & 29.5 m & - & [104] \\  & high-reflectivity & & & \\  & mirrors & & & \\ Circular & A polished & & & \\ multireflection cell & spherical reflecting & 105 cm & 300 ppm & [105] \\  & surface & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Some parameters of novel multipass cells.  
Based on FMS, TDLs offer remote detection of methane gas the opportunity to operate over distances of 10 m or more in high sensitivity [111]. In these systems, the laser beam aimed through the probed region is collected after one-way transmission or further reflection from a topographic target [112; 113]. The ambient methane detection using the FM technique at a frequency of 5.35 MHz was reported by [PERSON] et al. [112]. Then, [PERSON] et al. [113] described a portable methane detection sensor with a 1.65-\(\upmu\)m InGaAsP DFB laser. The tuning range of wavelength was 14 cm\({}^{-1}\).  
In addition to conventional lasers, new types of lasers such as QCL are also used for FMS in methane detection. [PERSON] et al. [114] developed a novel laser spectrometer, which relied on a QCL for detecting methane and nitrova oxide. Moreover, the research group firstly and thoroughly applied single-tone FM technique to QCL, detecting the same components under low pressure, as shown in Figure 4[65]. Two QCLs of 8.06 \(\upmu\)m and 7.3 \(\upmu\)m are applied for FMS on N\({}_{2}\)O and TTFMS on CH\({}_{4}\), respectively. For methane gas, the minimum detectable concentration can reach up to 400 ppt Hz\({}^{-1/2}\). Due to the tunability and sensitivity characteristics of the system, a mixture of gases can be monitored. As they summarized, QCL is an appropriate choice for frequency modulation spectroscopy in the mid-infrared region.