A Trace Methane Detector Based on Mid-infrared Tunable QCL at 7.5μm*

In order to detect trace methane (CH4) with non contact, the proposed manuscript describes high sensitivity trace methane detector using quantum cascaded laser (QCL) with centre wavelength at 7.5 μm, which is based on combination of Tunable Diode Laser Absorption Spectroscopy (TDLAS) and Wavelength Modulation Spectroscopy (WMS). Under room temperature, the spectrum of QCL can scan CH4 absorption line via adjusting the injection current of QCL. Meanwhile, a compact herriott cell (40 cm long and 800 ml volume) is utilized to achieve a total optical path with 16 meters length. The aforementioned detector is applied to detect CH4 with different concentrations, results show that the relative detection error is less than 7%, the lowest detection is 1×10. Meanwhile, the researchers can detect other gases through replace lasers with different wavelength.


Introduction
Compared to the traditional chemical analysis method, the detection method based on intermediate infrared absorption spectrum has advantages including convenience, efficiency and no damage to gas sample, so it has been widely used in fields such as petrochemical engineering, food safety inspection, public security, and so forth [1][2][3] . The lasing wavelength is intermediate infrared QCL when compared with the light source of traditional near-infrared spectroscopy.
And it has high light intensity and good coherence.
Moreover, we can select the lasing wavelength by adjusting the injection current to make it consistent with the spectral line of the largest absorption strength of the tested gas, so as to realize high-precision detection of hazardous gas in concentration of trace quantity [4][5] .
In 2005, R. Kormann et al detected the concentration of CH4 in the air under liquid nitrogen refrigeration by using QCL, and the lower limit of concentration in detection is 50×10 -6 [6] . In 2012, L. Dong et al detected CH4 with gas concentration of trace quantity by using lasing wavelength of 3.3 μm, with the minimum lower limit of detection of 10×10 -6 [7] . In recent years, many scientific research institutions and institutions of higher learning have also tried to detect harmful gas by QCL, The QCL lasing length of 7.5 µm is used by this detector, and the strong absorption spectral line of CH4 (1332.8 cm -1 ) can be found by adjusting the injection current under indoor temperature. And eventually, we obtain the detection limit of CH4 is 1×10 -6 .

Selection of absorption line
Molecule of the target gas CH4 detected by this detector has 4 natural vibrations, respectively corresponding with 4 basebands, and all of them locates in middle-infrared band [8] .
In the formula, I1 and I0 are respectively incident light intensity and emergent light intensity before and after passing through the even gaseous medium, s is the intensity of gaseous medium of gas corresponding with the specific emergent light beam, p is total air pressure,  is mole fraction of the absorbing substance,   is linear function, c is concentration of the detected gas, and L is the effective total optical distance of the system.

Configuration of detector
In order to effectively restrict the system noise and improve the lower limit of detection of the detector, this detector uses difference absorption method, that is to say, structure of single light source and double detector is used to establish the system. The overall block diagram is as shown in Figure 2.

Experiment
In the experiment, the independently designed QCL driving power and temperature controller are used to control current and temperature. During the operation, temperature controller is used to control temperature of QCL, to keep it working under 298 K. Overlay the high-frequency sine wave and a slowly changing current ramp signal to realize scanning of absorption line of QCL wave length through gas, by which we realize the adjustment of its output wave length by changing the mode of QCL injection current.
Meanwhile, we can obtain CH4 gas with different concentrations for measurement by detector through quantitative mixing of pure CH4 and N2 by using dynamic gas flowmeter.  Suppose SNR (Signal to Noise Ratio) is 1, and the detection sensitivity of this sensor system is 1×10 -6 .

Lower limit of detection of detector
The lower limit of detection of concentration of CH4 can confirm the quality of performance of detector, and it's an important detection index of the measurement. Results indicate that the practical maximum deviation of gas concentration of CH4 is ±0.5×10 -6 , gas concentration difference of CH4 is 1×10 -6 , and lower limit of detection of concentration of CH4 is 1×10 -6

Stability of detector
Carry out experiment of concentration detection of CH4 with concentration respectively 0.1‰ and 10% for 30 hours, and the results are as shown in Figure 5. When the gas concentration of CH4 is 0.1‰, the result of concentration detection is 0.0912～0.1090 (‰), with relative error less than 7%. When the gas concentration of CH4 is 10%, the result of concentration detection is 9.7～10.4 (%), with relative error less than 2.5%.

Conclusions
Based on detecting techniques with combination of TDLAS-WMS, QCL with central wavelength of 7.5 µm is used in this article to design and develop high-sensitivity trace detection detector. In the meantime, compact herriott gas chamber is used to make the total optical distance in detection reach 16m.
In the experiment, the performance of this detector is tested and results tell that the relative error of measurement is less than 7% and lower limit of detection is 1×10 -6 , so trace detection of CH4 is feasible.