A Gas Sensor System Based On Tunable Diode Laser Absorption Spectroscopy

With the rapid development of the oil, gas, coal and other industrial, the toxic gas is increasing rapidly, gas poisoning and explosion casualties continue to increase. Therefore, accurate and fast detection and warning for harmful gas is very important for environmental protection, manufacturing operation control and personal safety protection.Our gas detection technologies are lagged far behind western countries and are unable to meet the requirements of the actual detection due to technique problems, such as presence detection accuracy, real-time monitoring, gas selectivity, life, operating process, environmental impact, detection response and so on. In our country, the common gas concentration detection is achieved by detecting amplitude of the second harmonic of laser since it is in proportion to target gas concentration at relatively low level. However, the amplitude of the second harmonic is not proportional to the gas density anymore when the gas concentration is at a relatively high level. Besides, detection accuracy of this method is easily affected by environmental temperature change. The literature on the continuous measurement of high and low concentration by using this method is few. Tunable diode laser absorption spectroscopy is a new method widely used in many applications, such as environmental quality detection, spectrum measurement, analysis and measurement of harmful gas, etc. By using this kind of method, target gas concentration can be detected based on the output light intensity change of the tunable diode laser after it is absorbed by the target gas. The system has high sensitivity, high precision and fast response speed.In this paper, a gas sensor system based on tunable diode laser absorption spectroscopy is studied. Specifically, integrated real-time on-line detection system for methane at wide temperature and concentration range is researched and demonstrated. The gas detection technology of tunable diode absorption spectroscopy is studied in detail, the main contents of the study are as follows:1) The theory of molecular spectra is introduced, including the linear and the line width of the gas absorption spectrum, provides a theoretical basis for the detection technology of gas which is based on the gas Bill Lambert law. According to HITRAN database, the absorption line of methane molecule used in the system was selected to reduce cross effects of other gas molecules. 2) The detection technology of gas sensing system based on absorption spectrum technology is studied, including differential absorption detection, modulation and harmonic detection technology. And comparison of these two detection methods, the system based on the principle of wavelength modulation and harmonic detection is designed in this paper. 3) According to detection requirements, the narrow linewidth DFB laser at 1653.7nm is selected as light source, and select spectral response range of PIN 1100~1700nm photodiode for photoelectric signal conversion. Light path is composed of multiple reflection type gas absorption cell. And the temperature control circuit of the laser is designed. 4) The modulation signal and the phase locked reference signal are designed based on the function generator, laser automatic continuous tuning in a certain wavelength range is realized. Combined with all digital lock-in amplifier, a weak photoelectric signal detection circuit is designed. 5) With 0~100% of different concentrations of methane as the target gas, analysis and process the results of the two scanning harmonic at room temperature. First, extract effective data and analysis noise in the system, then take some measures like background noise deduction, smoothing filter, temperature compensation to improve detection accuracy, continuous real time detection in the whole concentration range by least square fitting is realized. 6) Take 5% of the density of methane as the target gas, analysis the test results of the system at different temperatures,and analysis of the system detection sensitivity and noise sources, realize the continuous measurement in the whole temperature range,ensure the system has the advantages of high sensitivity, stable work and so on.It is demonstrated by experimental results that the system error is better than + 0.02% when the gas concentration is between0 to 1%. The system error is better than + 2% when the gas concentration is between 1%~100%. The response time less than 10 s, which agree very well with testing standards. The long time test and high and low temperature test demonstrated that the system’s performance in a wide range of temperature(0-40℃), is stable and reliable. The system meets the detection requirements of coal mine and is able to provide a reliable protection for the prevention and control of gas outburst disaster.