Research On Related Technologies Of Methane Concentration Second Harmonic Detection

Methane explosion accident is always the main threat to the mine production. The two main physical factors of methane explosion are the concentrations of methane and oxygen reaching explosive limits and the existence of fire source. It is very important to measure the distribution of methane concentration in real-time. For a long time, scientists in many countries have been researching on technologies for methane detection. Among many methods of methane gas concentration measurement, second harmonic detection is one of more effective methods.First, the common measurement methods and the status of methane detection at home and abroad, as well as methane gas infrared absorption theory and gas absorption lineshape are introduced. Second harmonic detection technology is indicated as a better method with high sensitivity and strong interference immunity, and the second harmonic signal can be extracted by using lock-in amplifier. Second, lock in amplifying technology which is one of methods for weak signal detection is introduced, the related circuit is designed, and some parts of the circuit have been simulated. The correlator, one of the main part in lock-in amplifier, has been designed by using FPGA(Field programmable gate array).Distributed feedback laser diode (model: AB6A1052J510F) with central wavelength 1650nm is used in this research. Its characteristics have been measured by using the spectrum analyzer Q8344A. Two different experiment schemes have been adopted to measure methane concentration separately. In the first one, methane concentration is measured by using digital lock-in amplifier SR830. The second harmonic detection theory is validated with good coherence by analyzing the experiment result data. In the second scheme, methane concentration is measured by using the lock-in amplifier designed by ourselves to replace SR830. The objective of this scheme is to test the performance of the lock-in amplifier designed by ourselves.At last, factors which limit the detection resolution are analyzed, and the existing problems are pointed out.