Research On Mixing Fiber Gas Sensing Key Techniques Based On Tunable Diode Laser Absorption Spectroscopy Of Software Lock-in Demodulation Technique

In recent years, with the rapid development of science and technology, environmental pollution is becoming a growing topic of concern. The gas pollution harm to the industrial production safety and human health seriously. Tunable diode laser absorption spectroscopy（TDLAS） combining with wavelength modulation spectroscopy technique and harmonic detection technology with its short response time, high detection sensitivity and high stability, etc., are widely used in toxic gas concentration detection. Based TDLAS technique, this paper introduced the Software Lock-In Demodulation Technique（SLIDT） instead of hardware lock-in amplifier, and analyzed the mathematical model and algorithm of SLIDT in theory and experiment. Instrument design system can detect CO, CO₂ and CH4 gas concentration, temperature and pressure simultaneously, which to achieve multi-gas, multi-parameter mixing sensing of gas sensing system.The major innovations of this dissertation are as follows:1. Based on the theory of TDLAS, this article presented the theoretical model of Software Lock-In Demodulation Technique（SLIDT） for gas concentration detection. The influences of modulation depth, modulation frequency, phase shift and system noises on the amplitude of second harmonic were analyzed in theory. The theoretical values of every optimization parameter were obtained by simulation and theoretical analysis.2. Based on the Empirical Mode Decomposition（EMD） theory using in non-linear signals and non-stationary time-domain decomposition, this paper proposed the EMD-FCR algorithm, an improved EMD algorithm, for signal denoising system to improve the detection accuracy. The present algorithm with other conventional filtering algorithms are compared, the results show that our approach is an effective filtering algorithms and greatly improves the SNR of the demodulated signal, the minimum detection limit of the system reduced from the original 18 ppm to 2ppm, which reduced an order of magnitude.3. For the partially overlap phenomenon of gas absorption line, the paper constructed a mathematical model of overlapping spectral fitting, and proposed an overlapping spectral separation algorithm for separating overlapping spectra of CO and CO₂ to achieve the CO and CO₂ mixed gas concentration simultaneous detection under a single DFB laser with the current modulation.4. Based on the SLIDT, this paper proposed a method of temperature and pressure measurements by using gas absorption line width. By measuring the absorption line width under different temperature and pressure of gas, we can calculate the temperature and pressure of the test gas. Based on simultaneous detection of multiple gases above studies, as well as the detection of gas temperature and pressure, this paper can detect the multiple gases and multi-parameter simultaneously, which realized multi-gas, multi-parameter mixing sensing.