| Gas sensing and monitoring are widely used in many important fields such as industrial testing,environmental monitoring,and mine safety.At present,gas sensors are limited in their large-scale use in the field of safety testing due to their low detection sensitivity,short life,and high price.Along with the maturity of optical fiber and optical communication technology,the cost of optical fiber and related devices has been continuously reduced,and the research of optical fiber sensors has been gradually introduced.Compared with traditional gas sensors,fiber-optic gas sensors are more resistant to cross-interference,easier to form a functionally integrated fiber-optic sensor network system,and more secure.Therefore,the research of fiber-optic gas sensors is of great significance.Based on the theory of spectral absorption,this paper analyzes the theoretical analysis,simulation research and experimental verification of tunable laser absorption spectrometry(TDLAS).Based on the theory of harmonic detection,a methane gas detection simulation system is established in MATLAB.Based on the simulation system,three aspects of simulation research are performed: one is the methane gas detection technology that proposes an improved stretch smoothing nonlinear scanning waveform;the second is the introduction of The half-ranged scanning waveform is used instead of the traditional full range scanning waveform,and the half-wave scanning waveform is simulated and analyzed.The third is to consider the effects of temperature and pressure on the gas line shape,and simulate the changes of harmonic waveforms by temperature and pressure.Simulation results show that the improved non-linear scanning technology can enhance the system signal-to-noise ratio(SNR)and improve the methane detection sensitivity;the half-ranged scanning method can reduce the system sampling ratio.Furthermore,the temperature and pressure changes of the air chamber can be in-situ monitored,as well as the diode laser wavelength shift observed simultaneously.Combined with the non-linear scanning simulation analysis,this paper introduces the construction of a methane gas detection system based on harmonic detection technology,including device selection,circuit design,and software development.In order to achieve a multi-channel gas online monitoring solution,the methane detection system was introduced 1 × 8-channel optical switch,and software debugging of the optical switch.The methane gas detection system is established through experiments,and the methane gas concentration calibration test is performed to obtain the fitting relationship between the second harmonic and the gas concentration under different scanning signals.The experimental results show that the system noise floor of the system output signal after the stretching and smoothing scan is significantly reduced,and the signal-to-noise ratio of 1% methane concentration is increased by 15.49 d B,effectively improving the detection performance of the system.Compared with the traditional TDLAS technology,the proposed method provides the significant improvement of the measurement accuracy and sensitivity of the system with the same cost of the data algorithm.Furthermore,the proposed technique in this paper is universal for arbitrary saw-tooth wave or triangular wave scanning signals,showing prospective in the fiber optic gas sensing application. |
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