Research Of Mid-infrared Laser Methane Sensing Technique

In the past decade,there have been nearly 400 coal mine accidents in China,which have caused a lot of casualties and economic losses.The real-time monitoring of methane gas is essential because the main dangerous component of coal mine gas is methane.Further more,methane（CH₄）is the second most important greenhouse gas after carbon dioxide.Its increasing concentration has a negative impact on ecology and climate.In addition,as a flammable and explosive gas,methane is a safety hazard in natural gas production,distribution,storage,and transportation.Therefore,whether in the field of environmental science or the field of public safety production,the demand for accurate and sensitive methane detector is particularly urgent.Tunable diode laser absorption spectroscopy（TDLAS）is a common method to detect methane.It has the merits of high accuracy,long lifetime,good selectivity,fast response speed and it will not affect the measured gas composition.The commonly used techniques in the TDLAS system are direct laser absorption spectroscopy（DLAS）technique and wavelength modulation spectroscopy（WMS）technique.Wavelength modulation spectroscopy technique and lock-in amplification technique are the main methods to improve the detection accuracy of TDLAS system.In this dissertation,a digital quadrature lock-in amplifier was developed based on DSP（Digital Signal Processor）with low power consumption,low cost,and small size.Using TMS320F28335 as the main controller,a high precision and low noise ADC（Analog-to-Digital Converter）and DDS（Direct Digital Synthesizer）were choosen as the main chips.By optimizing the power tree,the efficiency of the power supply was improved and the power consumption of the system was reduced.When the averaging time was～2s,the detection accuracy of the mid-infrared methane sensor was 13ppbv using the DSP-based lock-in amplifier.In the DLAS-based system,if there is electrical-domain noise in the laser driver,the output of the laser source will eventually contain an optical noise,which will affect the measurement results and reduce the detection accuracy of the system.Therefore,in order to suppress the electrical noise in the laser driver,an electrical-domain adaptive mid-infrared laser methane sensor based on the Recursive Least-Squares（RLS）adaptive algorithm was developed.Firstly,numerical simulations were carried out to validate the function of the RLS adaptive algorithm and the the selection criteria of RLS adaptive algorithm parameters were discussed based on the MATLAB platform.Then,numerical simulations and measurements were given to verify the filtering performance of RLS algorithm in DLAS system.Using a room temperature,continuous,mid-infrared interband cascade laser（ICL）and mercury cadmium telluride（MCT）detector,the sensor was built combining with multi-reflection gas cell（MPGC）technique.Compared with an Allan deviation of～79 ppbv with a～6 s averaging time without the filtering algorithm,an Allan deviation of～44 ppbv with a～6 s averaging time was obtained with RLS self-adaptive（SA）filtering.Long-term indoor and outdoor atmospheric CH₄ measurements were conducted to validate the sensor as a reliable and robust sensor system.The demonstrated SA-DLAS sensor architecture shows the merits of faster response speed and higher accuracy,which will have great application prospects in the field of public safety production.Wavelength modulation spectroscopy technique focuses on the harmonic signal of the absorption which is a good method to suppress system noises at other frequency.However,if the frequency of the noise caused by temperature drift,electrical parts or optical parts is less than the scanning frequency,this kind of noise can’t be suppressed by WMS technique.Therefore,in order to suppress the slowly changing noise,a WMS-Least-Mean-Square（LMS）adaptive methane sensor was developed.Firstly,numerical simulations were carried out to validate the denoising performance of the LMS adaptive algorithm and the selection criteria of LMS adaptive algorithm parameters were discussed based on the MATLAB platform.By incorporating a room temperature,continuous mid-infrared interband cascade laser and a mercury cadmium telluride detector,a novel WMS-LMS self-adaptive CH₄ sensor was proposed.Slowly changing noise was obtained by introducing an optical reference channel to the system.The detection accuracy of the system was optimized from 25 ppbv to 16ppbv with a～1.9 s averaging time using the LMS adaptive filter.The innovations of this work are as follows:1.Using DSP as the main control unit,by optimizing the power tree of the system,the power efficiency was improved and the power consumption of the whole board was reduced.A digital lock-in amplifier was developed with low power consumption and small size which lays a foundation for further reducing the sensor volume and cost.2.In order to suppress the electrical-domain noise of laser driver in DLAS system,a noise-channel was created by direct feedback of the ICL drive signal to the DAQ card.Combined with the recursive least squares adaptive algorithm,the electrical-domain noise of the system was effectively suppressed and the detection accuracy of the system was improved.As the noise in the electrical-domain increases,the detection accuracy of the system will have more improvements.The proposed RLS self-adaptive denoising technique can be applied to various infrared gas sensing fields.3.In order to suppress the slow-varying noise of system whose frequency is less than the scanning frequency,an optical reference channel was added to the mid-infrared methane sensor.To verify the denoising performance of the system,a slowly changing noise was added to the laser drive signal to simulate the noise influences.With the least mean square adaptive algorithm,the slowly changing noise was suppressed.This technique can surpress the slow-changing noise with unknown statistical characteristics and improve the anti-interference performance of the system.