Design And Application Of Trace Gas Laser-based Sensor Using A Novel Multi-pass Absorption Cell

With the development of the economy and the maturity of the laser technology,gas detection based on laser spectroscopy has become a focus of research and industry.Because it has advantages in continuous and in-situ detecting concentration of trace gases,tunable diode laser absorption spectroscopy(TDLAS)has been used in a variety of fields,such as atmospheric environment,industrial processes,combustion diagnosis,and medical diagnosis.The signal to noise ratio of direct absorption spectroscopy is generally low due to the existence of noise.Using wavelength modulation spectroscopy(WMS),the spectrometer can achieve a sensitivity of ppm or even ppb.Moreover,multi-pass cell is often used to improve the amplitude of absorption signals.Wavelength modulation spectroscopy(WMS)technology and multi-pass absorption cell have been commonly used to improve the detection limit of TDLAS and the stability of the system.In this thesis,a palm-sized laser spectrometer has been developed for detecting trace gases based on TDLAS in combination with a novel double-layer toroidal cell.The physical size of the laser spectrometer is reduced to 24×15×16cm³ by the use of home-made electronic system and compact construction with high efficiency utilization of inner space of the sensor.A toroidal absorption cell,with 84 reflections in 2 layers for an effective optical path length of 8.35 m,was used to enhance the absorption signals of gaseous species.A home-made electronic system was designed for implementing the function of the distributed feedback(DFB)diode laser controller,analog lock-in amplifier,data acquisition and data communication.Calibration-free scanned wavelength modulation spectroscopy was employed to determine the concentration of the gas and to reduce the random fluctuations from electronical noise and mechanical vibration.The measurement of CH₄ in ambient air was demonstrated using a DFB laser at1.653?m.The rise time and fall time for renewing the gas mixture are approximately 16 s and 14 s,respectively.Vibration and temperature tests have been carried out for verifying the performance of the spectrometer.The standard deviations for 20 ppm CH₄ at different vibration frequencies and temperatures are0.38 ppm and 0.11 ppm,respectively.According to the Allan deviation analysis,the minimum detection limit for CH₄ can reach to 22 ppb with an integration time of 57.8 s.Continuous measurement of atmospheric CH₄ for two days was performed to validate the feasibility and robustness of our laser spectrometer.It is proven that the system provides a promising laser spectral sensor for deploying in unmanned aerial vehicles or mobile robots.In the TDLAS system combined with multi-pass cell,the etalon fringes can reduce the sensitivity of detection and the analysis of the signal.Therefore,a wavelet filtering method is presented to minimize the optical fringes.Firstly,the optimal wavelet function and decomposition level were analyzed and chosen.Then,the CO₂ signal superimposed with strong interference noise was denoised.It demonstrated the feasibility of the wavelet denoising method for the spectral denoising.Finally,the observed CO₂ signals were detected by the direct absorption spectroscopy(DAS)and WMS-2f,and they were denoised by the wavelet denoising method.The ratio of the signal to noise was improved about two orders,from 0.4 to 259.A good linear relationship was derived between the intensity of the signal and the CO₂concentration.The detect limit of concentration of CO₂ of this system can reach to 7?10^-6 using WMS-2f in combination of wavelet denoising.