Investigations Of Digital Filtering Algorithm Used In Tunable Diode Laser Absorption Spectroscopy

Tunable diode laser absorption spectroscopy（TDLAS）,which is used to derive the spectral information through detecting the intensity variation of transmitted laser,is with advantages of high sensitivity,high selectivity,fast response and non-intrusive measurement etc.Therefore,it is widely applied in trace gas detection of kinds of areas.However,the sensitivity of TDLAS,particularly for embedding multi-pass absorption cell system,is susceptive to optical interference.Digital filtering method is one of the most effective methods to suppress the optical etalon mixing with the spectral signals.In the thesis,Airy function that is the analytic description of optical etalon is employed to accurately describe interference which can be significantly reduced by wavelet transform and empirical mode demodulation（EMD）.Mean-square-root is introduced to judge the interference suppression.By comparison with haar,fk18,db02 and coif1 wavelets,fk18 is optimum at 13th level of decomposition in de-noising simulations.In addition,the parameter of multi-EMD denoising method is obtained.Direct absorption spectroscopy and wavelength modulation spectroscopy（WMS）-2f recording CO₂ spectra were subjected to Fourier low-pass filtering,multi-averaging,wavelet denoising and multi-EMD.It demonstrates that only wavelet denoising and multi-EMD effectively suppress the noise,including optical interference.After filtering by wavelet and EMD,they can achieve 7 ppm and 35 ppm CO₂minimum detection limits,respectively.For further testifying the feasibility of the approach,spectral signals at different pressures and temperatures were acquired and the etalon interference was substantially eliminated by both wavelet denoising and multi-EMD methods.Consequently,the sensitivity of system is improved.A portable laser sensor for trace ammonia（NH₃） is developed based on near-infrared laser absorption spectroscopy.TDLAS in conjunction with WMS is implemented by a system-on-chip based on field-programmable gate array.A variable temperature multipass cell with a 15 m optical path length is adopted to enhance the absorption path length and avoid the NH₃ adsorption.The physical dimension of the sensor is minimized into 43×18×16 cm³ on the benefit of home-made electronic system and compact optical design.A unique absorption line of NH₃ around 1.51μm covered by a distributed feedback diode laser is selected to reduce the influence from carbon dioxide,vapor and other trace gases.Calibration-free modulation spectroscopy is employed to avoid the random fluctuations due to laser noise and non-absorption transmission losses.Hence,the robustness of system is substantially improved.The performance of sensor including adsorption of NH₃,stability and flow of gas are examined and discussed in detail.For removing the adsorption of NH₃,the temperature-dependent curve is obtained by the direct absorption spectroscopy,and it shows that the adsorption is significantly suppressed at（or higher than）415 K.According to Allan deviation analysis,the minimum detection limit is 0.16 ppm at 184 s,and the rise time and the fall time of system are about 4.5 s and 3.7 s,respectively.The feasibility of the sensor is validated and deployed to monitor the NH₃ slip in selective catalytic reduction（SCR）of coke oven flue gas.It is proved that our laser spectroscopy sensor is capable for monitoring NH₃ slip.