【作者】 娄秀涛；

【导师】 张治国；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2009， 博士

【摘要】 对气体的有效探测在环境监测、工业控制和生物医疗等领域都具有重要意义。相比于化学方法,光学技术具有非接触、测量范围大等优点,其中可调谐二极管激光吸收光谱（TDLAS）和关联光谱（COSPEC）是两种主流技术。为了克服TDLAS技术成本高、稳定性差、可靠性低的缺点,本论文在开展了TDLAS和COSPEC技术研究的基础上,将COSPEC引入到TDLAS中,发展了新的可调谐多模二极管激光关联光谱（TMDL-COSPEC）技术,并用该技术开展了多种气体监测的研究。开展了TDLAS气体监测技术的研究。在以CO₂为样例气体的测量实验中,使用中心发射波长为1578.7nm的VCSEL型二极管激光器获得了CO₂的高分辨吸收光谱,通过应用波长调制光谱（WMS）技术显著提高了信噪比,采用2s的积分时间获得了130ppm的测量灵敏度,测量值与计算值的偏差小于1%。TDLAS技术的研究为后面气体监测研究工作的开展奠定了基础。采用发射波长在300nm附近的LED开展了COSPEC技术监测SO2的研究。通过测量多组不同浓度的SO2获得了具有很高线性度的浓度标定曲线,光强调制系数正比于参考气饱和度。使用Allan方差分析法评估系统的最佳积分时间大于60s,最佳灵敏度好于0.4 ppm。压强变化和NO2的存在对测量的影响很小。该研究表明采用COSPEC技术可实现对SO2的低成本、抗干扰、灵敏测量。提出了新的TMDL-COSPEC气体监测技术,该技术结合了TDLAS技术高灵敏和响应快速的技术优势与COSPEC技术抗干扰和坚固稳定的优点。从Beer-Lambert定律出发建立了多模激光吸收光谱理论模型。在TMDL-COSPEC技术测量气体的实验研究中所使用的光源是廉价的Fabry-Pérot（FP）型MDL,其发射光谱与目标气体吸收光谱具有很好的交叠,很容易获得丰富的目标气体的特征吸收信号。使用发射谱在1570nm附近的MDL在没有对光源做温度稳定控制的条件下实现了对CO₂的准确测量,获得的灵敏度为400ppm。样品气内混入的CO干扰气体没有对测量产生影响,证明了TMDL-COSPEC技术的选择性。使用发射波长在1520nm附近的MDL测量了C2H2气体,研究了测量灵敏度与可靠性的关系,实验结果表明,在采用多信号推演待测气体浓度时,灵敏度与可靠性间存在着此消彼长的辩证关系。开展了CO₂和CO的同时测量研究,利用两个分别装有已知浓度CO₂和CO的参考池实现了对两种目标气体的特异性识别和准确定量分析。在300⁴73K的范围内研究了温度对TMDL-COSPEC技术测量气体的影响。在以氧气为测量对象的实验研究中,从理论上分析了温度对气体体积和吸收线形的影响,实施了三种方案找到了在不同应用场景下的温度修正方法并比较了它们的优缺点。温度修正后的测量结果相对标准偏差小于1%,表明温度效应被有效修正。分别采用FP型的MDL和VCSEL型单模二极管激光器对TMDL-COSPEC和TDLAS技术进行了比较研究。以氧气为测量对象的实验研究结果表明,尽管TMDL-COSPEC技术的灵敏度比TDLAS技术要差大约3倍,但前者的稳定性远好于后者,并且前者具有对光源要求宽松和使用简单的优点。两种技术均表现出了水平相当的高线性度。应用TMDL-COSPEC技术在无人职守情况下对热电厂烟气中氧气含量实施了连续5天的实时监测,TMDL-COSPEC技术表现出了高度的测量稳定性和对复杂环境的适应性。在应用TMDL-COSPEC技术实施气体监测时,不必要求光源必须是单模发射,不必对每个二极管激光器做严格标定,不必做波长稳定控制。这些特性不仅大大降低了气体测量成本,提高了测量的稳定性,更使得气体监测系统更加易用,将有望促进以二极管激光器作为光源的气体监测技术的普及。更多还原

【Abstract】 Efficient gas detection is of great importance in many fields, including environmental monitoring, industrial control and biomedical applications. Optical techniques benefit from being non-contact and large-range compared with chemical gas sensing methods. Tunable Diode Laser Absorption Spectroscopy （TDLAS） and COrrelation SPECtroscopy （COSPEC） are two of the most widely-used optical techniques. In order to overcome the drawbacks of TDLAS– including high cost, poor stability and low reliability,– this dissertation presents how the concept of COSPEC was combined into TDLAS, and a novel gas sensing technique called Tunable Multimode Diode Laser COrrelation SPECtroscopy （TMDL-COSPEC） was developed and employed for monitoring of various gases.The gas monitoring technique of TDLAS was first employed to measure CO₂, where the high-resolution absorption spectrum of the gas was obtained by using a VCSEL type of diode laser with a centre wavelength of 1578.7 nm. Wavelength modulation spectroscopy was performed to increase the signal-to-noise ratio. A sensitivity of 130 ppm was obtained with an integration time of 2 s. The discrepancy between the measured and the calculated value was less than 1%. The investigation of the TDLAS method laid a foundation for the latter research work on gas monitoring.Sulfur dioxid monitoring was performed by COSPEC utilizing a LED with the emitted spectrum around 300 nm. A concentration calibration profile with high linearity was obtained by measuring different concentrations of SO2. The intensity modulation index was found propotional to the saturation of the reference gas. The system performance was evaluated by Allan variance methods, yielding a sensitivity better than 0.4 ppm and an optimum integration time longer than 60 s. The accuracy of the concentration measurements were not significantly affected by pressure changes or simultaneous presence of NO2. These results indicate that COSPEC for SO2 monitoring has the advantage of low cost, high specificity and high sensitivity.A novel TMDL-COSPEC technique for gas monitoring was proposed by combining the high sensitivity and speed of TDLAS and the insensitivity to gas interferences and robustness of COSPEC. A theoretical model of the multi-mode laser absorption spectroscopy was established on the basis of the Beer-Lambert law. Low-cost Fabry-Pérot-type MDLs were employed in TMDL-COSPEC experiments. Since the random radiation spectra of the MDLs greatly overlapped with the absorption spectra of the target gases, plenty of absorption signals were readily obtained. The CO₂ concentration was accurately measured with a sensitivity of 400 ppm by using a MDL emitting around 1570 nm without temperature stablization. The interfering CO gas in the sample did not affect the measurement, demonstrating the specificity of TMDL-COSPEC. The relationship between sensitivity and reliability was studied by measuring C2H2 employing a MDL with the emission spectrum aournd 1520 nm. The measurement results indicated that there is a trade-off between sensitivity and reliability for retrieving gas concentrations by multiple absorption signals. Simultaneous detection of CO₂ and CO was also performed. Both of the two target gases were specifically identified and accurately analyzed by using two reference gas cells containing well-calibrated concentrations of CO₂ and CO, respectively.Temperature effects on the gas monitoring using TMDL-COSPEC was investigated in the temperature range of 300-473 K,, where oxygen was used as the sample gas. The temperature effects on gas volume and absorption line shape were theoretically analysed. Temperature correction methods were found in three different application schemes, and their advantages and disadvantages were compared. The relative standard deviations of measurement results after temperature correction were less than 1%, indicating that the temperature effects were well corrected.The performances of TMDL-COSPEC and TDLAS were compared by measuring the oxygen gas concentration employing FP-type MDLs and a VCSEL-type single mode diode laser, respectively. Although the sensitivity of TMDL-COSPEC was worse than the TDLAS by a factor of about 3, the stability of the former was far better than the latter. Futhermore, the TMDL-COSPEC had the advantage of ease-of-use since the laser source did not require extensive characterization before empolyment. Both of the two techniques displayed nearly perfect linearity. By employment of TMDL-COSPEC, we performed 5-days unattended real-time monitoring of the oxygen content of exhaust gas in a coal-combustion-based power plant, demonstrating the high stability and adaptability of TMDL-COSPEC in a complex environment.By using TMDL-COSPEC, the diode laser source is not necessarily single mode, does not require extensive characterization and does not need temperature stablization. The above advantages of TMDL-COSPEC scale down the system cost, enhance the measurement stability and make the system easy to use, which is promising to the promotion and widespread application of diode-laser-based gas monitoring techniques.更多还原