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Tunable Multi-Mode Diode Laser Correlation Spectroscopy For Gas Monitoring

Posted on:2010-04-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:X T LouFull Text:PDF
GTID:1118360302465471Subject:Optics
Abstract/Summary:PDF Full Text Request
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.
Keywords/Search Tags:gas monitoring, absorption spectroscopy, correlation spectroscopy, laser spectroscopy, tunable multi-mode diode laser
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