二氧化碳雷射光聲光譜及其在氣體檢測之應用

CO2 Laser Photoacoustic Spectrometry and Its Application to Gas Detection

林雋文(Jim-Wein Lin)

指導教授 ： 蕭憲彥

國立清華大學/電機資訊學院/電機工程學系/博士(2009年)

摘要

以二氧化碳雷射光聲光譜法來偵測微量氣體的方法在過去已被廣泛地運用，包括汽機車及工業排放的廢氣檢測、大氣中的稀有氣體偵測及農作物的生長監測等。近年來，以雷射紅外光譜偵測微量氣體的技術更被應用到各種生物及醫學用途，如動植物(包含人體)的呼吸檢測、醫療診斷以及生化反應的監控等。其中亦不乏以二氧化碳雷射當作激發光源，透過相關反應物的偵測來完成其定性或定量分析，這些研究指出了二氧化碳雷射光聲光譜技術開發新應用的潛能。

本研究成功建置一套二氧化碳雷射光聲光譜量測系統，該系統可自動化測量各種氣體樣品之吸收光譜。二氧化碳雷射是採用射頻激發放電式的波導結構。相對於傳統開放式的光學共振腔雷射而言，波導式共振腔可以更有效利用放電區域增益介質，提高單位長度雷射增益並縮小整個雷射的體積；而射頻激發放電則可以避免使用高電壓設備，延長器件壽命並提升放電效率。由於它可以操作在較高的氣壓下，如此可增加雷射線寬並提高其與氣體吸收譜線的重合機率，適合作為光聲光譜儀之激發光源。至於光聲氣室的設計，我們嘗試使用一個簡單小巧的非聲波共振式結構來實現高靈敏度的偵測，方法是將該氣室置於雷射共振腔中，透過增強的雷射功率來提昇氣體吸收的能力。

除量測系統的建立外，本研究亦提出幾種可能的應用。首先，我們注意到此方法可用於酒精飲料之檢測。藉由測量純甲醇與純乙醇水溶液的吸收光譜，再依據各吸收譜線之吸收係數大小與吸收係數對比，並考量量測動態範圍與雷射的穩定性，建立了三條校正線。接著我們配置不同乙醇濃度的酒精溶液來模擬市面上常見的酒精飲料，再分別混合少量甲醇於其內。透過該吸收光譜與先前的校正曲線可以計算溶液中的甲醇與乙醇含量，並與理論值比較，有相當程度的吻合(誤差 < 5.89%)。此結果顯示二氧化碳雷射光聲光譜法的確有其可行性。相同的方法可以運用到一些揮發性有機化合物的偵測上，例如汽油添加甲醇的檢驗、水質偵測(汽油污染)、丙酮污染與監測、甚至酒駕執法所需的酒精濃度檢測等。相對於傳統的氣相層析法，我們的方法具有快速、簡單且成本低等優勢；相對於電化學式氣體感測器，我們的方法則能提供較準確與多種元素的分析。這些初步的實驗將有助於未來進一步研究的參考。

關鍵字

二氧化碳雷射 ； 光聲光譜

並列摘要

The CO2 laser-based photoacoustic (PA) spectrometry has played an important role in a wide variety of applications, such as the detection of motor-vehicle and industrial exhausts, air pollution monitoring, trace gas sensing in the growth of plants, and so forth. With development of laser-spectroscopic gas sensors, the infrared gas analysis techniques are attracting lots of interest in medicine and biology in recent years. These include breath analysis for animals or human beings, medical diagnostics, and biological processes in living organisms, etc. For some specific molecular species, the use of CO2 laser PA method was still proven to be valuable in sensitivity and multi-component analysis. The successful examples of CO2 laser PA detection may imply the possibilities for future potential applications.

This thesis reports the construction of an automatic CO2 laser PA spectrometer which uses a RF-excited waveguide CO2 laser. Operating in a waveguide mode can obtain higher gain per unit length, reduced physical size, and larger linewidth, thus increasing the probability of overlap between laser transitions and gas absorption spectrum. RF excitation provides additional advantages of low-voltage operation, improved lifetime and better overall efficiency. To increase the sensitivity, we put the small PA cell inside the laser cavity while operating in nonresonant mode.

In addition, a number of new applications for trace-gas detection by CO2 laser PA spectrometry have also been proposed in this study. The use of this PA method for detecting the methanol contents in alcohol beverages is first presented. The calibration curves for methanol and ethanol were established by selecting three laser transitions. A series of samples referring to adulterated alcoholic beverages were measured and the concentrations of methanol and ethanol could be derived by the PA spectra. These results show great promise for diagnosis of adulterated wine by using CO2 laser PA spectrometry (error < 5.89%). This idea can be applied further for the detection of some volatile organic compounds, such as methanol/gasoline blends, gasoline contamination in water, acetone pollution, and even alcohol breath analysis. Compared to traditional gas chromatography, the PA scheme provides fast and easy operation without pre-treatment of toxic chemicals. Also, the spectroscopic method offers reliable and multi-component analysis in comparison with common chemical gas sensors. The preliminary measurements can be an experimental reference for future work.

並列關鍵字

CO2 laser ； Photoacoustic spectroscopy

參考文獻

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