Abstract
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.