High Sensitivity Cavity Ring Down Spectroscopy For Trace Gas Detection

Direct absorption spectroscopy is widely applied in environmental monitoring, medical diagnosis, industrial process control, and security inspection. Tunable diode laser absorption spectroscopy (TDLAS) is simple and widely used in the combustion gas analysis, but the lack of sensitivity and insufficient data-acquisition rate make it hard for some applications. Cavity ring down spectroscopy (CRDS) is suitable for the trace gas detection owing to its high sensitivity, but the instrument is relatively more complicated, sensitive to vibrations and fluctuations of environment temperature. The thesis is devoted to develop absorption spectroscopy based instruments for trace detection applications. A wavelength-locked TDLAS system is established for rapid measurements of the temperature of gases without scanning the laser frequency, which reduces the measuring time as short as1μs. An auto-calibrated cavity ring down spectrometer based on DFB diode lasers is established for trace detection of gases including CO2、H2O、CO, with a sensitivity of5×10-12cm-1.The first chapter presents the principle of the TDLAS and spectral line-shape func-tions. The configuration of a wavelength-locked TDLAS system is also presented. In order to evaluate its performance, we applied it to determine the temperature of sample gas by measuring the absorption of water molecules. A fast readout rate of1μs per data has been demonstrated.In the second chapter, the principle of CRDS is introduced. The theory of mode matching between a laser and a high-finesse cavity is presented as well. A cavity ring down spectrometer based on DFB diode lasers is established, which is calibrated using the longitudinal modes of the cavity. The methods of spectral scanning and data acquisition are given. The performance of the spectrometer was tested through the measurement of the spectrum of CO2near1.6μm. A sensitivity of5×10-12cm-1has been demonstrated.In the third chapter, a CRDS system for detecting CO2or CO in air samples is introduced. The content of CO2(or CO) is determined by measuring its absorption spectrum near1.6μm (or1.56/μm). The quantitative capability of the spectrometer is verified using a series of standard gas samples with known CO2concentrations. The spectrometer has also been applied to determine the CO2(CO) concentration in ambient air.In the fourth chapter, a CRDS system for humidity measurement is presented. The water concentrations in gases of high humidity and ultra-low humidity are determined by measuring the water vapor spectrum at1.6μm and1.3μm, respectively. The pre-cision of the CRDS-determined humidity is better than1%, by comparing the results from a hygrometer. The capability of determining ultra-low humidity is demonstrat-ed by the measurements of the trace water concentration in a high-purity nitrogen gas sample.