Research On Gaseous Detection System Of Off-axis Cavity Enhanced Absorption Spectroscopy

In the last decade, the highly sensitive spectroscopy techniques, such as cavity ring-down spectroscopy and cavity enhanced absorption spectroscopy techniques have been developed rapidly and gained wide application. Due to the simplicity of experimental setup, low cost, high sensitivity and spectral accuracy, robustness and other advantages, cavity enhance absorption spectroscopy has widely been used in the study of materials, chemical reaction, atmospheric chemistry, spectroscopy, and bio-medical science et. And because of these unique advantages, cavity enhanced absorption spectroscopy has the potential and is valuable in the actual detection of gaseous pollutants. In this study, an off-axis incidence cavity enhanced absorption spectroscopy system (OA-CEAS) has established and some technical problems in OA-CEAS application are discussed.In this paper, the development of cavity enhanced absorption spectroscopy, the character of several highly sensitive detection techniques and their difference are briefly reviewed. A simulation of the resonance mode in an optical cavity is performed, and its impact on the spectral measurement is also discussed. the principles and methods of off-axis incidence cavity enhanced absorption spectroscopy are provided in detail. The theory of infrared spectroscopy (including vibrational spectra, rotational spectra, and vibration-rotation spectra), the principle of absorption spectrum, the mechanism and characteristics of spectral linewidth broadening are systemically introduced. The HITRAN IR spectrum database and application of the parameters in the database for the spectral simulation, the absorption intensity and spectral absorption cross-section calculation are discussed.A high-resolution, high-sensitivity and real-time gas detection system is developed based on the combination of the virtual instrument technology and off-axis cavity enhanced absorption spectroscopy. The system integrated the functions of main instruments used in OA-CEAS, such as ECDL, wavemeter and oscilloscope, and has a friendly visual interface. It's easy to operate, and meets the requirements in off-axis cavity enhanced absorption spectroscopy study.Based on the developed system, the reflectivity of the cavity mirrors was measured. The non-linear variation of wavelength in the wavelength scanning process of ECDL had been studied and was corrected automatically. The single spectra of CO2 near 6358.65cm-1 in on-axis and off-axis alignment condition were recorded respectively. A detailed anaysis of the two spectra showed that a much more dense cavity mode spectrum and a smaller fluctuation in light coupling are achieved under the condition of off-axis alignment, and so obtained a better signal to noise ratio and sensitivity～1.98×10-7cm-1. The characteristics of spectral intensity and linewidth in different pressure of CO2 gas are discussed along with different data processing. A vibration-rotational spectrum of CO2 in the wide range from 6450 to 6530cm-1 was recorded and compared with theoretical simulation. They were consistent with each other. The experimental results proved here demonstrated that the OA-CEAS system has high sensitivity and can be used for practical application of trace CO2 detection.