| Cavity enhanced absorption spectroscopy?CEAS?,a new type of direct absorption spectroscopy technology with high sensitivity,is widely used on air pollution monitor,combustion efficiency analysis and human breath gas diagnosis and more.CEAS is based on relaxation effect of high quality optical cavity.Depend on the resonance,it can increase absorption distanced over limited cavity length,and its enabling absorption paths could be kilometers to tens of kilometers,with greatly increasing system detection sensitive.It is different from traditional Cavity ring-down spectroscopy?CRDS?by detecting cavity's ring-down time for calculating cavity gas absorption spectrum,but directly measures the cavity transmission light intensity to analysis absorption spectrum of the medium in cavity.And it is unnecessary to quickly shut off the laser or obtain data with high-speed,which make the structure of spectrum system more concise and easier in detecting.Based on the analysis of performance characteristics of the typical CEAS system,this topic takes the diagnosis if human breath gas as the background and focuses on research of three different CEAS systems t improve the measure performance of such systems,laying the foundation of human breath gas diagnostic system as following.The main work of this subject includes:it is introduced the background and research status of CEAS and breath gas diagnosis;based on the fundamental theory of F-P cavity,it is focused on the key issues of broadband CEAS?BB-CEAS?and optical-feedback CEAS?OF-CEAS?;with these theoretical analyses,it is proposed/three kind of spectrum detection experiment programs for das detecting,and making related experiments.The main content includes:?1?the linear BB-CEAS system is designed and built,and the performances of this system is tested;?2?the short folded cavity BB-CEAS system is designed and built,and the performances of this system is tested;?3?the V-cavity OF-CEAS is designed and built,through the measurement of atmospheric water vapor,the system would be evaluated.The experimental results show that:in the exploring experimental schemes,because it is difficult to match narrow line-width laser with narrow cavity resonance frequency and the intensity of cavity enhanced absorption light signal would not be stable enough,the linear cavity BB-CEAS system's detection accuracy is poor and can't meet the needs of human breath gas diagnosis;the theory of short folded cavity BB-CEAS is feasible and the broadband transmission curve could be got from experiment.However,due to the low power density of the system's light source,the normal spectrometer could not support the high accuracy absorption spectrum curve of the gas medium in the cavity;because of the locking of optical feedback frequency and the effect of narrower line-width,the V-cavity OF-CEAS system solves the question of the laser resonance with cavity.So it has the best sign-to noise radio and the detection accuracy.A number of measurements over a long period shows that the repeatability of this system is better than 0.6%.Detecting the cavity's residue gas,the measurement spectrum curve is in good agreement with the theoretical gas absorption spectrum curve by Hitran,and the reliability of experimental results is verified from this.At last,the OF-CEAS system's Noise Equivalent Absorption Sensitivity is NEAS=8.78×10-9cm-1Hz-1/2,and the minimum detectable concentration of water vapor is 1.6ppm?part per million?,and it could be inferred that the system measures the minimum concentration of ammonia would be ppb,which lays the foundation for the detection about the concentration of ammonia as a breath gas maker in the next. |
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