Tdlas Gas Detection Sensitivity Enhancement And Application In Diffuse Reflectance Measurement

Effective gas monitoring is significant in the fields of environmental protection, industrial production and life safety. Tunable Diode Laser Absroption Spectroscopy (TDLAS) is popular for the advantages of high resolution, high speed, non-contact and on-line monitoring in gas monitoring techniques. In this thesis, we studied the low cost, high sensitive and miniaturized gas monitoring technique based on TDLAS, and the application for diffuse reflectance measurement.The research on low cost Tunable Multimode Diode Laser Absorption Spectroscopy (TMDLAS) for methane detection was developed. The low cost TMDLAS was achieved by using multimode laser instead of single mode laser. The1310nm multimode diode laser commonly applied for communication was used as light source. The experimental result indicated that the laser mode was stable. Each mode changed ruly, and the wavelength could be adjusted continuously with high repeatability. Methane was used as sample gas. The best measurement condition was ensured by comparing the emission spectrua of laser and transmission spectrum of methane. Single absorption signal which was superimposed by multipule absorption lines was obtained. The signal-to-noise ratio was improved by using wavelength modulation spectroscopy with second harmonic signal detection, and the sensitivity was increased obviously. The sensitivity of25ppm was obtained with1s integral time and the accuracy of0.06%.The research on using integrating sphere (IS) to enhance the sensitivity of TDLAS was developed (TDLAS-IS), and a method of integrating sphere effective optical path length calibration by using gas absorpion spectroscopy was presented. The results indicated that the incident light intensity, detecting voltage, incident angle and detecting angle did not affect the absorption signal in the TDLAS-IS, which decreased the mechanical vibration effect. There were no optical lens in the integrating sphere, which eliminated the interference fringes. The long optical path length of integrating sphere enhanced the abosprion signal, which increased the sensitivity of50times. From the experiment, we also found that the effective optical path length of integrating sphere was correlated with the gas absortpion, and a393.7cm effective optical path length was obtained for a8.4cm diameter integrating sphere when oxygen concentration was21%. However, according to the tranditional theory, the effective optical path length of an integrating sphere is436cm without taking gas absorption effect into account. The error was about11%, which will be increased with the increasing of concentration. The research on diffuse reflectance meaurement by using TDLAS-IS was developed. Gas absorption spectrum was connected with the diffuse reflectance of the diffusing coating in the inner of the integrating sphere. This connection allows us to obtain diffuse reflectance from gas absroption spectrum, which can avoid the low accuracy shortage of tranditional methods by measuring the light intensity. Two methods of using time-resoved spectroscopy and gas absorption spectroscopy were carried out in the experiments. Based on time-resolved spectroscopy, the measured diffuse reflectance of an integrating sphere at800nm was99.22%with the accuracy of0.06%. Based on gas absorption spectroscopy, the measured diffuse reflectacne at764nm was98.944%with the accuracy of0.005%. Better than10-6accuracy will be achieved by the optimization of system parameters. More than an order of magnitudes was improved comparing with the tranditional methods.