Cavity-enhanced Diode Laser Correlation Spectroscopy

Deteriorating global environment seriously affected people in various countriesof the world, so people take an active approach to deal with, not only to curbpollution emissions, but also to strengthen the contaminated gas detection. Theapproaches for detecting contaminant gases include chemical and optical methods.Because the area measured by chemical method, the reaction is slow, can notdirectly detect polluting gases such as shortcomings in depth, more and morecountries and companies use optical detection methods. In order to improve themeasurement accuracy, reduce testing costs, increasing the detection range, wecombine the technique of cavity enhanced spectroscopy and correlationspectroscopy, using a low-cost multimode semiconductor laser as light source, thuscreate a new gas detection technology. Based on the detection of oxygen in the air,we verify the feasibility of this new technology.Firstly, the theoretical basis from the principle of this experiment were given,the specific expressions to detect gas concentrations were analyzed, the minimumdetectable sensitivity of this device were evaluated. Considering the line broadeningdue to gas collisions, various specific expressions were given, the correspondingtheoretical simulations provide theoretical guidance.By adjusting the parameters of the experimental set up with the equipmentoptical path, we choose the best experimental parameters for the collection ofexperimental data, the sample chamber to enhance the optical path. In this article,on-axis and off-axis signals were detected separately. To eliminate interference, avariety of approaches were tried. Oxygen concentration in the air is relatively large,so in the signal acquisition process, the absorption saturation phenomenon occurs.Therefore, during the measurement, the measurement accuracy was ensured bycontrolling the cavity length. The linear relationship between the signal and theabsorption length of the cavity was obtained. A minimum detectable sensitivity of188ppm for direct absorption and22ppm for wavelength modulation technologywere achieved, thus proving the feasibility of this new technology.Thereafter, the paper also study the treatment of experimental data usingdifferent methods to verify the theoretical data. By no absorption baseline intensitysignal acquisition and processing, we developed a baseline fitting based approachwith no absorption signal with linear combination of absorption. By comparingdifferent signals with respect to the peak absorption ratio and area ratio, the latter ismore suitable for strong absorption and large noise condition, and the absorption peak ratio fits theory well at lower absorption condition.Finally, we study the conditions under theory, given the same initial intensity,given reasonable parameters on the experiment to verify the accuracy of theexperimental data. Through simulation program, using existing information fromHITRAN database, we discuss the absorption lines detected in the experiment forwhich of. And after verification, the source of the error between experiment andsimulation were evaluated, and finally a reasonable explanation was given.