Gaseous Pollutant Detection Based On Tunable Diode Laser

Based on their unique energy states structre,atoms and molecules have different absorption lines.These absorption lines act like "finger prints",and play an important role in atom and molecule detection.Since 1666,when Newton discovered spectrum,spectroscopy has become a fundamental tool for human beings to understand and explore the nature.After laser became reality in 1961,because of its wavelength purity,well collimation and high power characteristics,applications based on laser spectroscopy is boomingly developed and have been applied to fields like biology,medical treatment,agriculture,environmental monitoring and so on.Along with the remarkable development of China's economy and facility building,environmental problems are becoming increasingly serious.Unlike last century,when people were more concerned about water pollution,soil pollution and acid rain,what people nowadays worry about is haze.Air pollutions can spread easily to everywhere and affect everyone's daily life.Thus when air quality becomes a problem,everyone cares.Optical methods are key ways for gas detection.Tunable diode laser absorption spectroscopy(TDLAS)based gas detection system,due to its quick response,non-invasive measurement and high selectivity,is widely utilized to various gas detection systems,and taking place of wet chemistry and electrical-chemistry sensors.This thesis mainly includes three works:1.Simulation of absorption function,wavelength modulation signal and second harmonic signal using Matlab.Based on line strength,line profile and Beer-Lambert law,absorption function is successfully simulated.Though absorption function can be got from Hitran data base,problems of time delay,parameter limits and lack of API prevent its use in different circumstances.The wavelength modulation simulation relates absorption function to the output characteristics of lasers.Second harmonic signal simulation makes the process of wavelength modulation spectroscopy(WMS)all virtual available,so all the parameters can be set and tested before real experiments.2.Built and tested a prototype for elemental mercury vapor continuous monitoring.Mercury is extraordinary harmful to nervous system,kidney and lung.It vapors at room temperature.In China,as fossil fuels still dominate in producing energy,mercury contained in coal exhausts to the atmosphere when burnt.Mercury exhausts in stock emission requires continuous emission monitoring system(CEMs)to rule.Because the absorption line of mercury locates at ultraviolet region,where no commercial laser is available,our system generates 253.7nm laser by sum-frequency generation through nonlinear crystal employing one blue laser and a red laser.To purify the output modes of the blue laser,external cavity is employed to select a feedback wavelength.To overcome the cross-section fluctuation,a reference cell is added to the system to provide a real time calibration.A prototype is built after plenty of work including optics,electronics,gas pipes,mechanics and software programming.The system can reach a sensitivity of 0.1?g/m3 if a 1 meter long gas cell is utilized.3.Built a compact system for methane detection employing scattering material as gas cell.Methane is an explosive gas at room temperature.Developing a compact methane sensor is a hot point both in academic and industrial field.Towards getting good detection limit,a practical way is to enhance optical path length.Typically,there are two ways to increase optical path length.One needs careful adjustment,the other needs large system volume.To achieve small system volume as well as easy adjustment,scattering material based gas cell is developed for methane sensing.This technology is called gas in scattering material absorption spectroscopy(GASMAS).Since scattering material owns high scattering coefficient and low absorption coefficient in selected wavelength,the corresponding light will scatter back and forth inside the material,the result is an enhanced optical path length of the output beam.In this thesis,we developed an alumina ceramic material,and achieved a ppmv level methane sensor with physical thickness of 0.5cm.