Modulation Spectroscopy Based On TDLAS And Applications In Gas Detection

With the development of modern laser technology,tunable diode laser absorption spectroscopy?TDLAS?has been widely used in atmospheric environment monitoring,combustion diagnosis,hazardous gas leak monitoring,industrial process control and medical diagnostics,thanks to its high spectral resolution,good selectivity,high sensitivity and fast response.Wavelength modulation spectroscopy?WMS?and multipass absorption cells also have been commonly employed to improve the detection sensitivity and measurement stability of TDLAS systems.The TDLAS-based WMS and its applications in gas detection are investigated in this thesis.Firstly,the wavelength modulation theory is studied,and a WMS gas measurement system is built to achieve simultaneous measurement of CO₂ and CO in combustion with a single laser.Secondly,the theory of calibration-free WMS is deduced,and the immunity of the calibration-free technology to detecting light intensity and external interference is experimentally verified.Finally,the miniaturized TDLAS system is set up based on calibration-free WMS to realize real-time monitoring of CO and CH₄ in atmosphere by a single laser.The research results and innovations,herein,mainly include:1.The theory of WMS is studied,and a WMS gas absorption spectroscopy system is established.The spectral lines of CO and CO₂ at 1.578?m are measured by a single distributed feedback?DFB?laser.The white noise is further reduced by averaging technology of ten times in one second integration time.Based on an Allan variation analysis,the minimum detectable limit of CO₂and CO is 7.5 and 14 ppm,respectively.An in situ measurement of CO₂ and CO of exhaust of alkane combustion emission under different air intake is presented.2.It has been demonstrated that the calibration-free WMS using 1f normalized 2f signal is of immunity to the laser light intensity change,airflow,system vibration and other external disturbances.A miniaturized multipass TDLAS system based on calibration-free WMS has been built for real-time monitoring CO and CH₄ of atmosphere,and there is a physical size of 60?30?25cm³.The integrated FPGA control system and the new Herriott multipass absorption cell are used.A single DFB laser with a center frequency of 2.3?m is selected as the light source to measure atmosphere CO and CH₄ simultaneously.Through the analysis of Allan variance,the system can achieve a minimum detection limit of 0.73 ppb(10^-9)and 36 ppb for CO and CH₄,respectively.The continuously in-situ measurement of atmosphere CO and CH₄ had been performed for 48 hours,and the results were in good agreement with the reported from nearby environmental monitoring center.3.The frequency modulation?optical heterodyne?spectroscopy technique is researched.The?3,0?band in b⁴?50??-a⁴?47?system of NO radical was experimentally measured.A miniaturized multi-optical frequency modulation spectroscopy system has been established to measure the absorption line of C₂H₂gas near 1.5?m,and the interference noise suppression is achieved by selecting appropriate modulation frequency.It has been found in theoretical analysis and experimental research that the optical fringe is effectively repressed when the modulation frequency is an integer multiple or a half integer multiple of the interference free spectral range.Moreover,fast detection can be achieved with frequency modulation technology,the measurement period can be shortened to10?s.Based on Allan variance,the system has a minimum detection limit of 18ppb.