| Trace gas detection technology has been played an important role in the airpollution monitoring, industrial production, medical diagnostics and other fields, inwhich quartz enhanced photoacoustic spectroscopy (QEPAS) is a gas detectiontechnology without background light absorption completely. It has been widely usedbecause of its small size, high sensitivity, fast response speed and suitable for real-timeonline measurement, etc. But the current gas detection techniques still have somelimitations in some extremely harsh environments, such as high temperature, highhumidity, strong electromagnetic interference environment, or measurement forflammable, explosive and toxic gases. In this paper, a new all-optical quartz enhancedphotoacoustic spectroscopy system based on fiber Fabry-Perot was developed, whichused a new demodulation method to obtain the vibration signal of tuning fork to replacethe traditional method of obtaining tuning fork piezoelectric signal. Meanwhile, theFabry-Perot sensor operating point stabilization technology was used to further improvethe demodulation accuracy and stability of the QEPAS system.Firstly, based on the theory of quartz enhanced photoacoustic spectroscopy, thispaper detailedly analysed the gas photoacoustic theory and the relationship between theconcentration of gas and the vibration amplitude of tuning fork. To improve the signalto noise ratio, the necessity of wavelength modulation and harmonic detectiontechnology was analyzed. The fiber Fabry-Perot sensor was employed to obtain thevibration signal of tuning fork, and the relationship between the vibration amplitude oftuning fork and the output signal of FP sensor was given according to the theory ofmultiple-beam interference, and analysed the operating point drift impact on the resultsof the demodulation by simulation and experiment. Meanwhile, combined withwavelength compensation technique, the Fabry-Perot sensing technology stabilized theoperating point in the orthogonal position, to obtain the best demodulation results.The fiber Fabry-Perot demodulation sensor based on QEPAS system was designed,which mainly included the source of fiber laser and the operating point feedback controlmodule. The principle of light amplifier in erbium-doped fiber was described. A tunablering cavity fiber laser based on FP filter was developed which obtained output power upto10mW, wavelength tuning range of1525nm~1565nm,3dB bandwidth less than0.00051nm. Fiber Fabry-Perot sensor module with feedback was designed based on LabVIEW. The output signal of FP sensor by sampling、filtering and taking DCcomponent was converted to DC voltage. The difference between the DC voltage andthe static operating voltage was worked as the error feedback signal. And the detectionmethod based on digital PID was employed to lock the operating point to the quadraturephase point in the system for improving the stability of the fiber Fabry-Perot sensor, soimproving the detection accuracy of the photoacoustic signal.In order to verify the feasibility of Fabry-Perot sensing system based on QEPAS,the experiment for detection of water vapour in the open environment was carried outand a normalized noise equivalent absorption coefficient1.81×10-7cm-1W/Hz1/2wasachieved, which was about4times higher than that of conventional QEPAS. It had theadvantages such as low cost, high efficiency detection, immunity to electromagneticinterference, safely in flammable and explosive situations, and remote sensing. |
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