Multipass Cell Gas Seneing Based On Frequency Modulated Continuous Wave

As a kind of distributed optical fiber sensing technology,optical frequency domain reflectometry has the advantages of high spatial resolution,high signal-to-noise ratio,and wide dynamic range.The system includes a linearly tuned laser source,and the wavelength tuning range covers the absorption band of gas molecules,which has a high sensitivity and selectivity for characteristic spectrum detection of the target gas.Combined with traditional laser absorption spectrum technology and detection devices,distributed gas sensing can be applied to atmospheric environment monitoring,industrial production control and other applications.The main focus of this thesis is to launch research and application exploration on multipass gas sensing combined with frequency modulated continuous wave technology and tunable laser absorption spectroscopy.From the signal model of optical frequency domain reflectometry,the principle of multipass gas sensing is introduced.In view of the spatial resolution blurred by the nonlinear tuning of the laser source,the principle and limitations of nonlinear phase compensation methods for laser source are demonstrated separately.According to the Beer-Lambert absorption law,the absorption line profile of gas molecules under different conditions is listed,which is selected for the spectral fitting.On the study of multipass gas sensing method,we start from the physical meaning of absorption coefficient and optical path length in transmission function,derive the expression of interference signal containing gas absorption information,simulate and analyze the FFT and IFFT results of the signal.The accurate ranging measurement of the optical path of gas absorption is performed under FFT,as well as the performance of the inversion of gas transmission spectrum under IFFT.Considering the real situation,the artificial factors such as FFT leakage and the selection of the IFFT rectangular window with limited size on the transmission spectrum are investigated.Feasible measures for realizing distributed gas sensing is then put forward.Combined with the principle and method of multipass gas sensing,the test using multipass cell is performed.In experiment,the absorption light path of the gas cell and the transmission spectrum is measured simultaneously.By retrieving the transmission spectrum of the internal reflections of the multipass cell,we realize the detection of the gas concentration in the different optical path length in the multipass cell,with result of the improvement of the dynamic range of the multipass cell.Specifically,in the test of acetylene gas with low concentration of 100 ppm at atmospheric pressure,the absorption path length of multipass cell is precisely measured at 11.2468 m in one way.The transmission spectrum of acetylene gas near 1520 nm is also retrieved,corresponding to a detection limit of 5 ppm.In experiments with acetylene gas at a relatively high concentration of 1600 ppm at atmospheric pressure,the concave mirror inside the multipass cell forms multi-level reflections at different absorption paths,so the upper limit of the sensing range is improved by retrieving the transmission spectrum of the reflection peak under a short optical path.