Investigation Of Mixed Gas Sensing Based On Laser Intracavity Absorption Using C+L Band Fiber Ring Laser

Aiming at the requirement of monitoring gas pollutants in real time in the fields of industrial production and environmental protection,we investigate the gas sensing technique based on fiber laser intracavity absorption spectroscopy.After conducting the theoretical analysis of intracavity gas sensing,we construct the mixed gas sensing system which has a ring cavity structure and operates in C+L band.By combining intracavity absorption with wavelength sweep and modulation techniques,we realize the sensitive detection of the mixed gases of C2H2,CO and CO₂.To solve the problem of spectral overlap in mixed gas sensing,we propose a method to separate the overlapping absorption lines,so their respective absorbance distributions can be retrieved.Considering the nonlinear and creeping characteristics during wavelength tuning when using our scanning laser source,we use the wavelength reference gas cell for laser wavelength calibration.The gas cell is filled with multiple gases and the absorption lines of each gas locate in one band of the laser source.The absorption wavelengths of multiple gases are used as the real-time wavelength reference and have good temperature stability.As a result,the accurate wavelength calibration of the scanning laser source is realized in the wide tuning range and temperature range.The innovative work of this paper is listed as follows:1.On the basis of laser transmission equation and gas absorption law,we conduct the theoretical investigation of mixed gas sensing using fiber laser intracavity absorption spectroscopy.The influence of system parameters on mixed gas sensing is analyzed.We propose the sensitivity enhancement method and software lock-in method for different intracavity gas detection techniques of direct absorption and wavelength modulation respectively.The influence of environmental temperature change on gas concentration measurement is also investigated and the corresponding temperature compensation method is proposed.2.The mixed gas sensing system which is based on fiber laser intracavity absorption and works in C+L band is constructed.Intracavity absorption is combined with wavelength sweep and modulation techniques.In this way,a wider spectral coverage and a lower gas concentration detection limit are obtained.When detecting the mixed gases of C2H2,CO and CO₂,the effective absorption length is about 50 times more than the single absorption length,and their minimum detectable concentrations are 0.6 ppm,17.4 ppm and 19.2 ppm respectively.3.In order to solve the spectral overlapping problem in mixed gas sensing,we propose a method to separate the overlapping absorption lines.The method consists of three steps,which are continuous wavelet transform,linear regression analysis and chaos particle swarm optimization.Then the individual absorbance distributions of the overlapping lines can be retrieved.When using the method to separate the overlapping absorption lines of CO and CO₂ for gas concentration measurement,their respective maximum errors are reduced to 35% and 42% of the errors without using the method.4.Considering the nonlinear and creeping wavelength tuning characteristics of the scanning laser source in our system,we adopt the wavelength reference gas cell for laser wavelength calibration.The gas cell is filled with mixed gases which have absorption bands corresponding to each band of the laser source.The absorption lines' wavelengths are used as wavelength reference and realize the accurate wavelength calibration of the C+L band laser source in different temperature conditions.When using the laser wavelength calibration results to locate the absorption wavelengths of C2H2,CO and CO₂,their mean square root errors of wavelength positioning are no more than 3.3 pm in the temperature range from 0? to 60?.