Sensitivity Analysis of Greenhouse Gas Detection System Based on Array Fabry-Pérot Interferometer
Objective In response to the urgent demand for high-precision global greenhouse gas(GHG)emissions monitoring,essential for carbon inventories and enforcement,achieving low-cost,high-resolution detection has become a key research focus.The array Fabry-Pérot(F-P)spectrometer,with its compact structure,lack of moving parts,and ability to account for both the sampling density and range of optical path differences,presents an effective solution for achieving accurate and cost-efficient GHG detection.The parameters of the array F-P interferometer are critical to the system's optical performance and directly affect detection accuracy.To establish optimal detection parameters,we explore the effects of variables such as F-P interval thickness,interferometric cavity reflectivity,F-P quantity,and adjacent F-P optical path difference sampling interval on system sensitivity.By analyzing the variation in integral sensitivity with changes in GHG volume fraction,we determine the optimal parameters for spectrometer design,providing a theoretical foundation for further research on array F-P spectrometers for GHG detection.Methods Using the upwelling radiance spectra of GHGs at varying concentrations as input,we propose a simulation model for raw interferometric data from the array F-P spectrometer.The influence of spectrometer parameters on system detection sensitivity is analyzed using this model.To maximize integral sensitivity,the analysis focuses on how varying the thickness of the F-P intervals affects integral sensitivity and determines the optimal thickness of the F-P plates.To achieve maximum normalized sensitivity for the detection system,the relationship between signal-to-noise ratio(SNR),spectral resolution,detection sensitivity,and interferometric cavity reflectivity is analyzed,confirming the optimal reflectivity value.In addition,the effect of the number of F-P cavities and the adjacent F-P optical path difference sampling interval on integral sensitivity is evaluated.Results and Discussions This analysis quantitatively evaluates how integral sensitivity varies with F-P interval thickness,cavity reflectivity,F-P numbers,and the sampling interval of the adjacent F-P optical path difference.Specific parameters are confirmed for both carbon dioxide and methane detection systems.To thoroughly assess the influence of interferometric cavity reflectivity on SNR and detection sensitivity,the normalized sensitivity for various reflectivities is simulated(Fig.12).For both the carbon dioxide and methane systems,normalized sensitivity exceeds 0.98 at reflectivities between 0.35 to 0.49 and 0.39 to 0.50,respectively,with optimal values observed around 0.42 and 0.47.The influence of F-P numbers on integral sensitivity is shown(Fig.14).As the number of cavities increases,the sampling range of the optical path difference increases linearly,leading to a corresponding increase in integral sensitivity.The influence of the adjacent F-P optical path difference sampling interval on both the sampling range and integral sensitivity is simulated(Fig.17).As the adjacent F-P optical path difference sampling interval decreases,the overall optical range decreases;however,both the sampling density of the interferometric signal and the integral sensitivity increase.When the adjacent F-P optical path difference sampling interval is reduced to λ/4 or less,further reductions have minimal effect on integral sensitivity.Conclusions In this paper,we introduce the fundamental principles of the array F-P spectrometer and its application in GHG detection.By analyzing the magnitude of the Fourier expansion term coefficients in relation to variations in the reflectivity of the interfering cavity,we confirm that the reflectivity of the F-P flat plate approximation for double-beam interferometry falls within the range of 0.3 to 0.7.A raw data simulation model for the array F-P interferometer is developed using the upwelling radiance spectra of greenhouse gases with varying concentrations as a system input.Based on this model,we conduct a simulation analysis to assess the effects of F-P spectrometer parameters on detection sensitivity,defining the guiding principle for parameter selection and determining their optimal values.The simulation results indicate that the interferometric cavity reflectivity for the carbon dioxide and methane systems are 0.42 and 0.47,respectively,at which point the system's normalized sensitivity reaches its maximum.The integral sensitivity of the detection system is positively correlated with the number of F-P cavities.When the adjacent F-P optical path difference sampling interval is set to a quarter-wavelength,the system achieves high integral sensitivity and a broad optical path difference.
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