Nonlinear Fitting of Ring-down Time for the Fabry-Perot Cavity in FPGA Devices

H 2 S is an important characteristic decomposition component of SF6 insulated gas, and the evaluation of its concentration is significant for the fault detection of SF6 insulated electrical equipment. Experimental data were obtained by detecting the absorption line of H2S at 1578.1nm by Cavity Ring-Down Spectroscopy (CRDS). Based on the high degree of freedom programming and parallel running characteristics of FPGA, ring-down time constant of Fabry-Perot cavity resonant signal was calculated accurately with exponential function fitting by Levenberg-Marquardt algorithm. Comparison experiment was conducted between FPGA and Software under different gas components, and result shows that the coefficient of determination R-Square =0.9802 within 40 iterations, and the experiment meets the requirements of ppm scale concentration measurement. This research achieves fast and accurate calculation of ring-down time, which is suitable for the online detection of H2S in integrated SF6 insulated equipment.