An Optimized Optical System for Trace Detection of Characteristic Components in Electrical Insulation Gases

Cavity ring-down spectroscopy (CRDS) has been widely used in trace gas detection due to the ultrahigh detection sensitivity. Optical detector performance and data processing accuracy contribute to the sensitivity of CRDS. An optimized transimpedance amplifier circuit for the photodetector is proposed. A carefully designed Sallen-Key low-pass filter is added to improve the signal-to-noise ratio performance substantially. An online data sampling and nonlinear fitting structure based on FPGA is proposed to calculate ring-down time and invert concentration values in real time. The corresponding printed circuit boards are made, and the gas detection platform is set up in the laboratory to verify the noise level. The noise-equivalent absorption coefficient of the system is measured as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$9.22 \times 10^{-12} ~\text{cm}^{-1} \cdot ~\text{Hz}^{-1 / 2}$</tex> by Allan variance analysis. The detection sensitivity of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$7.05 \times 10^{-10} ~\text{cm}^{-1}$</tex> for trace gas detection can be achieved in an integration time of 50 s.