Vibrating cavity off-axis integrated cavity output spectroscopy combined with new uniform phase empirical mode decomposition for CH4 measurement system

Abstract

In order to realize CH₄ measurement, a vibrating cavity off-axis integrated cavity output spectroscopy (VC-OA-ICOS) CH₄ sensing system was developed in this paper. The inclusion of a vibration module in the system can effectively suppress cavity mode noise. To further eliminate characteristic cavity mode noise and improve system stability, we proposed a new uniform phase empirical mode decomposition (NUPEMD) method. This method enables the selection of the appropriate number of phases and the amplitude of the masking signal, thereby reducing the impacts of mode mixing and residual noise. The self-contained fitting and mutual correlation signal reconstruction technique employed in NUPEMD is particularly effective in eliminating medium- and high-frequency noise. Analysis of the Fourier frequency domain plots of the system noise revealed that the combination of vibration with the NUPEMD method significantly suppresses medium- and high-frequency residual cavity mode noise. Experimental results demonstrated that utilization of this method enabled the VC-OA-ICOS system to achieve a minimum detection limit (MDL) of 0.381 ppmv within 2 s, resulting in a 5.62-fold improvement in measurement accuracy compared to traditional methods. Furthermore, continuous measurements using this enhanced VC-OA-ICOS system on standard CH₄ samples over a period of 12 h yielded a long-term detection error of no more than 0.47 %, representing a 1.98-fold improvement in stability. The effectiveness and superiority of both the proposed system and the methodology for CH₄ measurement are thoroughly validated through these findings.