Multi-gas photoacoustic sensor using multi-mode demodulation

Modulation technology is the necessary means for generating periodic acoustic waves in photoacoustic gas detection, primarily including intensity modulation and wavelength modulation. In multi-gas detection, when multiple lasers employ the same modulation technique, current technologies include time-division multiplexing (TDM) for measurements at different times and frequency-division multiplexing (FDM) for simultaneous measurements; when multiple lasers employ different modulation techniques, the only available technology is TDM with measurements conducted at different times, and whether simultaneous measurement can be achieved has not yet been verified. We propose, for the first time, a multi-gas photoacoustic sensor using multi-mode demodulation. This sensor employs multi-mode frequency division multiplexing (MMFDM) technology to separate and demodulate the multi-mode photoacoustic signal, thereby enabling the simultaneous measurement of multiple gases under different modulation techniques. To demonstrate the feasibility of this method, we used SO${}_2$ and HF, the SF${}_6$ decomposition products in gas-insulated switchgear (GIS), as target gases and simultaneously detected their mixture using different modulation modes. Experimental results show that when the frequency difference is 10 Hz, multi-mode photoacoustic signal can be successfully separated, with the minimum detection limits for SO${}_2$ and HF reaching 117.9 ppb and 65.5 ppb, respectively. This study is the first to validate the separability of multi-mode photoacoustic signal and achieve multi-gas simultaneous measurement under multi-mode modulation, thereby eliminating the limitations of modulation mode in simultaneous photoacoustic multi-gas detection.