Fast response and real-time simultaneous dual-component LITES sensor based on mode division multiplexing of IPSFFM and OPSFFM with a single QTF

A fast-response and real-time simultaneous dual-component light-induced thermoelastic spectroscopy (LITES) sensor based on mode division multiplexing (MDM) of a single quartz tuning fork (QTF) was proposed for the first time, to the best of our knowledge, in this paper. The out-plane symmetric fundamental flexural mode (OPSFFM) and in-plane symmetric fundamental flexural mode (IPSFFM) of a single QTF were excited and demodulated simultaneously to achieve dual-component detection. Acetylene (C₂H₂) and methane (CH₄) were selected as target gases to validate the sensor's performance. Compared with the dual-component LITES sensors based on traditional time division multiplexing (TDM) and frequency division multiplexing (FDM) techniques, this proposed method based on MDM offers a compact structure and real-time simultaneous measurement. Minimum detection limits (MDLs) of 1.82 ppm and 37.72 ppm were obtained for C₂H₂ and CH₄ detection, respectively. The concentration response, long-term stability, and repeatability of this sensor were demonstrated to be excellent.