Disturbance-immune, fast response LITES gas sensor based on out-plane vibration mode employing micro Fabry-Perot cavity with heterodyne phase demodulation

The disturbance-immune, fast response light-induced thermoelastic spectroscopy (LITES) gas sensor based on out-plane vibration (OPV) mode employing micro Fabry-Perot cavity with heterodyne phase demodulation was firstly reported in this paper. Compared to the traditional in-plane vibration (IPV) mode, the OPV mode has a lower resonance frequency (f0) to enhance the energy accumulation time of the system. The OPV mode based H-LITES (OPV-H-LITES) sensor and the IPV mode based H-LITES (IPV-H-LITES) sensor were established using a standard quartz tuning fork (QTF) at the same conditions for performance comparison. Acetylene (C2H2) was selected as the target gas to verify the sensor performance. In comparison to the IPV-H-LITES sensor, there has been a 3.5-fold improvement in signal-to-noise ratio (SNR) of OPV-H-LITES sensor. In order to further improve the sensor performance, a novel self-designed low-frequency QTF was adopted to replace the standard QTF, leading to a 2.3-fold increase in the SNR of the OPV-H-LITES sensor. Finally, a minimum detection limit (MDL) of 7.22 ppm for C2H2 detection was obtained. The proposed technique holds the merits of high sensitivity, power disturbance resistance, wavelength independence, excellent linear concentration response and long-term stability.