All-Optical Anti-Interference Light-Induced Thermoelastic Spectroscopy Gas Sensor Based on Normalized Harmonic Demodulation

An all-optical anti-interference light-induced thermoelastic spectroscopy (LITES) gas sensor is proposed for the first time. In this all-optical LITES gas sensor, the Fabry-Perot (F-P) sensing technology was applied to demodulate the vibration of quartz tuning fork (QTF), replacing the measurement of piezoelectric signal in traditional LITES sensors. To overcome the F-P detection instability caused by the environmental interference, the wavelength modulation spectroscopy with the second harmonic normalized by the first harmonic (WMS-2f /1f ) demodulation method was adopted for F-P sensing. Acetylene (C₂H₂) was chosen as the target gas to verify the performance of the proposed sensor. Theoretical analyses and experimental validations demonstrated that this novel F-P-based WMS- 2f/1f demodulation method is immune to the disturbance of excitation laser, probe laser, and F-P cavity length. Moreover, it exhibits excellent long-term stability with an improvement of over 60 times compared to traditional 2f demodulation. This all-optical anti-interference LITES gas sensor is expected to play an important role in remote and non-contact gas sensing.