Cavity-Enhanced Beat Frequency Light-Induced Thermoelastic Spectroscopy Using Differential-Frequency Demodulation

The calibration of the resonant frequency and <i>Q</i> factor of a quartz tuning fork (QTF) in light-induced thermoelastic spectroscopy (LITES) is essential, but time-consuming and disruptive to ongoing measurements. We report on a beat-frequency (BF) method based on differential-frequency demodulation to achieve precise and real-time calibration of QTF resonant frequency and <i>Q</i> factor while maintaining the full excitation of the QTF in LITES. This distinctive property leads to a double improvement in response amplitude compared to the well-optimized LITES operated with the widely used differential-frequency modulation. The tight locking of a near-infrared laser to a high finesse (∼12000) optical cavity further enhances the laser-gas interaction, and then the system's sensitivity. By targeting the R(4) transition of hydrogen sulfide (H₂S), we achieved a minimum detection limit of 157 parts per billion in concentration and a normalized noise equivalent absorption coefficient of 1.25 × 10^-12 cm^-1·W·Hz^-1/2. Comparison with state-of-the-art LITES sensors confirms the superior performance achieved in this work.