Electrical excitation beat-aided light-induced thermoelastic spectroscopy for high-robustness trace gas measurements

Heterodyne-based light-induced thermoelastic spectroscopy (HLITES) is capable of correcting measurement errors by evaluating the parameters of the quartz tuning fork (QTF). However, the correcting performance of HLITES will deteriorate under low concentration levels or laser power due to the weakened QTF transient response. Therefore, we propose an electrical excitation beat-aided LITES (EEBA-LITES), which is realized by optical and electrical excitation to the QTF utilizing the time-division multiplexing technique. Gas concentration and QTF parameters can be measured quasi-simultaneously. By normalizing the first harmonic (1f) signal with the background signal, a normalized 1f-signal was obtained to correct measurement errors resulting from power fluctuation and focus position change. The obtained nonlinear response of the normalized 1f-signal was used to correct the measurement error resulting from the resonant frequency shift. Unaffected by both gas concentration and laser power change, EEBA-LITES achieves the highest precision in beat-frequency-based HLITES and quartz-enhanced photoacoustic spectroscopy (QEPAS) techniques, with 1σ detection limits of ∼ 0.016 Hz for resonant frequency and ∼ 63 for quality factor for QTF, respectively. With the same hardware configuration, as the average laser power drops from 7.7 mW to 0.9 mW, EEBA-LITES enhances the detection limit of gas concentration and QTF resonant frequency by ∼ 3-7.8 times and ∼ 2.3-16.5 times, respectively, compared to previously reported self-correlated HLITES (SC-HLITES). Due to good resistance to interference factors, the EEBA-LITES demonstrates good potential for long-term, high-robustness field gas detection scenarios.