Calibration-free quartz tuning fork enhanced laser spectroscopy for trace gas detection

In this paper, a calibration-free quartz tuning fork enhanced laser spectroscopy (QTFELS) gas sensing system is presented for trace gas detection. To demonstrate the proposed gas detection technique, a near-infrared distributed feedback (DFB) semiconductor laser operating at 1653 nm was combined with a 30-meter multi-pass cell was specifically employed for CH₄ gas measurements using the first harmonic-normalized second harmonic signal of wavelength modulation spectroscopy (WMS-2f/1 f). A standard spectral model and least-squares-based calibration-free algorithm were proposed for real-time inversion of gas concentrations. This algorithm is designed to compensate for signal amplitude fluctuations in QTF detectors induced by ambient light intensity variations. Experimental results demonstrate that the 2 f/1 f signal ratio exhibits an 8.31-fold enhancement in response sensitivity for concentration retrieval compared to the standalone 2 f signal. The CH₄ detection system achieves a maximum error below 3.3%, with its precision reaching 20 ppb at an optimal Allan deviation integration time of 308 s. Finally, the proposed gas sensing system was employed for approximately 24-hour long-term measurements of ambient CH₄ concentrations outside the laboratory, demonstrating the sensor's exceptional monitoring performance.