Ultra-sensitive CH4 monitoring using QEPAS and a 3.3-μm interband cascade laser

In this study, a highly sensitive CH₄ detection system based on clamp-type Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) is proposed and demonstrated, aiming to address the critical need for reliable monitoring of dissolved gases in transformer oil. The system employs a novel clamp-type quartz tuning fork with a high quality factor, combined with a coaxial acoustic micro-resonator (AmR) and a 3.3 μm interband cascade laser (ICL) that targets the strongest absorption band of CH₄ in the mid-infrared region. Compared to near-infrared wavelengths, the mid-infrared band provides a significantly higher absorption cross-section, thereby enhancing detection sensitivity by two to three orders of magnitude. The experimental setup was carefully optimized to ensure effective overlap between the laser emission and the key CH₄ absorption features. Spectroscopic measurements revealed two prominent photoacoustic signal peaks corresponding to closely spaced CH₄ absorption lines. The system demonstrated excellent linearity between the signal amplitude and gas concentration, confirming its reliability for quantitative analysis. At a CH₄ concentration of 100 ppm, a maximum signal amplitude of 9.93×10^-4 V was recorded at 3368.68 nm. By introducing high-purity nitrogen (N₂) to evaluate the noise baseline, the background signal was measured to be 5.05×10^-6 V, yielding a signal-to-noise ratio (SNR) of 196.8 and a minimum detection limit (MDL) of 508.1 ppb. These results highlight the superior performance of the proposed QEPAS sensor in terms of sensitivity, selectivity, and real-time response. The use of a cost-effective clamp-type quartz tuning fork design significantly lowers system cost while preserving high signal fidelity, thereby laying a solid technological and engineering foundation for applying photoacoustic spectroscopy to one of its most critical industrial use cases—dissolved gas detection in transformer oil within the power industry.