Quartz-enhanced photoacoustic spectroscopic methane sensor system using a quartz tuning fork-embedded, double-pass and off-beam configuration

Development of a methane (CH4) sensor system was reported based on a novel quartz-tuning-fork (QTF)-embedded, double-pass, off-beam quartz-enhanced photoacoustic spectroscopy (DP-OB-QEPAS). A simplified and accurate numerical model was presented to optimize the DP-OB-QEPAS spectrophone and to enhance the detection sensitivity. A compact and fiber-coupled acoustic detection module (ADM) with a volume of 3 × 2×1 cm3 and a weight of 9.7 g was fabricated. A continuous-wave distributed feedback diode laser was used to target the CH4 absorption line at 6046.95 cm−1. With the combination of wavelength modulation spectroscopy (WMS) and second harmonic (2f) detection technique, the CH4 sensor system reveals a 1σ detection limit of 8.62 parts-per-million in volume (ppmv) for a 0.3 s averaging time with an optimized modulation depth of 0.26 cm−1. The proposed CH4 sensor shows a similar or even lower level in the normalized noise equivalent absorption coefficient (NNEA) (1.8 × 10−8 cm−1∙W/√Hz), compared to previously reported QEPAS-based CH4 sensors.