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

Development of a methane (CH(4)) 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 cm(3) and a weight of 9.7 g was fabricated. A continuous-wave distributed feedback diode laser was used to target the CH(4) absorption line at 6046.95 cm(−1). With the combination of wavelength modulation spectroscopy (WMS) and second harmonic (2f) detection technique, the CH(4) 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 CH(4) 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 CH(4) sensors.