Ppb-level all-optical off-axis QEPAS gas sensor based on dual-wavelength demodulation of out-of-plane vibration mode

In Quartz Enhanced Photoacoustic Spectroscopy (QEPAS), due to the strained material and piezoelectric effect of the Quartz Tuning Fork (QTF), the in-plane mode of QTF is generally detected, requiring micron-scale precise and stable optical alignment and convergence. It poses a significant challenge for the use of light sources and has become a bottleneck for practical implementations of QEPAS. In this paper, a ppb-level all-optical off-axis QEPAS gas sensor based on dual-wavelength demodulation of out-of-plane mode is proposed and experimentally demonstrated. A dual-wavelength Fabry-Perot (FP) Interferometer with Ellipse Fitting-Differential Cross Multiply (EF-DCM) algorithm is designed and employed to demodulate the out-of-plane mode of off-axis QTF, which is approximately 80 times stronger than the conventional in-plane mode. This work established a novel ultra-sensitive off-axis QEPAS-based hydrogen sulfide (H2S) sensor with a 1.5 W fiber-amplified near-infrared diode laser. The minimum detection limit and normalized noise equivalent absorption of the developed H2S sensor is 74.19 ppb with 1 s integration time and 2.68 × 10−9 W∙cm−1∙Hz−1/2 respectively, beyond previously reported QEPAS-based H2S sensors. This technique effectively utilizes the out-of-plane mode of QTF and realizes the simultaneous enhancement of off-axis location and out-of-plane vibration mode detection in QEPAS, allowing a novel approach for the development of ultra-sensitive QEPAS gas sensors with notable practical application prospects. •An out-of-plane vibration mode based QTF was used for off-axis Quartz Enhanced Photoacoustic Spectroscopy (QEPAS).•The dual-wavelength FPI demodulation and the EF-DCM algorithm was proposed to detect the QTF surface vibration.•The developed QEPAS system has reduced the requirement of light source, which can match imperfect beam.•The performance of the system was evaluated using a 1.5 W fiber amplified near infrared diode laser.•The one order of magnitude sensitivity improvement was realized compared with traditional off-axis QEPAS.