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.