Dual-Gas Intra-Cavity QEPAS Sensor Based on Frequency Division Multiplexing Using Two Acoustic Microresonators

A dual-gas intra-cavity quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on frequency division multiplexing (FDM) of a quartz tuning fork (QTF) using two acoustic microresonators (AmRs) was demonstrated in this paper. The two AmRs were constructed into two fiber ring lasers with C-band and L-band erbium-doped fibers (EDFs) and two pump sources, eliminating the model competition of dual-wavelength and improving the stability of the wavelength. Two laser beams modulated with different frequencies f₁ and f₂ from two fiber ring lasers with different wavelengths are focused at two different positions between the QTF prongs to excite two photoacoustic signals, and two AmRs of on-beam structure achieved the enhancement of two photoacoustic signals respectively. The piezoelectric transducer (PZT) and acousto-optic modulator (AOM) are used to scan and modulate the gas absorption wavelength respectively, and different modulation frequencies f₁ and f₂ ensure that the QEPAS signals generated by different target gases will not be interfered with each other. Under 1 atm and 296 K atmosphere, the acetylene (C 2 H 2 ) and carbon dioxide (CO 2 ) selected as target gases were simultaneously detected at 1530.37 nm, 1572.66 nm to demonstrate the performance of the sensor. The experimental results achieved the minimum detection limits (MDLs) of 492.71 ppbv, 173.36 ppmv, and R² of 0.9986, 0.9992 by the linear fits of the signal with different gas concentrations for the C 2 H 2 and CO 2 , respectively. It shows that the dual-gas QEPAS sensor system proposed in the paper possessed high sensitive and real-time detection of various gases.