Fiber Ring Laser Intracavity QEPAS Gas Detection Based on Improved Heterodyne Demodulation of Lock-In Amplifiers

In this study, an improved heterodyne demodulation quartz-enhanced photoacoustic spectroscopy (IHD-QEPAS) gas sensor is demonstrated, employing an erbium-doped fiber (EDF)-ring laser for the real-time and uninterrupted measurement of acetylene (C2H2) concent- ration at ppb level. The improved heterodyne demodulation method breaks through the original modulation frequency requirement that it must be equal to the demodulation and not equal to the resonance. It is found that the output signal peak value is influenced by the modulation frequency, with an increase observed as it approaches the resonance frequency of the quartz tuning fork (QTF). The attenuation frequency of the output signal is affected by the demodulation frequency, decreasing as it approaches the resonance frequency of the QTF. The concentrations of C2H2 exhibit a strong linear relationship with the photoacoustic signal (with R-squared values of 0.99947), and the minimum detection limit (MDL) is 305.7 ppb at 1531.59 nm, demonstrating the accuracy and robustness of the intracavity gas sensor.