Trace Gas Detection System Based on Photoacoustic and Photothermal Spectroscopy Using Ring Fiber Laser and Quartz Tuning Fork

In this article, a dual-spectroscopy gas sensing system based on a ring cavity laser and quartz tuning fork (QTF) is designed. A 15 m erbium-doped fiber (EDF) with 1500 ppm doping concentration is used as a gain medium, and a piezoelectric transducer (PZT) and acousto-optic modulator (AOM) are used as detection methods. The first fiber collimator (FC) emits a laser beam; the laser is received by the second FC after passing through an off-beam acoustic micro-resonator (AmR) for quartz-enhanced photoacoustic spectroscopy (QEPAS) signal excitation and enhancement. The received laser is transmitted through the optical fiber and then incident on the QTF prongs by the third FC, which excites and quartz-enhanced photothermal spectroscopy (QEPTS) signal. Combined with the ring intracavity fiber laser and wavelength scanning and intensity modulation technology, dual-spectroscopy gas detection is realized. Compared with the single detection of QEPAS or QEPTS signals, the detection limit is enhanced. Under the conditions of 400 mW pump power and 2500 ppmv acetylene (C2H2) gas concentration, the signal-to-noise ratio (SNR) of QEPAS, QEPTS, and dual-spectroscopy signals are 838.07, 1117.47, and 2096.26, respectively, and the minimum detection limits (MDLs) of 2.98, 2.24, 1.19 ppmv are achieved, respectively.