All-optical QEPAS trace gas senor employing an optical amplification technique

Quartz-Enhanced Photoacoustic Spectroscopy (QEPA) technology has become a kind of trace gas detection technology due to its advantages of high sensitivity, strong immunity to noise and compact size. An all-optical QEPA trace gas detection system was designed. A quartz tuning fork with a resonant frequency of 32.7 kHz was employed as the acoustic sensor, and the system signal amplitude was greatly enhanced by employing an optical power amplification technology. A fiber-optic Fabry-Perot interferometer with self-stabilizing characteristics was used as the vibration demodulation unit, significantly improving system stability. Acetylene (C ₂H ₂) was selected as the target gas for detection. A tunable semiconductor laser with a output wavelength of 1.5 μm was used as the excitation light source. By combining wavelength modulation and second harmonic demodulation techniques, the optimal modulation depth was determined to be 0.171 cm ^-1, and the optimal distance between the laser position and the bottom of the tuning fork was found to be 0.8 mm. The QEPAS signal exhibited linear variation with C ₂H ₂concentration, with a linearity of 0.998. The system detection limit was 30.3 ppb, corresponding to a normalized equivalent noise coefficient of 2.51×10 ^-8cm ^-1·W·Hz ^-1/2. Finally, under the same conditions, a continuous working test for one hour was conducted, verifying the excellent stability of the system.