Resonant Frequency Tracking of a Cantilever-Enhanced Fiber-Optic Photoacoustic Gas Sensor Based on Solid Absorption

The cantilever-enhanced fiber-optic photoacoustic (PA) sensor (FOPAS) has the advantages of intrinsic safety, small size, and high sensitivity. However, the resonance frequency of the FOPAS is easily affected by temperature, which leads to gas concentration measurement errors. Herein, the cantilever-enhanced FOPAS with resonance frequency tracking is presented to improve the stability of gas detection. A gold-plated silicon wafer reflects the excitation light to double-enhance the second harmonic (2f) PA signal resulting from gas absorption. The excited light is incident on the PA cell wall, and the solid absorption generates the first harmonic (1f) PA signal. Both the 1f and 2f PA signals were detected by a silicon cantilever fiber-optic spectrophone. The 1f PA signal obtained through frequency scanning is used for real-time tracking the resonance frequency of the cantilever. Moreover, the ambient temperature can also be obtained by the tracked resonance frequency. The experiment showed that the minimum detection limit of C₂H₂ reached 27 ppb. The resonant frequency track can be obtained through a fast scan in just 5 s. Moreover, the standard deviation of the multiple tracking results is 1.9 Hz. When the temperature increases from 30 to 70 °C, the gas concentration detection error can be reduced from 28 to 4% by resonant frequency tracking and temperature compensation.