Cantilever-Enhanced Fiber-Optic Photoacoustic Spectrophone for Low-Pressure Gas Detection

A cantilever-enhanced fiber-optic photoacoustic (PA) spectrophone is reported for trace gas detection at a low-pressure environment. A cantilever-based fiber-optic Fabry-Perot (F-P) interferometer (FPI) is utilized for simultaneous measurement of air pressure and PA pressure. Since the cantilever resonance frequency follows air pressure linearly, the fundamental frequency intensity modulation (1f-IM) technique is applied to scan the frequency response of the solid PA signal from tube wall absorption for tracking the cantilever resonance frequency in real time. The second-harmonic wavelength modulation spectroscopy (2f-WMS) technique is used to measure the gas PA pressure wave at the cantilever resonance. According to the inverse restriction relationship of air pressure on the PA excitation and cantilever detection, the measured gas PA signal at the low-pressure environment is enhanced. The target gas concentration is corrected by the measured air pressure, which makes the spectrophone generally applicable under any pressure. The experimental results indicate that the normalized noise equivalent absorption (NNEA) coefficients of the spectrophone in the standard atmospheric pressure and the low-pressure environment of 60 kPa are 2.2 x 10-9 and 2.0 x 10-9 cm-1<middle dot>W<middle dot>Hz-1/2, respectively. 0.1 ppm acetylene (C2H2) can be detected at any air pressure. The detected maximum relative error of 10 ppm C2H2 gas under different pressures is less than +/- 9% and the error is reduced to less than +/- 2% when the concentration rises to 70 ppm. In the pressure range of 60-100 kPa, the cantilever-enhanced fiber-optic PA spectrophone has extremely high accuracy and pressure stability, covering the pressure range of most ground gas detection scenes.