Cantilever enhanced fiber-optic photoacoustic microprobe for diffusion detection of sulfur dioxide

A cantilever enhanced fiber-optic photoacoustic microprobe with the cavity volume of only 119 μL is designed to detect SO2, which has the obvious advantages of passive sensing, anti-electromagnetic interference and low cost. A UV multi-mode fiber and a single-mode fiber are applied to transmit excitation light emitted by UV-LED and probe light, respectively. The lens assembly based on a collimating lens and a focusing lens is added to improve the light energy of UV-LED coupling into UV-fiber. Based on theoretical analysis and finite element method, a photoacoustic cavity with a radius of 1.5 mm and a length of 15 mm is designed, which is conducive to the realization of photoacoustic enhancement and rapid response at high frequencies. SO2 gas diffuses into the PA cavity through a hole with the diameter of 0.5 mm and a gap of the cantilever with the width of 0.5 µm. A fiber-optic cantilever based on F-P interference is adopted to detect sound pressure. The frequency characteristic and response time of the sensor in different backgrounds of SF6 and N2 are analyzed experimentally. Compared with N2, the photoacoustic microprobe operates in SF6 background with lower resonance frequency and longer response time, which is due to the high density and low thermal diffusivity of SF6 gas. With the flow rate of 250 SCCM in the external cavity, the response time of the photoacoustic microprobe to SO2/SF6 and SO2/N2 are 96 s and 40 s, respectively. The detection limits of SO2 in SF6 and N2 background are 35 ppb and 5 ppb, respectively.