Fiber-Optic Photoacoustic Gas Microprobe Based on Linear Spot-Type Multipass Cell

A linear spot-type multipass cell-enhanced fiber-optic photoacoustic gas microprobe is proposed. To further reduce the volume of the gas chamber and enhance the photoacoustic signal, we designed the cross section of the photoacoustic tube as a slit with a height of 10 mm and a width of 1.5 mm. The volume of the gas chamber is only 210 μL. Through theoretical analysis and multiphysical field simulation, the photoacoustic intensity of the slit cross section is more than 6 times that of traditional circular cross section. A cantilever beam is integrated into the photoacoustic microprobe and forms a Fabry-Perot (FP) interferometer with an optical fiber, achieving enhanced photoacoustic detection. By adjusting the angle of the collimator, the light passes through the photoacoustic cell 26 times, and 13 linear spots are formed on the reflecting surface. The photoacoustic signal with multiple linear reflections is 12.4 times that of a single reflection. The gas enters the sensor in a freely diffused manner, and the response time of the sensing system is 54 s. When the averaging time is 400 s, the detection limit of the designed sensor for acetylene gas reaches 1.5 ppb. The minimum detectable absorption coefficient (α_min) and the normalized noise equivalent absorption coefficient (NNEA) are 1.7 × 10^-8 cm^-1 and 1.9 × 10^-9 Wcm^-1/Hz^1/2, respectively.