Miniaturized differential Helmholtz resonators for photoacoustic trace gas detection

Photoacoustic cells are a type of optical sensors than can be used to detect trace gases. The photoacoustic spectroscopic technique is based on the absorption of photons by the molecules of interest and the subsequent creation of acoustic waves. It is extremely sensitive but the sensors bulkiness and cost prevent its mass deployment in several applications, such as greenhouse gases survey or indoor air monitoring. A trend towards miniaturization is thus engaged. In this paper, two centimeter-sized photoacoustic cells are presented. After an initial design stage by the finite element method, the cells have been fabricated and characterized. The experimental limit of detection reached with a centimeter-size cell made by direct metal laser sintering and an interband cascade laser delivering an optical power of 2.2 mW at 3.36 μm wavelength, is estimated at 92 ppbv methane in nitrogen.