Combining optical ray tracing and acoustical finite elements for the simulation and optimization of a photoacoustic cell

Photoacoustical trace gas analysis exploits the photoacoustic phenomenon for the measurement of the concentration of different gas species. Resonators are applied in the detection for amplifying the acoustical signal and thus enhancing the signal to noise ratio. The present contribution focuses on the simulation of a photoacoustic cell by means of the acoustical finite element method and optical ray tracing for modeling the excitation of the cell by the incoming light. The combination of these two techniques allows for the computation of the spatially distributed source term and the acoustical response of the cell. The model also includes viscothermal losses at the walls of the resonator. The paper introduces the approach for combining the optical ray tracing with the finite element model. Beam patterns of an optical system consisting of a high power UV LED and lenses are simulated by means of geometrical ray tracing. A photoacoustic cell is examined using both 2D axisymmetric and 3D finite element models with the aim of optimizing the geometry to attain a high photoacoustic signal while keeping the background signal at a moderate level. Finally, the results of both optical and acoustical simulations are compared to measurements.