Beam folding analysis and optimization of mask-enhanced toroidal multipass cells.

We present computational and experimental investigations of the beam folding properties and fringe suppression capabilities in monolithic toroidal multipass cells (MPCs) when combined with absorption masks. Coherent field simulations based on the Fresnel-Huygens theory were performed to understand the effect of multiple field truncations in such an optically semi-unstable mirror arrangement. The explicit numerical calculation of the radiation field at each reflection allows detailed optimization and performance analysis. We experimentally verified the evolving irradiance distributions and identified optimal initial field configurations. Furthermore, we suggest a proxy to estimate the noise level for specific initial conditions. These insights pave the way to a better optical performance and, thus, to even more lightweight and compact designs of this MPC type.