Numerical analysis of coupling efficiency limits in single-photon sources to fiber systems via idealized lens models

We present an analysis on the limitations of free-space micro-optical coupling between single photon sources and standard single mode fibers. By combining dedicated FDTD simulations with idealized lens models, we evaluate how different far-field emission profiles impact achievable coupling efficiencies. Our results highlight that broader angular distributions impose tighter alignment requirements, with coupling differences of up to 25% for the same magnitude of spatial offset purely as a consequence of Étendue conservation. The presented framework enables tolerance-aware coupling analysis without requiring the design of any specific optical interface.