Lossless tapers, Gaussian beams, free-space modes: Standing waves versus through-flowing waves

It was noticed in the past that, to avoid physical inconsistencies, some ba sic features of waves flowing through Marcatili's lossless tapers must be d ifferent from those of standing waves in the same structures. In this paper , we first present numerical results (based on an extended BPM algorithm) w hich reconfirm this statement. Next, we explain this surprising behavior as straightforward consequences of Maxwell's equations. Finally, we show that similar situations occur for Gaussian beams in a homogeneous medium, and f ree-space modes expressed in terms of Bessel functions.