  A novel approach to study transmission through waveguides in terms of optical
streamlines is presented. This theoretical framework combines the computational
performance of beam propagation methods with the possibility to monitor the
passage of light through the guiding medium by means of these sampler paths. In
this way, not only the optical flow along the waveguide can be followed in
detail, but also a fair estimate of the transmitted light (intensity) can be
accounted for by counting streamline arrivals with starting points
statistically distributed according to the input pulse. Furthermore, this
approach allows to elucidate the mechanism leading to energy losses, namely a
vortical dynamics, which can be advantageously exploited in optimal waveguide
design.
