The wavelength-independent, non-reciprocal splitting properties of the asymmetric, multimode Y-junction make it an ideal device for use as a low-loss, low-power tap in a concatenated linear bus. The properties of a two-dimensional slab model of the junction are investigated and the results of ray-tracing, modal and numerical beam propagation method (BPM) techniques are compared. Minimization of splicing loss between the ports of a square-core, three-dimensional buried channel waveguide version of the device and a multimode fibre is also addressed.