The minimisation of phase errors in MMI devices

Multimode interference (MMI) devices have been widely used as passive couplers in fibre communications systems. MMI devices rely on approximating a parabolic spacing between the propagation constants of the modes supported by the multimode structure. A parabolic mode spectrum ideally enables the formation of direct, mirrored or multiple self-images at appropriate imaging planes. In practice, planar MMI structures cannot achieve the ideal parabolic mode spectrum, so it is important for the designer to ascertain the deviation from this ideal. The new design formulae presented in this paper will lead to simple approaches for minimising the phase errors of planar step-index MMI structures having low index contrast.