New scaling rules for MMI devices

Multimode Interference (MMI) devices have found wide use in photonic integrated circuits as couplers, multiplexers, switches and filters, as well as performing other useful signal processing functions. In the past, first order design formulae for such devices have relied on using simplifying assumptions that are not very accurate. The designer typically then uses repeated field simulations, varying the geometrical and material parameters in order to improve the predicted performance. The results are usually limited to a single specific device and therefore cannot easily be generalised. The purpose of this paper is to look at MMI structures in a more general way and so derive a minimal parameter set that still adequately describes the behaviour of the device. This is achieved without making over-simplified approximations. Using this new scaling approach 1) identifies the most important parameters; 2) provides a better starting point for the design process; 3) enables the designer to gain deeper insight into the effect on performance of parameter variations; and 4) leads to new methods for optimising the design of MMI devices.