Improved self-imaging characteristics in 1×N multimode couplers

A 1×32 tapered coupler has been designed based on the self-imaging theory. Fields inside the taper can be calculated using the local modes theory. It can be clearly seen that when varying the taper angle the modal interference spectrum can be controlled. An optimum spectrum allows us to optimise the ratio excess loss against dimensions. This is the way to solve one of the major problems of multimode interference (MMI) devices. The splitter exhibits excess loss of 0.40 dB and a uniformity of 0.56 dB at a wavelength of 1.55 μm. The bandwidth is 45 nm long when the limit conditions are 1.4 dB of loss and 2.0 dB of uniformity. Fabrication tolerances in the rectangular multimode area, which is the most sensitive one, are ±3 μm for the width and ±115 μm for the length. The device is almost completely insensitive to changes in polarisation, and reflected power is negligible. The results achieved show a remarkable improvement with respect to those of classical multimode interference power splitters