A multimode interference device and method of making the same comprises at least
one input port, at least one output port, a multimode interference region separating
the input port from the output port, and at least one subregion in the multimode
interference region, wherein a self-image length within the multimode interference
region is reduced by a factor that is approximately equal to one plus a number
of subregions configured in the multimode interference region, wherein the at least
one subregion is configured to have an effective width and effective refractive
index running longitudinally through the multimode interference region. Additionally,
the subregion may comprise at least one slot.