  We demonstrate for the first time flat-top interleavers based on cascaded
Mach-Zehnder interferometers (MZIs) which use only single multimode
interferometers (MMIs) as power splitters. Our previous designs were based on
4-stage cascades of MZIs, where we used single MMIs and double MMIs to achieve
85:15 splitting ratio and 31:69 splitting ratio respectively. This time, we
propose instead a greatly simplified 2-stage configuration using only single
MMIs, including a standard 50:50 MMI, and two tapered MMIs to achieve 71:29 and
92:08 splitting ratios. We have designed the interleaver based on its
geometrical representation on the Bloch sphere, then confirmed by efficient 2D
simulations of the building blocks and of the whole structure, based on the
eigenmode expansion method. We show how important is to take into account the
phase relations between the outputs of all MMIs in order to make a working
design. We have successfully fabricated devices with different channel spacing
on our micron-scale silicon photonics platform, and measurement results
confirmed their expected flat-top operation on a broad band. Using only single
MMI splitters we can not only greatly outperform the bandwidth achieved by
standard directional couplers, but we can also ensure much higher robustness to
fabrication errors, also compared to previous demonstrations based on double
MMIs. Indeed, when compared to those previous attempts, the new results prove
tapered MMIs to be the most robust approach to achieve arbitrary splitting
ratios.
