High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler

We propose a multimode interference coupler (MMI) design for high-index-contrast technologies based on a shallowly etched multimode region, which is, for the first time to our knowledge, directly coupled to deeply etched input and output waveguides. This reduces the phase errors associated with the high-index contrast, while still allowing for a very compact layout. Using this structure, we fabricate a $2\times 4$ MMI operating as a $90°$ hybrid, with a footprint of only $0.65\mathrm{mm}\times 0.53\mathrm{mm}$, including all the structures necessary to couple light to a fiber array. We experimentally demonstrate a common mode rejection ratio better than $-20\mathrm{dBe}$ and phase errors better than $\pm 5°$ in a $\sim 50\mathrm{nm}$ bandwidth.