High performance multimode interference couplers for coherent communications in silicon

We review the need for coherent optical communication systems and briefly describe the receiver frontend they require. The key element of the receiver is an optical 90° hybrid, which combines the received signal with the local oscillator laser with the adequate amplitude and phase relations. Silicon technology offers several advantages for the realization of such devices: compactness and CMOS compatibility, among others. However, the realization of high-performance hybrids is hampered by the high index contrast of this technology. We show that using multimode interference couplers with fully etched access waveguides and a shallowly etched multimode region, ultra-compact, high performance 90° hybrids can be realized. We experimentally demonstrate a device with a phase error below 5° and a common mode rejection ratio better than -20dBe in a ~50nm bandwidth.