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.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.