A COHERENT RECEIVER OPTICAL SUB-ASSEMBLY

A coherent receiver optical sub-assembly C-ROSA of the invention is disposed as a photonic integrated circuit PIC upon a substrate which comprises a single multimode interference MMI coupler comprising a main body, input access waveguides and output access waveguides configured to operate as a 90-degree optical hybrid, a plurality of photodetectors configured as balanced photodetectors and integrated optical waveguides. In the C-ROSA PIC the MMI coupler is disposed upon the PIC substrate in a "flip" arrangement, that is, with its output access waveguides facing the plurality of photodetectors. The "flip" arrangement enables the direct interconnection of the MMI coupler output access waveguides to the plurality of photodetectors via non-intersecting integrated optical waveguides requiring no further optical coupler component other than the MMI coupler.