Integrated coherent receiver having a single MMI coupler optical hybrid, non-intersecting optical paths and uniform electrical paths

A coherent receiver optical sub-assembly photonic integrated circuit (PIC) comprising:

• a substrate for photonic integration upon which the following elements are disposed:
• two input ports for respectively delivering a phase modulated optical signal(S) and a continuous wave local oscillator (LO) optical signal, the S and LO optical signals providing mixing products;
• four electrical output ports for delivering electrical signals corresponding to the mixing products of S and LO optical signals;
• four photodetectors forming two pairs of balanced photodetectors arranged in the vicinity of the electrical output ports, the four photodetectors being arranged along an arrangement axis on the substrate parallel to an output facet of the PIC and connected to the electrical output ports via uniform electrical lines;
• a single multi-mode interference (MMI) coupler configured to operate as a 90-degree optical hybrid, the MMI couple having a main body, a plurality of input access waveguides facing the photodetectors, a plurality of output access waveguides, and an optical axis perpendicular to the arrangement axis of the photodetectors, two of the plurality of input access waveguides being configured to respectively receive the S and LO optical signals from the two input ports and guide light to the main body of the MMI coupler main body, and the plurality of output access waveguides being configured to collect multi-mode interference signal products from the main body of the MMI coupler and deliver the mixing products of the S and LO optical signals;
• and
• four integrated optical waveguides respectively interconnecting four of the plurality of output access waveguides to four of the plurality of photodetectors in a non-intersecting manner to provide four optical paths between the corresponding output access waveguides and photodetectors, wherein:
• a first one of the four integrated optical waveguides interconnecting a first one of the four output access waveguides to a first one of the four photodetectors is routed in an area in front of the plurality of output access waveguides and through an area along an upper portion of the PIC and above the MMI coupler main body;
• and
• second, third and fourth ones of the four integrated optical waveguides interconnecting respective second, third and fourth ones of the ones of the output access waveguides to respective second, third and fourth ones of the four photodetectors are routed in an area in front of the plurality of output access waveguides and through the area along an lower portion of the PIC and below the MMI coupler main body.