Silicon photonics 90° optical hybrid design for coherent receivers

The recent rapid development of silicon photonics technology has spurred the process of on-chip integration of all kinds of opto-electronic components. One of the most common components of such type is the opto-electrical receiver. The monolithic implementation of the receiver could potentially have lower power consumption, higher sensitivity and bandwidth due to very short diode to amplifier connection length, which has very low parasitic capacitance and series resistance. The SiGe photodiode itself is also very compact, thus lowering the junction capacitance and improving its bandwidth. Among the different optical communication systems, coherent transmission lately received a lot of attention due to the rising requirements of the optical link capacity, and it was shown that this particular approach could benefit greatly from the monolithic integration, since the major component required for the demodulation on the receiver side – 90° optical hybrid – could be implemented fully passive and directly on the same chip as the receiver itself, together with digital post-processing circuitry. Despite the initial complexity of the modulation scheme, advanced silicon photonics components like this optical hybrid could make coherent transmission attractive even for short-range optical links. I would like to present the actual designs, implementation and measurement results of 90° fully passive optical hybrids, implemented in the IHP SG25PIC (passive photonics IC) technology. One of the designs is based on 4x4 multimode interferometer (MMI). The other one is based on two separate 2x2 MMIs with additional delay element. The final designs didn’t require any additional tuning after fabrication and have shown sufficient precision and performance for a coherent system design. The results of this work were later used for the design of monolithic coherent receiver.