Silicon-Integrated Coherent Optical Receiver Realized by a Semi-inverse-Designed 90° Hybrid

Coherent optical communication is a key communication technology featuring high sensitivity and capacity. As the key component of a coherent optical receiver, the 90° hybrid has been realized by several conventional designs, showcasing high performance and still room for improvement in compactness. While inverse design is a promising method for improving the compactness of the photonic devices, the previous inverse-designed 90° hybrids have limited performance, needing the high-precision lithography process. Here, we present a compact and fabrication-friendly 90° hybrid developed by semi-inverse design. It exhibits simulation performance of an excess loss ≤0.9 dB, a common mode rejection ratio better than −24 dB, and a phase error better than ±4° in the C band, matching the state-of-the-art conventional design counterparts; meanwhile, the footprint of 4.5 × 32.5 μm2 is 4-fold smaller. Importantly, this 90° hybrid fabricated by a commercial 130 nm photolithography process exhibits practical measured performance even in the presence of a −20 nm waveguide sidewall offset. Its experimental performance is on par with that of the state-of-the-art inverse-designed counterpart fabricated by e-beam lithography. Further, a coherent optical receiver was experimentally demonstrated based on it and four SiGe photodetectors, featuring a bit error rate of 5.21 × 10–4 (below the FEC limit) when receiving a 50 Gbps QPSK optical signal with a signal power of −11.8 dBm. This work can promote the application of inverse design in developing complex functional photonic devices with high compatibility with the commercial semiconductor foundry.