Compact polarization diversity Kramers-Kronig coherent receiver on silicon chip

We design and fabricate a compact silicon photonic integrated circuit (PIC) for polarization diversity heterodyne coherent detection. This PIC integrates two optical gratings for fiber coupling and polarization diversity, two germanium single-ended photodetectors (PDs), and three multimode interferometers (MMIs) for power splitting and optical hybrid. The device is highly compact with a footprint of 0.68mm × 0.9mm. We test this PIC with heterodyne detection experiments of polarization division multiplexed (PDM) 32Gbaud quadrature phase shift keying (QPSK) and 16-ary quadrature amplitude modulation (16QAM) signals. The signal-signal beat interference due to square-law detection is separately mitigated with the Kramers-Kronig (KK) scheme for each of the two orthogonal polarizations. To our best knowledge, we report the first PDM-KK coherent receiver in PIC with a capability of detecting 256Gb/s 16QAM signals, which shows the most compact size among the silicon coherent receivers ever reported.