High-Efficiency, Broadband, and Polarization-Diversity Multicore Fiber-Chip Interfaces for Optical I/O

Driven by the substantial data transfer demands of artificial intelligence clusters and high-performance computing, moving optics into on-board and in-package interconnects is emerging as a long-term solution. Although specialized optical fibers, such as multicore fibers (MCFs), have the potential to enable ultradense I/O interfaces and decrease fiber cable counts, the random dynamic polarization states across the individual fiber cores hinder their use in receiver applications and polarization diversity scenarios. Here, we present an efficient and broadband chip-fiber interface for 7-core MCFs integrated with silicon photonic chips, while facilitating automatic polarization control for each fiber core. Our approach integrates a perfectly vertical coupling two-dimensional grating coupler (2DGC) array with compact spot-size converters to provide direct fan-in/fan-out between a 7-core MCF and a silicon photonic chip. This configuration achieves a peak coupling efficiency of −2.4 dB, a 3-dB bandwidth of 55 nm, and polarization-dependent loss below 0.7 dB. To accommodate polarization variations in each fiber core, the outputs of the 2DGC are routed to an on-chip 4 × 4 Mach–Zehnder interferometer mesh, which implements arbitrary unitary transformations and performs automatic polarization unscrambling via a gradient-descent feedback loop. Using this interface, we demonstrate coherent transmission of 64-GBaud 16-QAM signals across eight WDM channels from 1540 to 1570 nm through all seven fiber cores, achieving bit-error rates on the order of 10 –3, well below the 7% hard-decision FEC threshold. This indicates that broadband polarization-diverse MCF interfaces with integrated MZI meshes hold great promise for ultrahigh-bandwidth-density optical interconnects.