[Paper Review] Compact Dual-Polarization Silicon Integrated Couplers for Multicore Fibers

Julián L. Pita, Lucas G. Rocha+6 more|arXiv (Cornell University)|Feb 17, 2021

Photonic and Optical Devices22 references27 citations

TL;DR

This paper presents a compact dual-polarization silicon-on-insulator (SOI) coupler that enables efficient, simultaneous coupling to all seven cores of a multicore fiber with 32 µm core-to-core spacing. Using optimized 2D gratings and ultra-compact tapers, the device achieves −4.3 dB coupling efficiency, 48 nm 3 dB bandwidth, and crosstalk below −42.7 dB, marking the first experimental demonstration of full dual-polarization coupling to all cores in a dense multicore fiber on a standard SOI platform.

ABSTRACT

Compact fiber-to-chip couplers play an important role in optical interconnections, especially in data centers. However, the development of couplers has been mostly limited to standard single-mode fibers, with few devices compatible with multicore and multimode fibers. Through the use of state-of-the-art optimization algorithms, we designed a compact dual-polarization coupler to interface chips and dense multicore fibers, demonstrating, for the first time, coupling to both polarizations of all the cores, with measured coupling efficiency of -4.3 dB and with a 3 dB bandwidth of 48 nm. The dual-polarization coupler has a footprint of 200 mu m(2) per core, which makes it the smallest fiber-to-chip coupler experimentally demonstrated on a standard silicon-on-insulator platform. (C) 2021 Optical Society of America

Motivation & Objective

• Address the lack of compact, dual-polarization couplers for dense multicore fibers in integrated photonics.
• Enable high-capacity optical interconnects by supporting both polarization states across all cores simultaneously.
• Overcome limitations of existing single-polarization or non-uniform couplers that restrict capacity in space-division multiplexing systems.
• Achieve high coupling efficiency and low crosstalk in a footprint compatible with high integration density.

Proposed method

• Designed a dual-polarization coupler using two superimposed single-polarization 2D gratings per core, optimized via 3D finite-difference time-domain (FDTD) simulations.
• Employed ultra-compact tapers (10 µm × 5 µm) to connect each grating to a 450 nm-wide silicon waveguide, minimizing insertion loss.
• Optimized the grating pitch (623 nm) and fill factor (0.7) through simulation to maximize coupling efficiency and bandwidth.
• Fabricated the device using electron-beam lithography and reactive ion etching on a 250 nm SOI wafer with 3 µm buried oxide, followed by PECVD SiO₂ cladding.
• Used a fan-out configuration for characterization, with single-mode fibers coupled via conventional focused grating couplers on the opposite side of the chip.
• Measured coupling efficiency and crosstalk using a tunable laser, power meter with integrating sphere, and optical spectrum analyzer with piezo-controlled alignment.

Experimental results

Research questions

• RQ1 Can a compact dual-polarization coupler be designed to interface all cores of a dense multicore fiber with 32 µm spacing on a standard SOI platform?
• RQ2 What is the achievable coupling efficiency and 3 dB bandwidth when both polarization states are supported across all cores?
• RQ3 How does fabrication-induced non-uniformity affect coupling efficiency and polarization-dependent loss in such a compact device?
• RQ4 Can crosstalk between adjacent cores be suppressed below −42 dB while maintaining high efficiency and wide bandwidth?
• RQ5 Is the footprint of 200 µm² per core sufficient to enable high-density integration in multicore fiber interconnects?

Key findings

• The dual-polarization coupler achieved a maximum coupling efficiency of −4.3 dB for core 2, with a minimum of −5.4 dB for core 1, resulting in a 1.1 dB variation across cores.
• The 3 dB bandwidth of the device was experimentally measured at 48 nm, ranging from 1505 nm to 1553 nm, with a slight blue shift compared to simulation.
• The worst-case inter-core crosstalk was measured at −42.7 dB at 1508 nm, which is 34.7 dB below the coupling efficiency at that wavelength.
• Polarization-dependent loss was measured at a maximum of 0.54 dB, indicating minimal sensitivity to device asymmetries or waveguide bending.
• The footprint of 200 µm² per core is the smallest experimentally demonstrated for a fiber-to-chip coupler on a standard SOI platform.
• The device enables full dual-polarization coupling to all seven cores of a multicore fiber, representing the first experimental demonstration of its kind for such a dense core arrangement.