Bridging the Graded-Index Few-Mode Fibre with Photonic Integrated Circuits via Efficient Diffraction Waveguide Gratings

Spatial-division multiplexing (SDM) employing few-mode fibres (FMF) has attracted much interest for increasing the capacity of optical fibre communication systems. Graded-index few-mode fibres (GI-FMF) have low modal differential group delay (DGD), allowing extended transmission distance and bandwidth. In this paper, we describe novel efficient waveguide grating couplers fabricated on the silicon photonic platform for selectively launching different modes in the GI-FMF. The waveguide grating couplers may be integrated with transceivers for implementing mode division multiplexing in the fibre communications. The single-polarization diffraction waveguide gratings is measured to have coupling efficiencies into the LP01 mode and LP11 mode of – 2.8 dB and – 3.88 dB, respectively. The measured 3-dB bandwidth of the two modes are 61.2 nm and 59.0 nm. A twodimensional grating based on the effective medium theory (EMT) is also demonstrated, supporting totally four optical channels including the two orthogonal polarizations of the LP01 mode and LP11 mode. The obtained peak coupling efficiency is – 4.89 dB for LP01 mode and – 5.71 dB for LP11 mode. The 3-dB bandwidth of the two modes are 23.2 nm and 21.6 nm, respectively.