Coherent few mode demultiplexer realized as a 2D grating coupler array in silicon.

We demonstrate an on-chip coherent mode scrambling demultiplexer for polarization multiplexed few mode signals. The device has been fabricated in the standard silicon-on-insulator platform. The mode demultiplexer consists of an array of 2D grating couplers for dual polarization few mode fiber-to-chip coupling and optical hybrids realized by 4×4 MMIs. The array of perfect vertical 2D grating couplers allows us an efficient fiber-to-chip coupling with experimental peak coupling efficiencies of -5.2 dB and -9.0 dB at 1570 nm for LP and LP modes, respectively, while simulated coupling efficiencies at 1550 nm are -3.6 dB and -3.3 dB for LP and LP, respectively. We successfully performed a back-to-back three LP modes division multiplexing transmission experiment with single polarization 32 Gbaud QPSK signals using the fabricated mode demultiplexer relying on offline MIMO DSP techniques.