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We discuss our recent work on mode division multiplexing (MDM) silicon devices for on-chip optical interconnects. First, we demonstrate a 4-channel MDM device based on directional couplers. The crosstalk values are below -13.3 dB at 1550 nm for the TE0, TM0, and TM1 channels. The TE1 channel experiences the largest crosstalk at 1553 nm of -6.5 dB and the highest loss of 2.6 dB. Four channels of 40Gb/s signals in the form of 20-Gbaud quadrature phase shift keying (QPSK) are multiplexed through MDM to test the device The 160-Gb/s data is transmitted through a multi-mode waveguide and then de-multiplexed for bit-error ratio (BER) measurement. By using multiple-input multiple-output (MIMO) digital processing, error free operation is achieved. Second, to further increase the capacity, we design and implement an 11-channel MDM device by employing subwavelength grating (SWG) couplers. The main limiting factor in the number of MDM channels is the susceptibility of the effective waveguide refractive index to the width variation due to the fabrication errors, and the index curves of for the multi-mode bus waveguide and the single mode access waveguide are of different slopes. Through index engineering by duty cycle optimization of the SWG access waveguide, the index curves of the two different waveguides can be matched and thus the phase matching condition in the MDM process is maintained with the similar fabrication errors imposed on the two waveguides. The crosstalk values of such a SWG based 11-channel MDM device remain lower than -7 dB for all channels, and the loss is below 13.7 dB for the worst channel. We then use 30-Gbaud 16-quadrature amplitude-phase modulation (QAM) signals to test the device, and the BERs are below the 7% feedforward error correction threshold after MIMO processing. A total transmission capacity of 1.23 Tb/s on a single wavelength is achieved.
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