On-Chip 4×10 GBaud/s Mode-Division Multiplexed PAM-4 Signal Transmission

Emerging 5G mobile networks and cloud computing applications are driving the demand for an ever-increasing capacity of short-reach optical communications. To meet this demand, mode-division multiplexing (MDM) has been proposed to scale up the bandwidth density by leveraging the spatial modes of an optical waveguide for transmitting multiple optical signals. On the other hand, the use of multi-level pulse amplitude modulation (PAM) can also increase the transmission bandwidth. Therefore, on-chip MDM in conjunction with PAM is an approach to enhance the transmission capacity in a photonic integrated circuit. In this paper, we report a silicon photonic integrated four-channel MDM circuit for high data rate on-chip communications with a low channel crosstalk and small insertion loss. To make the circuit have a small size that supports broadband operation, a mode multiplexer and demultiplexer are realized with the use of cascaded asymmetrical directional couplers on rib waveguides. By incorporating the MDM circuit in an optical communications system, the transmission of a 4 x 10 GBaud/s OOK and PAM-4 signal is experimentally demonstrated. The performance in terms of eye diagrams and power penalties is evaluated. The power penalties for the four-channel OOK transmission are 4.31, 2.38, 1.44 and 3.5 dB at a BER of 10^-9. For the four-channel PAM-4 transmission, the power penalties are 7.98, 1.10, 0.66 and 5.14 dB. The required received optical power at a BER (<3.8 × 10^-3) of the 7% overhead-hard decision FEC is-2.6 dBm. The key advantage of the approach is that high-capacity on-chip communications is enabled by the photonic integrated MDM circuit with a small footprint. Since the MDM circuit is implemented on a silicon photonic platform, the system holds high potential for full integration on a single chip.