Mode-Division Multiplexed PAM-4 Signal Transmission in a Silicon Photonic Chip

Emerging 5G mobile networks and cloud computing applications are driving the demand for an ever-increasing capacity for short-reach optical communications. To meet this demand, mode-division multiplexing (MDM) is being employed to scale up the bandwidth capacity by leveraging the spatial modes of an optical waveguide for transmitting multiple optical signals. On the other hand, the use of PAM-4 can also increase the transmission bandwidth since PAM-4 offers four amplitude levels in a symbol and thus its bit rate is twice as high as OOK at a same baud rate. Therefore, on-chip MDM with PAM-4 is an approach to enhance the transmission capacity in a photonic integrated circuit. In this paper, we report a photonic integrated four-channel MDM circuit that is implemented on a silicon platform with a small size and low channel crosstalk. By using the MDM circuit, a 80 Gbit/s mode-division multiplexed PAM-4 signal transmission system is experimentally demonstrated. The advantage of the approach is that high data capacity on-chip communications is enabled by the photonic integrated MDM circuit with a small footprint. The MDM circuit can be employed in a radio over fiber (RoF) system to increase the data transmission capacity of the RoF link.