High-density optical interconnects within large-scale systems

Next-generation high-end data processing systems such as Internet switches or servers are approaching aggregate bandwidths in excess of 1 Terabit per second (Tbps). In the case. of Internet switches, the increase of fiber bandwidth that is caused by the introduction of Dense Wavelength Division Multiplexing (DWDM) leads to an increase of system size from single-shelf to multi-rack configurations. Infra-system interconnects will therefore span from centimeters (on-board) up to tens of meters (rack-to-rack). The task of providing hundreds of individual links at speeds in excess of 10 Gigabit per second (Gbps) over these distances becomes increasingly difficult for conventional copper-based technology. Using a packet switch system as an example application, we define a set of interconnect requirements for future large-scale systems. Distinguishing three interconnect classes (on-board, card-to-card over a backplane, rack-to-rack), we study the expected limits of copper-based solutions from an application point of view. After an overview of the state of the art in optical interconnect technology, we compare available technologies with the initially defined requirements. From this, we deduct key focus areas for future optical interconnect research. Finally, we present some of our, recent activities in the field of waveguide and free-space based board-to-board optical interconnects.