  Significant effort in optical-fiber research has been put in recent years
into realizing mode-division multiplexing (MDM) in conjunction with
wavelength-division multiplexing (WDM) to enable further scaling of the
communication bandwidth per fiber. In contrast almost all integrated photonics
operate exclusively in the single-mode regime. MDM is rarely considered for
integrated photonics due to the difficulty in coupling selectively to
high-order modes which usually results in high inter-modal crosstalk. Here we
show the first demonstration of simultaneous on-chip mode and wavelength
division multiplexing with low modal crosstalk and loss. Our approach can
potentially increase the aggregate data rate by many times for on-chip
ultra-high bandwidth communications.
