NASA

Free-space laser links used for satellite communications are currently limited in modulation speeds due to the high power-per-bit consumption of the optical transceivers. In this project we take advantage of recent breakthrough 3D monolithic integration of photonic structures, particularly high-speed graphene-silicon devices on CMOS electronics in order to create CMOS-compatible high-bandwidth transceivers for ultra-low power terabit-scale optical communications. The target signal integrity and end-to-end energy-per-bit are set as the figure of merits for the inter-satellite free-space link.  The parameters subject to optimization in the design space are the modulator’s bandwidth, thermal control of optical devices, and the tradeoff between required net coding gain (NCG) and modulation overhead.

The graphene modulator’s operation bandwidth is not limited by the carrier transport in graphene, but it is bottlenecked by the parasitic components of the modulator circuit. These parasitics are the capacitor formed by the graphene-dielectric-graphene stack, graphene-sheet resistance, and graphene-metal contact resistance. While it is possible to design the device footprint in such a way to reduce the capacitance and the sheet resistance, and simultaneously optimize both modulator performance and operation bandwidth, the contact resistance is fundamentally related to how the metal-graphene interface is formed. Previously, surface contacts have been employed to interface the metal and the graphene, where the metal is deposited at the graphene’s top surface in order to form contacts. The metal and graphene interaction in the surface contacts approach occurs perpendicularly to the graphene molecular two-dimensional (2D) plane. Since graphene’s vertical orbital hybridization is weak, surface contacts are fundamentally incapable of maximizing metal and graphene interaction. In this project our team is aiming at exploring novel ways to break through this barrier and achieve ultra-high bandwidth graphene-based (and other 2D material-based) silicon modulators.

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