Multiplexing and switching for mode division multiplexed optical interconnects

We present some basic functional elements for mode division multiplexed optical interconnects including an inverse designed mode multiplexer, a thermo-optic based mode add-drop switch, and integrated modulators for mode division multiplexed interconnects. Optical interconnects for on-chip global interconnects have attracted research interest in recent years because they provide a potential solution to the problem of thermal limitations of copper interconnects, which already severely constrain the maximum clock frequencies that may be used in microprocessors [1]. While state of the art CMOS transistors can switch at frequencies of many tens of gigahertz desktop microprocessors today are still limited to ∼ 5 GHz clock frequencies because of the extreme heat density and associated practical problem of dissipating the heat generated if the dense electrical interconnects were operated at higher frequencies. The use of dense wavelength division multiplexing (DWDM) can provide an order of magnitude improvement in on-chip data throughput at the same power consumption as electrical interconnects [2]. DWDM was necessary because of the limited area available for on-chip optical interconnects and the high density of interconnects that are needed. However DWDM is expensive because of its requirement of precise wavelength laser arrays (and temperature control of the wavelength selective elements). In this paper we review our work towards the development of functional elements for implementing on-chip optical interconnects that can adopt the lower cost approach of using a single wavelength source with mode division multiplexing (MDM) as an alternative to DWDM. We have developed some of the basic functional elements of MDM including a novel inverse designed mode multiplexer and demultiplexer [3], mode switches for MDM networks [4], and the integration of modulators for a MDM network on chip [5]. Experimental performance on some of the fabricated devices will be presented at the conference.