Electro-optic Mode-controlling Waveguide Devices for Reconfigurable Mode-division Multiplexing Systems

The continuous growth in the Internet traffic coupled with the rapid developments in bigdata, cloud computing, and Internet of things is putting much pressure on the bandwidththat can be provided by the existing wavelength-division multiplexing (WDM)technology, where a single-mode optical fiber is allowed to carry many close wavelengthchannels. Simply increasing the fiber count without increasing the transmission capacityof each fiber is not economical. There is intense world-wide research effort in thepursuit of new technologies to increase the transmission capacities of optical fibers.Mode-division multiplexing (MDM) that allows different modes in a multimode fiber tocarry different channels is a particularly promising technology. This proposal aims atdeveloping key photonic components to advance the MDM technology.Like the existing WDM networks, the future MDM networks will need to bereconfigurable, where different modes in a fiber can be switched or routed into differentfibers. However, there are no effective active mode-controlling devices like modeswitches and reconfigurable mode multiplexers available for MDM applications at thistime. The goal of the project is to develop new mode-controlling devices based onlithium-niobate (LN) three-mode waveguides, where fast electrical mode switching isachieved by exploiting the electro-optic (EO) effect in LN. The project will investigatetwo types of EO mode-controlling devices: (1) mode switch that can switch any modeinto any other mode along the same waveguide, and (2) mode (de)multiplexer that canswitch a selected mode between two waveguides. The mode switch is based on thegeneration of an EO grating in an LN waveguide. Because of the mode symmetry, twodifferent designs of mode switches are needed to achieve switching among the threemodes. The mode (de)multiplexer is based on the configuration of two parallel LNwaveguides with EO controlled coupling between the two waveguides. Only one design ofthe EO mode (de)multiplexer for a specific mode is needed, as the device can be used inconjunction with the mode switches to achieve space switching for the other modes. Theproject will also study the integration of these mode-controlling devices to achieve moreadvanced switching functions.The PI has over 30 year experience in the development of waveguide (including LN)devices. CityU is equipped with the necessary facility for the fabrication of LN waveguides and EO gratings. The project will lead to experimental demonstration of new mode-controlling devices with potential applications in reconfigurable MDM networks.