Metasurfaces as a Platform for Space-Division Multiplexing

Abstract

The impending capacity crunch in optical fiber communications is driving the need for a new technological breakthrough in multiplexing technology. Multiplexing is the technique used to encode information on the five orthogonal degrees of freedom of the electromagnetic wave. These are the amplitude, phase, polarization, wavelength, and spatial mode of light. As current commercial communications have exhausted the first four of these degrees of freedom, space is set to be the next frontier for increasing the channel capacity of the optical fiber. Therefore, there is major interest in space-division multiplexing (SDM) and the new generation of optical devices that must facilitate its implementation. The spatial multiplexer (MUX) is the optical device that bridges the gap between single-mode fibers and the fiber that will be the backbone of SDM. There are two possible types of fiber that can allow SDM: (1) multimode fiber and (2) multicore fiber. Both fibers have uses in long-haul submarine links to short-reach data centers. As a result, there has been intensive research on the design of spatial MUXs for both fiber types. However, the technologies used to build these MUXs fall short in efficiency, crosstalk, and scalability that hinder their adaptation as the platform that enables spatial MUXs. In this dissertation, the metasurface is tested as a potential technology that could allow efficient and scalable spatial MUXs. An inverse design technique known as adjoint optimization is used to design the metasurfaces. The mathematical theory behind this method is developed for the multiplexing problem and is related to familiar phase-retrieval techniques which provides physical insight. Metasurface-based spatial MUXs are designed for both multimode fiber and multicore fiber. For the former, measured fidelity of converted modes are as high as 1.42 dB. For the latter, measured insertion losses and crosstalk are as low as 1.2 dB and -40 dB, respectively. The work presented here demonstrates the viability of metasurfaces as an ideal optical platform for SDM.