ECE PhD Prospectus Defense: Vineetha Ashok

ECE PhD Prospectus Defense: Vineetha Ashok

Title: Optimizing Multimode Fibers for High-Capacity Space-Division Multiplexing via Topological Confinement

Presenter: Vineetha Ashok

Advisor: Professor Siddharth Ramachandran

Chair: Professor Miloš Popović

Committee: Professor Siddharth Ramachandran, Professor Miloš Popović, Professor Tianyu Wang, Dr. Poul Kristensen

Google Scholar Link: https://scholar.google.com/citations?hl=en&user=XvNHi4gAAAAJ

Abstract: Fiber-optic networks have been the backbone of global communication since the 1980s, enabling high-capacity data transmission over single optical fibers. In recent years, however, the achievable capacity of a single optical fiber has approached fundamental limits due to optical nonlinearities. Together with the continuous growth in bandwidth demand, there is a critical need to reconfigure optical fiber communication links. Over the past decade and a half, space-division multiplexing (SDM) has emerged as a promising approach to address this challenge by exploiting multiple spatial channels—such as fiber cores or modes—to increase data capacity, in principle linearly, while improving energy efficiency through integration and shared hardware. Despite its potential, conventional SDM implementations are constrained by fabrication and packaging limitations when scaling to large channel counts, as well as by intermodal and intercore crosstalk, limiting system scalability and performance. More recently, an alternative waveguiding technique which enables guidance beyond the total internal reflection (TIR) cutoff condition has been developed and investigated. This centrifugal barrier effect, known as topological confinement, naturally resists mode mixing and has demonstrated the ability to scale the number of unmixed channels in a multimode fiber (MMF) to roughly four times previously reported counts over lengths up to ~1 km, highlighting a promising route for high-capacity SDM.

My proposed dissertation aims to study the scaling laws of topological confinement and to develop fiber designs optimized for high-channel-count, low-crosstalk SDM. Preliminary experiments were conducted on a particular multimode ring-core fiber (RCF) design to determine the optimal core size, revealing consistent trends with simulations. An unmixed channel count of 88 was demonstrated across five wavelengths in C-band, in both orbital angular momentum (OAM) basis and linearly polarized (LP) basis, in a device length fiber of ~75 m, representing the highest unmixed channel count to date. The availability of unmixed spatial modes opens an extensive toolbox for promising applications, including both linear and nonlinear intermodal interactions. We leverage the properties of topologically confined modes (TCMs) to explore the propagation of optical skyrmions through fibers, previously hindered by intermodal crosstalk in multimode platforms. Higher-order optical skyrmions up to