Exceptional Points of Degeneracy in Periodic Coupled Waveguides and the Interplay of Gain and Radiation Loss: Theoretical and Experimental Demonstration

TL;DR

This paper introduces a new approach to dispersion engineering in coupled transmission lines using exceptional points of degeneracy, with theoretical development and experimental validation at microwave frequencies, highlighting robustness despite losses and tolerances.

Contribution

It develops a theory for fourth-order EPDs in coupled waveguides, experimentally demonstrates the degenerate band edge, and explores gain-loss regimes to maintain EPDs despite radiation and dissipation.

Findings

First experimental verification of a fourth EPD in microwave CTLs.

EPD features are observable despite radiation and losses.

Gain and loss balancing can recover EPDs without PT-symmetry.

Abstract

We present a novel paradigm for dispersion engineering in coupled transmission lines (CTLs) based on exceptional points of degeneracy (EPDs). We develop a theory for fourth-order EPDs consisting of four Floquet-Bloch eigenmodes coalescing into one degenerate eigenmode. We present unique wave propagation properties associated to the EPD and develop a figure of merit to assess the practical occurrence of fourth-order EPDs in CTLs with tolerances and losses. We experimentally verify for the first time the existence of a fourth EPD (the degenerate band edge), through dispersion and transmission measurements in microstrip-based CTLs at microwave frequencies. In addition, we report that based on experimental observation and the developed figure of merit, the EPD features are still observable in structures that radiate (leak energy away), even in the presence of fabrication tolerances and dissipative losses. We investigate the gain and loss balance regime in CTLs as a mean of recovering an EPD in the presence of radiation and/or dissipative losses, without necessarily resorting to Parity-Time (PT)-symmetry regimes. The versatile EPD concept is promising in applications such as high intensity and power-efficiency oscillators, spatial power combiners, or low-threshold oscillators and opens new frontiers for boosting the performance of large coherent sources.