Impact of coordinate frames on mode formation in twisted waveguides

Off-axis twisted waveguides possess unique optical properties such as circular and orbital angular momentum (OAM) birefringence, setting them apart from their straight counterparts. Analyzing mode formation in such helical waveguides relies on the use of specific coordinate frames that follow the twist of the structure. In this paper, the differences between modes forming in high-contrast off-axis twisted waveguides defined in the three most important coordinate systems-the Frenet-Serret, the helicoidal, or the Overfelt frame-are investigated through numerical simulations. We explore modal characteristics up to high twist rates (pitch: 50 mu m) and clarify a transformation allowing us to map the modal fields and the effective index back to the laboratory frame. In case the waveguide is single-mode, the fundamental modes of the three types of waveguides show significant differences in terms of birefringence, propagation loss, and polarization. Conversely, the modal characteristics of the investigated waveguides are comparable in the multimode domain. Furthermore, our paper examines the impact of twisting on spatial mode properties. At high twist rates, a separation of modes with different spins is observed, suggesting a potential influence of the photonic spin Hall effect. Additionally, twisting induces OAM-dependent changes in the intensity distribution, indicating the presence of the photonic orbital Hall effect. Lastly, modes of single-mode helical waveguides were found to exhibit superchiral fields on their surfaces. These findings provide a comprehensive basis for further research into the physics of twisted off-axis waveguides. Implementation approaches such as 3D nanoprinting or fiber-preform twisting open the doors to potential applications of such highly twisted waveguides, including chip-integrated devices for broadband spin- and OAM-preserving optical signal transport, as well as applications in chiral spectroscopy or nonlinear frequency conversion.