  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, making the waveguide invariant along one of the new
coordinates. In this study, 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 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 study examines the impact of twisting on spatial mode
properties with the results suggesting a potential influence of the photonic
spin Hall and orbital Hall effects. Additionally, modes of single-mode helical
waveguides were found to exhibit superchiral fields on their surfaces.
  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.
