  Optical fiber is a ubiquitous and indispensable component in communications,
sensing, biomedicine and many other lightwave technologies and applications.
Here we propose topological one-way fibers to remove two fundamental mechanisms
that limit fiber performance: scattering and reflection. We design
three-dimensional~(3D) photonic crystal fibers, inside which photons propagate
only in one direction, that are completely immune to Rayleigh and Mie
scatterings and significantly suppress the nonlinear Brillouin and Raman
scatterings. A one-way fiber is also free from Fresnel reflection, naturally
eliminating the needs for fiber isolators. Our finding is enabled by the
recently discovered Weyl points in a double-gyroid~(DG) photonic crystal. By
annihilating two Weyl points by supercell modulation in a magnetic DG, we
obtain the photonic analogue of the 3D quantum Hall phase with a non-zero first
Chern number~($C_1$). When the modulation becomes helixes, one-way fiber modes
develop along the winding axis, with the number of modes determined by the
spatial frequency of the helix. These single-polarization single-mode and
multi-mode one-way fibers, having nearly identical group and phase velocities,
are topologically-protected by the second Chern number~($C_2$) in the 4D
parameter space of the 3D wavevectors plus the winding angle of the helixes.
This work suggests a unique way to utilize higher-dimensional topological
physics without resorting to artificial dimensions.
