  Optical fibers confine and guide light almost unattenuated and thus convey
light forces to polarizable nano-particles over very long distances. Radiation
pressure forces arise from scattering of guided photons into free space while
gradient forces are based on coherent scattering between different fiber modes
or propagation directions. Interestingly, even scattering between
co-propagating modes induces longitudinal forces as the transverse confinement
of the light modes creates mode dependent longitudinal wave-vectors and photon
momenta. We generalize a proven scattering matrix based approach to calculate
single as well as inter-particle forces to include several forward and backward
propagating modes. We show that an injection of the higher order mode only in a
two mode fiber will induce a stationary tractor force against the injection
direction, when the mode coupling to the lower order mode dominates against
backscattering and free space losses. Generically this arises for non-absorbing
particles at the center of a waveguide. The model also gives improved
predictions for inter-particle forces in evanescent nanofiber fields as
experimentally observed recently. Surprisingly strong tractor forces can also
act on whole optically bound arrays.
