  The anisotropy of the Fe-based superconductors is much smaller than that of
the cuprates and the theoretical calculations. A credible understanding for
this experimental fact is still lacking up to now. Here we experimentally study
the magnetic-field-angle dependence of electronic resistivity in the
superconducting phase of iron-based superconductor
CaFe$_{0.882}$Co$_{0.118}$AsF, and find the strongest anisotropy effect of the
upper critical field among the iron-based superconductors based on the
framework of Ginzburg-Landau theory. The evidences of energy band structure and
charge density distribution from electronic structure calculations demonstrate
that the observed strong anisotropic effect mainly comes from the strong ionic
bonding in between the ions of Ca$^{2+}$ and F$^-$, which weakens the
interlayer coupling between the layers of FeAs and CaF. This finding provides a
significant insight into the nature of experimentally observed strong
anisotropic effect of electronic resistivity, and also paves an avenue to
design exotic two dimensional artificial unconventional superconductors in
future.
