  We report on the experimental investigation of the polarization-dependent
energy splitting in the lower exciton-polariton branches of a 1D microcavity.
The splitting observed for the lowest branch can reach up to 1 meV. It does not
result from low temperature thermal constraints but from anisotropic mechanical
internal strains induced by etching. Those strains remove the degeneracy both
in the photonic ($\delta E_{\mathrm{ph}}$) and excitonic ($\delta
E_{\mathrm{exc}}$) components of the polariton but also in the photon-exciton
coupling ($\delta\Omega$). Those three contributions are accurately infered
from experimental data. It appears that the sign and magnitude of the
polarization splitting as well as the linear polarization of the corresponding
polariton eigenstates can be tuned through the bare exciton-photon detuning.
Moreover, no dependence on the width of the wire (from 3 to 7 $\mathrm{\mu}$m)
is observed. We propose a mechanical model explaining the universality of those
observations paving the way to the engineering of polarization eigenstates in
microwires exciton-polaritons.
