  $Cu_3TeO_6$ is a modest frustrated $S=1/2$ spin system, which undergoes an
anti-ferromagnetic transition at $T_N\sim61$ $K$. The anti-ferromagnetic spin
alignment in $Cu_3TeO_6$ below $T_N$ is supposed to induce a magneto-elastic
strain of the lattice. The complete absorption spectrum of $Cu_3TeO_6$ is
obtained through Kramers-Kronig transformation of the optical reflectivity,
measured from the far-infrared up to the ultraviolet spectral range as a
function of temperature ($T$). Below $T^*\sim 50$ $K$, we find a new mode at
208 $cm^{-1}$. The spectral weight associated to this additional mode increases
as $\propto (T^*-T)^{1/2}$ with decreasing $T$ below $T^*$. The implication of
the optical findings will be discussed in relation to the magnetic phase
transition at $T_N$.
