  The doping effect of the folded phonon mode at 98 cm$^{-1}$ was investigated
on the Si-doped CuGeO$_3$ by magneto-optical measurements in far-infrared (FIR)
region under high magnetic field. The folded phonon mode at 98 cm$^{-1}$
appears not only in the dimerized (D) phase but also in the
dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO$_3$. The splitting
was observed in the incommensurate (IC) phase and the antiferromagnetically
ordered incommensurate (IAF) phase above $H_C$. The split-off branches exhibit
different field dependence from that of the pure CuGeO$_3$ in the vicinity of
$H_C$, and the discrepancy in the IAF phase is larger than that in the IC
phase. It is caused by the interaction between the solitons and the impurities.
