We performed broadband optical transmission measurements of Bi₂Se₃ and In-doped (Bi_1-xIn_x)₂Se₃ thin films, where in the latter the spin-orbit coupling (SOC) strength can be tuned by introducing In. An optical absorption peak located at E=1eV in Bi₂Se₃ becomes completely suppressed at the critical x=0.06 in correlation with the topological surface state (SS) quenched at the same x due to TI-NTI transition. When Bi₂Se₃ is electrically gated, the 1eV-peak becomes stronger(weaker) when electron is depleted from (accumulated into) the SS. These observations combined together demonstrate that under the 1eV photo illumination, electron is excited from a bulk band into the SS of Bi₂Se₃. The bulk-to-surface optical transition or equivalently the optical population of the SS, is the first kind of such phenomena observed in TI's, is not only of fundamental significance but also offers an opportunity for optoelectronic application.