Under strong laser radiation, a Dirac material, the topological insulator (TI) Bi<inf>2</inf>Te<inf>3</inf>, exhibits an optical transmittance increase as a result of saturable absorption. Based on an open-aperture Z-scan measurement at 1550 nm, we clearly show that the TI, Bi<inf>2</inf>Te<inf>3</inf> under our investigation, is indeed a very-high-modulation-depth (up to 95%) saturable absorber. Furthermore, a TI based saturable absorber device was fabricated and used as a passive mode locker for ultrafast pulse formation at the telecommunication band. This contribution unambiguously shows that apart from its fantastic electronic property, a TI (Bi<inf>2</inf>Te<inf>3</inf>) may also possess attractive optoelectronic property for ultrafast photonics. ©2012 American Institute of Physics.