A novel fiber optical tupered coupler is proposed with its waveguide refractive index gradually changing, induced by germania defects under the UV irradiation. The refractive index change An of 6 ×10-3 in the, single- mode communications fiber is achieved in our experiment by means of pre-treating the fiber in high-pressure and lowtemperature environment. A particular index tuper profile function is obtained by delicately controlling the exposure conditions, and the mode field radius (MFR) is reduced from 4.8 μm to 3.2 μm in single-mode fibers. The coupled wave theory analysis show that the tapered coupler is capable of excellent transmission for fundamental mode with very small mode conversions.