AbstractGradient functionalization is a widely applied process in materials science that involves generating gradient distribution of composition or structure to yield materials with new functions or improved properties. Here an interfacial engineering is described for construction of gradient fiber, which exhibits gradient distribution of active centers, bonding configuration, and the resultant optical response. It is shown that the interfacial diffusion of protype niobium (Nb) in the core and cladding region of the fiber can be tuned via rational control of the viscosity of melt and the subsequent thermal activation. This leads to gradient distribution of Nb species and active [NbO6]7− centers within 10–20 µm region. As a result, unique optical properties including the tunable energy transfer between active centers and special annular emission beam can be obtained. These results highlight a fundamental principle to develop novel photonic fibers and provide a great step in expanding the scope of gradient materials.