One most efficient gain media in solid-state lasers is the crystal YAG doped with Nd. But fabricating an Nd:YAG optical fiber has proven to be particularly challenging because of thermal expansion and diffusion. The aim of this study was to fabricate an Nd:YAG silica fiber with a majority content of YAG in the fiber’s core to subsequently utilize the fiber as a gain medium. By fabricating the fiber through the molten-core method, the elemental content of silicon in the core was estimated to be 8.5 ± 2.6 wt%—the highest core purity yet achieved, to the best of our knowledge. Simulations of the Nd:YAG optical fiber as a gain medium have provided the optimal parameters, such as the fiber length, for the currently performed study of a laser based on this particular Nd:YAG optical fiber.