High-index optical fibers are specialty fibers engineered with a core refractive index that is intentionally higher than that of conventional single-mode transmission fibers. This is achieved by precisely controlling dopant profiles in the glass preform, most commonly with germanium or other index-raising materials. Due to this elevated refractive index, high-index fibers support tighter optical confinement, altered mode field diameters, and tailored numerical aperture characteristics compared to standard telecom fibers.