This document discusses different types of optical fibers. It describes single mode fibers as having a small diameter that supports only one propagation mode, while multimode fibers have a larger core diameter supporting multiple modes. Index profiles can be step index, where the core and cladding have uniform but different refractive indices, or graded index, where the core index decreases from the center outward. Single mode fibers typically have a step index profile, while multimode fibers can be either step or graded index. The document provides illustrations and explanations of step index and graded index fiber structures and their light propagation characteristics.