What is the difference between single-mode and multimode fiber?

There are a specific number of ray paths that can efficiently propagate through a fiber. These ray paths are called modes. The amount of modes a fiber can support is a function of the core size, in addition to other factors. The core diameters of single-mode fibers are smaller than multimode fibers. A multimode fiber can support many modes a single-mode fiber only allows one mode to be transmitted over large distances without great loss, because this mode travels without any reflections from the core-cladding interface. Edmund Optics' Optical Grade and Communications Grade fibers are all multimode.