Before we discuss each type of fiber, here are some definitions:
Optical fiber: The glass portion of a fiber optic cable – no jacketing or strength members included. An optical fiber is made up of a light carrying core surrounded by cladding. The cladding prevents light from escaping the core, effectively keeping the signal moving down the glass.
Singlemode fiber: a fiber featuring a small light-carrying core of about 9 micrometers (µm) in diameter. For reference, a human hair is closer to 100 µm. The core is surrounded by a cladding that brings the overall diameter of the optical fiber to 125 µm.
Multimode fiber: a fiber with a core of 50 µm or above. A larger core means multiple modes (or rays of light) can travel down the core simultaneously. Just like single mode, the core is surrounded by a cladding that brings the overall diameter of the optical fiber to 125 µm.