The Basic Structure of Optical Fiber

This article is part of our Basics of Fiber Series. Other blogs in this series include fiber benefits, the differences between single-mode and multimode and intrinsic and extrinsic attenuation in fiber optic cables.

What are fibers made of?

Have you ever thought about the structure of optical fiber? How can a thin strand of glass, about the width of a human hair, transmit vast amounts of data across great distances? Optical fiber is composed of three elements – the core, the cladding and the coating. These elements carry data by way of infrared light, thus propagating signal through the fiber.


The core is at the center of the optical fiber and provides a pathway for light to travel. In multimode fiber, the core size is either 62.5 or 50 microns (µm), and approximately 8.3 microns in a single-mode fiber. The larger core size in multimode fiber provides various pathways for light to travel, whereas the small core size in single-mode fiber provides a single pathway.

Next is the cladding, which is 125 microns. The cladding holds the light inside the core and controls the direction in which light is spread through the fiber. When light enters the fiber at the appropriate approach angle, also known as the critical angle, it will reflect and stay inside the core, thus achieving a process called “total internal reflection”. If it’s not at the perfect angle, the light will refract and the signal will be lost.

Lastly, we have the coating, which acts as the primary buffer. It has a diameter that, historically, has been 250 µm. It cushions and protects the fibers from humidity and hostile environments. Two layers of urethane Acrylate (plastic) make up the coating, also known as the “soft” and “hard” layers. The soft layer cushions the fiber and the hard layer provides abrasion resistance. The coating also has a higher index of refraction than the core and the cladding which allows for unwanted light to refract from the cladding.

Additionally, there are new fibers being release to the industry that have a reduced diameter of 200 µm. These allow for smaller cable diameters in ultra-high density fiber optic cables, like 3456 fiber and 6912 fiber cables. 

You can learn more about optical fiber in this presentation from Patrick Dobbins, the Director of Applications Engineering and Field Engineering for AFL.