How Fiber Optics Work

By: Craig Freudenrich, Ph.D. & Chris Pollette  | 

What Are Fiber Optics?

The structure of a typical single-mode fiber
The structure of a typical single-mode fiber: 1: core, 2: cladding, 3: buffer, 4: jacket.
Bob Mellish/Wikipedia

Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.

If you look closely at a single optical fiber, you will see that it has the following parts:


  • core - thin center of the fiber where the light travels
  • cladding - outer optical material surrounding the core that reflects the light back into the core
  • buffer - a protective plastic coating applied directly to the optical fiber
  • jacket - protective outer layer of the cable that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables.

Optical fibers come in two types:

  1. single-mode fibers
  2. multi-mode fibers

Single-mode fibers have small cores (about 3.5 x 10-4 inches or 9 microns in diameter) and transmit infrared laser light (wavelength = 1,300 to 1,550 nanometers or nm). Multi-mode fibers have larger cores (about 2.5 x 10-3 inches or 62.5 microns in diameter) and transmit infrared light (wavelength = 850 to 1,300 nm) from light-emitting diodes (LEDs).

Some optical fibers can be made from plastic. These fibers have a large core (0.04 inches or 1 millimeter diameter) and can be used with silicon chips. Glass fiber doesn't work well with silicon and is costly to adapt.

Let's look at how an optical fiber works.