Core - Thin glass center of the fiber where the light travels

Core - Thin glass center of the fiber where the light travels
Cladding - Outer optical material surrounding the core that reflects the light back into the core
Buffer coating - Plastic coating that protects the fiber from damage and moisture
Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering, called a jacket.

Optical fibers come in two types:

Single-mode fibers
Multi-mode fibers
See Tpub.com: Mode Theory for a good explanation.
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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). 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 mm diameter) and transmit visible red light (wavelength = 650 nm) from LEDs.

Let's look at how an optical fiber works.