Brittle Things: Optical Fiber

An optical fiber is a thin, flexible medium capable of conducting optical rays. Optical fibers use reflection to guide light through a channel. In optical fibers, a cladding of less dense glass or plastic surrounds a glass or plastic core. Due to the difference between the density of core and cladding, the light moving through the core is reflected off the cladding instead of being refracted into it.

Optical fiber

Propagation Mode

According to the mode of propagation, optical fiber is classified into two: single-mode and multi-mode.

Optical fiber propagation mode

Multi-mode

Multimode fiber optic cable has a large diameter core that allows multiple modes of light to propagate. How these different light beams are move within the optical fiber depends on the construction of the core.

In step-index optical fiber, the density of the core is remains same constant through out the core. That means the density is constant from center to edges of the core. When a light reaches the interface of the core and the cladding, there is a sudden change due to change in the density. The following figure shows a step index optical fiber.

Step index

In the case of graded index fiber, density is highest at the center of the core and decrease slowly to its lowest at the edge. There fore the light beams are propagate as shown below,

Graded Index

Single-Mode

Single-mode fiber optic cable has a small diameter and small dense core that allows only one mode of light to propagate. The decrease in density results in a critical angle that is close to 90 degree to make the movement of the beams horizontally. All beams arrive at the destination together and it is the responsibility of the receiver to distinguish them.

Single Mode

Connectors

There are three types of connectors for fiber-optic cable.

The subscriber channel (SC) connector is used for cable TV. It uses a push/pull locking system.

The straight-tip (ST) connector is used for connecting cable to networking devices. It uses a bayonet locking system and is more reliable than SC.

MT-RJ is a connector that is the same size as RJ45.