Basic Elements of Optical Fiber

Cross Section of Optical Fiber

Core

This is the light transmission area of the fiber, either glass or plastic. The larger the core, the more light that will be transmitted into the fiber

Cladding

The function of the cladding is to provide a lower refractive index at the core interface in order to cause reflection within the core so that light waves are transmitted through the fiber.

Coating

Coatings are usually multi-layers of plastic applied to preserve fiber strength, absorb shock and provide extra fiber protection. These buffer coatings are available from 250 microns to 900 microns.

Fiber Size

The size of the optical fiber is commonly referred to by the outer diameter of its core, cladding and coating. Example: 50/125/250 indicates a fiber with a core of 50 microns, cladding of 125 microns, and a coating of 250 microns. The coating is always removed when joining or connecting fibers. A micron (Ám) is equal to one-millionth of a meter. 25 microns are equal to 1/1000 of an inch. (A sheet of paper is approximately 25 microns thick).

 

Fiber Types

Fiber can be identified by the type of paths that the light rays, or modes, travel within the fiber core. There are two basic types of fiber: multimode and single-mode. Multimode fiber cores may be either step index or graded index.

Step index multimode fiber derives its name from the sharp step like difference in the refractive index of the core and cladding.

In the more common graded index multimode fiber the light rays are also guided down the fiber in multiple pathways. But unlike step index fiber, a graded index core contains many layers of glass, each with a lower index of refraction as you go outward from the axis.

The effect of this grading is that the light rays are speeded up in the outer layers, to match those rays going the shorter pathway directly down the axis.

The result is that a graded index fiber equalizes the propagation times of the various modes so that data can be sent over a much longer distance and at higher rates before light pulses start to overlap and become less distinguishable at the receiver end.

 

Picture

Picture

Picture

Graded index fibers are commercially available with core diameters of 50, 62.5 and 100 microns. The single mode fiber allows only a single light ray or mode to be transmitted down the core. This virtually eliminates any distortion due to the light pulses overlapping. The core of the single mode fiber is extremely small, approximately five to ten microns.

The single mode has a higher capacity and capability than either of the two multimode types. For example, undersea telecommunications cables can convey 60,000 voice channels on a pair of single mode fibers.