FIBER OPTIC BASIC FACTS

Optical fiber consists of three basic structures: the core, cladding and coating. Transmission occurs through the glass or plastic core. Light waves are transmitted down the length of the fiber by way of the interaction between the core and the cladding. The cladding provides a lower refractive index at the core interface which causes reflection of light exclusively within the core. Multi-layers of plastic are applied to provide fiber protection. The coating on the glass fibers enables them to be handled and processed into cable form.There are two types of fiber relative to the paths that the light rays, or modes, travel within the fiber core:

Multimode and single mode. Multimode fiber cores are classified as either step or graded index, depending on the refractive index. Step index multimode fiber has a uniform core and an abrupt change in refractive index at the core/cladding interface. In graded index multimode fiber the core contains many layers of glass, each with a lower index of refraction.

The light modes taking longer paths travel faster than the modes taking the straighter paths. This allows for all the light modes to be received at the same time. The main characteristic of single modefiber is that only a single light light ray, or mode, is transmitted down the core. Since the light pulses do not overlap, distortion is eliminated.

Graded index fibers are commercially available with core diameter/ cladding diameters of 50/125, 62.5/125, and 100/140 microns. Single mode fibers have a smaller core, typically 8.5µm, but also have 125µm cladding diameter. Coatings are available from 250 microns to 900 microns.




A Graphic Representation of How Light Rays Travel in Three Fiber Types