Fiber optic cable is a communication medium made of glass or plastic that can transmite signals in the form of light. In optics there is a principle which states that when a beam of light travels from denser medium to rerer medium then it deviates away from the normal drawn at the point of incidence at the junction of the two media.
If the angle of incidence is such that the angle of refraction is 90 degree then the angle of incidence is called critical angle. Now if the angle of incidence is greater then critical angle then the refracted ray would re-enter into the denser medium. This phenomenon is called total internal reflaction.
In optical fiber light signals travel by following the principle of total internal reflaction. In an optical fiber, a glass or plastic core is surrounded by a cladding of less dense glass or plastic. This difference in density facilities total internal reflection for a light signal that travels through the cable.
Current technology provides two modes of light propagation through an optical fiber. These two modes are MONO MODE and MULTI MODE. Further fiber optic cable has two varieties STEP INDEX and GRADED INDEX.
In Step index fiber, core and cladding has a definite boundary. In graded index fiber, density of the fiber is gradually decreases from inner core to the outer boundary.
In mono mode, step index fiber is used. Mono mode fibers are of the order of 8-10 microns in diameter. Mono mode fibers are costly and they can transmit light signal to a longer distance then multi mode fiber. A highly focused beam of light is injected into the fiber at an incident angle close to the horizontal line. Its density is also much lower so that the critical angle is almost equal to 90 degree. Therefore the reflected beam also propagates in almost horizontal line. The delay is minimum here.
Multi mode operation can transmit multiple beams of light signal at a time. These fibers are thicker in diameter then the mono mode fiber, of the order of 50 microns. These are cheaper but can transmit signal to a shorter distance. In multi mode both step index and graded index cables are used.