Current technology supports two modes for propagating light along optical channels.
1. Multimode.
2. Single mode.
Multimode can be implemented in two form:
1. Step index.
2. Graded index.
Multimode :
Multiple beams from a light source move through the core in different paths.
How these beams move within the cable depends on the structure of the core as shown in the fig.
In multimode step-index fiber, the density of the core remains constant from the center to the edges.
A beam of light moves through this constant density in a straight line until it reaches the interface of the core and the cladding.
At the interface, there is an abrupt change due to a lower density, this alters the angle of the beam‟s motion.
The term step index refers to the suddenness of this change, which contributes to the distortion of the signal as it passes through the fiber.
A multimode graded index fiber decreases this distortion of the signal through the cable.
In this, density is highest at the center of the core and decreases gradually to its lowest at the edge.
Following fig shows the impact of this variable density on the propagation of light beams.
Single mode:
The single mode fiber is manufactured with a much smaller diameter than that of multimode fiber and with substantially lower density.
The decrease in density results in a critical angle that is close enough to 900 to make the propagation of beams almost horizontal.
In this case propagation is almost identical and delays are negligible.
All the beams arrive at the destination together and can be recombined with little distortion to the signal.