Propagation Modes
There are two types of modes of propagation of light in optical fibers
And there are two types of multi modes
Multimode is named because multiple beams from a light source move through the core in different paths.
And these beams move within the cable depends on the structure of the core.
Multimode Step Index:
In multimode step index fiber, the density of the core remains constant from the center to the edge.
A beam of light moves through this constant density in a straight line until it reaches the interface of the core and cladding.
The light ray travelling the straight path through the centre and reaches the receiving end before the other rays, which follow a zigzag path.
This difference in path length means, the different beams arrive at the destination at different times.
As these different beams are recombined at the receiver, they result in a signal that is no longer an exact replica of the signal that was transmitted.
This is known as modal dispersion.
Multimode Graded Index:
It decreases the distortion as compared to step index fiber.
A graded index fiber is one with varying densities.
Density is highest at the centre of the core and decreases gradually to its lowest at the edge and this is due to light rays travel faster through the lower index of refraction, the light at the fiber core travels more slowly than the light nearer the surface.
Therefore, both light rays arrive at exit point at almost the same time, thus reducing modal dispersion.
Single Mode:
It uses step index fiber and a highly focused source of light that limits beams to a small range of angles, all close to the horizontal.
The single mode fiber is manufactured with a much smaller diameter than that of multimode fibers and with substantially lower density.
Light sources for optical cable:
To occur transmission, the sending device must be equipped with a light source and the receiving device with a photosensitive cell (called a photodiode) capable of translating the received light into current usable by a computer.
The light source can be either a light emitting diode (LED) or an infection laser diode (ILD).
LEDs are the cheaper source, but they provide unfocused light that strikes the boundaries of the channel at uncotrollable angles.
Lasers, can be focused to a very narrow range allowing control over the angle of incidence.
Laser signals preserve the character of the signal over considerable distances.
