Showing posts with label intramodal and intermodal dispersion. Show all posts
Showing posts with label intramodal and intermodal dispersion. Show all posts

24 Dec 2018

Dispersion in Optical Fiber - Intramodal Dispersion (Chromatic Dispersion) and Intermodal Dispersion

Dispersion in optical fibres

Broadening of the transmitted light pulses take place as the light rays move along the optical fibre. This broadening of light pulses is known as dispersion.


Intersymbol Interference (ISI) in Optical Fibers

Let's understand the dispersion with help of diagram given below-


Dispersion in Optical Fiber, Intersymbol Interference in optical fiber, ISI in optical fiber
Dispersion in Optical Fiber (Intersymbol Interference ISI)

You can see in this diagram that the light pulses that are sharp before transmission, gets broadened after travelling through the optical fibre. This Increase in width of the pulses makes it very difficult to distinguish them at the receiving end. Because of this light pulse broadening, these pulses overlap with its neighboring light pulses and it becomes hard to identify them as separate pulses at the receiving side. This effect is known as the Intersymbol Interference (ISI).
Now observe the same diagram carefully. Due to this dispersion effect (broadening of light pulses) the digital bit pattern 1011 at the input side is not indistinguishable at the output side as the same bit pattern. Because of this effect, '0' level is missing at the output side.

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Types of Dispersion in optical fibers

Dispersion in optical fibers can be categorized into two parts -
#Intramodal Dispersion (Chromatic dispersion) and
#Intermodal Dispersion (Modal or Mode dispersion)

Intramodal Dispersion (Chromatic Dispersion)

Intramodal dispersion may occur in all types of optical fibers. As we know that optical sources emit a band of frequencies so do not emit just a single frequency. Therefore different spectral components present in the optical source take different propagation delay while travelling through the optical fiber. This phenomena results in the broadening of each transmitted mode and is responsible for the intramodal dispersion. Intramodal dispersion is also popular by another name 'chromatic dispersion'.
Intramodal dispersion (chromatic dispersion) is found more in LED sources in comparison to LASER sources.
This delay difference may be caused by the dispersive properties of the material of the waveguide (material dispersion) and also guidance effects within the fibre structure (waveguide dispersion). 

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Material Dispersion

Pulse broadening because of material dispersion is caused due to different group velocities of various spectral components that are launched into the fibre from the optical source.
It occurs when the phase velocity of a plane wave that is propagating in the dielectric medium varies non-linearly with wavelength.


Waveguide Dispersion

Intramodal dispersion may also be caused due to waveguiding of the optical fibre. As the group velocity varies with change in wavelength for a particular mode, the waveguide dispersion takes place.
When the angle between the ray and the fibre axis varies with wavelength then it results in different transmission times for the rays which is responsible for dispersion.
Now we will discuss another kind of dispersion known as intermodal dispersion.

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Intermodal Dispersion (Modal or Mode Dispersion)

Intermodal dispersion is found in multimode optical fibres. Multimode fiber are the fibres that allow various modes to propagate through it. Therefore it is not observed in single mode fibers as only a single mode is allowed to propagate through the single mode fibre. But single mode fibres suffer from the intramodal dispersion (chromatic dispersion). 
The intermodal dispersion results due to propagation delay difference between various modes propagating through the optical fibre.


Intermodal Dispersion in Step Index Fibers, Intermodal Dispersion, Dispersion in Step Index Fibers
Intermodal Dispersion in Step Index Fibers  

Intermodal dispersion is found more in case of multimode step index fibres in comparison to graded index fibres. As in case of multimode step index fibres, the refractive index of the core is uniform. Because of this same refractive index throughout the core of the multimode step index fibre, different modes propagating through the core travel with same speed. Because of this same speed, different light rays launched into the optical fibre at different angles at the transmitting end takes different times to reach to the other end of the optical fibre as their propagation path (path length) changes with change in angle while launching.

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You can observe this phenomena in the diagram shown above. Because of this, intermodal dispersion is found more in multimode step index fibres.


Propagation of Light Ray inside Graded Index Fibers, Total Internal Reflection in optical fiber
Propagation of Light Ray inside Graded Index Fibers (Total Internal Reflection)

On the other hand, graded index fibres offer less intermodal dispersion as the refractive index of the core is not uniform in it. Refractive Index is maximum at the core axis and decreases as we move away from the core axis. So the refractive index is maximum at the core axis in case of graded index fibers. The refractive index of the cladding is uniform.
But how does this non-uniform refractive index of the core in graded index fibres help in reducing intermodal dispersion?
To understand it, carefully observe the diagram shown below.


Intermodal Dispersion in Graded Index Fibers, Dispersion in optical Fibers
Intermodal Dispersion in Graded Index Fibers 

As the light rays travel slower in denser mediums (high refractive index) and we also know that refractive index in case of graded index fibers is maximum at the core axis and decreases as we move radially away from the core axis towards the core-cladding interface. 

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We can transmit light rays at different angles into the optical fibre. So the light rays that travel near the core axis move slower in comparison to the light rays that travel near the core-cladding interface. 

You can easily understand this, that the light rays that travel near the core axis have to cover smaller distance in comparison to the rays that are close to the core-cladding interface. This creates a compensating effect in dispersion of light rays.

This phenomena is responsible for lower dispersion (broadening of light pulses) in case of graded index fibres in comparison to step index fibres because the light rays travelling at different angles in graded index fiber reach at the receiving end, almost at the same time. Because the light rays that need to travel longer distances (moving close to core-cladding interface) propagate at high speed because of lower refractive index (rarer medium) near the core-cladding interface. This reduces broadening (dispersion) of the light pulses.

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