As pulse travels down the fiber, dispersion causes pulse spreading. This limits the distance travelled by the pulse and the bit rate of data on optical fiber. In a fiber three distinct types of distortion are observed: 1. Intramodal dispersion: Pulse broadening within a single mode is called as intramodal dispersion or chromatic dispersion. Since this phenomenon is wavelength dependent and group velocity is a function of wavelength, it is also called as group velocity dispersion GVD.
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This broadening of light pulses is known as dispersion. This Increase in width of the pulses makes it very difficult to distinguish them at the receiving end. Now observe the same diagram carefully.
Due to this dispersion effect broadening of light pulses the digital bit pattern at the input side is not indistinguishable at the output side as the same bit pattern. 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. 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. 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.
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 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.
Because of this, intermodal dispersion is found more in multimode step index fibres. 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 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.
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. Read More-.
Material dispersion 2. Waveguide dispersion Intramodal Dispersion Chromatic or intramodal dispersion may occur in all types of optical fiber and results from the finite spectral linewidth of the optical source. Since optical sources do not emit just a single frequency but a band of frequencies in the case of the injection laser corresponding to only a fraction of a percent of the center frequency, whereas for the LED it is likely to be a significant percentage , then there may be propagation delay differences between the different spectral components of the transmitted signal. This causes broadening of each transmitted mode and hence intramodal dispersion. The delay differences may be caused by the dispersive properties of the waveguide material material dispersion and also guidance effects within the fiber structure waveguide dispersion.
What is Optical Fiber Dispersion?
The two definitions do not coincide in general, so the reader has to understand the context. Dispersion control[ edit ] The result of GVD, whether negative or positive, is ultimately temporal spreading of the pulse. This makes dispersion management extremely important in optical communications systems based on optical fiber, since if dispersion is too high, a group of pulses representing a bit-stream will spread in time and merge, rendering the bit-stream unintelligible. This limits the length of fiber that a signal can be sent down without regeneration.
What is Optical Fiber Dispersion? Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. Dispersion in optical fiber includes model dispersion, material dispersion and waveguide dispersion. Each type is discussed in detail below. Model Dispersion in Multimode Fibers Multimode fibers can guide many different light modes since they have much larger core size. This is shown as the 1st illustration in the picture above.