Compare dispersion effects single mode and multimode fibre, Electrical Engineering

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Compare dispersion effects in single mode and multimode fibres?

The optical fibres were categorized within two according to the number of modes it passes as:
• Single mode fibres
• Multi mode fibres

The single mode fibre is which one that passes just the axial ray (fundamental mode) and the multimode fibre passes number of modes by it.

Dispersion of the transmitted optical signal causes distortion for both digital and analog signal transmission by the fibre. When considering the main implementation of the optical fibre transmission that includes some form of digital modulation, the dispersion mechanism inside the fibre causes the broadening of the transmitted light pulse as they travel along the cannel. The dispersion can be separated within two as:

• Intermodal dispersion
• Intramodal dispersion/ chromatic dispersion

The purpose for the intermodal dispersion is number of modes within the fibre. This is the main source of dispersion in multimode fibres.
The Intramodal dispersion occurs in all kinds of optical fibres and results from the finite spectral width of the optical source. Because optical sources do not emit just a single frequency, other than a band of frequencies, then there will be propagation delay differences among various spectral components of the transmitted signal. This causes the broadening of each transmitted mode and therefore intramodal dispersion. The delay differences might be caused through the dispersive properties of the waveguide material (material dispersion) and also the guidance effect inside the fibre structure (waveguide dispersion).

The intermodal dispersion within single mode fibre is extremely low. But in multimode fibres the intermodal dispersion is extremely prominent. Since in multimode fibres number of modes passes by the fibre. The different modes cover various distances and thus reach at the destination at different times. Therefore an input pulse is get as a broadent pulse at the destination. To avoid this problem in multimode fibres the refractive index is varies slowly in the core so that the different rays travels about equal distance and therefore reaches at the destination at the same time. These fibres are known as multimode graded index fibres. Therefore we say in which the dispersion for a graded index fibre is smaller compared to multimode step index fibre other than it is higher compared to single mode fibre. These are describing as shown below:

2134_single mode and multimode fibres.png


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