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0477-2013

Introduction to Fibre Optics (II)

Description: 

The new information technologies demand more stringent requirements, especially regarding speed of packet data transmission. If we add the digitization of video services, including high definition, the transmission speed and data multiplexing are the features that are setting the latest trends in electronics communication design.
The optical fibre suits perfectly to these needs. It’s the ideal transmission medium to achieve high transmission rates because of its minimal losses and its bandwidth.
The frequency response of an optical fibre is shown in the figure below. The operating band is the one with less losses. In that area you can see a deformation that years ago prevented the use of certain signals in the band of 1400nm. This response deformation is called water peak. Currently, the technology used in the manufacturing of fibre optics can

  have “zero water peak-fibres”.
Note that, in optical communications, the frequency is not used as a parameter characterizing the transmission band.
The parameter used is the wavelength. The reason is obvious: a 1300nm transmission is equivalent to a frequency near the 230.000GHz.
Therefore, the larger the wavelength the smaller the attenuation.
There are two types of fibre optics: multimode and singlemode. They are identified by the cladding diameter and its core.


In a multimode fibre, the light signal travels through the core in many rays. Each one is called “mode”. This fibre, compared with the one in

 

single-mode has a larger diameter in order to support multiple transmission modes.

The single-mode optical fibre has a smaller diameter, so that light travels in a single beam ("mode"). It is characterized by having a bigger bandwidth than the one of multimode fibres.

Single-mode fibres are usually used in 1310 and 1550 nm windows, where the attenuation is lower, reason why this type of fibre is used in long distance transmssions

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