Optical fibre cable construction & working

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Optical Fibre Cable

Optical fibre is at the heart of the communication networks. It took over from copper cables in the 70s and revolutionized communications. Today Optical fibres run through millions of kilometres over land and on sea beds connecting every corner of the world. In 2019 alone there was enough fibre to us to pull ourselves to the moon and back a good 430 times!!

So how did optical fibres come to be?

Well since the invention of telegraphs and telephones, copper cables have been connecting the world. However, they carried information in the form of electric forces which resulted in heat generation and high losses. This required installation of repeaters at short distances to regenerate the last electrical signal. with the rapid expansion of communication networks. There was a need for a more efficient mode of transmission. So by the early seventies, engineers have developed flexible strands of pure glass as thin as the human hair called optical fibres

What made the world choose optical fibre cable over copper cables?

These fibres had higher bandwidth capacity. Could help carry signals over longer distances faster and at much lower signal loss. They were thus most space and cost-efficient. One optical fibre today carries multiple signals of different wavelengths in the same time in the form of light each representing separate data channels. Hundreds and thousands of these optical fibres have been bundled to form an optical fibre cable.

How exactly does light travel inside the strand of a glass?

As light moves from one medium to another. It bends a little if the light hits the surface of a particular angle while moving from a denser medium to a rare medium, complete reflection of light takes place. This phenomenon is called internal reflection and it’s the principle by which optical fibres work.

How is optical fibre cable made?

Optical fibre is made up of the core and two the cladding which surrounds the core. Glass used in the core has a higher refractive index than the glass used in the cladding making the core denser than cladding using the principles of total internal reflection, light is trapped inside the core as is guided along the length of the optical fibre. However, while travelling through the core, it is also expressed through a slightly larger area including the inner edge of the cladding. This effective area is the fibres mode field diameter or MFD. There can be two types of optical fibres, single mode fibres, and multi mode fibres.

Single mode fibres:

This is the most widely used fibre. Having a very narrow core of around nine microns. It is ideal for long haul signal transmission applications such as across campuses, underseas or in remote offices. Currently, single mode fibres are optimized to operate as 1310 and 1550 nm in 1625 nm wavelengths.

Multi mode fibres:

These have higher light gathering ability but due to a larger core diameter of 15 or 62.5 microns. They simply connect the applications like data centres about a certain wavelength noses cut off for events. A fibre supports only a single mode. Multi mode fibres operates at 1850 and 1300 nanometers.

Does that mean optical fibres can carry signals without any losses?

No, any mode of signal transmission would have losses, but digression of signals inside an optical fibre is much lower over long distances in comparison to electrical and radio signals. The optical fibre major causes of losses would be attenuation, which happens due to absorption and scattering of light inside the core or micro bends with some axial distortions and core cladding interface caused majorly by the local mechanical stress placed in the cable to a manufacturing or packaging or largely due to macro bends. Macro Bend is the result of light leaking due to cable bends beyond the specified bend radius during installation.

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