Optical Fibre Structure.

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Optical Fibre Structure is composed of a core, a cladding, and a buffer coating.

Also known as the three C’s.

  • Core being the most inner part of the fibre guides the light.
  • Cladding that surrounds the core completely.
  • Coating which protects the core and cladding from any potential harm.


fibre cable


The refractive index of the core is designed to be higher than that of the cladding. So, light in the core that strikes the boundary with the cladding at an angle shallower than the critical angle will be reflected back into the core by total internal reflection.


Internal reflection



For instance, the most common glasses used in the telecommunications and data communications markets are 50 micron and 62.5 micron multimode glasses and 8-9 micron Singlemode glasses. These glasses are typically written as:

Firstly, 50/125 or 62.5/125 for Multimode and Secondly, 8/125 or 9/125 for Single Mode




Therefore, the first number is the measurement of the core and the second number is the measurement of the cladding.

Both of these measurements are in microns.

So as an example when we state 50/125, the size of the core is 50 micron and the size of the cladding is 125 micron.


What is a Micron?

A Micron is a unit of measurement that measures length. Its’ other name is “micrometre,” which is one thousandth (10-3), of a millimetre or one millionth (10-6) of a meter. There are 1,000 microns in a millimetre and 10,000 microns in a centimetre.

The symbol used for a micron is μm. The Greek letter μ or legacy micro symbol µ. It is a unit prefix denoting a factor of 10−6 (one millionth)

On average, the human eye cannot see particles that are smaller than 50 to 60 micron. However we can see things as small as 40 microns.

We often refer to a human hair when comparing the size of the core and cladding in a fibre. A human hair is approximately 70 microns, plus or minus 5 microns given the thickness of the individual’s hair. And a human red blood cell in comparison is 8 microns. This is nearly the same size as a Singlemode fibre core, being approximately 9 microns.



diameter of human hair


These microns are often measured in Nano meters, which are one thousandth of a micron, or one billionth of a meter

So, a micron is really quite small but it has a big impact!


What is the Core?

According to the dictionary the word core can have two meanings:

 1. Tough central part of various fruits, containing the seeds.

 2. Part of something that is central to its existence or character.

The core of an optical fibre is definitely central to its character. It is a cylinder of glass that runs the whole length of the fibre. This is the area of the fibre that allows the light transmission to take place.


How does it work?

You start from SiCl4 and O2 in their gas state, and use heat or a catalyst to make the reaction go:

SiCl4 + O2 ———-> SiO2 + 2 Cl2

That is the scientific formula but what does that all mean? In simple terms;

It is made by careful construction of a large diameter glass tube known as a preform which is pulled to form the long thin optical fibre we know.

The core becomes the optically pure centre of the fibre. These cores are precisely centred. As already mentioned we have several core sizes to consider. The performance of the fibre cable is based around the core size and operations are limited by the core size as well.





What is Cladding?

The cladding, typically 125 microns in diameter for glass fibre, surrounds the core. It has a lower refractive index than the core to ensure all light stays within it.

For the cladding, knowing that the refractive index of the glass core must be higher than the cladding, the procedure is easy as well.

In the manufacturing process, dopants such as Germania (GeCl4), phosphorous pentoxide (P₄O₁₀) or alumina (Al₂O₃) are used to increase the refractive index, conducted under very strictly controlled conditions.

As a result of this, the refractive index of the core glass is increased. However, the attenuation (loss) is not affected. By the same token, you can add a bit of boron or fluorine to reduce the refractive index of the cladding glass. Both methods increase the variation in the refractive index, which is the key prerequisite for good light transmission!

A core refractive index might be 1.473 at 1300nm while the cladding may have a refractive index of 1.45. These numbers, however, are wavelength dependent. The core of the same fibre will have a different refractive index at different wavelengths.

index of refraction chart 



What is Coating?

The Coating is a layer of protection that wraps around the glass. You will most commonly find that the coating on any single fibre is 250-micron. This is often referred to as bare fibre. Fundamentally this 250-micron coating is used in all fibre cable constructions.

This initial coating is applied directly onto the cladding. A Loose Tube Cable construction simply keeps its original 250- micron coating.


anderson wire


Whereas with Tight Buffered Cable yet another coating is applied over the top of this 250-micron coating, to build it up to 900 microns, this protects the cable even more.

Patch Cables consist of the original 250-micron coating, plus the 900 Micron coating with an additional 2 or 3mm coating provided over the top.

This coating is soft and provides support for the optical fibre when it is bent. Additional coatings are mostly harder than the primary coating and provide a hard outer surface.



The three “C”s are the basis of any Fibre Optic Cable. Without any single one of these components, you would not be able to function.

1. Core; which carries the light signals;

2. Cladding; with a lower refractive index that surrounds the core and contains the light;

3. Coating; which protects the fragile fibre within it.

When all is said and done it is a simple as that!