Fibre Optics

The Properties of Light Applied to Fibre Optics

One of the basic properties of light is that it travels in straight lines and can have it's

direction changed. This is the property which is applied to achieve the purpose of Fibre

Optics which is to carry light from one place to another. The way this is done can be

explained by the principle of refraction. This is the principle which governs the behaviour

of light as it passes from one transparent substance to another. It states; if a ray of light

travelling in air make contact with a glass block at a slant, some light is reflected back

but most enters the block and is refracted (bent) away from the surface. The ray is called

the incident ray before it reaches the bend and the reflected ray after it has been refracted.

Light also refracts as it leaves the block but back

towards the surface. This refraction is the result of a change in the speed of the light as it

travels through the air and the glass block. Although this is true, when dealing with

optical fibres the speed of the light is not usually referred to. Instead, the measure used is

the material's refractive index. This is calculated for each material by dividing the speed

of light in a vacuum by the speed of light in the material. The relative refractive index

between two materials is calculated by the speed of the incident ray divided by the speed

of the refracted ray. If this calculation works out to less than one, then the light will be

refracted towards the surface when it is leaving the material. (eg. light from glass block

into air) At a certain angle of the incident ray, the angle of the refracted ray becomes 90

degrees as the light runs along the surface. If the angle of the incident ray is larger at this

point then the light can't escape from the block as it is reflected back inside. This is called

a total internal reflection and this is how light is trapped inside optical fibres.

The light inside an optical fibre travels in a zigzag manner. The incident ray emerges

slightly before being totally internally reflected because this supplies the change in light

speed which is needed for refraction to occur. To aid this process, an optical cladding was

developed. This cladding has a lower refractive index and is used to protect the surface of

the core which is the middle of the fibre. The light now emerges slightly into the cladding

instead of onto the surface of the fibre to avoid being affected by any dirt or scratches

which could be on it. This idea of cladding was the most important development which

has made fibre optics practical.

When trying to get light into an optical fibre, you need to know which light rays will

be trapped in the fibre and which will escape through the cladding. There are two

different measure required to determine this. The first is the acceptance-cone half angle.

This is the angle of the largest cone of light which will be trapped inside the fibre. Any

other light rays which enter the fibre at larger angles then this one pass through the

cladding and become either trapped in the cladding or escape the fibre. The second is

numerical aperture. The numerical aperture of a fibre is the sine of it's acceptance-cone

half angle above which light can enter the core but will not be guided by the cladding. A

high NA fibre is able to accept more light than a lower NA fibre. Normally, the NA of a

glass fibre is 0.64 which means an acceptance angle of 40 or 80 degrees. Although this is

true, not all the light within the acceptance cone gets into the fibre. Approximately 8% of

the light is reflected away when entering and leaving the fibre through something called

Fresnel reflection.

Optical Properties of Optical Fibres

It is hard to define and detail the optical properties of fibres because they are greatly

varied. This is because of the many application areas and the fact that optical properties

have been changed to suit these certain applications. One of the most common optical

fibres is glass fibre bundles so we will relate the optical properties to this.

Attenuation is a very important optical property. It is when light is inside a fibre and

it becomes dimmer as it travels along the fibre. Lambert's Law is the application of this.

The law states: equal lengths of material cause equal amounts of attenuation. This means

that attenuation is exponential or as the then length of the material increases, so does the

amount of attenuation at the same rate. The attenuation of a fibre is measured in decibels

per kilometre of fibre (dB/km). The level of attenuation depends on three main things;

the fibre's construction, the colour of light and the frequency at which the intensity of the

light is varied.

Mechanical Properties of Optical Fibres

Materials such as glasses are strong by nature. When they are made into fibres, their

strength can decrease due to flaws on the surface of the material. This can cause the

defectiveness of fibres when they are put under great amount of pressure of tension. This

problem greatly increases as the length of the fibre increases. To help offset the failure of

these fibres, different materials are used for different applications depending on which are

more suited for the task. An example of this is telecommunications fibres which are

almost always silica with a cladding diameter of 125 microns. This allows the fibre to

endure high vibration frequency.

When a fibre cable is designed, it is always taken into account what the application

of it will be and the amount of strength and what properties it will need. This is apparent

with things such as how a fibre can withstand heat. Glass fibres have a range of between -

60 to 400 degrees Celsius, silica fibres can reach 800 degrees Celsius and plastic fibres

can reach 800 degrees Celsius.

Optical Fibre Bundles

Glass fibres which are made into bundles are usually manufactured by a process

called the rod-in-tube method. This is done by placing a rod of core glass inside a tightly

fitting cladding glass tube. The two are suspended and slowly moved into a furnace which

causes the tube to collapse onto the rod and the diameter of the two are reduced.

Fibre bundles are charcaterized by many different things such as the packed bundle

diameter, the individual fibre diameter, the fibre numerical aperture, the fibre attenuation

or the core to cladding ratio.

By: Christine Phelan

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