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
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