Fibre Optics

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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

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