Optical Fibres relies heavily on two concepts of physics, the concepts of refraction, refractive indexes, critical angles and Total Internal Reflection. The concepts of refraction states that a ray of light travelling from a medium with a higher refractive index to a medium with a lower refractive index would bend away from the normal. With this in mind, it also states that a critical angle would be reached when the ray of light increases to an angle that will bend it 90 deg. away from the normal. The concept of Total Internal Reflection is apparent when the ray of light travelling from a higher refractive index medium to a lower refractive index medium has an angle so great it is able to refract the light greater than the critical angle, resulting in the ray reflecting back into the first high refractive index medium.

The main function for Optical Fibres is to send information through it by transmitting a beam of light from one end to another, trying to have as little quality loss as possible. The design of modern Optical Fibres reflect this and relies much on the physics involved.

Optical Fibres are more commonly used in surgery as a means of a tiny camera for surgeons in a patient's body and used together in their thousands in Fibre Optic communication cables for telephones and high-speed Internet connections. Each Optical Fibres has a core of thin glass, which is wrapped then by a cladding (usually a plastic with a lower refractive index than glass) and is then finally all wrapped in a harder plastic buffer to prevent damage. Refer to Diagram 4.

Diagram 4

When transmitting a signal, an L.E.D. or laser is used to send a beam of light in on/off pulses (digital) down the Optical Fibre to be received on the other end. It is either transmitted straight down the Optical Fibre, resulting in longer distance travel, or it is transmitted at an angle which is calculated to be greater than critical angle of the glass to the plastic, resulting in Total Internal Reflection and the ability for the beam of light to turn corners. Refer to Diagram 5.

Diagram 5

The angle of transmission of the beam into the glass is usually greater than 82 deg, which is required to achieve Total Internal Reflection when the light hits the plastic cladding. If the angle is less than 82 deg, the beam of light would be refracted out of the Optical Fibre. Refer to Diagram 6.

Diagram 6

That concludes the project on Optical Fibres, and other relevant information. To find out more, go to the next section 'Relevant Links'.

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