Optical Fibres

• Optical Fibres are thin, flexible tubes of glass or plastic that carry information in the form of light signals.
• The optically dense core is surrounded by a cladding with a lower optical density, allowing total internal reflection to occur.
• Signal degradation can be caused by the following: 1. Absorption. 2. Modal dispersion. 3. Material dispersion.
• Absorption is when part of the signals energy is absorbed by the fibre, reducing the amplitude of the signal, leading to a loss of information.
• Modal dispersion is when light rays enter at different angles, and thus, take paths of different lengths as they reflect. This means they arrive at different times, causing the signal to broaden.
• Material dispersion is when light of different wavelengths is used, meaning they travel at different speeds and therefore arrive at different times, causing pulse broadening.
• Modal dispersion can be reduced by using a narrower core that reduces the possible path differences of the light rays.
• Monochromatic light can eliminate the wavelength differences that cause material dispersion.
• Both absorption and dispersion can be reduced by using an optical fibre repeater that regenerates the signal as it travels along the fibre.