3.5.3 Fibre Optics

• Fibre optics utilise the phenomenon of total internal reflection to send high speed light signals over large distances
• These have many important uses, including:
  • Communications, such as telephone and internet transmission
  • Medical imaging, such as endoscopes

A light ray is totally internally reflected down an optical fibre

• There are three main components that make up optical fibres:
  • An optically dense core, such as plastic or glass
  • A lower optical density cladding surrounding the core
  • An outer sheath

• Since the refractive index of the core is more than the refractive index of the cladding, this allows TIR to occur
  • n_cladding < n_core

• The outer sheath:
  • Prevents physical damage to the fibre
  • Strengthens the fibre
  • Protects the fibre from the outside from scratches

• The cladding is also required because:
  • It protects the core from damage
  • It prevents signal degradation through light escaping the core, which can cause information from the signal to be lost
  • It keeps signals secure and maintains the quality of the original signal
  • It prevents scratching of the core
  • It keeps the core away from adjacent fibre cores hence preventing crossover of information to other fibres
  • It provides the fibre with strength and prevents breakage given that the core needs to be very thin

• Material dispersion, or spectral dispersion, occurs when white light is used instead of monochromatic light
• This is because different wavelengths of light travel at different speeds
  • Blue light travels slower than red light due to the greater refractive index
  • Therefore, the red light reaches the end before the blue light

• This results in pulse broadening, and to prevent this, monochromatic light must be used
• Modal dispersion occurs when the light pulses in the optical fibre spread out due to the different angles of incidence in the original pulse
• This is more prominent in wider cores as the light travelling along the axis of the core travels a shorter distance than light undergoing total internal reflection at the core-cladding boundaries
  • To prevent modal dispersion, the core needs to be very narrow

• This also causes pulse broadening as the pulses emerging are longer than they should be

• The advantages of using a narrow core are:
  • Less light is lost by refraction out of the core
  • There is a smaller change in angle between each reflection, so the angle of incidence is less likely to fall below the critical angle
  • Less overlapping pulses hence reduction of modal dispersion
  • The quality of the signal will be better and less distorted
  • The signal will be transferred quicker leading to improved data and information transfer

• Absorption occurs when
  • Part of the signal’s energy is absorbed by the fibre
  • The signal is attenuated by the core

• This reduces the amplitude of the signal, which can lead to a loss of information
• Pulse broadening is caused by modal and material dispersion
• The consequence of pulse broadening is that different pulses could merge, resulting in a completely distorted final pulse

Reducing Pulse Broadening & Absorption

• To reduce absorption:
  • Use a core which is extremely transparent
  • Use of optical fibre repeaters so that the pulse is regenerated before significant absorption has taken place

• To reduce pulse broadening:
• • Make the core as narrow as possible to reduce the possible differences in path length of the signal
  • Use of a monochromatic source so the speed of the pulse is constant
  • Use of optical fibre repeaters so that the pulse is regenerated before significant pulse broadening has taken place
  • Use of single-mode fibre to reduce multipath modal dispersion