AQA A Level Physics

Revision Notes

3.5.3 Fibre Optics

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

Optical Fibre, downloadable AS & A Level Physics revision notes

  • 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
    • ncladding < ncore
  • 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 & Modal Dispersion

  • 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

Material & Modal Dispersion (1), downloadable AS & A Level Physics revision notes

Material & Modal Dispersion (2), downloadable AS & A Level Physics revision notes

  • 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

Pulse Broadening & Absorption

  • 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

Absorption & Pulse Broadening, downloadable AS & A Level Physics revision notes

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

Worked Example

A cross-sectional view of a step-index optical fibre is shown in the diagram.

Optic Fibres Worked Example (1), downloadable AS & A Level Physics revision notes

The ray of light enters the end of the fibre and refracts along the core-cladding boundary

Calculate the angle of incidence, θ, of the ray at the point of entry to the fibre.

The speed of light in the core is 2.027 × 108 m s–1

The speed of light in the cladding is 2.055 × 108 m s–1

Optic Fibres Worked Example (2), downloadable AS & A Level Physics revision notes

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Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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