Optics and Fibre Optics

• What are the conditions for total internal reflection (TIR) to occur?

  Incident light speed must be slower in first material
• What is the role of the core in a step-index optical fibre?

  The core transmits electromagnetic waves via TIR
• Why is cladding used in optical fibres for communications?

  Cladding provides a lower refractive index for TIR
• What does modal dispersion mean in an optical fibre?

  Spreading of a pulse due to different travel times
• What happens to the critical angle when cladding is used?

  It increases
• How does cladding protect the core of an optical fibre?

  It prevents scratches and signal loss
• What is the effect of using a smaller diameter core in a fibre optic cable?

  It reduces modal dispersion and improves data transfer
• Why does red light have a shorter transit time than blue light?

  Red light has a lower refractive index than blue
• How does the difference in refractive index affect the pulse of white light in a fibre?

  It causes different colours to exit at different times
• What changes can be made to reduce material pulse broadening in fibres?

  Use monochromatic light and shorter repeaters
• What problems are associated with modal dispersion in optical fibres?

  It causes pulse broadening and limits bandwidth
• How can modal dispersion problems be reduced?

  Use monomode fibres with narrow cores
• What happens to the angle of incidence when bending a fibre optic cable?

  It decreases, increasing chances of light escaping
• What are bending losses in optical fibres?

  Attenuations due to bending the fibre
• What are the advantages of using cladding in optical fibres?

  • Provides a lower refractive index for TIR
  • Protects the core from scratches and damage
  • Prevents cross talk between fibres
  • Reduces pulse broadening and modal dispersion
• What are the impacts of bending a fibre optic cable?

  • Decreases angle of incidence on core-cladding boundary
  • Increases chances of light escaping into cladding
  • Causes bending losses leading to signal attenuation
• What are the effects of modal dispersion on signal transmission?

  • Causes pulse broadening
  • Limits bandwidth of the signal
  • Results in overlapping of pulses
• What are the benefits of using a smaller diameter core in fibre optics?

  • Reduces range of path lengths
  • Decreases modal dispersion
  • Improves data transfer rates