Fibre Optics

• What principle is used to reflect light along optical fibres?

  Total internal reflection
• What happens to light when it enters an optical fibre?

  It refracts at one end

  
• How does light travel through an optical fibre?

  It undergoes repeated total internal reflection
• What occurs when light reaches the other end of the optical fibre?

  It is refracted back out
• What is the benefit of using optical fibres for light signals?

  They travel long distances without losing information
• What condition must be met for total internal reflection to occur in optical fibres?

  Angle of incidence must be greater than critical angle

  
• What are the main uses of optical fibres?

  • Communications (telephone, internet)
  • Medical imaging (endoscopes)
• What are the three main components of optical fibres?

  • Optically dense core tube (plastic/glass)
  • Lower optically dense cladding
  • Outer sheath
• What happens to a light ray at the core-cladding boundary in an optical fibre?

  It is totally internally reflected

  
• What is a step-index fibre?

  It has increasing refractive index from outside to center
• What is the role of the cladding in optical fibres?

  Protects core and prevents signal degradation
• How does the cladding maintain signal quality?

  It prevents light from escaping the core
• What is material dispersion in optical fibres?

  It causes pulse broadening when using white light

  
• What happens to white light in an optical fibre?

  It is separated into all colours of the spectrum
• Which colour of light travels the slowest in an optical fibre?

  Violet light
• Why does violet light take longer to travel down the fibre?

  It undergoes more reflections
• What is the relationship between angle of incidence and angle of reflection?

  Angle of incidence equals angle of reflection
• What causes modal dispersion in optical fibres?

  Different angles of incidence cause spreading

  
• How does the width of the fibre core affect modal dispersion?

  Wider cores increase modal dispersion

  
• How can modal dispersion be prevented?

  By using a very narrow core
• What is pulse broadening?

  It is caused by modal and material dispersion

  
• What effect does absorption have on a signal in an optical fibre?

  It reduces the amplitude of the signal
• What can result from pulse broadening?

  Merging of pulses distorts information
• How can pulse broadening and absorption be reduced?

  • Use an extremely transparent core
  • Use optical fibre repeaters
  • Use a narrow core
  • Use a monochromatic source
  • Use single-mode fibre
• How can you calculate the angle of incidence in an optical fibre?

  Using the speeds of light in core and cladding

  
• What is the speed of light in the core of the optical fibre?

  $2.027 \times 10^8 \text{ m/s}$
• What is the speed of light in the cladding of the optical fibre?

  $2.055 \times 10^8 \text{ m/s}$
• What is the formula for calculating the angle of incidence?

  Based on the speeds of light

  
• What happens to the light ray as it enters the optical fibre?

  It refracts along the core-cladding boundary

  
• What is the effect of using white light in an optical fibre?

  It causes dispersion into spectral components
• What is the significance of the critical angle in optical fibres?

  It determines the conditions for total internal reflection

  
• How does the angle of incidence affect the speed of light in the fibre?

  It influences the number of reflections
• What is the relationship between wavelength and speed in an optical fibre?

  Different wavelengths travel at different speeds
• What is the effect of using a single-mode fibre?

  It reduces multipath modal dispersion
• What happens to the signal if significant absorption occurs?

  Information transmitted may be lost
• What is the purpose of optical fibre repeaters?

  To regenerate the pulse before absorption
• How does the core diameter affect pulse broadening?

  Narrower cores reduce pulse broadening
• What is the impact of modal dispersion on signal transmission?

  It causes signals to arrive at different times

  
• What is the effect of using monochromatic light in optical fibres?

  It maintains a constant speed of the pulse
• What is the relationship between pulse broadening and signal clarity?

  Pulse broadening distorts the final pulse