3.3.2.3

Material and modal dispersion cause pulse broadening

Refraction & Total Internal Reflection — AQA A-Level Physics

Key Definition
Pulse broadening — The lengthening (spreading out) of signal pulses as they travel through an optical fibre, caused by material and/or modal dispersion.
Material dispersion
  • Occurs when white light (multiple wavelengths) is used instead of monochromatic light.
  • Different wavelengths travel at different speeds in the fibre.
  • Violet light (shortest wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m).) travels slowest, so it undergoes more reflections and takes longer.
  • The pulse spreads out because different colours arrive at the other end at different times.
Modal dispersion
  • Occurs even with monochromatic light.
  • Different parts of the wavefront enter at different angles, giving different path lengths.
  • Some rays travel nearly straight; others bounce at steeper angles and travel longer paths.
  • More prominent in wider cores (more possible paths).
  • Solution: use a very narrow core to limit path differences.
Related:Diffraction
Refraction & Total Internal Reflection Overview
Optical fibres use total internal reflection to transmit light signals 7 of 9 Absorption and pulse broadening degrade signals -- but can be reduced