3.3.2.3

Absorption and pulse broadening degrade signals -- but can be reduced

Refraction & Total Internal Reflection — AQA A-Level Physics

Absorption
  • The fibre absorbs part of the signal's energyThe capacity to do work. Measured in joules (J)., reducing its amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m)..
  • This can cause loss of information if the signal becomes too weak to detect.
Reducing signal degradation
  • Use an extremely transparent core to reduce absorption.
  • Use optical fibre repeaters at intervals to regenerate the signal before significant absorption or broadening occurs.
  • Use a narrow core to reduce modal dispersion.
  • Use a monochromatic source to eliminate material dispersion.
  • Use a single-mode fibre (only one wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). and one path) to minimise both types of dispersion.
Common Mistake MEDIUM
Students often: Confusing material and modal dispersion.
Instead: Material dispersion is about different wavelengths travelling at different speeds (use monochromatic light to fix). Modal dispersion is about different paths through the fibre (use a narrower core to fix).
Related:Diffraction
Refraction & Total Internal Reflection Overview
Material and modal dispersion cause pulse broadening 8 of 9 Worked example: calculating the angle of incidence at an optical fibre entry