Optical Fibres


  • Optical fibre consists of a core(thin glass centre) which is surrounded by a cladding layer with a lower refractive index than that of the core to reflect the light back to the core.
  • The optical fibre is surrounded at the outer layer by the buffer coating which is a plastic coating that protects the fibre from damage and moisture.
  • Two main types of optical fibres: monomode and multimode.


Single Mode Fibre

single mode fibre

  • The core size of single mode fibre is small(typically around 5 to 10 micrometer) hence only the straight through transmission path is possible(one mode).
  • Disadvantages: Require laser diodes, core to core alignment becomes critical during fibre splicing.
  • Advantages: Lower signal loss and higher bandwidth as compared to multimode fibres due to low fibre dispersion


Multimode Fibre

multimode fibre

  • Miltimode optical fibres has a much larger core diameter which allows for a larger number of modes.
  • Multimode fibres consist of step index fibre and graded index fibre.
  • Advantages: Light is launched into the fibre with more ease, larger core size makes it easier to make fibre connections, light emitting diodes(cheaper, less complex and last longer) can be used instead of laser diodes(single mode fibre).
  • Disadvantages: As the number of modes increases, the effect of modal dispersion increase. Modal dispersion affects system bandwidth. Modal dispersion (intermodal dispersion) means that modes arrive at the fibre end at slightly different transit times have the effect of changing the shape of the pulses of light that represent the signal, and causes the light pulse to spread.


Step-index Fibre vs Continuous-index Fibre

step index vs continuous index

Step index fibre

  • Step index fibres have an uniform core with one index of refraction, and the refractive index changes abruptly to a cladding with a smaller index of refraction.
  • Three different lightwaves travel down the fibre. One mode travels straight down the center of the core. A second mode travels at a steep angle and bounces back and forth by total internal reflection. The third mode exceeds the critical angle and refracts into the cladding. The second mode travels a longer distance than the first mode. Since the rays travel different paths, the different modes to arrive at separate times. This disparity between arrival times of the different light rays is known as dispersion, and the result is a muddied signal at the receiving end. The output pulse is spread out in time compared to the input pulse.

Graded index fibre (continuous-index)

  • In graded index fibre, the refractive index gradually decreases farther away from the core, hence making the boundary between cladding and core indistinct.
  • The higher index of refraction in the centre of the core slows the speed of some light rays, allowing all the light rays to reach the receiving end at approximately the same time, reducing dispersion and increasing bandwidth.


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