Types of Fibres
are two main fibre types:
Step index (multimode, single mode)
Graded index (multimode)
Step index fibre is so called because the refractive index of the fibre 'steps" up as we move from the cladding to the core of the fibre. Within the cladding the refractive index is constant, and within the core of the refractive index is constant.
Figure 1 - Step Index Fibre
Single Mode Step Index Fibres
Because its core is so narrow , a Single Mode fibre can support only one mode. This is called the "Lowest Order Mode".
Figure 2-Single mode Step Index Fibre
ii) Multimode Step Index Fibres
Although it may seem from what we have said about Total Internal Reflection
that any ray of light can travel down the fibre, because of the wave nature
of light, only certain ray directions can actually travel down the fibre.
These are called the "Fibre Mode". In a multimode fibre many different
modes are supported by the fibre.
Figure 3- Multimode Step Index Fibre
2) Graded Index Fibre
Graded Index Fibre has a different core structure from single mode and multimode step index fibres. Whereas in a step-index fibre the refractive index of the core is constant throughout the core, in a graded index fibre the value of the refractive index changes from the centre of the core onwards. In fact it has a Quadratic Profile. This means that the refractive index of the core is proportional to the square of the distance from the centre of the fibre.
Figure 4- The Graded Index Fibre