Differences between step index fibres and graded index fibres:

Step index fibre 

1. In step index fibres the refractive index of thecore medium is uniform through and undergoesan abrupt change at the interface of core and cladding.
 2. The diameter of core is about 50 to 200micrometers in case of multi mode and 10micrometers in single mode fibre
 3. The transmitted optical signal will cross thefibre axis during every reflection at the corecladding boundary.
 4. The shape of propagation of the optical signalis in zigzag manner.
5. Attenuation is more for multi mode step index fibres but Attenuation is less in single mode step index fibres
6. Numerical aperture is more for multi mode step index fibres but it is less in single mode step index fibres

  Graded index fibre
 1. In graded index fibres, the refractive index of the core medium is varying in the parabolic manner such that the maximum refractive indexis present at the center of the core.
 2. The diameter of the core is about 50 micro meters.
3. The transmitted optical signal will never cross the fibre axis at any time.
4. The shape of propagation of the optical signal appears in the helical or spiral manner
 5. Attenuation is very less in graded index fibres
 6. Numerical aperture is less in graded index fibres