Step Index and Graded Index Fibre - Comparison between Step Index and Graded Index Fibre (Optical Fiber Cable)

Here is the comparison between step index and graded index fibres on the basis of various parameters like basic structure, working principle, types, refractive index profile, propagation of light inside fibers, dispersion in optical fibers, advantages and disadvantages-

1. In step index fibers, sudden (step) change takes place in the refractive index at the core-cladding interface. The refractive index of the core is uniform
While in case of graded index fibers, refractive index of the core is non-uniform. It is maximum at the core axis and then it decreases (generally parabolically) with increasing distance from the core axis.

2. Step index fibers are of two types on the basis of modes-
#Single mode step index fiber
#Multimode step index fiber

Multimode and Single Mode Step Index Fibers

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While on the basis of modes, only one type of graded index fibre is present i.e. Multimode graded index fibre.

Graded Index Fiber (Refractive Index Profile and Propagation of Light with TIR)

3. In step index fibres, light rays propagate in a zigzag manner (on zigzag path) inside the fiber Core. These rays travel as meridional rays. It means that the rays cross the axis of fibre for each reflection while propagating.

Step Index and Graded Index Fiber Video

 

But in case of graded index fibres, the light rays propagate as skew rays or helical rays. It means that while travelling inside the core these days do not cross the fibre axis.

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4. In case of step index fibers the refractive index profile may be defined as-

Refractive Index Profile of Step Index Fiber

And in case of graded index fibers, the refractive index profile is defined as -

Refractive Index Profile of Graded Index Fiber

5. Modal dispersion is present that affects signal quality in case of step index fibres.
But the graded index fibres have very low or zero dispersion as the time taken by each mode to propagate is same. As the velocity of each mode is different due to change in refractive index.

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Optical fibers in communication: covers all important points
Optical fiber: structure and working principle
Optical fiber sources (desirable properties)
Advantages of optical fiber communication 
Step index optical fiber (multimode and single mode step index fibers)
Single-mode optical fiber advantages