Difference between Step Index and Graded Index Fiber

Single Mode Graded Index Fiber

• Not a commonly used standard type of fibers.

• Used only for special applications.

Difference between Step Index and Graded Index Fiber

	Step Index Fiber	Graded Index Fiber
1	Refractive index of the core and that of the cladding are constants.	Refractive index of the cladding is constant. But RI of the core is not a constant and varies radically outwards.
2	Index profiles are in the shape of step.	Index profiles is in the shape of a parabolic curve (for α = 2).
3	For multimode fiber (MMF), rays travel in zig - zag manner in the core.	Rays travel along smooth parabolic curves.
4	Pulse broadening and inter modal dispersion is present.	No Pulse broadening and inter modal dispersion due to periodic self focusing.
5	Maximum number of modes that can be propagated. M = V2/2	Maximum number of modes that can be propagated, M= V2/4 (for α = 2)
6	A part of energy is lost due to attenuation	Loss of energy due to attenuation is very less (negligible)
7	Propagation is through TIR. That is, they are reflective type.	Propagation is through refraction and TIR. That is, they are refractive type
8	NA is constant	NA is a function of the distance from the axis of the core.
9	Less expensive	Highly expensive
10	Used for short distance communication	Used for long distance communication
11	This fiber has lower bandwidth	Higher bandwidth
12	Meridional rays are the propagation light ray. It crosses through fiber axis	Here, Skew rays and it will not cross fiber axis

Types of Optical Fiber and Refractive Index Profile

Optical fiber can be classified based on:

Based on number of mode that can propagate through fiber

• Single Mode Fiber can propagate only fundamental mode.

• Multi Mode Fiber can propagate hundreds of mode.

Based on refractive index profile: The value of RI as a ‘function of radial distances of the core’

• Step Index Fiber:

Refractive Index of the core is uniform, a step change of sudden decrease of RI at core-cladding interface.

• Graded Index Fiber:

Refractive index of core is not a constant, but that of cladding is constant. n₁ greater than n₂ and n₁ is varying. It is maximum along the axis of the core and decreases radially outwards with a distance from axis core and is minimum at core boundary. That is, ‘RI is graded’.