What is Graded Index Fiber : Working and Its Applications

We know that multimode fiber is also known as step-index fiber, where the function of the radial position is the refractive index i.e, it is stable in some areas & exhibits steps at certain positions. So these are also known as graded-index fibers otherwise gradient index fibers because the refractive index changes easily within the radial direction. This can be attained through fabrication techniques of fiber The design of a graded-index fiber includes a parabolic shape from the axis of fiber that is away to a certain radial location. This article discusses an overview of graded-index fiber, working, and its differences.

What is the Graded Index Fiber?

Definition:  In optical fiber communication, a graded-index optical fiber has a refractive index. When the radial distance is increased from the fiber axis then the refractive index will decrease. Since the core parts are nearer to the axis of the fiber then that has a high refractive index compared with the parts close to the cladding, rays of light will follow sinusoidal lanes under the fiber.

The most frequent refractive index used in a graded-index fiber is parabolic which results in frequent refocusing of the emissions within the core & reduces modal dispersion. The designing of multimode fiber optic can be done using step-index otherwise graded-index.

The main benefit of the graded index as compared with the step-index is the great decrease within modal dispersion. Further, this dispersion can be reduced by selecting a lesser core size to form a step-index fiber in a single mode. This kind of fiber is regulated through the ITU (International Telecommunications Union) at G.651.1recommendation.

Graded Index Fiber Diagram

Under ITU (International Telecommunication Union), it is also known as G.651.1. It is one kind of fiber where the radial distance increases then the refractive index will be decreased slowly. In contrast, what we usually observed is a G.652.D fiber has a profile of a step-index refractive index. The graded-index fiber diagram is shown below.

Graded Index Fiber
Graded Index Fiber

In graded-index fiber, the refractive index of the core is not stable but reduces slowly from its utmost value (n1) at the center of the core to its least value (n2) at the interface of core-cladding that is illustrated in the following image. The main intension of designing Graded-index fibers is to have almost quadratic reduce & is examined through α-profile that is given by the following formula.


Graded Index Fiber Formula
Graded Index Fiber Formula

In the above equation,

‘ρ’ is the radial position

‘a’ is the radius of the core

‘α’ is the profile parameter,

‘Δ’ is the difference between the relative refractive number

Δ = n12-n22/2n12 = n1-n2/n1

Here the parameter like ‘α’ verifies the index profile and the profile of step-index fiber is move toward in the boundary of large ‘α’. A parabolic-index fiber communicates to α = 2.

It is very easy to understand why multipath dispersion and intermodal are decreased in these fibers. In the above diagram, we can observe that, the three rays in the fiber transmitted in different paths. For more angled rays, the path is longer. But, the velocity of the ray will change along with the path due to differences within the refractive index.

More specifically, the beam circulating along the axis of fiber will take the shortest lane, however, transmits slowly because the index is main along this lane.

Alternatively, angled rays take a large path, although they include a huge part of their lane through the low refractive index, therefore they move faster. So, it is possible for all signals to appear at once at the end of the fiber, providing that we select a proper selection of the α (refractive-index profile).

Graded-Index Multimode fiber

In this type of fiber, the core diameter ranges from 50 to 100 micrometers. When the core has a large diameter, then it allows numerous rays to circulate throughout the fiber. When the light signal travels in the fiber, then it will change its behavior through the time when traveling within it. Because we have already discussed that the core’s refractive index at the axis is relatively higher compare with the other part in it.

So once the light signal is permitted then it will circulate in the fiber, after that it transmits from low dense medium to high dense medium. So, the light signal in spite of being reflected, it gets refracted in the core.

Therefore, the transmitting light gets constantly refracted & bends. So in multimode fiber case, the light signals do not circulate by tracking a straight line, rather they track parabolic lane because of non-uniformity within the refractive index in the core.

But, some of the modes will transmit in a straight path or have low parabolic nature. As a result, these light signals will circulate slowly because of progress in high refractive index regions compare with which follow a highly parabolic lane.
The light signals which propagate throughout the region will leave from the axis that moves during the low refractive index area & transmits long distances but circulates quickly. As a result, the time taking to circulate will reduce at another side of the fiber. Therefore all the signals will travel through different lanes. This removes the probability of spreading in the core.

Difference between Step Index and Graded Index Fiber

The main differences between these two fibers are discussed below.

Step Index Fiber

Graded Index Fiber

In this fiber, the core’s refractive index is stable throughout the core.In this fiber, the core’s refractive index of the graded-index fiber is utmost at core, center & then it reduces in the direction of the core-cladding interface.
The propagation of  light is in a zigzag wayThe propagation of light is in a helical way.
It has low bandwidthIt has a high bandwidth
These are two types like mono mode & multi-modeThis is only one type like multi-mode fiber


For each reflection, the ray crosses the axis of the fiber.The rays in this fiber will not cross the axis of the fiber.
The process of manufacturing is easyThe process of manufacturing is complex.


The advantages of graded-index fiber include the following

  • By using this fiber, a large amount of data can be transmitted
  • Compare with step-index, the distortion is relatively small


The disadvantages of graded-index fiber include the following

  • It has less light coupling efficiency.
  • It is costly as compared with step-index fiber.

Applications of Graded-Index Fiber

The applications include the following.

  • Generally, graded-index multimode fiber is used in comparatively less bandwidth and short-haul applications LANs (local area networks) which runs at 1 Gbps otherwise less.
  • SMF or Step-index single-mode fiber is used in high BW and long haul applications like carrier backbones.

Thus, this is all about an overview of graded-index fiber. From the above information finally, we can conclude that, in this fiber, the information signal which is transmitted can be circulated well and the chances of dispersal are also fewer in this case. Here is a question for you, what is optical fiber?

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