Difference between Step-Index Fiber and Graded-Index Fiber

Optical fiber technology is defined as technology that is capable to transmit information in the form of light pulses with the help of a high-quality glass or plastic material. It looks like a transparent filament. In this article, we will go through optical fiber, its types, and the differences between them. So, let's begin with the definition of optical fiber.

What is an Optical Fiber?

Optical fiber is thin a thin or flexible glass or plastic material used to transmit information in the form of light. Glass or plastic serves as a medium to allow the passage of information up to long distances within a very short instant of time.

Parts of an Optical Fiber

There may be different numbers of glass fibers. For its construction, we have the following main components:

• Core

• Cladding

• Buffer Tube

• Jacket Layer

Cladding covers the core of glass fiber. For the protection of the cladding layer, we have a buffer tube. For the final cover, optical fiber has a jacket layer for each strand of optical fiber. Light is present in the core layer with the help of a process called total internal reflection. This process reflected off the light in core ad cladding at different angles and it enables the optical fiber to carry large amounts of data without large loss or any large interference. This is how optical fiber works to carry large information.

Optical fibers are widely used due to their large transmit speed and larger bandwidth. They are commonly making their usage in telecommunication, internet connectivity, cable television, and in telephones. For instance, Google uses fiber optics in its Google Fiber services to provide large internet speed to its users around the world.

Types of Optical Fiber

Optical Fiber is classified into two types depending on the number of modes and refractive index. Here are these two types:

• Single-Mode Fiber - This type of optical fiber has only path or mode to transmit the light signal.

• Multi-Mode Fiber - This type of optical fiber provides multiple modes or paths to transmit multiple rays of light at the same time.

In Multi-Mode Fiber, we have further two types:

• Step-Index Fiber

• Graded Index Fiber

These two types of multi-mode fiber are based on the refractive index distribution. Now, we will discuss these two types in detail.

What is a Step-Index Fiber?

In the case of step-index fiber, the refractive index of the core and cladding is fixed or constant. The light ray passes through optical fiber and crosses the fiber axis in every reflection that occurs in the core-cladding boundary. In the case of step-index fiber, the light wave passes in the form of a zigzag shape according to the phenomenon of total internal reflection.

The incident light and input light of optical fiber have a difference in the time of reaching the output of the optical fiber. This difference is called model dispersion. Due to this effect, light pulse wide severely and spreads more while traveling along the optical fiber. Digital communication uses these light pulses to communicate long distances with the length of the optical fiber.

When a light pulse will transmit many modes in optical fiber, it will more spread out due to the dispersion effect. So, its bandwidth will decrease.

Step index will cause the following different limitations:

• The dispersion effect in step-index will cause interference between the pulses and it will drop the transmission capacity of optical fiber.

• Bit error also increased due to this effect and it is not helpful in step-index fiber.

• The distance for digital communication will also be affected by step index fiber due to the dispersion effect.

Due to these limitations, the step-index fiber is used where short-distance communication is required. Step index will provide lower speed and it has less cost.

What is a Graded-Index Fiber?

In the case of graded-index optical fiber, the refractive index is a bit higher at the surface of the core as compared to the core-cladding surface. It decreases from the core surface to the core-cladding surface. For more clarity, we should say, it decreases from its center.

Here, the process of change in refractive index cause refraction instead of total internal reflection in step-index mode. Because when the light will enter with a lower refractive index, it will be folded back to the fiber axis. Hence due to the folding back property, the total internal reflection will not be produced.

You can see light is traveling in the form of a sin wave. The speed of light varies due to changes in refractive index. When the light will move away from the center, its speed will increase more. For long distances, speed differences can be neglected or compensated. Hence the mode of dispersion is nullified or reduced to a great extent in graded index fiber. So, the graded index has large bandwidth as compared to step-index fiber.

The graded index can be used for medium or longer distances with a higher speed as compared to step-index fiber.

Difference between Step-Index Fiber and Graded-Index Fiber

The important differences between step-index fiber and graded-index fiber are given in the following table:

Parameter	Step-Index Fiber	Graded-Index Fiber
Definition	Step index fiber offers a constant refractive index and works with total internal reflection.	Graded index fiber offers different refractive indices at the core ad core-cladding boundary and works with folded-back reflection.
Bandwidth	Step-index fiber offers less bandwidth.	Graded index fiber offers larger bandwidth.
Effect of model dispersion	Step-index fiber has a severe effect by dispersion mode and has limitations on relay functions.	Graded index fiber has reduced or removed the dispersion mode effect and offers a larger bandwidth.
Core diameter	Step-index fiber has a 50-200 µm diameter.	Graded index fiber has nearly 55 µm diameter.
Cost	Step-index fiber has a lower cost due to the simple manufacturing process.	Graded index fiber has a higher cost due to the complex manufacturing process.
Communication distance	The step index is used for shorter distances and offers less speed.	The graded index is used for medium or longer distances and offers high speed than step index fiber.
Operation speed	Step-index fiber provides less operational speeds.	Graded index fiber provides higher operational speed.
Data wave shape	Step-index fiber provides a zigzag shaped path for light transmission.	Graded index fiber has a smooth sin wave shaped path for light wave to propagate.
Pulse spreading	In step-index fiber, more pulse spreading occurs.	In graded index fiber, comparatively less pulse spreading occurs for long distances.

Conclusion

Optical fiber is widely used in the modern communication systems due to its advantages over copper wires and other types of signal transmission mediums. We have read thoroughly about its types and the key differences between them. From the above discussion, it is clear that the step-index fiber is used for short distance communication while the graded index fiber is preferable for large distances with higher speed.