The refractive index measures how much the speed of light is slowed down as it travels through a material. The refractive index of the core is typically higher than the refractive index of the cladding, which causes the light to be confined to the core.
When we talk about classification based on the refractive index profile, we look at the specific relationship between the core’s refractive index and the cladding’s refractive index. By understanding this relationship, we can better understand the properties of the fiber and how it will affect the transmission of light through it.
Table of Contents
What are the types of optical fiber based on the refractive index?
There are two types of optical fibers based on the refractive index, and those can be divided into two subcategories. Here is the classification.
1. Step-index fibers:
- Step-index multimode fiber
- Step-index single-mode fiber
2. Graded-index fibers:
- Graded-index multimode fiber
- Graded-index single-mode fiber
Step-index fibers
The core has a uniform refractive index in step-index fibers. Its cladding has a lower index of refraction. Because of this, the cladding and core create a sharp boundary. We can call it a step.
Because of that step surface, the light reflects back into the core. By this reflection, the light can be propagated along the fiber core. There are two types of step-index fibers: 1. Multimode and 2. Single mode.
Step-index multimode fiber
Step-index multimode fiber is one of the earliest fiber designs. It has been used extensively in early fiber optic systems. It consists of a core made entirely of one type of optical material and a cladding made of another material with different optical characteristics.
In other words, the refractive index of the core is uniform throughout, while the cladding has a lower index of refraction.
However, the core’s large diameter makes step-index multimode fiber prone to modal dispersion. Different modes of light travel through the fiber in different paths. It will result in different arrival times at the receiver. This can lead to signal dispersion and limit the bandwidth and distance of the transmission.
Step-index multimode fiber has higher attenuation and lower bandwidth than other types of fiber optic cables. As a result, it is not widely used today in high-speed data communication applications. However, it is still used in some low-speed applications, such as short-distance data transmission and sensing systems.
In addition, step-index multimode fiber can be used in fiber optic lighting and decoration applications. It has a large core diameter. As a benefit of the large core diameter, it can easily couple with light sources. So, step-index multimode fibers are mostly used for illumination in decorative lighting applications.
Step-index multimode fiber was the initial fiber design used extensively in early fiber optic systems. Its high attenuation, dispersion, and lower bandwidth have limited its use in high-speed data communication applications. However, it still has some niche applications, such as short-distance data transmission and fiber optic lighting.
Step-index single-mode fiber
Step-index single-mode fiber is a fiber optic cable designed to transmit only one mode of light. As a result, the modal dispersion is minimal, significantly reducing the impact on the transmission distance. This makes single-mode step-index fiber ideal for long-distance communication applications.
The light ray propagates through the fiber through reflections. It will result in a signal that is virtually free from modal dispersion. Typical core sizes range from 2 to 15 µm. That’s why single-mode fiber is also known as fundamental or mono-mode fiber.
Step-index single-mode fiber can transmit only one mode and does not suffer from mode delay differences. It is an important advantage over multimode fiber. It is primarily developed for the 1300 nm window. But it can also be used effectively with time multiplex and wavelength division multiplex systems operating in the 1550 nm wavelength region.
In addition, step-index single-mode fiber is widely used in telecommunications, data centers, and other high-speed communication applications.
Its low attenuation and high bandwidth make it an ideal choice. It is also used in fiber optic sensing systems, where its ability to transmit light over long distances with minimal signal distortion is critical.
The core fiber of a step-index single-mode fiber is very narrow compared to the wavelength of light to be used. Therefore, only one path exists through the cable core through which light can travel. This makes single-mode fiber an attractive option for high-speed communication applications, particularly those that require long-distance transmission.
Graded-index fibers
The refractive index gradually decreases from the center of the core to the cladding in graded-index fibers. Because of this phenomenon, the light follows a curved path as it propagates through the fiber.
The graded index profile of fiber helps to reduce the dispersion of light. Then, it will allow high bandwidth and transmit over long distances. There are also two types of graded index fibers: 1. Multimode and 2. Single mode.
Graded-index multimode fiber
Graded-index multimode fiber is a fiber optic cable that uses variations in the composition of the glass in the core to compensate for the different path lengths of the modes. Unlike step-index multimode fiber with a uniform core index, graded-index multimode fiber has a non-uniform index profile.
The index profile of graded-index multimode fiber is highest at the center and gradually decreases until it matches the cladding. This gradual change in a refractive index reduces modal dispersion.
It will allow higher bandwidth and longer transmission distances than step-index multimode fiber. The gradual change in the index also eliminates the sharp break between the core and the cladding, which reduces reflection and signal loss.
Graded-index multimode fiber offers hundreds of times more bandwidth than step-index fiber. It is an ideal choice for high-speed data transmission applications.
It is primarily used for premises networks, LANs, fiber to the desk, CCTV, and other security systems. It is also used in data centers and other high-speed applications where the cost of single-mode fiber is prohibitive.
In addition, graded-index multimode fiber has applications in sensing and biomedical imaging systems, where the ability to transmit light over long distances with minimal signal distortion is critical.
Overall, graded-index multimode fiber is a versatile and widely used fiber optic cable. It offers higher bandwidth and longer transmission distances than step-index multimode fiber. Its applications range from high-speed data communication to sensing and biomedical imaging systems.
Graded-index single-mode fiber
Graded-index fibers are typically associated with multimode fibers. It has a larger core diameter that allows for multiple modes of light to propagate.
However, single-mode fibers can also have a graded refractive index profile, which can be tailored for specific applications, particularly those related to chromatic dispersion.
The graded refractive index profile of single-mode fibers may be non-parabolic. It means it is not a smooth curve and may be shaped like a triangle, trapezoid, or Gaussian distribution.
This is particularly important for dispersion-shifted fibers, which are designed to minimize the effects of chromatic dispersion at specific wavelengths.
Comparison table of optical fiber based on the refractive index
| Feature | Step-index multimode fiber | Step-index single-mode fiber | Graded-index multimode fiber | Graded-index single-mode fiber |
|---|---|---|---|---|
| Refractive Index | Uniform | Uniform | Graded | Graded |
| Core Diameter | Large | Small | Large | Small |
| Modal Dispersion | High | Minimal | Reduced | Minimal |
| Bandwidth | Low | High | High | High |
| Attenuation | High | Low | Low | Low |
| Distance | Short | Long | Medium | Long |
| Applications | Short distance data transmission, fiber optic lighting | Telecommunications, high-speed communication applications, fiber optic sensing systems | Premises networks, LANs, fiber to the desk, CCTV, and other security systems | Data centers, high-speed applications, sensing and biomedical imaging systems |
| Advantages | Large core diameter, easy to couple with light sources | Virtually free from modal dispersion, low attenuation, and high bandwidth | Higher bandwidth, longer transmission distances than step-index multimode fiber | Offers a good balance of performance and cost for high-speed data transmission |
| Disadvantages | Limited use in high-speed data communication applications due to high attenuation, dispersion, and lower bandwidth | Limited use in short-distance data transmission and sensing systems | Limited use in long-distance transmission due to modal dispersion | Higher cost than multimode fiber for high-speed transmission applications |
Summary
Fiber classifications depend on refractive index profiles. Step-index fiber uniformly structures the core while graded-index gradually decreases the core’s index from center to cladding.
Step-index multimode fiber accommodates multiple light modes but suffers high dispersion. Used for short-distance tasks like lighting, its large core easily couples light. Graded-index multimode fiber also conveys multiple modes through its refractive index gradient that offsets path differences and boosts bandwidth beyond step-index limits.
Step-index single-mode fiber conducts a sole light ray with minimal dispersion, ideal for long telecom runs. Graded-index single-mode variants further optimize dispersion for specific applications.
Each category exemplifies refractive index designs correlating to distinctive bandwidth, attenuation, distance, and use properties. Skillfully engineered profiles for single or multi-mode transmission underpin diverse modern communication and imaging solutions from data centers to security and biomedical technologies.
FAQ
Which is the most beneficial index profile in optical fibers?
The most beneficial index profile in optical fibers is the graded index. Graded index fibers gradually decrease the refractive index from the core center to the edge, ensuring that light rays travel at the same speed, minimizing dispersion, and enhancing signal quality. The benefits of graded index fibers include reduced dispersion, improved signal quality, and wider bandwidth than step-index fibers, allowing for more efficient data transmission.
What is the refractive index difference of optical fiber?
The refractive index difference of an optical fiber is the distinction between the refractive index of its core and cladding. This difference, typically around 0.01, enables light to be guided through the fiber via total internal reflection. However, the refractive index difference between fiber type and light wavelength can vary. Step-index fibers typically have a 0.01 difference, while graded-index fibers can be as low as 0.001. The wavelength also affects the difference, with shorter wavelengths exhibiting a larger disparity.
What is the relation between the refractive index and the optical index?
The refractive and optical indexes are terms related to light propagation in materials, but they have distinct meanings. The refractive index measures how much light bends when passing from one material to another, while the optical index measures how much light a material absorbs. Generally, materials with a high refractive index also have a high optical index, but exceptions exist. The refractive index describes the properties of optical fibers, allowing light guidance through total internal reflection. The optical index describes materials used in optical devices, like lenses and filters, where high optical index materials absorb more light, enabling filtering of specific wavelengths.
