Why multimode optical fiber commonly not used in telecom?

Why multimode optical fiber commonly not used in telecom? : I will discuss this issue in this article. To understand why multimode optical fiber is not used in telecom sector, we need to know about the structure of multimode fiber. We also need to know what are the characteristics of multimode fiber.

Whenever we talk about the characteristics of optical fiber, we do not miss to say that " optical fiber is high carrying capacity ".Would it be okay to say for multimode fibers that it is a high carrying capacity?Maybe it would not be very right to say for multimode optical fiber.

Why multimode optical fiber commonly not used in telecom

Multimode Optical Fiber

Multimode Optical Fiber

Multimode optical fiber, due to its large core diameter, allowing multiple mode of light to propagate through the fiber simultaneously. That's why this fiber is enough Sensitive to modal dispersion.

There are few advantages of multimode fiber is ease of pairing in light sources and other optical fibers, reducing the cost of light sources (transmitters), easy for connection and splicing.

However, it’s relative higher attenuation and/ or low bandwidth limit it is used for short distance (approx 2KM). Such as LAN communication.The main reason for being used for a short distance is that it has very high loss and very low bandwidth capacity.

Types of Multimode Optical Fiber

There are two type of multimode optical fiber:
1. Step Index Multimode Fiber
2. Graded-Index Multimode Fiber

1. Step Index Multimode Fiber : Multimode step-index fiber guides light rays on the boundary between core and cladding through the total reflection.In the core it has uniform refractive index. The minimum core diameter of multimode step-index fibers is 52.5 μm and 62.5 μm,numerical aperture between 0.2 and
0.5,cladding diameter approx100/140 μm.

Due to modal dispersion it has very low bandwidth, we can say that this is the drawback to this design. its bandwidth-length expressed as product in MHz x km.The bandwidth of this type of multimode fiber is approximately 20 MHz x km.It means it is suitable for carrying a 20 MHz optical signal only a distance of 1 km, or a 10 MHz signal a distance of 2 km, or a 40 MHz signal a distance of 0.5 km, etc.

Multimode step-index fibers have been applied in plastics; Their application area mostly in short-range links that can accommodate high attenuation.

2. Graded-Index Multimode Fiber : Multimode Graded-index (GI) fibers core are non-uniform refractive index, The refractive index is gradually decreasing from the central axis to the cladding. In a sinusoidal manner index variation of the core forces the rays to progress in the fiber.

Multimode Graded-Index attenuation are : 3 dB/km at 850 nm 1 dB/km at 1300 nm.
Multimode Graded-Index fiber numerical aperture is about 0.2.
Multimode Graded-index fiber bandwidth-length product is approximately:
160 MHz x km at 850 nm
500 MHz x km at 1300 nm.

Typical values of the group index :
1.49 for 62.5 mm at 850 nm
1.475 for 50 mm at 850 nm
1.465 at 1300 nm.

You can also READ : How optical attenuation can be determined?

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