Which fiber is preferred for long distance communication?

Question

Question

This question was previously asked in

IPRC ISRO Technical Assistant Electronics 10 Dec 2016 Official Paper

Answer 1

Single-mode step indexed fibers are widely used for wideband communications are preferred for long-distance communication.
Single-mode step-index fiber is used to eliminate modal dispersion during optical communication.
In this fiber, a light ray can travel on only one path so minimum refraction takes place hence, no pulse spreading permits high pulse repetition rates.

Advantages of single-mode fiber:

1) Low signal loss

2) No modal dispersion

3) Does not suffer from modal dispersion

4) Can be used for higher bandwidth applications

5) Long-distance applications

6) Cable TV ends

7) High speed local and wide area network

Single-mode means the fiber enables one type of light mode to be propagated at a time. This is explained with the help of the following diagram:

Single-mode fiber core diameter is much smaller than multimode fiber.

2) For single-mode fiber, the B.W ranges from 50 to 100 GHz/km

Multimode fibers:

Fibers that carry more than one mode are called multimode fibers. There are two types of multimode fibers:

1) Step Index

2) Graded Index

The comparison of the refractive index profile for step and graded fibers are respectively shown as:

The multimode step-index multimode fiber suffers from Modal dispersion.

Rays of light enter the fiber with different angles to the fiber axis. The limit is the fiber’s acceptance angle.
Rays that enter with a shallow angle travel a more direct path and arrive sooner than those that enter at steeper angles.
This arrival of different modes of light at different times is called modal dispersion.