Question

1. Optical fibers can be classified according to the number of modes and their refractive index profiles. Explain what you know about optical fibers with this classification! Why is singlemode optical fiber widely used as a backbone optical fiber communication system?

Answer 1

According to number of modes optical fiber are classified into two types. One is (i) single-mode optical finer and other is (ii) multi-mode optical fiber.

(i) Single-mode optical fiber:- A single-mode optical fiber is designed to carry only a single mode of light- the transverse mode.The diameter of this type of fiber is relatively low, about 8.3 to 10 microns and it operates at wavelength of 1310nm and 1550 nm.This type of optical fiber has less degradation of signal, low dispersion and well suited for long distance communication.But the price of single-mode optical fiber is higher. ​

(ii) Multi-mode optical fiber:- A multi-mode optical fiber is a optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angel within the optical core. This type of optical fiber has larger core about 50 to 100 microns and is usually operates at 850nm and 1300 nm.Multi-mode optical fiber has more degradation of signal, more dispersion and is suitable for short range communication. Prince of multi-mode optical fiber is low comapre to single-mode optical fiber.

# According to refractive index profile optical fibers are also classified into two types. One is (i) Step-index optical fiber and (ii) Graded-index optical fiber.

(i) Step-index optical fiber:- Step index optical fiber are the type of optical fiber in which refractive index within the core is uniform and is sharply decreases at the core cladding interface so that the cladding is of lower refractive index. Thus in other words it has two type of refractive index, the refractive index of core is greater than that of the cladding.

(ii) Graded-index optical fiber:-​​​​​​ A graded-index optical fiber is defined as that optical fiber whose core has a refractive index that decreases with increase in radial distance from the optical axis of the fiber. Because parts of the core closer to the fiber axis have a higher refractive index than parts near to cladding, the light rays follow a Sinusoidal paths down the fiber.

## The single-mode optical fiber are widely used because they have many facilities over multi-mode optical fiber. The multi-mode optical fibers has more degradation of signal and more distortion. But in single-mode optical fiber signal degradation and distortion are less. Also they transmit data at a higher rate. So single-mode is suitable for long range communication. In modern life we need to communicate all over the world i,e long range communication is needed and thus single-mode is appropriate for that.For these reasons singlemode optical fiber widely used as a backbone optical fiber communication system.