Optical Fiber Communication: Principles and Ray Optics, Cheat Sheet of Natural Resources

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Optical Fiber Communication

EC- 4313

Lecture 02

Subject Taught By:

Dr. Surjeet Raikwar

Assistant Professor

The general system Vs Optical Fiber System

Figure (a) The general communication system. (b) The optical fiber system

In the following block diagram: Initially, the input digital signal from the information source is suitably encoded for optical transmission. The laser drive circuit directly modulates the intensity of the semiconductor laser with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable. The avalanche photodiode (APD) detector is followed by a front-end amplifier and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth reduction. Finally, the signal obtained is decoded to give the original digital information. The various elements of this and alternative optical fiber system configurations are discussed in detail in the following chapters. However, at this stage it is instructive to consider the advantages provided by light wave communication via optical fibers in comparison with other forms of line and radio communication which have brought about the extensive use of such systems in many areas throughout the world. Figure. digital optical fiber link using a semiconductor laser source and an avalanche photodiode (APD) detector

Ray Optics

Basic laws of ray theory/geometric optics

• The basic laws of ray theory are quite self explanatory
• In a homogeneous medium, light rays are straight lines. Light may be absorbed or reflected.
• Reflected ray lies in the plane of incidence and angle of incidence will be equal to the angle of reflection.
• At the boundary between two media of different refractive indices, the refracted ray will lie in the plane of incidence. Snell’s Law will give the relationship between the angles of incidence and refraction.

Refraction

1. Refraction occurs when light ray passes from one medium to another i.e. the light ray changes its direction at interface. Refraction occurs whenever density of medium changes. E.g. refraction occurs at air and water interface, the straw in a glass of water will appear as it is bent. The refraction can also observed at air and glass interface.

2. When wave passes through less dense medium to more dense medium, the wave is refracted (bent) towards the normal. Fig. shows the refraction phenomena. 3.The refraction (bending) takes place because light travels at different speed in different mediums. The speed of light in free space is higher than in water or glass.

Refractive Index The amount of refraction or bending that occurs at the interface of two materials of different densities is usually expressed as refractive index of two materials. Refractive index is also known as index of refraction and is denoted by n. Based on material density, the refractive index is expressed as the ratio of the velocity of light in free space to the velocity of light of the dielectric material (substance). The refractive index for vacuum and air is 1.0 for water it is 1.3 and for glass refractive index is 1.5.

The critical angle can be calculated from Snell's law, putting in an angle of 90° for the angle of the refracted ray θ2. This gives θ1 : Since

So

sin(θ2) = 1

Then

θc = θ1 = arcsin( n 2/ n 1)

Critical angle

Total internal reflection

• If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection.

Q.1. A light ray is incident from medium- 1

to medium-2. If the refractive indices of

medium-1 and medium-2 are 1.5 and 1.

respectively then determine the angle of

refraction for an angle of incidence of 30º.

(Answer=33.46º)