General optical communication

 

 THE GENERAL COMMUNICATION SYSTEM AND ANALOG

OPTICAL COMMUNICATION SYSTEM

Diagram 1 represents the general communication system. The function is to

convey the information from the information source over the transmission medium og destination. 

OPTICAL COMMUNICATIONS:-

1) Information source:-

 Any communication system serves to communicate either a

massage or information, this massage originates in the information source.

The message can be in the form of words, group of words, code symbols

etc., out of these only the desired message is selected and conveyed. The

part of the message which is conveyed is called information.

2) Transmitter:-

 The message from the information source may or may not be in

electrical nature. If the message is not an electrical signal, then it has to

be converted into an electrical signal by means of transducer.

 For example, in radio broadcast transmitter, a microphone converts

the information in the form of sound waves into a corresponding

electrical signal.

3) Transmission medium (or) Channel:-

 The term transmission medium or channel implies the medium

through which the message travels from the transmitter to receiver. For

example, pair of wires, a coaxial cable or radio link through a free space.

4) Receiver:-

 Receivers in the communication system are of different types

depending on the system requirements such as modulation system used,

operating frequency, range of the system etc. Receiver do the exact

reverse process of the transmitter does. For example, in radio broadcast

receiver, loud speaker converts the form of electrical signal into a sound

waves.

 The diagram 2 represents block schematic of the different elements

in an optical fiber communication system. The carrier is modulated using

analog information signal. The variation of light emitting from the

optical source is a continuous signal. The information source provides an

electrical signal to the transmitter. The transmitter comprises electrical

stage. The electrical stage (circuits) drives an optical source. 

 At the receiver the optical signal is detected by the optical

detectors such as PIN diode and Avalanche photodiode.

 Sometimes photo transistors and photo conductors are used for

converting an optical signal into electrical signal. The electrical signal

is again processed and given to the transducer to get the original

information.

 Digital Fiber optical Communication System

 Figure shows a schematic of a typical digital optic fiber link. 

The input is given as digital signal from the information source and it is 

encoded for optical transmission in the encoder. The encoder, encodes 

or modulates the digital signal as in the case of simple communication 

system where we are using a message signal in which the signal is in 

analog form, but here the signal is in digital form which is encoded i.e., 

modulated in the encoder. The laser drive circuit directly modulates the 

intensity of semiconductor laser with the encoded digital signal. Hence 

a digital optical signal is launched into the optical fiber cable. At the 

receiver we have to decode the digital optical signal for which we are 

using another Avalanche Photo Diode (APD) as detector. The 

avalanche photo diode detector is followed by a front-end amplifier and 

equalizer or filter to provide gain as well as linear signal processing and 

noise bandwidth reductions. Then the signal is passed through the 

decoder to get original digital information which is transmitted.

ADVANTAGES OF OPTICAL FIBER COMMUNICATIONS

1. Information bandwidth is more.

2. Optical fibers are small in size and light weighted.

3. Optical fibers are more immune to ambient electrical noise, 

electromagnetic interference.

4. Cross talk and internal noise are eliminated in optical fibers.

5. There is no risk of short circuit in optical fibers. source output is a light which is intensity modulated by the information.

The optical source converts the electrical signal into an optical signal.

The source may be either semiconductor laser or Light Emitting Diode

(LED). The intensity modulated light signal is coupled to fiber. The fiber

which is made up of a glass acts as a channel between the transmitter and

receiver.