Optical communication introduction

 HISTORICAL DEVELOPMENT

 In 1880 Alexander Graham Bell reported the transmission of speech

using light beam. The photophone proposed by Bell just four years after the

invention of telephone modulated sunlight with a diaphragm giving speech

transmission over a distance of 200m.

 However, some investigation of optical communications continued in

early 1900s but its use was limited to low capacity communication links. It was

due to lack of suitable light sources and the problem that light is restricted to

line of sight and is severely affected by rain , fog, dust and atmospheric

turbulence. Therefore, in ensuring years, an increasingly larger portion of the

electromagnetic spectrum was utilized for conveying information transfer

through radio and microwaves which provide to be most suitable.

 From radio and microwave communication systems, we expect more

capacity. The information carrying capacity is directly linked to bandwidth

frequency extent of the modulated carrier. The more the frequency of the

carrier, the more the information it can carry. Therefore radio communication

was developed in VHF and UHF range and latterly millimeter wave

transmission.

 With the invention of laser in 1960, interest in optical communication

was renewed. Optical fibers and lasers together increase the capacity of

communication system by 100000 times the conventional system. However, it

is increasingly replacing wire transmission lines in communication system. By

the late 1970s, every major telephone communication company was rapidly

installing new and more efficient fiber systems.

 Unguided optical communication systems (non-fibe) were developed

shortly after the discovery of laser. Communication over light beams travelling

through atmosphere was easily accomplished. The disadvantages of these

systems include.

1) Dependence on a clear atmosphere.

2) The need for a line of sight path between transmitter and receiver.

3) Possibility of eye damage to persons who unknowingly look into the

beam.

 Therefore, to overcome these disadvantages, an interest aroused in

fiber optic system that would guide the light beam, this led to guide.