26-12-2012, 06:08 PM
OVERVIEW OF OPTICAL FIBER COMMUNICATION
OVERVIEW OF OPTICAL FIBER.pptx (Size: 1.28 MB / Downloads: 75)
Introduction
Transfer of information from one point to other is known as communication
In a communication system long distance transmission of information is achieved by modulating the information on a electromagnetic wave of higher frequency known as the carrier wave
At the receiving end, the electromagnetic wave is first received and then demodulated by which the original information can be retrieved
The electromagnetic career signals can have frequency ranging from radio waves, microwaves, millimetre waves, or may even have optical frequencies.
Historical Development
Optical waves or light has been common for many years as a signalling system (Signal fires, reflecting mirrors and signalling lamps) but the information transfer was limited
In 1880, Alexander Graham Bell reported the transmission of speech using a light beam. The prototype of this was developed which used a telephone with sunlight modulation and a diaphragm which was so called as the Photophone
The use of optical signals were limited due to lack of suitable light source and were also affected by rain, fog, snow, dust and other electromagnetic noises. It is also restricted to the line of sight.
Because of these disadvantages, the career waves (Radio and microwaves) which were less affected by the above factors proved useful.
Theoretically, greater the frequency of the carrier wave, higher the information carrying capacity. So, optical frequencies have higher data capacity compared to radio and microwaves.
In context, it may also be noticed that optical frequencies supports higher bandwidth by a factor 104 over microwave transmission.
Also at high frequencies in a communication system, the available power can be concentrated within the available signal. Thus the system performance can be improved.
A Digital optical fiber link
Initially, the input digital signal from the information source is suitably encoded for optical transmission.
The laser drive circuit modulates the intensity of the semiconductor laser with the encoded digital signal.
Hence a digital optical signal is launched into the optical fiber.
The avalanche photodiode (APD) detector is followed by a front-end amplifier and equalizer to provide gain as well as noise bandwidth reduction.
Finally, the signal obtained is decoded to give the original digital information.
Advantages of O.F.C.
Long Distance Transmission
Large Information Capacity
Small Size and Low Weight
Immunity to electrical interference
Enhanced safety
Increased signal security
Reliable and ease of maintenance
Potential low-cost
Disadvantages of O.F.C
Lack of Bandwidth Demand
Lack of Standards
Radiation Darkening
High Investment cost
Difficulty in splicing( connecting)
Complex testing procedure
OPTICAL FIBER WAVEGUIDE
Optical fiber is a cylindrical waveguide operating at optical frequencies.
The inner cylindrical structure is called as core and has a refractive index n1.
The inner core is surrounded by an external cladding of glass having slightly lower refractive index n2.