17-01-2013, 02:43 PM
Optical fiber communication —An overview
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Abstract.
This paper deals with the historical development of optical communication systems and
their failures initially. Then the different generations in optical fiber communication along with their
features are discussed. Some aspects of total internal reflection, different types of fibers along with
their size and refractive index profile, dispersion and loss mechanisms are also mentioned. Finally
the general system of optical fiber communication is briefly mentioned along with its advantages and
limitations. Future soliton based optical fiber communication is also highlighted.
Introduction
Now we are in the twenty first century, the era of ‘Information technology’ [1-6]. There is
no doubt that information technology has had an exponential growth through the modern
telecommunication systems. Particularly, optical fiber communication plays a vital role
in the development of high quality and high-speed telecommunication systems. Today,
optical fibers are not only used in telecommunication links but also used in the Internet and
local area networks (LAN) to achieve high signaling rates.
Historical perspective of optical communication
The use of light for transmitting information from one place to another place is a very old
technique. In 800 BC., the Greeks used fire and smoke signals for sending information
like victory in a war, alertting against enemy, call for help, etc. Mostly only one type of
signal was conveyed. During the second century B.C. optical signals were encoded using
signaling lamps so that any message could be sent. There was no development in optical
communication till the end of the 18th century. The speed of the optical communication
link was limited due to the requirement of line of sight transmission paths, the human
eye as the receiver and unreliable nature of transmission paths affected by atmospheric
effects such as fog and rain. In 1791, Chappe from France developed the semaphore for
telecommunication on land. But that was also with limited information transfer.
Unguided optical communication
The optical communication systems are different from microwave communication systems
in many aspects. In the case of optical systems, the carrier frequency is about 100 THz
and the bit rate is about 1T bit/s. Further the spreading of optical beams is always in the
forward direction due to the short wavelengths. Even though it is not suitable for broadcasting
applications, it may be suitable for free space communications above the earth’s
atmosphere like intersatellite communications. For the terrestrial applications, unguided
optical communications are not suitable because of the scattering within the atmosphere,
atmospheric turbulence, fog and rain.
The birth of fiber optic systems
To guide light in a waveguide, initially metallic and non-metallic wave guides were fabricated.
But they have enormous losses. So they were not suitable for telecommunication.
Tyndall discovered that through optical fibers, light could be transmitted by the phenomenon
of total internal reflection. During 1950s, the optical fibers with large diameters
of about 1 or 2 millimetre were used in endoscopes to see the inner parts of the human
body.
Optical fibers can provide a much more reliable and versatile optical channel than the
atmosphere, Kao and Hockham published a paper about the optical fiber communication
system in 1966. But the fibers produced an enormous loss of 1000 dB/km. But in the
atmosphere, there is a loss of few dB/km. Immediately Kao and his fellow workers realized
that these high losses were a result of impurities in the fiber material. Using a pure silica
fiber these losses were reduced to 20 dB/km in 1970 by Kapron, Keck and Maurer. At
this attenuation loss, repeater spacing for optical fiber links become comparable to those of
copper cable systems. Thus the optical fiber communication systembecame an engineering
reality.
Different types of fibers
We know that the light or the optical signals are guided through the silica glass fibers by
total internal reflection. A typical glass fiber consists of a central core glass (50 mm)
surrounded by a cladding made of a glass of slightly lower refractive index than the core’s
refractive index. The overall diameter of the fiber is about 125 to 200 mm. Cladding is
necessary to provide proper light guidance i.e. to retain the light energy within the core as
well as to provide high mechanical strength and safety to the core from scratches.
Based on the refractive index profile we have two types of fibers (a) Step index fiber (b)
Graded index fiber.
(a) Step index fiber: In the step index fiber, the refractive index of the core is uniform
throughout and undergoes an abrupt or step change at the core cladding boundary. The
light rays propagating through the fiber are in the form of meridional rays which will cross
the fiber axis during every reflection at the core cladding boundary and are propagating in
a zig-zag manner as shown in figure 1a.
(b) Graded index fiber: In the graded index fiber, the refractive index of the core is made
to vary in the parabolic manner such that the maximum value of refractive index is at the
centre of the core. The light rays propagating through it are in the form of skew rays or
helical rays which will not cross the fiber axis at any time and are propagating around the
fiber axis in a helical (or) spiral manner as shown in figure 1b.
Basic optical fiber communication system
Figure 2 shows the basic components in the optical fiber communication system. The input
electrical signal modulates the intensity of light fromthe optical source. The optical carrier
can be modulated internally or externally using an electro-optic modulator (or) acoustooptic
modulator. Nowadays electro-optic modulators (KDP, LiNbO3 or beta barium borate)
are widely used as external modulators which modulate the light by changing its refractive
index through the given input electrical signal.
In the digital optical fiber communication system, the input electrical signal is in the
form of coded digital pulses from the encoder and these electric pulses modulate the intensity
of the light from the laser diode or LED and convert them into optical pulses. In the
receiver stage, the photo detector like avalanche photodiode (APD) or positive-intrinsicnegative
(PIN) diode converts the optical pulses into electrical pulses. A decoder converts
the electrical pulses into the original electric signal.
Dispersion and losses in fibers
Dispersion in the fiber means the broadening of the signal pulse width due to dependence
of the refractive index of the material of the fiber on the wavelength of the carrier. If we
send digitized signal pulses in the form of square pulses, they are converted into broadened
gaussian pulses due to dispersion. The dispersion leads to the distortian (or) degradation
of the signal quality at the output end due to overlapping of the pulses. There are two
kinds of dispersion mechanisms in the fiber: (i) Intramodal dispersion and (ii) Intermodal
dispersion.
Dispersion-shifted single mode fibers
Generally in single mode fibers, zero dispersion is obtained at a wavelength of about
1.3 mm. Since there is a finite loss in the silica fiber at 1.3 mm, today the fibers are designed
such that there is zero dispersion at 1.55 mm with a minimum loss. At 1.55 mm, the material
dispersion in single mode fiber is positive and large, while the waveguide dispersion
is negative and small. So to increase the waveguide dispersion equal to that of material
dispersion, the relative refractive index difference ‘D’ may be slightly increased by adding
more Ge O2 in the core (which increases the refractive index of the core) or adding more
fluorine in the cladding (which decreases the refractive index of the cladding) or instead
of parabolic refractive index profile, a triangular refractive index profile can be designed.
Thus the dispersion-shifted fibers have minimum loss and zero dispersion at 1.55mm.
Optical amplifiers
In the long distance optical fiber communication systems, the repeaters are situated at an
equal distance of 100 km. These are used to receive and amplify the transmitted signal to
its original intensity and then it is passed on to the main fiber. Previously it was done by
conversion of optical energy into electrical energy and amplification by electrical amplifiers
and then reconversion of electrical energy into optical energy. Such methods not only
increase the cost and complexity of the optical communication system but also reduce the
operational bandwidth of the system. But today it is done by erbium doped optical fiber
amplifiers in an elegant manner by inserting a length of 10 m fiber amplifier for every 100
km length of main fiber. By this, the signal to noise ratio is greatly improved due to optical
domain operation only [8,9].
Conclusion
At present there are many optical fiber communication links throughout the world without
using optical solitons. When we introduce optical solitons as light pulses through the
fibers, we can achieve high quality telecommunication at a lower cost. We can expect a
great revolution in optical fiber communication within a few years by means of solitons.