OPTICAL FIBER COMMUNICATION SYSTEM
– THEORY & ANALYSIS
By
Prasenjit Pal
Electronics & Communication
Engineering Department
Asansol Engineering College
Kanyapur, Sen Raleigh Road, Asansol 713304, Burdwan, India
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CHRONOLOGICAL
DEVELOPMENT OF OPTICAL FIBER COMMUNICATION :
The
visible optical carrier waves or light has been commonly used for
communication purpose for many years. Alexander Graham Bell transmitted
a speech information using a light beam for the first time in 1880. Just
after four years of the invention of the telephone Bell proposed his
photophone which was capable of providing a speech transmission over a
distance of 200m. In the year 1910 Hondros and Debye carried out a
theoretical study and in 1920 Schriever reported an experimental work.
Although in the early part of twentieth century optical communication
was going through some research work but it was being used only in the
low capacity communication links due to severe affect of disturbances in
the atmosphere and lack of suitable optical sources. However, low
frequency (longer wavelength) electromagnetic waves like radio and
microwaves proved to be much more useful for information transfer in
atmosphere, being far less affected by the atmospheric disturbances. The
relative frequencies and their corresponding wavelengths can be known
from the electromagnetic spectrum and it is understandable that optical
frequencies offer an increase in the potential usable bandwidth by a
factor of around 10000 over high frequency microwave transmission. With
the LASER coming into the picture the research interest of optical
communication got a stimulation. A powerful coherent light beam together
with the possibility of modulation at high frequencies was the key
feature of LASER. Kao and Hockham proposed the transmission of
information via dielectric waveguides or optical fiber cables fabricated
from glass almost simultaneously in 1966. In the earlier stage optical
fibers exhibited very high attenuation (almost 1000 dB/km)which was
incomparable with coaxial cables having attenuation of around 5 to
10dB/km. Nevertheless, within ten years optical fiber losses were
reduced to below 5dB/km and suitable low loss jointing techniques were
perfected as well. Parallely with the development of the optical fibers
other essential optical components like semiconductor optical sources
(i.e. injection LASERs and LEDs) and detectors (i.e. photodiodes and
phototransistors) were also going through rigorous research process.
Primarily the semiconductor LASERs exhibited very short lifetime of at
most a few hours but by 1973 and 1977 lifetimes greater than 1000 hr and
7000 hr respectively were obtained through advanced device structure.
The first
generation optical fiber links operated at around 850 nm range. Existing GaAs based optical sources, silicon photo detectors, and multimode
fibers were used in these links and quiet understandably they suffered
from intermodal dispersion and fiber losses. With the advent of optical
sources and photo detectors capable of operating at 1300 nm, a shift in
transmission wavelength from 850nm to 1300nm was possible which inturn
resulted in a substantial increase in the repeaterless transmission
distance for long haul telephone trunks. Systems operating at 1550nm
provided lowest attenuation and these links routinely carry traffic at
around 2.5Gb/s over 90 km repeaterless distance. The introduction of
optical amplifiers like Erbium-doped fiber amplifiers (EDFA) and
Praseodymium-doped fiber amplifiers (PDFA) had a major thrust to fiber
transmission capacity. The use of Wavelength Division Multiplexing along
with EDFA proved to be a real boost in fiber capacity. Hence
developments in fiber technology have been carried out rapidly over
recent years. Glass material for even longer wavelength operation in the
mid-infrared (2000 to 5000nm) and far-infrared (8000 to 12000nm) regions
have been developed. Furthermore, the implementation of active
optoelectronic devices and associated fiber components (i.e. splices,
connectors, couplers etc.) has also accelerated ahead with such speed
that optical fiber communication technology would seem to have reached a
stage of maturity within its developmental path.
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A to Z of OPTICAL FIBERS :
Optical fiber is a dielectric
waveguide or medium in which information (voice, data or video) is
transmitted through a glass or plastic fiber, in the form of light. The
basic structure of an optical fiber is shown in figure 1. It consists of
a transparent core with a refractive index n1 surrounded by a
transparent cladding of a slightly less refractive index n2. The
refractive index of cladding is less than 1%, lower than that of core.
Typical values for example are a core refractive index of 1.47 and a
cladding index of 1.46. The cladding supports the waveguide structure,
protects the core from absorbing surface contaminants and when
adequately thick, substantially reduces the radiation loss to the
surrounding air. Glass core fibers tend to have low loss in comparison
with plastic core fibers. Additionally, most of the fibers are
encapsulated in an elastic, abrasion-resistant plastic material which
mechanically isolates the fibers from small geometrical irregularities
and distortions. A set of guided electromagnetic waves, also called the
modes of the waveguide, can describe the propagation of light along the
waveguide. Only a certain number of modes are capable of propagating
through the waveguide.
Figure 1.
2.1 Principle of ray
propagation :
This is the most
interesting thing about optical fiber cables. Such an indispensable part
of modern day communication system works on an extremely simple property
of light ray i.e. Total Internal Reflection. As we all know that
when light ray is passing from denser (refractive index is higher)
dielectric medium to a rarer (refractive index is lower) dielectric
medium then from the point of incidence at the interface it bends away
from the normal. When the incidence angle is sufficiently high such that
the angle of refraction is 90º then it is called critical angle. Now if
light ray falls at the interface of the two mediums at an angle greater
than the critical angle then the light ray gets reflected back to the
originating medium with high efficiency (around 99.9%) i.e. total
internal reflection occurs. With the help of innumerable total internal
reflections light waves are propagated along the fiber with low loss as
shown in figure2. In this context, two parameters are very crucial
namely Acceptance Angle and Numerical Aperture.
Figure 2.
Acceptance
angle is the maximum angle at which light may enter the fiber in order
to be propagated and is denoted by θa in figure3. The relationship
between the acceptance angle and the refractive indices of the three
media involved-core, cladding and air, leads to the definition of
Numerical Aperture which is given by –
NA =
(n1²-n2²)½ = n0 sin θa where n0 is the refractive index of
air.
The light ray shown in
figure3 is known as a meridional ray as it passes through the axis of
the fiber. However, another category of ray exists which is transmitted
without passing through the fiber axis and follows a helical path
through the fiber.
Figure 3.
2.2 Modes in optical fibers :
The electromagnetic wave
theory must be taken into account for getting an improved model for
propagation of light through optical fibers. The optical waveguide can
be considered to be either a planer guide or a cylindrical guide.
Electromagnetic field comprises of a periodically varying electric field
E and magnetic field M which are oriented at right angle to each other.
When the electric field is perpendicular to the direction of propagation
and hence Ez=0, but a corresponding magnetic field component is in the
direction of propagation, that mode is known as Transverse Electric
(TE) mode. But when the reverse thing happens then it is termed as
Transverse Magnetic (TM) mode. Now when total field lies in the
transverse plane, Transverse electromagnetic (TEM) waves exist
where both Ez and Hz are zero. The formation of modes in a planer
dielectric guide and the interference of plane waves are shown in
figure4. Here the stable field distribution in the x direction with only
a periodic z dependence due to sinusoidally varying electric field in z
direction is known as a mode. In a cylindrical fiber transverse electric
(TE) and transverse magnetic (TM) modes are obtained which is bounded in
two dimensions. Thus two integers (l & m) are necessary to specify the
modes. Hybrid modes may also occur in the cylindrical fibers. These
modes result from skew ray propagation and are designated by HElm when H
makes a larger contribution to the transverse field and EHlm when E
makes larger contribution to the transverse field.
2.3 Transmission Characteristics of Optical
Fiber Cables:
The
transmission characteristics of optical fiber cables play a major role
in determining the performance of the entire communication system.
Attenuation and bandwidth are the two most important
transmission characteristics when the suitability of optical fiber for
communication is analysed. The various attenuation mechanisms are
linear scattering, non linear scattering, material absorption and
fiber bends etc. The bandwidth determines the number of bits of
information transmitted in a given time period and is largely limited by
signal dispersion within the fiber.
Figure 4.
2.3.1
Attenuation in Optical Fibers :
Attenuation
is defined as the loss of optical power over a set distance, a fiber
with a lower attenuation, will allow more power to reach to the receiver
than a fiber with higher attenuation. Signal attenuation within optical
fiber is usually expressed in decibel per unit length (i.e. dB/km).
Loss in decibel (dB) = 10
log₁₀(Pi/Po)
where Pi and Po are the
transmitted and output optical power respectively. Figure5 shows optical
fiber attenuation as a function of wavelength.
Figure 5.
2.3.2 Linear scattering
losses :
Through this mechanism a portion/total optical power within one
propagating mode is transferred to another. Now when the transfer takes
place to a leaky or radiation mode then the result is attenuation. It
can be divided into two major categories namely Mie scattering
and Rayleigh scattering.
2.3.2.1 Mie
Scattering :
Non perfect cylindrical structure of the fiber and imperfections like
irregularities in the core-cladding interface, diameter fluctuations,
strains and bubbles may create linear scattering which is termed as Mie
scattering.
2.3.2.2 Rayleigh
Scattering :
The dominant reason behind Rayleigh scattering is
refractive index fluctuations due to density and compositional variation
in the core. It is the major intrinsic loss mechanism in the low
impedance window. Rayleigh scattering can be reduced to a large extent
by using longest possible wavelength.
2.3.3 Non linear scattering losses :
Specially at high
optical power levels scattering causes disproportionate attenuation, due
to non linear behaviour. Because of this non linear scattering the
optical power from one mode is transferred in either the forward or
backward direction to the same, or other modes, at different
frequencies. The two dominant types of non linear scattering are :
a) Stimulated Brillouin Scattering and
b)
Stimulated Raman Scattering.
2.3.4 Material Absorption losses :
When there happens
to be some defect in the material composition and the fabrication
process of optical fiber, there is dissipation of optical power in the
form of heat in the waveguide. Here also there are two types of
absorption losses in the fiber such as intrinsic absorption and
extrinsic absorption. When the absorption is caused by
interaction with one or more components of glass it is termed as
intrinsic absorption whereas if it is due to impurities within the glass
like transition metal or water then it is called the extrinsic one.
2.3.5 Dispersion :
It is defined as the
spreading of the light pulses as they travel down the fiber. Because of
the spreading effect, pulse tend to overlap, making them unreadable by
the receiver which is a critical problem to deal with. It creates
distortion for both digital and analog transmission. Dispersion limits
the maximum possible bandwidth attainable within a particular fiber.
Pulse broadening is a very common problem created by dispersion in
digital transmission. To avoid it, the digital bit rate must be less
than the reciprocal of the broadened pulse duration.
2.3.5.1 Intermodal Dispersion :
The propagation
delay difference between different modes within multimode fibers is
responsible for intermodal dispersion and hence pulse broadening. In
fact, the different group velocities with which the modes travel through
the fiber creates the main problem. Multimode step index fibers exhibit
a large amount of intermodal dispersion whereas in a pure single mode
fiber there is no intermodal dispersion. By adopting an optimum
refractive index profile (parabolic profile in most graded index
fibers), we can drastically reduce intermodal dispersion.
2.3.5.2 Intramodal Dispersion :
This type of dispersion takes place due to the fact that optical sources
do not emit a single frequency but a band of frequencies and there
happens to be propagation delay differences between these spectral
components. This kind of pulse broadening occurs in almost every type of
optical fibers. When the dispersive characteristics of the waveguide
material are responsible for the delay differences then it’s known as
material dispersion. On the other hand if imperfect guidance effect
is behind the pulse broadening then it’s termed as waveguide
dispersion. There is almost zero waveguide dispersion in multimode
fibers.
2.3.6 Fiber bending losses :
Light energy gets
radiated at the bends on their path through the fiber and eventually is
lost. This is the mechanism known as fiber bend losses. There are two
types bending causing this loss namely micro bending and macro bending.
If the fiber is sharply bent so that the light traveling down the fiber
can not make the turn and gets lost then it’s macro bending as shown in
figure 6(a). When small bends in the fiber created by crushing,
contraction etc causes the loss then it is called micro bending as shown
in figure 6(b). These bends are not usually visible with naked eye.
Figure
6a
Figure
6b
2.4 Types of Optical
Fibers :
According
to the refractive index profile optical fibers can be divided into two
categories namely Step index fibers and Graded index fibers
which are described below.
2.4.1 Step
index fibers :
If the
refractive index profile of a fiber makes a step change at the core
cladding interface then it is known as step index fiber. A multimode
step index fiber is shown in figure7(a), the core diameter of which is
around 50µm. Some physical parameters like relative refractive index,
index difference, core radius etc determines the maximum number of
guided modes possible in a multimode fiber. A single mode fiber has a
core diameter of the order of 2 to 10µm and the propagation of light
wave is shown in figure7(b). It has the distinct advantage of low intermodal dispersion over multimode step index fiber. On the other hand
multimode step index fibers allow the use of spatially incoherent
optical sources and low tolerance requirements on fiber connectors.
Figure7.
2.4.2 Graded index fibers :
The graded index fibers
have decreasing core index n(r) with radial distance from a maximum
value of n1 at the axis to a constant value n2 beyond the core radius a
in the cladding as shown in figure8. The graded index fiber gives best
results for multimode optical propagation for parabolic refractive index
profile. Due to this special kind of refractive index profile multimode
graded index fibers exhibit less intermodal dispersion than its
counterpart i.e. multimode step index fibers.
Figure 8.
3. GENERAL
OVERVIEW OF OPTICAL FIBER COMMUNICATION SYSTEM :
Like all other
communication system, the primary objective of optical fiber
communication system also is to transfer the signal containing
information (voice, data, video) from the source to the destination. The
general block diagram of optical fiber communication system is shown in
the figure9.
The source
provides information in the form of electrical signal to the
transmitter. The electrical stage of the transmitter drives an optical
source to produce modulated light wave carrier. Semiconductor LASERs or
LEDs are usually used as optical source here. The information carrying
light wave then passes through the transmission medium i.e. optical
fiber cables in this system. Now it reaches to the receiver stage where
the optical detector demodulates the optical carrier and gives an
electrical output signal to the electrical stage. The common types of
optical detectors used are photodiodes (p-i-n, avalanche),
phototransistors, photoconductors etc. Finally the electrical stage gets
the real information back and gives it to the concerned destination.
It is notable that
the optical carrier may be modulated by either analog or digital
information signal. In digital optical fiber communication system the
information is suitably encoded prior to the drive circuit stage of
optical source. Similarly at the receiver end a decoder is used after
amplifier and equalizer stage.
4. PRIMARY ELEMENTS OF
OPTICAL FIBER COMMUNICATION SYSTEM :
Figure10 shows the
major elements used in an optical fiber communication system. As we can
see the transmitter stage consists of a light source and associated
drive circuitry. Again, the receiver section includes photodetector,
signal amplifier and signal restorer.
Additional
components like optical amplifier, connectors, splices and couplers are
also there. The regenerator section is a key part of the system as it
amplifies and reshapes the distorted signals for long distance links.
Figure 10.
4.1
Transmitter section :
The main parts of
the transmitter section are a source (either a LED or a LASER),
efficient coupling means to couple the output power to the fiber, a
modulation circuit and a level controller for LASERs. In present days,
for longer repeater spacing, the use of single mode fibers and LASERs
are seeming to be essential whereas the earlier transmitters operated
within 0.8µm to 0.9µm wavelength range, used double hetero structure
LASER or LED as optical sources. High coupling losses result from direct
coupling of the source to optical fibers. For LASERs, there are two
types of lenses being used for this purpose namely discrete lenses
and integral lenses.
4.1.1
LED vs LASER as optical source :
A larger fraction of the output
power can be coupled into the optical fibers in case of LASERs as they
emit more directional light beam than LEDs. That is why LASERs are more
suitable for high bit rate systems. Figure11 enlightens how light output
power depends on input drive current in case of LASERs and LEDs. From
that it is obvious that LASER is more temperature dependent than LED.
LASERs have narrow spectral width as well as faster response time.
Consequently, LASER based systems are capable of operating at much
higher modulation frequencies than LED based systems. Typical LEDs have
lifetimes in excess of 10^7 hours, whereas LASERs have only 10^5 hours
of lifetime. Another thing is that LEDs can start working at much lower
input currents which is not possible for LASERs. So, according to the
situation and requirements either LED or LASER can be utilized as an
optical source.
Figure 11.
Now there are a
number of factors that pose some limitations in transmitter design such
as electrical power requirement, speed of response, linearity, thermal
behavior, spectral width etc.
4.1.2
Drive circuitry :
These
are the circuits used in the transmitters to switch a current in the
range of ten to several hundred miliamperes required for proper
functioning of optical source. For LEDs there are drive circuits like
common emitter saturating switch, low impedance, emitter coupled,
transconductance drive circuits etc. On the other hand for LASERs, shunt
drive circuits, bias control drive circuits, ECL compatible LASER drive
etc are noticeable.
4.2
Receiver section :
Figure12 enlightens
the general structure of a receiver section. It is clear that it
includes Photodetector, low noise front end amplifier, voltage amplifier
and a decision making circuit to get the exact information signal back.
High impedance amplifier and Trans impedance amplifier are the two
popular configurations of front end amplifier, the design of which is
very critical for sensible performance of the receiver. The two most
common photodetectors are p-i-n diodes and avalanche
photodiodes. Quantum efficiency , responsivity and speed of response
are the key parameters behind the decision of photodetectors. The most
important requirements of an optical receiver are sensitivity,
bit rate transparency, bit pattern independence, dynamic
range, acquisition time etc. As the noise contributed by
receiver is higher than other elements in the system so, we must put a
keen check on it.
Figure 12.
5.
BENEFITS OF OPTICAL FIBER COMMUNICATION SYSTEM :
Some of the
innumerable benefits of optical fiber communication system are:
-
Immense
bandwidth to utilize
-
Total
electrical isolation in the transmission medium
-
Very
low transmission loss,
-
Small
size and light weight,
-
High
signal security,
-
Immunity
to interference and crosstalk,
-
Very
low power consumption and wide scope of system expansion etc.
These are the main advantages that
have made optical fiber communication system such an indispensable part
of modern life.
6. FIELD OF APPLICATION :
Due to its variety of advantages
optical fiber communication system has a wide range of application in
different fields namely :
-
Public
network field which
includes trunk networks, junction networks, local access networks,
submerged systems, synchronous systems etc.
-
Field
of military applications
,
-
Civil,
consumer and industrial applications,
-
Field
of computers which
is the center of research right now.
7.
CONCLUSION :
Though there are some negatives of
optical fiber communication system in terms of fragility, splicing,
coupling, set up expense etc. but it is an un avoidable fact that
optical fiber has revolutionized the field of communication. As soon as
computers will be capable of processing optical signals, the total arena
of communication will be opticalized immediately. |