UNIVERSITY OF KENT
NETWORKS AND NETWORK SECURITY
ACHAL LEKHI
al333@kent.ac.uk
OPTIC FIBRE DATA
TRANSMISSION
A Report on Optic Fiber data Transmission which include the structure type and mechanism of
fiber optics and defined all the principle behind the working of fiber optics with its advantages
over other cables. Wave division multiplexing is also being explained with its types and working.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
INTRODUCTION
OPTIC FIBRES are long, thin strands of very pure glass and are arranged in bundles
that used transmit light signals over long distances. These are the system that
makes communication system very strong and reliable over a medium. These uses
light instead of electronic signal so less drop of code and provide large coverage.
Optical fibres carries much more information than conventional copper wire as these
are the glass fibres so subject to less interference, and more protection because of
glass. In fibre optic system only the difference from the conventional system is that
they provide optic source and optic detector via optic cables.
OPTIC FIBRE
SYSTEM
SOURCE
TRANSMITTER
DESTINATION
RECIEVER
OPTIC FIBRE SYSTEM
OPTIC
SOURCE
OPTICAL
FIBRE CABLE
OPTICAL
DETECTOR
FIGURE 1: OPTIC FIBRE SYSTEM
As from the above diagram the optical fibre system works in same way until signal
reaches to optic source, here it provide electrical-optical conversion by
semiconductor laser or LED(Light Emitting Diode), and in the end by Optical detector
which again drives a further electrical stage and hence provide demodulation to the
optical carrier.
Now days telephone service (POTS) uses optic fibre across the nation. Local
exchange carriers use fibre to carry the same services between the office and the
local stations (FTTH is also used when comes to provide high speed services for
home users). The high bandwidth provided by fibre makes it the perfect choice for
transmitting broadband signals, such as high-definition television (HDTV) telecasts.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
MECHANISM AND STRUCTURE
Optic fibre works on the principle of reflection of light, i.e. whenever light travels form
one medium to another it either moves into the same medium or away from the
medium. Light travels in same line unless change in medium occurs, if angle of
incidence I is less than the critical angle, the ray refracts and moves closer to the
surface. If the angle of incidence is equal to the critical angle, the ray reflects and
travels again in the denser substance. This is how the signal travels in the core of
the optical fibre based on the principle of refraction and especially on the critical
angle property.
Less Dense
Less Dense
Less Dense
More Dense
More Dense
More Dense
I
I
I
I< Critical Angle, refraction
I=Critical Angle, refraction I> Critical Angle Reflection
FIGURE 2: Bending of Signals in OPTIC FIBRE
Based on the above fact signal travels in the core based on the above principle
hence optical fibre is made in such a way that I>Critical Angle Reflection always and
hence the signal propagate throughout the medium. Medium is very important in
case of light as light as different wave of propagation throughout the different
medium is different and based on the refractive index of the medium so propagation
speed varies according to the medium of travelling. Optic fibre works on the light
principle hence if the refractive index varies from cladding to core it may affect the
core or the signal as all components are independent but during the propagation of
signals all are linked together. Hence it works on the principle of TOTAL INTERNAL
REFLECTION. In the Optic fibre system it consist of following components, based on
these components it transmit signal over the medium.




Transmitter- Produce and encodes the signal over the medium.
Fiber Cable- Carry the signal through the end.
Regenerator- Boost the signal while propagating.
Receiver- Receive and decode the signals.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
STRUCTURE
Fibre optics is not functioning independently it consist of following partsCORE- Thin class centre of fibre where the light (Signal) travels, as optic fibre
uses guide light so the signals usually travel in a specific direction in the core.
CLADDING- Core’s surrounding and it’s of less dense glass or plastic. It
basically reflect back the light to the core because of difference in the density
of the core and cladding as the principle of optic fibre is based on the
reflection of light, hence light is always refracted into the core instead of
reflected outwards.
BUFFEER COATING- A coating that protects the cladding surface from being
damage; hence provide more durability to the optic fibre cable.
CLADDING
CORE
SENDER
RECIEVER
FIGURE 3: FIBRE OPTIC STRUCTURE
This is how Fibre optics cable works using light as a signal. Fibre optics has two
types of propagation modes i.e. modes that require for the propagation of light along
optical channel. They perform differently with respect to both attenuation and time
dispersion these ares MULTIMODE
 STEP INDEX
 GRADED INDEX
 SINGLE MODE
 STEP INDEX
 DUAL STEP INDEX
Light has a dual nature and can be viewed as either a wave phenomenon or a
particle phenomenon, let’s consider the wave mechanics of light. When the light
wave is guided down a fibre-optic cable, it exhibits certain modes of propagation.
These modes are usually numbered from there degree of performance that is
highest to lowest. In a very simple sense, each of these modes can be thought of as
a ray of light. For a given fibre-optic cable, the number of modes that exist depends
on the size and structure off cable and the variation of the indices of refraction of
both core and cladding across the cross section of cable.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
MULTI MODE
It is so named because multiple beams from a light source move through the core in
different paths. Here a large group of beams can travel across the optic cable
without the interference.
MULTIMODE STEP INDEX FIBRE, the density of the core remains constant form
the center to the edges (Diameter of the core is very large as compared to the
cladding). A beam of light moves through the cable in straight line till it reaches to the
interface of the core and the cladding within the constant density. At the interface
there is change in the density i.e. lower density this change in density change the
angle of the signal. Different order modes travel in different order like lowest-order
mode travels straight down the center and higher order mode bounce back and forth
to the center of core to the end of cable.
SOURCE
DESTINATION
FIGURE 4: MULTIMODE STEP INDEX
Fibre-optic cable that exhibits multimode propagation with a step index profile is
thereby characterized as having higher attenuation and more time dispersion than
the other propagation candidates. However, it is also the least costly and is widely
used in the premises environment.
Multimode Graded Index
In this mode density plays a vital role in propagation of the signals, because index
means density and it varies throughout the cable and it’s higher in the center of the
core and decrease gradually to the edges, that’s why it got higher refractive index in
the core it decreases when it extends to outwards. There is no sharp discontinuity in
the indices of refraction between core and cladding. The core is much larger than the
single-mode step index. They appear in the form of ellipses, as higher the path is
confined less the attenuation due to leakage. Popular graded index fibre-optic cables
have core diameters of 50, 62.5, and 85 microns.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
They have a cladding diameter of 125 microns. This type of fibre-optic cable is
extremely popular in premise data communications applications. In particular, the
62.5/125 fibre-optic cable is the most popular and most widely used in premise data
communications applications and generally glass used to fabricate multimode
graded index fibre-optic cable. There is limited attenuation and time dispersion, but
not much as compared to the multimode step index fibre-optic cable.
SOURCE
DESTINATION
FIGURE 5: Multimode Graded Index
Single-Mode Step Index
It uses highly focused source of light that limits beams to the small range of angles,
all close to the horizontal. It has much smaller diameter than cladding and the lower
density (Refractive Index). The decrease in density results in the very closeness of
critical angle (900) to make the propagation of beans almost horizontal, in this
scenario propagation of the different beans is almost identical and the delays are
negligible. As no energy is lost to heat through the leakage of the higher modes into
the cladding as they are not present and all energy is confined to the single, lowerorder mode, hence attenuation is not significance. Single-mode propagation exists
only above a certain specific wavelength called the cut off wavelength, i.e. the
smallest operating wavelength when it propagates in fundamental mode. Here it’s
less attenuation and time dispersion. However, these are also the most costly in the
premises environment. That’s why it has been used more with metropolitan- and
wide-area networks than with premises data communications. Single-mode fibreoptic cable has also been getting increased attention when local area network is
extended or merged with some other networks then the attenuation rate will
increase. The core diameter for this type of fibre-optic cable is exceedingly small,
ranging from 8 microns to 10 microns. The standard cladding diameter is 125
microns.
SOURCE
DESTINATION
FIGURE 6: SINGLE MODE STEP INDEX
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
Single-Mode Dual-Step Index
These are single mode but the difference is that they have a dual cladding. A
depressed-clad fibre is also called dual-step index, it has an advantage that they
have less macro bending losses, also has two zero-dispersion points and low
dispersion over a much wider wavelength. Cladding has a lower refractive index than
the core, and the outer cladding has more refractive index from the inner one.
OUTER CLADDING
INNER CLADDING
SOURCE
DESTINATION
FIGURE 7: SINGLE MODE DUAL STEP INDEX
FACTORS THAT GOVERNES FIBER OPTIC EFFICIENCY
Attenuation
It affects the propagation of waves and signals in fibre optics. It is an exponential
function of the path length through the medium, basically a transmission loss i.e. the
reduction in intensity of the light beam with respect to distance travelled through a
transmission medium. It is an important factor limiting the transmission of a digital
signal across large distances. So it defined the efficiency of the fibre.
Attenuation in fibre optics can be quantified using the following equation:
Dispersion
It is a phenomenon of which a phase velocity of wave depends on the frequency of
the signal. It causes the wave to light to split up into different forms when not
provided with the proper surface. It determines how much data can be transported
on a single fibre. When a broad range of frequencies (a broad bandwidth) is present
in a single wave packet, such as in ultra short or other forms of spread
spectrum transmission, it may not be accurate to approximate the dispersion by a
constant over the entire bandwidth, and more complex calculations are required to
compute effects such as pulse spreading.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
Regeneration
It is the phenomenon of generating the signal when it loose its power, so it is been
regenerated by the system to propagate through the medium. Recent advances in
fibre and optical communications technology have reduced signal degradation so far
that regeneration of the optical signal is only needed over distances of hundreds of
kilometres. This has greatly reduced the cost of optical networking globally and
increased the efficiency of the technology worldwide.
Material Absorption
Material absorption occurs as a result of the imperfection and impurities in the fibre.
The basic reason of this problem is production issue because if something went
wrong during the production it will affect on whole lot of production. The most
common impurity is the hydroxyl (OH-) molecule. The presence of hydroxyl radicals
in the cable material causes an increase in attenuation, so this cause decrement in
fibre efficiency and if the product is pure then less attenuation and more throughput.
WAVE DIVISION MULTIPLEXING (WDM)
It’s a technology which multiplexes a number of optical carrier signals to a optic fibre
using different wavelength of light (laser). This is a duplex mode of communication
over one fibre and provides a range of capacity while transmitting.
Each communication channel is allocated to a different frequency and multiplexed
onto a single fibre. At the destination wavelengths are spatially separated to different
receiver locations. Hence a high carrier bandwidth is utilized to transmit up to a
greater extent.
SYSTEM STRUCTURE & WORKING
It uses a multiplexer and the transmitter to joins the signals together and the
demultiplexer at the receiver to split them apart again to process them and deliver to
the different locations. The first WDM systems combined only two signals. Modern
systems can handle up to 160 signals and can thus expand a basic 10 Gbit/s system
over a single fibre pair to over 1.6 Tbit/s. This system is more popular because it can
expand it is capacity without laying more cables. We can easily expand the capacity
link just by upgrading the multiplexer and demultiplexers at each ends. Most of the
WDM systems operate on single-mode fibre optical cables, which has 9 µm of core
diameter but still certain forms of WDM can also be used in multi-mode fibre cables
which has 50 µm of core diameter or may be greater.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
CONVENTIONAL WAVE
DIVISION MULTIPLEXING
WAVE DIVISION MULTIPLEXING
DENSE WAVE DIVISION
MULTIPLEXING
FIGURE 8: WDM Different Wavelength Patterns.
CWDM (CONVENTIONAL WAVE DIVISION MULTIPLEXING), provide 8 channels
on a simple fibre while DWDM (DENSE WAVE DIVISION MULTIPLEXING) it also
uses the same transmission window but with more dense spacing. CWDM usually
increase the spacing to allow less disrubtance while DWDM uses same spacing but
utilize more channels and provide higher speed, but WDM, DWDM and CWDM are
based on the same concept of using multiple wavelengths of light on a single fibre,
but differ in the spacing of the wavelengths, number of channels, and the ability to
amplify the multiplexed signals in the optical space.
CWDM also called as coarse wavelength division multiplexing, its example is
Ethernet LX-4 10 Gbits/s it used to carry 10 Gbit/s of aggregate data. Currently
based on new standard signals are not spaced for amplification, hence now the
optical span of the CWDM is increased and hence used in metropolitan applications.
It also uses in the television for upstream and downstream signals.
DWDM, it multiplex within the 1550 nm band so handle the capacities with the help
of effective wavelength. They use EDFAs (erbium doped fibre amplifier) for
amplification. It can amplify many optic signals and multiplexed them in the
amplification band. DWDM has terminal multiplexer that contains one wavelength
converting transponders for each wavelength signal they carry, intermediate line
repeater that helps to retain the power in optic signal it uses EDFA, intermediate
optical terminal that amplifies the multi-wavelength signal while traversing and
signals may be dropped or added in the optic fibre using localized and enhanced
system, terminal demultiplexer that breaks the multi-wavelength signal back into
individual signal and outputs them on separate fibre for the clients
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
λA
λB
al333@kent.ac.uk
M
De
U
M
X
λC
λA
λB
λC
λA
λB
U
X
λC
FIGURE 9: WDM Transmission through Single Optical Fibre.
In above configuration the high carrier bandwidth is utilized to a greater extent to
transmit multiple optical signals through a single optical fibre. Here whenever the
signal with different wavelength arrive to the receiver it automatically taken control by
the WDM and hence all are based on the spacing of the cable it transmit the signal
over the cable, multiple signals are being transmitted over the medium and
multiplexer multiplex all the signal over the medium start point and transmission
takes place and at receiver end demultiplexer duplex the signal and again transmit to
the appropriate destination.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
ADVANTAGES OF USING OPTIC FIBRE FOR DATA
TRANSMISSION
Enormous Potential Bandwidth – The optical fibre carrier frequency
in the range 1013 to 1016 Hz yields a far greater potential transmission
bandwidth than metallic cable systems or even radio systems.
Small size and weight – They have very small diameter less than the
human hair. Even if considering the coating they are lighter and small
in size as compared to any cable across the usage.
Electrical Isolation – As they are made up of glass and sometimes
the plastic polymers hence they are electrical insulators, and they don’t
exhibit earth loop and interface problems.
Immunity to interference and crosstalk – They form a dielectric
waveguide and therefore free from electromagnetic interference (EMI),
radio-frequency interference (RFI). Hence these are unaffected by
transmission through an electrically noisy environment.
Signal security – It does not radiate significantly and hence provide
higher degree of security. Any tampering to the cable leads to the
detection of the error in the system, so highly secure for signalling.
Low transmission loss and flexible – It facilitates the implementation
of communication links with extremely wide optical repeater or amplifier
spacing, thus reducing cost and providing flexibility to the system with
less transmission loss, as signals are more modulated and they has a
capability to travel across large distance.
System reliability and ease of maintenance – As it reduces the
requirement of repeaters or amplifiers to boost the signal. Hence with
fewer optical repeater or amplifiers system reliability is enhanced and
now it’s easy to maintain the system.
CONCLUSION
Based on the above report on optic fibre, it’s been clearly defined that optic fibre
provide a great flexibility and advance features over any other cable with better
speed and security feature though it’s expensive but it wide range of features makes
its unique and more usable. Today more than 80 percent of the world's long-distance
traffic is carried over optical-fibre cables, covered globally. It’s basically an
international standard of many industries and worldwide implemented.
OPTIC FIBRE DATA TRANSMISSION
ACHAL LEKHI
al333@kent.ac.uk
REFERENCES
[1] Senior, JM 1885, OPTICAL FIBER COMMUNICATIONS: PRINCIPLES AND
PRACTICE, Third edition, Pearson, England.
[2] “Wavelength-division multiplexing”, (November 2011), (wikipedia.org),
Available: http://en.wikipedia.org/wiki/Wavelength-division_multiplexing
(Accessed: 2011, November 14).
[3] “Optical fibre” (November 2011), (wikipedia.org),
Available: http://en.wikipedia.org/wiki/Optical_fiber
(Accessed: 2011, November 14).
[4] Alwayn V., 2004, Fiber-Optic Technologies, accessed 14th November 2011
from http://www.ciscopress.com/articles/article.asp?p=170740&seqNum=4
[5] Frenudenrich, C, How Fibre Optics Work, Howstuffworks,
Accessed 15 November 2011 from
http://communication.howstuffworks.com/fiber-optic-communications/fiberoptic7.htm