FIBRE OPTIC TECHNOLOGY

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					FIBRE OPTIC TECHNOLOGY

INTRODUCTION

              Interest in the use of light as a carrier for information in the
1960’s with the advent of the laser as a source of coherent light. Initially the
transmission distances were very short, but as manufacturing techniques for
very pure glass arrived in 1970, it became feasible to use optical fibres as a
practical transmission medium. At the same time developments in semi-
conductor light sources and detectors meant that by 1980 worldwide
installation of fibre optic communication systems had been achieved.


What are Optical Fibres?

             Optical fibres are fibres of glass, usually about 120 micrometers
in diameter, which are used to carry signals in the form of pulses of light
over distances up to 50 km without the need for repeaters. These signals may
be coded voice communications or computer data.

             Fibre Optic (or “Optical Fibre”) refers to the medium and the
technology associated with the transmission of information as light impulses
along a glass or plastic wire of fibre. Fibre Optic wire carries much more
information than conventional copper wire and is far less subject to
electromagnetic interference. Most telephone company long-distance lines
are now fibre optic.

             Transmissions on fibre optic wire requires repeating at distance
intervals. The glass fibre requires more protection within an outer cable than
copper. For these reasons and because the installation of any new wiring is
labor-intensive, few communities yet have fibre optic wires or cables from
the phone company’s branch office to local customers.

Reflection and Refraction of Light

             When light traveling in a transparent meets the surface of
another transparent material two things happen some of the light is reflected,
some of the light is transmitted into the second transparent material. The
light which is transmitted usually changes direction when it enters the
second material. This bending of light is called refraction and it depends
upon the fact that light travels at one speed in one material and at a different
material. As a result each material has its own Refractive index that we use
to help us calculate the amount of bending which takes place. Refractive
index is defined as


                          C
                    n=
                          V

Where
     n is the refractive index.
     c is the speed of light in a vacuum
     v is the speed of light in the material

Two possibilities cases exist.

      These are: 1. Where light goes from a material with a low refractive
index to one with a high refractive index.
                  2. Where light goes from a material with a high refractive
index to one with a low refractive index.

             DIAGRAMS

Total Internal Reflection

             In the second case above, n2 is always greater than n1. So, as
we increase n1, eventually n2 will reach 900 before n1 does. At this point
where n1 has reached a value called critical angle (qc). The transmitted ray
now tries to travel in both materials simultaneously for various reasons this
is physically impossible so there is no transmitted ray and all the light
energy is reflected. This is true for any value of n1, the angle of incidence,
equal to or greater than qc. This phenomenon is called Total Internal
Reflection (TIR).

              We can define the two conditions necessary for TIR to occur
the refractive index of the first medium is greater than the refractive index of
the second one. The angle of incidence, n1, is greater than or equal to the
critical angle, qc.

              The phenomenon of TIR causes 100% reflection. In no other
situation in nature, where light is reflected, does 100% reflection occur. So
TIR is unique and very useful.

OPTICAL FIBRES

Structure of Fibre

              Optical Fibres are very fine fibres of glass. They consist of a
glass core, roughly 50 micrometers in diameter, surrounded by glass “optical
cladding” giving an outside diameter of about 120 micrometers. They make
use of TIR to confine light with the core of the fibre.

             DIAGRAM

              The core has a higher refractive index than the cladding.
Although the cladding does not carry light, it is nevertheless an essential part
of the fibre. The cladding is not just a mere covering. It keeps the value of
the critical angle constant throughout the whole length of the fibre.

             DIAGRAM

             Optical Fibres are optical waveguides. This means that
wherever the fibre goes the light, which is confined to the core of the fibre,
also goes. So optical fibres can be used to make light bend round corners.

Propogation of light in the fibre

              The angle q is the Acceptance Angle. Any light entering the
fibre at less than this angle will meet the cladding at an angle greater than
qc. If light meets the inner surface of the cladding (the core – cladding
interface) at greater than or equal to qc then TIR occurs. So all the energy in
the ray of light is reflected back into the core and none escapes into the
cladding. The ray then crosses to the other side of the core and, because the
fibre is more or less straight, the ray will meet the cladding on the other side
at an angle which again causes TIR. The ray is then reflected back across the
core again and the same thing happens. In this way the light zig zags its way
along the fibre. This means that the light will be transmitted to the end of the
fibre.

             DIAGRAM

             In reality the light which enters the fibre is a focused beam,
consisting of many millions of rays behaving in a similar way. They are all
zig zag along the core of the fibre, crossing over each other, and filling up
the core with light. A pulse of light traveling along the core of the fibre is
really a bundle of these rays.

Fibre Types

             There are two main fibre types:
                   1. Step index (multimode, single mode)
                   2. Graded index (multimode)

Step Index Fibre:

             Step index is so called because the refractive index of the fibre
steps up as we move from the cladding to the core of the fibre. Within the
cladding the refractive index is constant, and within the core of the refractive
index is constant.

             DIAGRAM

Multimode:
              Although it may seem from what we have said about total
Internal reflection that any ray of light can travel down the fibre, In fact,
because of the wave nature of light, only certain ray directions can actually
travel down the fibre. These are called the Fibre Mode. In a multimode fibre
many different modes are supported by the fibre.

             DIAGRAM

Single Mode:
           Because its core is so narrow single mode fibre can support
only one mode. This is called the Lowest Order Mode. Single mode fibre has
some advantages over multimode fibre which we will deal with later.

             DIAGRAM

Graded Index Fibre:
              Graded Index Fibre has a different core structure from single
mode and multimode fibre. Whereas in a step-index fibre the refractive
index of the core is constant throughout the core, in a graded index fibre the
value of the refractive index changes from the centre of the core onwards. In
fact it has what we call a Quadratic Profile. This means that the refractive
index of the core is proportional to the square of the distance from the centre
of the fibre.


             DIAGRAM

            Graded index fibre is actually a multimode fibre because it can
support more than one fibre mode. But when we refer to multimode fibre we
normally mean step index multimode.

PULSE SPREADING
 PULSE SPREADING

       The data which is carried in an optical fibre consists of pulses of light
energy following each other rapidly. There is a limit to the highest
frequency, i.e. how many pulses per second which can be sent into a fibre
and be expected to emerge intact at the other end. This because of a
phenomenon known as pulse spreading which limits the “bandwidth” of the
fibre.

Diagram


       The pulse sets off down the fibre with an nice square wave shape. As
it travels along the fibre it gradually gets wider and the peak intensity
decreases.
Cause of pulse spreading

      The cause of pulse spreading is dispersion. This means that some
components of the pulse of light travel at different rates along the fibre.
There are two forms of dispersion.
      Chromatic dispersion
      Modal dispersion

Chromatic dispersion

       Chromatic dispersion is the variation of refractive index with the
wavelength of the light. Another way of saying this is that each wavelength
of light travels through the same material at its own particular speed which is
different from that of other wavelengths.

       For example, when light passes through a prism some wavelengths of
light bend more because their refractive index is higher, i.e. they slower this
is what gives us the “spectrum” of white light. The “red” and “orange” light
travel slowest and so are bent most while the “violet” and “blue” travel
fastest and so are bent less. All the other colours lie in between.
       This means that different wavelengths traveling through an optical
fibre also travel at different speeds. This phenomenon is called “chromatic
dispersion”.

                   Diagram


Modal dispersion

           In an optical fibre there is another type of dispersion called
“Multimode dispersion”.

       More oblique rays travel a shorter distance. These correspond to rays
traveling almost parallel to the centre line of the fibre and reach the end of
fibre sooner. The more zig-zag rays take a longer route as they pass along
the fibre and so reach the end of the fibre later. Now:-

Total dispersion = chromatic dispersion + multimode dispersion
       Or put simply: for various reasons some components of a pulse of
light travelling along an optical fibre move faster and other components
move slower. So, a pulse which starts off as a narrow burst of light gets
wider because some components race ahead while other components lag
behind, rather like the runners in a marathon race.

Consequences of pulse spreading

      Frequency limit(Band width)
                                        The further the pulse travels in the
fibre the worse the spreading gets

                    Diagram

       Pulse spreading limits the maximum frequency of signal which can be
sent along a fibre. If signal pulse follows each other too fast then by the time
they reach the end fibre they will have merged together and become
indistinguishable. This is unacceptable for digital systems which depend on
the precise sequence of pulses as a code for information. The Bandwidth is
the highest number of pulses per second, which can be carried by the fibre
without loss of information due to pulse spreading.

Distance Limit

        A given length of fibre, as explained above has a maximum
frequency which can be sent along it. If we want to increase the bandwidth
for the same type of fibre we can achieve this by decreasing the length of the
fibre. Another way saying this is that for a given data rate there is a
maximum distance which the data can sent.

Bandwidth Distance Product (BDP)

            We can combine the two ideas above into a single term called
the bandwidth distance product (BDP). It is the bandwidth of a fibre
multiplied by the length of the fibre. The BDP is the bandwidth of a
kilometer of fibre and is a constant for any particular type of fibre. For
example, suppose a particular type of multimode firbre has a BDP of 20
MHz. km, then:-
             1 km of the fibre would have a bandwidth of 20 MHz
             2 km of the fibre would have a bandwidth of 10 MHz
             5 km of the fibre would have a bandwidth of 4 MHz
             4 km of the fibre would have a bandwidth of 5 MHz
             10 km of the fibre would have a bandwidth of 2 MHz
             20 km of the fibre would have a bandwidth of 1 MHz

The typical B.D.P. of the three types of fibres are as follows:-

             Multimode 6 – 25 MHz.Km
             Single Mode 500 – 1000 MHz.km
             Graded Index 100 – 1000 MHz.km (read as megahertz
kilometers). They are not MHz/Km (read as megahertz per kilometers). This
is because the quantity is a product (of bandwidth and distance) and not a
ratio.

Choice of Fibre

Multimode Fibre
            Multimode fibre is suitable for local area networks(LAN’s)
because it can carry enough energy to support all the subscribers to the
network. In a LAN the distances involved, however, are small. Little pulse
spreading can take place and so the effects of dispersion are important.

Single Mode Fibre
             Multimode Dispersion is eliminated by using Single Mode
fibre. The core is so narrow that only one mode can travel. So the amount of
pulse spreading in a single mode fibre is greatly reduced from that of a
multimode fibre. Chromatic dispersion however remains even in a single
mode fibre. Thus even in single mode fibre pulse spreading can occur. But
chromatic dispersion can be reduced by careful design of the chemical
composition of the glass.

              The energy carried by a single mode fibre , however, is much
less than that carried by a multimode fibre . For this reason single mode fibre
is made from extremely low less,very pure glass.
              Single mode low absorption fibre is ideal for
telecommunications because pulse spreading is small.

GRADE INDEX FIBRE
             In graded index fibre rays of light follow sinusoidal paths. This
means that low order modes, i.e. oblique rays stay close to the center of the
fibre, high modes spend more time near the edge of the core.

             Graded index fibre has the advantage that it can carry the same
amount of energy as multi mode fibre. The disadvantage is that this effect
takes place at only one wave length.

                          DIAGRAM

FIBRE MANUFACTIRE

             The most common method of making fibre is known as
Modified Chemical Vapour Dispersion (MCVD).An outer glass “bait tube”
isheated by a traversing burner. Through this tube a mixture of gases is
passed at a steady rate , which when heated undergoes a chemical reaction.
The gas mix contains compounds of silicon , metal halides ,oxygen and
dopant material which will determine the refractive index of the glass core.
The solid end products of the reaction are deposited on the interior of the
bait tube as soot. This soot will eventually form the core of the fibre while
the bait tube will form the cladding . when enough soot is deposited the gas
flow is stopped and the heat is turned up so that soot melts to form a sintered
glass. This sintered glass is heated to form a solid rod. This rod is held
vertically and passed through a oven which softens its ends. This end is now
stretched to form a glass fibre.


SPLICING

                    Optical fibres have to be joined to make longer lengths of
fibre or existing lengths which have been broken have to be repaired. Also
the ends of fibre have to be fitted with convenient connectors (terminations)
to allow them to be easily plugged into equipment such as power meters,
data transmitters ,etc.

                    Splicing is the process of joining the two bare ends of
two fibres together. The ends of the fibre must be precisely lined up with
each other, otherwise the light will not be able to pass from one fibre across
the gap to another to another fibre.
                   There are mainly four alignment of optical fibres. There
are :

                   LATERAL
                   AXIAL
                   ANGULAR
                   POOR END FINISH


There are mainly two types of splicing :

                        Fusion splicing
                        Mechanical splicing

FUSION SPLICING

                    In fusion splicing the ends of the fibre are aligned either
manually using micro-manipulators and a microscope system for viewing
the slice or automatically either using cameras or by measuring the light
transmitted through the slice and adjusting the positions of the fibre. The
ends of the fibre are then melted together using a flame or more commonly
an electric arc.

MECHANICAL SPLICING

                   In mechanical splicing the two fibre ends are held
together in a splice. This consists of some device usually made of glass
which by its internal design automatically brings the two fibres into
alignment. The openings at each ends of the device are usually fluted to
allow the fibre to be guided into the capillary where the alignment takes
place.

AREAS OF APPLICATION

TELECOMMUNICATIONS

                   Optical fibre are now the standard point to point cable
link between telephone sub stations.
                    A telecommunication link is the simplest of the fibre
optic system. It consists basically of a transmitter , a fibre link and a
receiver. The transmitter will normally be equipped with a laser diode,
usually with an output wave length of 1300nm or 1500 nm. The fibre link
will be made of single lengths of single mode optical fibre of length 2km
fusion spliced together. The link will be able to carry thousands of telephone
conversations simultaneously.

LOCAL AREA NETWORKS(LAN’S)

                    Multimode fibre is commonly used as a “back bone” to
carry signals between the hubs of LAN’s from where copper coaxial cable
takes the data to the desktop. Fibre links to the desk tops are also however
common.

CABLE TV

                    As mentioned above domestic cable tv networks use
optical fibre because of its very low power consumption.

CCTV

                 Closed Circuit Television security systems use optical
fibre because of its inherent security as well as the other advantages
mentioned above.

OPTICAL FIBRE SENSORS

                   Many advances have been made in recent years in the use
of optical fibres as sensors. Gas concentration , chemical concentration,
pressure , temperature , and rate of rotation can all be sensed using optical
fibre. Much work in this field is being done at the University Of Strathclyde.
ADVANTAGES


CAPACITY

                    Optical fibres carry signals with much energy loss than
copper cable and with a much higher bandwidth. This means that fibres can
carry more channels of information over longer distances and with fewer
repeaters required.

SIZE AND WEIGHT

                   Optical fibre cables are much lighter and thinner than
copper cables with the same bandwidth. This means that much less space is
required in underground cabling ducts. Also they are easier for installation
engineers to handle.

SECURITY

                    Optical fibre are much more difficult ot tap information
from undetected; a great advantage for banks and security installations. They
are immune to Electro magnetic interference from radio signals, explosive or
flammable atmospheres, for example , in the petro chemical industries or
munitions sites ,without any risk of ignition.

RUNNING COSTS

                    The main consideration in choosing fibre when installing
domestic cable tv networks is the electric bill. Although copper coaxial cable
can handle the bandwidth requirement over the short distances of a housing
scheme, a copper system consumes far more electrical power than fibre ,
simply to carry the signals.
DISADVANTAGES

PRICE

                    In spite of the fact that the raw material for making
optical fibres and sand is abundant and cheap , optical fibres are still more
expensive per meter than copper. Having said this one fibre can carry more
signals than a single copper cable and the large transmission distances mean
that fewer expensive repeaters are required.

SPECIAL SKILLS

                    Optical fibres cannot be joined together as easily as
copper cable ,and requires additional training of personnel and expensive
precision splicing and measurement equipment


CONCLUSION

                    We have looked at how the optic fibres work and how
they are made. The usage of these optical fibres is gradually increasing in all
the fields. The communication can be passed on more quickly and safely as
the usage of these optical fibres increases.