Post on 26-Jun-2020
ECE 4105
Optical Fiber Communications
Prof. Dr. Monir Hossen ECE, KUET
Department of Electronics and Communication Engineering, KUET
Department of Electronics and Communication Engineering, KUET 2
Communication may be broadly defined as the transfer of
information from one point to another.
When the information is to be conveyed over any distance a
communication system is usually required.
Electromagnetic spectrum of communication
Definition
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Why Fiber Optics?
Fiber can support huge bandwidth demand, because:
At implementation level:
Present capacity is 100 Gbps (single fiber network).
IEEE currently investigating for 400 Gbps or 1 Tbps
(due after 2017).
At research level:
Technical University of Denmark (DTU)
investigated 1 Tbps in 2009.
Karlsruhe Institute of Technology (KIT) investigated
26 Tbps in 2011.
Recently, the DTU investigated 43 Tbps.
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Development of Communications 1. Visual optical communications:
The mainstream of communications was the visual optical
communication using visible optical carrier waves or light
for thousands years.
Simple system such as beacon-fires in ancient Egypt,
ancient China, old Greek and Rome was developed to
color-combination signal fires with binoculars.
Other techniques were semaphore flags, reflecting mirrors
and signaling lamps.
They have been disappearing since the Mores telegraph
invention (1837).
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2. Electric communications
Electric communication started from the invention of
telegraph.
First transatlantic cable (between London and New York) in
1866: Queen Victoria’s message to President Buchanan.
It took the Queen 1.5 hours to receive a reply from President
Buchanan.
Chief engineer of the project: Lord Kelvin.
1940 - first coaxial cable system, 3MHz system with 300 voice
channels or a single television channel, the bandwidth is
limited by the frequency dependent cable losses which
increases rapidly beyond 10MHz.
Development of communications Contd.
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1975 - the most advanced coaxial cable system, 274Mb/s with
repeater spacing of 1km. Very expensive to operate.
Development of Communications Contd.
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3. Communication using high-frequency electromagnetic
waves: Inventions of Bell’s telephone (1876) and Marconi’s wireless
communication (1896) were two epoch-making events.
Information was carried and transmitted by electromagnetic
(EM) carrier wave.
Since then the frequency of EM carrier wave has been rising.
Depending on their wavelengths these EM carriers can be
transmitted over considerable distances but are limited in the
amount of information they can convey by their frequencies.
The information-carrying capacity is directly related to the
bandwidth or frequency extent of the modulated carrier,
which is generally limited to a fixed fraction of the carrier
frequency.
Development of Communications Contd.
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The greater the carrier frequency is, the larger the available transmission bandwidth. Thus the system has information-carrying capacity of the communication.
So, radio communication was developed to higher frequencies, i.e., VHF and UHF, leading to the introduction of the even higher frequency microwave and latterly, millimeter wave transmission.
Development of Communications Contd.
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Until 1960, the coherent wave generation together with the
communication technology increases the used frequency by
one order of magnitude every six years in average.
However, they got stagnant for about twenty years.
There seemed to be an invisible wall at around 1 mm
wavelength.
Photoelectronic era started by a leap from 300 GHz (mm
wave) to 300 THz (μm wave).
The leap A in the fundamental technology occurs with the
aid of the invention of laser and the leap B in the
communication technology with the practicability of optical
fibers.
Development of Communications Contd.
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The communication at optical frequencies offers an increase
in the potential usable bandwidth by a factor of around 104
over high frequency microwave transmission.
An additional benefit of the use of high carrier frequencies is
the general ability of the communication system to
concentrate the available power within the transmitted
electromagnetic wave, thus gives an improved system
performance.
Development of communications Cond.
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4. Optical communication
Using the light as the carrier signal in communication
system is nothing new.
In 1880 Alexander Graham Bell invented the photophone
just fourth year after his invention of telephone.
He demonstrated that speech could be transmitted on a
beam of light.
Bell focused a narrow beam of sunlight onto a thin mirror
on a diaphragm.
When the sound waves of human speech caused the
diaphragm with mirror to vibrate, the beam of sunlight was
modulated and the amount of energy transmitted to the
light detector varied correspondingly.
Development of Communications Contd.
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The light reaching the selenium detector caused its resistance,
therefore the intensity of the current in a telephone receiver,
to vary, setting up speech waves at the receiver end.
Bell managed to give speech transmission over a distance of
200 m by using his ingenious invention.
Problems for optical communication were: (a) the lack of
suitable light source; (b) the severe influence of disturbances
such as rain, snow, fog, dust and atmospheric turbulence.
From 1930 to 1940s, research on optical communication
between warships was carried out in some countries.
The signal source was a high-power bulb with current
modulation. The receiver was a paraboloidal mirror and the
signal was demodulated by photoelectric valve.
Development of Communications Contd.
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It did not reach practicability.
What is new today are the techniques available for generating
a light beam that can be modulated at extremely high rates
and, equally important transmitted through a low-loss optical
fiber several miles long with acceptable loss of energy.
Modern light-wave communication had its birth in the 1960s.
Laser provided a powerful coherent light source, together with
the possibility of modulation at high frequency.
In addition, the low beam divergence of the laser made
enhanced free space optical transmission a practical possibility,
which was still restricted by the atmospheric conditions and
made limited success.
Development of Communications Contd.
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Some links have been implemented for applications such as
the linking of TV camera to a base vehicle and for data links
of a few hundred meters between buildings.
Similar techniques could also be used for inter-satellite
communication in outer space.
For optical communications, because light gets absorbed on
foggy or rainy days, early work was aimed at beams of light
propagating inside tubes running in similar ducts to the
proposed millimetric waveguide system.
Development of Communications Contd.
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5. Light transmission path
(a) Optical fiber
The essence of optical fiber is a homogeneous dielectric rod
although a communication optical fiber with good light
transmission characteristics possesses complex internal
distribution of refractive indices.
Light transmits in the fiber through multi-total internal
reflection.
British physicist John Tyndall demonstrated light transmission
in 1870 in water flow from an opening under a big tank.
Such structure of light waveguide was used in stomach
endoscope in fifties but few considered the possibility to apply
optical fiber for telecommunication.
Development of Communications Contd.
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There were probably two reasons:
i. The loss factor was too large (10-8). The attenuation owing to dielectric itself reaches 500 dB/km.
ii. Large dispersion causes distortion of transmission along optical fibers, e.g. it broadens transmitted pulse and finally overlaps it with its neighbors.
(b) Dielectric thin-film waveguide supported by a frame (1965)
(c) Lens array waveguide
(1962-64) and gas lens
waveguide (1964-65)
Development of Communications Contd.
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6. Optical fiber came forth
Reliable information transfer using light wave can be achieved
via dielectric waveguides or glass optical fibers to avoid
degradation of the optical signal by the atmosphere.
Proposal for telecommunication with optical fibers were made
in 1966 by Kao and Hockham.
They analyzed various causes of light loss in glass and showed
that low-loss optical fibers for telecommunications could be
fabricated through improving technologies.
They also discussed a structure of core-cladding with minute
difference in refractive index (named weakly guiding fiber in
1970 by Gloge).
Development of Communications Contd.
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Technologies developed rapidly.
In 1970 Corning Glass Works, produced a silica fiber with
a signal-power transmission of better than 1% over a
distance of 1 km (i.e., an attenuation of 20 dB/km), which
was comparable to existing copper electrical system.
During the next two decades, the transmission rose to
about 96% over 1 km (i.e., an attenuation of only 0.16
dB/km).
Bell Laboratory developed a MCVD (chemical vapor
deposition) technology, which became a standard in optical
fiber production.
Development of communications Cond.
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Applications of Optical Fibers
o Image Transport
o Optical Communications
o Optical Fiber Sensors
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Advantages of Optical Communications Comprehensive economic aspects
• Enormous potential bandwidth: frequency (~ 1014 Hz) much
higher than micro-wave so that the available transmission
bandwidth could be 104 larger (~ 3 GHz);
• Low loss transmission: Extremely low attenuation, minimum
attenuation ~ 0.2 dB/km c.f. 5-10 dB/km of the coaxial cables;
• Small size and weight: Extremely small fiber size (diameter ~
100-150 μm) and ~ 30 g/km;
• Ruggedness and flexibility: Good flexibility (bendable to mm
curvature radius);
• Cheap and abundant material: Made of quartz (SiO2, sand)
instead of Cu.
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Security
• Electrical isolation (non-conductive, non-radiative and non-
inductive): no earth loop and interface problem; no effects of
lightening, surge current, or power cables on transmitting
signals of light.
• Immunity to interference and crosstalk: free from
electromagnetic interference, crossing, disturbance fire (endure
1000°C high temperature), no spark.
• Unparallel signal security: The light from optical fibers does
not radiate significantly.
Ultrapure glass fibers have become the premier
communication medium.
Advantages of Optical Communications Contd.
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Transmission Characteristics of Different Communication Medias
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Basics of the Optical Fiber Communication
System
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First Generation of Optical Fiber Communication System
In 1977, field trails on optical fiber communications were held,
and proved to be very successful! Operating wavelength was ~
0.8 μm, 50 - 100 Mb/s, repeater spacing 10 km.
Around this time another major development: Material
dispersion in silica fiber is zero at 1.3 μm, with OH- free glass,
attenuation ~ 0.5 to 1 dB/km at 1.3 μm, and 0.2 db/km at 1.55
μm.
Initially no laser and detector
work at such wavelengths!
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2nd Generation of Optical Fiber Communication System
In 1980, operating at 1.3 μm started appearing with multimode
fiber the bit rate is limited to 100 Mb/s and the repeater spacing
is ~ 20km.
In 1981, laboratory demonstration of 2 Gb/s over 44 km of
single mode fiber, and it is clear that single mode fiber offers
much wider bandwidth.
There was a worldwide rush to develop single mode fiber optic
systems, with monthly report of advances. Repeater spacing
stretched from 30 km to 200 km, and bit rate increased from
140 Mb/s to Gb/s.
In 1987, second generation of 1.3 μm fiber optic systems
employing single mode fibers, with bit rate up to 1.7 Gb/s and
repeater spacing of 50 km.
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3rd Generation of Optical Fiber Communication System
Repeater spacing is limited by the fiber loss (0.5 dB/km at 1.3
μm). Since loss in silica fiber is 0.2 dB/km at 1.55 μm, there is
an increased interest to operate at this wavelength.
System development was delayed due to the large material
dispersion in single mode fiber; the solution is to use either
dispersion shifted fiber or single longitudinal mode laser.
In 1985, laboratory demonstration - 4Gb/s over 100 km.
In 1990, 2.4 Gb/s system is commercially available.
Development of 10 Gb/s commercial system is underway.
Design and implementation of inter-city and undersea systems
of up to 200 km long, trans-oceanic fiber optic systems as well
as massive reduction in the cost of traffic.
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4th Generation of Optical Fiber Communication System
Increases the bit rate using wavelength division multiplexing (WDM)
or frequency division multiplexing (FDM).
Increase in repeater spacing using optical amplification.
Coherent communication using homodyne or heterodyne detection
system: first demonstrated in 1981/82, showed that one could exploit
the optical spectrum in an analogous manner to that of the radio
frequency. At 1.55μm, a spectral width of 20000 GHz is available!
In 1990, 2.5 Gb/s over 2223 km without repeaters, loss over fiber is
compensated by using optical fiber amplifiers in every 80 km.
In 1991, 2.5 Gb/s over 4500 km and 10 Gb/s over 1500 km
demonstrated. By using a recirculating loop - 2.4 Gb/s over 21000
km and 5 Gb/s over 14300 km.
Optical fiber amplifier provides trans-oceanic communication
system of 9000-10,000 km without repeaters!
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Soliton transmission - optical pulses that preserve their shapes during
propagation in a lossless fiber by counteracting the effect of dispersion
through the fiber nonlinearity.
Basic idea proposed in 1973. Laboratory demonstration in 1988 over
4000 km of fiber by compensating the fiber loss through Raman
scattering.
Since 1989, all experiments use optical fiber amplifiers. 10 Gb/s over
1000 km and 20 Gb/s over 350 km. By using recirculating loop, 2.4
Gb/s over 12,000 km.
Optical fiber communication systems: long term cost trends will favor
the use of fiber to the home for videophone, HDTV etc.
Development of Optoelectronic integrated circuits (OEIC's) will
further reduces the cost and makes deployment more cost effective.
Optical interconnects - optics is already tackling the backplane wiring
problem.
5th Generation of Optical Fiber Communication System
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Ray Theory of Transmission A transparent dielectric rod, typically of silica glass with a
refractive index of around 1.5, surrounded by air, proved to be an impractical waveguide due to its unsupported structure, especially when very thin waveguides were considered in order to limit the number of optical modes propagated, and the excessive losses at any discontinuities of the glass-air interface.
It was required that the smooth surface of a single fiber kept clean of moisture, dust, oil, etc. to prevent leakage.
Light also might leak from one fiber to another (cross-talk) as large numbers of fibers are packed in close proximity.
Applications of glass optical waveguides as optical imaging and medical diagnosis led to proposals for a clad dielectric rod in mid-1950s in order to overcome these problems.
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This structure is illustrated in Fig. 2.1, which shows a
transparent core with a refractive index n1 surrounded by a
transparent cladding of slightly lower refractive index n2.
The cladding supports the waveguide structure whilst also, when
sufficiently thick, substantially reducing the radiation loss into
the surrounding air.
At the cladding-air interface, the light field decays to negligibly
small.
Figure 2.1 Optical fiber waveguide with n2 ≤ n1.
Ray Theory of Transmission Cntd.
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Total Internal Reflection
,
The refractive index of a dielectric medium is defined as the
ratio of the velocity of light in a vacuum to the velocity of light in
the medium.
When a ray is incident on the interface between two dielectrics
of differing refractive indices, e.g. glass-air as shown in Fig. 2.2,
it travels more slowly in the optically dense glass than in the less
dense air, and the refractive angle is larger than the incident
angle. Total internal reflection (TIR)
occurs provided that the incident
angle is greater than the critical angle ,
, i.e., the glancing
angle is sufficiently small.
12
1 /sin nnc
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Total Internal Reflection Cntd.. A meridional ray (passes through the axis of an optical fiber)
passes through the axis of the fiber core.
This type of ray is the simplest to describe and is generally
used when illustrating the fundamental transmission
properties of optical fiber. A meridional ray might undergo
several thousand reflections per meter, .
Typically, D ~ 10μm, (seldom used in size much smaller than
this, available as small as 2μm), the glass core has an index n1
of 1.62 and the cladding n2 ~ 1.52.
tan/ DLN
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Acceptance Angle We now concern about light rays entering the fiber from air.
Only rays with a sufficiently shallow grazing angle at the core-
cladding interface are transmitted by TIF.
It is clear that not all rays entering the fiber core will continue
to be propagated down its length.
There is a maximum θa (a for air), for which the internal ray
will impinge at the critical angle ɸc.
Any rays B incident on the input face at an angle to the fiber
axis larger than θa will strike the internal core-cladding
interface at angle less than and will not have TIF.
B is refracted into the cladding
and eventually lost by
radiation. This situation is
shown by Fig.2.4.
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Acceptance Angle Contd.
The maximum angle θa to the fiber axis at which light may
enter the fiber in order to be propagated is often referred to as
the (total) acceptance angle for the fiber.
Start with (θf is the refractive
angle, f for fiber)
i.e.,
and .
This is a relationship of the acceptance angle and the refractive
indices of the three media involve, namely the core, cladding
and air.
ffc nn 2
12 sin1)90sin(/sin
an
nnn 2
2
1
2
012 sin1/
2
2
2
1
0
1sin nn
na
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Numerical Aperture
The quantity n0sinθa is defined as the numerical aperture, or
NA
NA = n0sinθa .
The NA is independent of the fiber core diameter and will
hold for diameters as small as 8 μm.
When interference phenomena are considered it is found that
only rays with certain discrete characteristics propagate in
the fiber core.
Thus the fiber will only support a discrete number of guided
modes.
This becomes critical in small core diameter fibers which
only support one of a few modes.
Hence, electromagnetic mode theory must be applied in these
cases.
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Numerical Aperture Contd..
The index difference of a waveguide is usually characterized by a
normalized parameter
The numerical aperture can be expressed as
For typical multimode fibers, we have Δ ~ 1 – 3 %, NA ~ 0.2 – 0.4
It has been defined the relative aperture as D/f, the ratio between
the aperture diameter D and the focal length f.
Its inverse is the focal ratio or f-number, often written f/ as a
single symbol, that is
The NA should clearly relate to the f-number of the system, and,
in fact,
In the air ni = 1 and the largest value of NA is 1, NA ≤ 1. Fibers
with a wide variety of numerical apertures, from about 0.2 up to
and including 1.0, are commercially obtainable.
Dff //#
)(2
1/#
NAf
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Skew Rays Besides the meridional rays in the
optical waveguide, there is
another category of ray exists
which is transmitted without
passing through the fiber axis.
These rays, which greatly
outnumber the meridional rays,
follow a helical path through the fiber, as illustrated in Fig. 2.6,
and are called skew rays.
When projecting into the cross-section, each reflection gives a
change in the direction of 2γ as shown in Fig.2.6(b).
When the light input to the fiber is nonuniform, skew rays will
therefore tend to have a smoothing effect on the distribution of
the light as it is transmitted, giving a more uniform output.
More reflec-tions, better the smoothing.
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Skew Rays Contd… To calculate the acceptance angle for
a skew ray it is necessary to define
the direction of the ray in two
perpendicular planes.
The geometry of the situation is
shown in Fig.2.7.
A skew ray is incident on the end face of a fiber core at the
point A at an incident angle θs.
The normal at the point A is AT parallel to the core axis SR.
The ray is refracted at the air-core inter-face before traveling
to the point B in the same incident plane ABT.
However, the normal at the point B is BR along the core radius
and the incident plane is ABR which makes an angle γ at the
cross-section RBT.
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Skew Rays Contd… The incidence (and reflection) angle at the point B is , and >
C .
The new incident angle satisfies
For the critical angle c,
The maximum input axial angle for skew rays is
The acceptance condition for skew rays are:
If cosγ = 1, θas reduces to θa like meridional rays. Thus θa defines
the maximum conical half angle for the acceptance of meridional
rays but it defines the minimum input angle for skew rays.
cossincos
2/12
12 )/(1coscossin nnc
2/12
12
0
1
0
1
0
1 )/(1coscos
cossinsin nn
n
n
n
n
n
n cas
cossin2
2
2
10 nnnNA as
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Skew Rays Contd… As shown in Fig.2.6(b), skew rays tend to propagate only in
the annular region near the outer surface of the core, and do
not fully utilize the core as a transmission medium.
However, they are complementary to meridional light-
gathering capacity of the fiber.
This increased light-gathering ability may be significant for
large NA fibers, but for most communication design purposes
meridional rays are considered adequate.
Self Study: All related Examples of Chapter 2
Assignment: A typical relative refractive index difference for an optical fiber designed for long
distance transmission is 1%. Estimate the NA and the solid acceptance angle in air
for the fiber when the core index is 1.46. Further, calculate the critical angle at the
core-cladding interface within the fiber. It may be assumed that the concepts of
geometric optics hold for the fiber.
Thanks for Your Kind
Attention
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