Transcript William Stallings, Data and Computer Communications, 8/e

EEE440
Modern Communication Systems
Optical Fibre Communication
Systems
En. Mohd Nazri Mahmud
MPhil (Cambridge, UK)
BEng (Essex, UK)
[email protected]
Room 2.14
Semester 1 2011-2012
Optical Fibre
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composed of two concentric layers: the core and the cladding
have different indices of refraction with the core having n1 and the cladding
n2.
an additional coating around the cladding called the jacket usually consists
of one or more layers of polymer to protect the core and cladding from
shocks
the cladding guides the light along the core by using the method of total
internal reflection
Semester 1 2011-2012
Optical Fibre
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Fiber optic cable sizes are usually expressed by first giving the core size followed by
the cladding size.
50/125 indicates a core diameter of 50 microns and a cladding diameter of 125
microns
The larger the core the more light can be coupled into it
However, larger diameter cores may actually allow too much light in and may cause
receiver saturation problems.
The 8/125 cable, is often found when a fiber optic data link operates with single-mode
propagation.
The 62.5/125 cable, is often found in a fiber optic data link that operates with multimode propagation.
Semester 1 2011-2012
Optical Fibre
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light propagates down the fiber optic cable and is confined to the core
the core and cladding have different indices of refraction with the index of the core,
n1, always being greater than the index of the cladding, n2
Light can be guided down the fiber optic cable if it enters at less than the critical angle
This angle is fixed by the indices of refraction of the core and cladding and is given by
the formula:
• Qc = arc cosine (n2 /n1).
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The critical angle is measured from the cylindrical axis of the core. if n1 = 1.446 and
n2= 1.430 then the critical angle is 8.53 degrees.
Semester 1 2011-2012
Optical Fibre
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If the light ray strikes the core-to-cladding interface at an angle greater than
the critical angle then it is reflected back into the core.
Since the angle of incidence is always equal to the angle of reflection the
reflected light will again be reflected
The light ray will then continue this bouncing path down the length of the
fiber optic cable.
If the light ray strikes the core-to-cladding interface at an angle less than the
critical angle then it passes into the cladding where it is attenuated very
rapidly with propagation distance.
Semester 1 2011-2012
Optical Fibre
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When light wave is guided down a fiber optic cable it exhibits certain modes
Mode can be one of two types, multi-mode or single-mode
the number of modes that exist depend upon the dimensions of the cable
and the variation of the indices of refraction of both core and cladding
across the cross section
Semester 1 2011-2012
Optical Fibre
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For the multi-mode propagation with a step index refractive index profile
– the diameter of the core is fairly large relative to the cladding
– a sharp discontinuity in the index of refraction between core and
cladding.
when light enters the fiber optic cable on the right it propagates down
toward the left in multiple rays or multiple modes.
the lowest order mode travels straight down the center.
The higher modes represented by rays, bounce back and forth, going down
the cable to the left.
The higher the mode the more bounces per unit distance down to the left.
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Optical Fibre
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the output pulse is significantly attenuated relative to the input pulse and suffers
significant time dispersion.
The higher order modes, the bouncing rays, tend to leak into the cladding as they
propagate down the fiber optic cable.
They lose some of their energy into heat and results in an attenuated output signal.
The bouncing rays and the lowest order mode, traveling down the center axis, are all
traversing paths of different lengths from input to output.
Consequently, they do not all reach the right end of the fiber optic cable at the same
time.
When the output pulse is constructed from these separate ray components the result
is time dispersion.
Semester 1 2011-2012
Optical Fibre
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Fiber optic cable that exhibits multi-mode propagation with a step index
profile is thereby characterized as having higher attenuation and more time
dispersion than the other propagation modes.
However, it is also the least costly
It is especially attractive for link lengths up to 5 km.
Usually, it has a core diameter that ranges from 100 microns to 970
microns.
It can be fabricated either from glass or plastic.
Semester 1 2011-2012
Optical Fibre
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For the single-mode propagation with a step index refractive index profile
– the diameter of the core is fairly small relative to the cladding.
– Typically, the cladding is ten times thicker than the core.
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When light enters the fiber optic cable on the right it propagates down
toward the left in just a single ray
This lowest order mode is confined to a thin cylinder around the axis of the
core.
All energy is confined to this single, lowest order, mode
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Semester 1 2011-2012
Optical Fibre
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The higher order modes are absent.
Consequently, there is no energy lost to heat by having these modes leak
into the cladding.
Attenuation is not significant.
Also, since the input signal is confined to a single ray path, that of the
lowest order mode, there is little time dispersion
Less time dispersion means higher bandwidth and this is in the 50 to 100
GHz/ km range.
Semester 1 2011-2012
Optical Fibre
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However, it is the most costly for local applications
It has been used more with Wide Area Networks than with local data
communications.
It is attractive for link lengths go all the way up to 100 km.
The core diameter for this type of fiber optic cable is exceedingly small
ranging from 5 microns to 10 microns. The standard cladding diameter is
125 microns.
Single-mode fiber optic cable is fabricated from glass.
Because of the small thickness of the core, plastic cannot be used to
fabricate single-mode fiber optic cable
Semester 1 2011-2012
Optical Fibre
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For multi-mode propagation with a graded index refractive index profile
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the variation of the index of refraction is gradual
the core is much larger than in the single-mode step index case
Multi-mode propagation exists with a graded index.
However, the paths of the higher order modes are somewhat confined
The attenuation through them due to leakage is more limited than with a
step index.
The time dispersion is more limited than with a step index
Fiber optic cable that exhibits multi-mode propagation with a graded index
profile is thereby characterized as having attenuation and time dispersion
properties somewhere between the other two.
Semester 1 2011-2012
Optical Fibre
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Likewise its cost is somewhere between the other two.
Popular graded index fiber optic cables have core diameters of 50, 62.5 and
85 microns.
They have a cladding diameter of 125 microns - the same as single-mode
fiber optic cables.
This type of fiber optic cable is extremely popular in local data
communications applications.
In particular, the 62.5/125 fiber optic cable is the most popular and most
widely used in these applications.
Glass is generally used to fabricate multi-mode graded index fiber optic
cable. However, there has been some work at fabricating it with plastic.
Semester 1 2011-2012
Fibre Attenuation
• Mainly due to absorption, scattering and radiative loss.
• Important in determining the maximum distance between transmitter
and receiver
• The more light that can be coupled into the core the more light will
reach the receiver and the lower the BER.
• The lower the attenuation in propagating down the core the more
light reaches the receiver and the lower the BER.
• The less time dispersion realized in propagating down the core the
faster the signaling rate and the higher the end-to-end data rate
Semester 1 2011-2012
Fibre attenuation
• Optical losses of a fiber are usually expressed in decibels per
kilometer (dB/km).
• The expression is called the fiber’s attenuation coefficient α and
the expression is
• where P(z) is the optical power at a position z from the origin, P(0) is
the power at the origin.
• For a given fiber, these losses are wavelength-dependent
• The value of the attenuation factor depends greatly on the fiber
material and the manufacturing tolerances
Semester 1 2011-2012
Fibre Attenuation
• Attenuation varies with the wavelength
• The three principal windows of operation correspond to wavelength
regions where attenuation is low and matched to the ability of a
transmitter to generate light efficiently and a receiver to carry out
detection.
Semester 1 2011-2012
Fibre Attenuation
• Absorption is due to the fibre material whereas scattering is due to
the structural imperfection in the optical waveguide
• Intrinsic Material Absorption
– caused by interaction of the propagating lightwave with one more more major
components of glass that constitute the fiber’s material composition.
– These looses represent a fundamental minimum to the attainable loss and can
be overcome only by changing the fiber material.
– An example of such an interaction is the infrared absorption band of Silica.
– However, in the wavelength regions of interest to optical communication (0.80.9um and 1.2-1.5um), infrared absorption tails make negligible contributions.
• Extrinsic Impurity Ions Absorption
– Extrinsic impurity ions absorption is caused by the presence of minute quantity of
metallic ions (such as Fe2+, Cu2+, Cr3+) and the OH- ion from water dissolved in
glass.
Semester 1 2011-2012
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Fibre Attenuation
Impurity Ion
Fe2+
Fe2+
Cu2+
Cr3+
V4+
OHOHOH-
Semester 1 2011-2012
Loss due to 1ppm Absorption Peak
of impurity
Wavelength (um)
(dB/km)
0.68
1.1
0.15
0.4
1.1
0.85
1.6
0.625
2.7
0.725
1.0
0.95
2.0
1.24
4.0
1.38
Fibre Attenuation
Semester 1 2011-2012
Fibre attenuation
A 30km long optical fibre that has
attenuation coefficient of 0.8dB/km at
1300nm wavelength. What is the optical
output power if 200 microwatt of optical
power is launched into the fibre?
Semester 1 2011-2012
Fibre link budget
• Short haul point-to-point link
• Long haul multistage point-to-point link
with amplifier
Semester 1 2011-2012