Reference Books:
Keiser G., optical fiber communications, McGraw-hill
Gowar J., optical communication systems, PHI.
Keiser G.,Optical Communication Essentials, McGrawhill,2008
Agrawal G.P.,Fiber-optic Communication Systems, 3rd ed.
Wiley
Senior J., optical fiber communications, principles &
practice, PHI
P S PANDEY THDC IHET
September 14
?? (Venice)
1609 Galileo (Italy)
1626 Snell (Holland)
1668 Newton (UK)
1870 Tyndall (UK)
1873 Maxwell (UK)
1888 Hertz (Germany)
1897 Rayleigh (UK)
1899 Marconi (Italy)
1902 Marconi (Italy)
1930 Lamb (Germany)
1936-40 USA
1951 Heel, Hopkins, Kapany (UK)
1958 Goubau et.al. (USA)
1958-9 Kapany et.al. (UK)
1960 Maiman et.al. (USA)
1960 Javan et.al. (USA)
1961 Kapany and Snitzer (UK)
1962 USA
1964 Goubau and Christian (USA)
1966 Kao and Hockham (UK)
transmission
1969 Uchida et.al. (Japan)
1970 Kapron and Keck (USA)
1972 Gambling et.al. (UK)
1975 Payne and Gambling (UK)
Decorative flowers made of glass fibers
Galileian telescope
Snells Law
Reflection telescope
Light guiding in a thin water jet
Electromagnetic waves
Confirmation of EM waves and relation to light
Analysis of waveguide
Radio communication (UK-Continent)
Invention of radio detector
Experiments with silica fiber
Communication using a waveguide
Image transmission w. fiber bundles
Experiments with the lens guide
Optical fiber with cladding
First laser (ruby laser)
Operation of He-Ne laser
mode analysis of optical fiber
Operation of semiconductor laser
light guide with periodic lenses
Suggestion of using optical fibers for long-distance
graded index optical waveguides
Fiber transmission loss <20dB/km
Gigahertz bandwidth over 1km
Prediction of zero material dispersion at 1.3 um
P S PANDEY THDC IHET
September 14
1- Overview of Photonic Communications
2- Optical Fiber: Wave-guiding, Propagation Modes
3- Signal Degradation in Optical Fibers
4- Photonic Sources & Transmitters: LED & Laser
Diodes
5- Laser-Fiber Connections (Power Launching &
Coupling)
6- Photodetectors
7- Digital Photonic Receivers & Digital Transmission
systems
8- WDM & Photonic Networks
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September 14
Optical Communication Bands
Original Band (O-band)
: 1260 to 1360 nm
Extended Band (E-band)
: 1360 to 1460 nm
Short Band (S-band)
: 1460 to 1530 nm
Conventional Band ( C-band)
: 1530 to 1565 nm
Long Band (L-band)
: 1565 to 1625 nm
Ultralong Band(U-band)
: 1625 to 1675 nm
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September 14
Electrical
Signal Input
Format
Bandwidth
Protocol
Comm.
Channel
Optical
Transmitter
Modulation
Characteristics
Power
Wavelength
Considerations:
Wavelength: 0.85, 1.3, 1.55,
DWDM
Transverse mode: SM vs. MM
Longitudinal mode: DFB, VCSEL
vs. FP, DBR
Modulation: Direct vs. external
vs. integrated modulator
Loss
Dispersion
Noise
Crosstalks
Optical
Receiver
Output
Bandwidth
Responsivity
Sensitivity
Noise
Wavelength
Considerations:
Wavelength: 0.85, 1.3, 1.55,
DWDM
Transverse mode: SM vs. MM
Dispersion
Loss
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12
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13
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14
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Examples of Information Rates for some typical Voice,
Video and Data Services
Type of service
Data Rate
Voice on demand/Interactive TV
1.5 to 6 mbps
Video games
1 to 2 Mbps
Remote education
1.5 to 3 Mbps
Electronic shopping
1.5 to 6 Mbps
Data transfer or Telecommunication
1 to 3 Mbps
Video conferencing
0.384 to 2 Mbps
Voice (single channel)
64 kbps
P S PANDEY THDC IHET
September 14
18
Optics involve generation, propagation & detection
of light.
Three major developments are responsible for
rejuvenation of optics & its application in modern
technology:
1- Invention of Laser
2- Fabrication of low-loss optical Fiber
3- Development of Semiconductor Optical Device
As a result, new disciplines have emerged & new
terms describing them have come into use, such as:
- Electro-Optics: is generally reserved for optical
devices in which electrical effects play a role, such
as lasers, electro-optic modulators & switches.
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19
Optoelectronics: refers to devices & systems that are
essentially electronics but involve lights, such as
LED, liquid crystal displays & array photodetectors.
Quantum Electronics: used in connection with
devices & systems that rely on the interaction of
light with matter (lasers & nonlinear optical devices).
Quantum Optics: Studies quantum & coherence
properties of light.
Lightwave Technology: describes systems & devices
that are used in optical communication & signal
processing.
Photonics: in analogy with electronics, involves the
control of photons in free space and matter.
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20
Extremely wide bandwidth: high carrier frequency ( a
wavelength of 1552.5 nm corresponds to a center frequency
of 193.1 THz!) & consequently orders of magnitude increase
in available transmission bandwidth & larger information
capacity.
Optical Fibers have small size & light weight.
Optical Fibers are immune to electromagnetic interference
(high voltage transmission lines, radar systems, power
electronic systems, airborne systems, )
Lack of EMI cross talk between channels
Availability of very low loss Fibers (0.25 to 0.3
dB/km), high performance active & passive
photonic components such as tunable lasers, very
sensitive photodetectors, couplers, filters,
Low cost systems for data rates in excess of Gbit/s.
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21
Type &
applications
Format
Uncompressed
Compressed
Voice, digital
telegraphy
4 kHz voice
64 kbps
16-32 kbps
Audio
16-24 kHz
512-748 kbps
32-384 kbps
(MPEG, MP3)
Video conferencing
176 144 or 352
2-35.6 Mbps
288 frames @ 10-30
frames/s
Data transfer, Ecommerce,Video
entertainment
64 kbps-1.544 Mbps
(H.261 coding)
1-10 Mbps
Full-motion
broadcast video
720 480frames @
30 frames/s
249 Mbps
2-6Mbps (MPEG-2)
HDTV
1920 1080
frames@ 30 frames
/s
1.6 Gbps
19-38 Mbps
(MPEG-2)
September 14
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Digital link consisting of time-division-multiplexing (TDM) of
64 kbps voice channels (early 1980).
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
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23
SONET level
Electrical level Line rate
(Mb/s)
SDH
equivalent
Common rate
name
OC-1
STS-1
51.84
OC-3
STS-3
155.52
STM-1
155 Mbps
OC-12
STS-12
622.08
STM-4
622 Mbps
OC-24
STS-24
1244.16
STM-8
1.25 Gbps
OC-48
STS-48
2488.32
STM-16
2.5 Gbps
OC-96
STS-96
4976.64
STM-32
5 Gbps
OC-192
STS-192
9953.28
STM-64
10 Gbps
OC - 768
STS-768
39,813.12
STM-256
40 Gbps
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
September 14
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P S PANDEY THDC IHET
September 14
25
Newton believed in the particle theory of light. He explained the
straight-line casting of sharp shadows of objects placed in a light
beam. but he could not explain the textures of shadows
Wave theory: Explains the interference
where the light intensity can be
enhanced in some places and diminished
in other places behind a screen with a slit
or several slits. The wave theory is also
able to account for the fact that the edges
of a shadow are not quite sharp.
This theory describes: Propagation,
reflection, refraction and attenuation
P S PANDEY THDC IHET
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26
1864 James Clerk Maxwell
His mathematical theory of electromagnetism led to the view that
light is of electromagnetic nature, propagating as a wave from the
source to the receiver.
1880s Heinrich Hertz
Discovered experimentally the existence of electromagnetic waves at
radio-frequencies.
Wave theory does not describe the absorption of light by a
photosensitive materials
1900-20 Max Planck, Neils Bohr and Albert Einstein
Invoked the idea of light being emitted in tiny pulses of energy
P S PANDEY THDC IHET
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27
Light behaviour can be explained in terms of the amount of energy
imparted in an interaction with some other medium. In this case, a
beam of light is composed of a stream of small lumps or QUANTA of
energy, known as PHOTONS. Each photon carries with it a precisely
defined amount of energy defined as:
Wp = h*f
Joules (J)
where; h = Plank's constant = 6.626 x 10-34 J.s, f = Frequency Hz
The convenient unit of energy is electron volt (eV), which is the kinetic
energy acquired by an electron when accelerated to 1 eV = 1.6 x 10-19 J.
Even although a photon can be thought of as a particle of energy it still has a
fundamental wavelength, which is equivalent to that of the propagating wave
as described by the wave model.
This model of light is useful when the light source contains only a few photons.
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29
The wave is composed of a combination of mutually perpendicular
electric and magnetic fields the direction of propagation of the wave
is at right angles to both field directions, this is known as an
ELECTROMAGNETIC WAVE
EM wave move through a vacuum at 3.0 x 108 m/s ("speed of light")
E E (r , )e j (t z )
H H (r , )e j (t z )
Speed of light
in a vacuum
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c f
September 14
30
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31
vp
P S PANDEY THDC IHET
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32
Solutions to Maxwells equations:
phase fronts
Plane wave:
Ee
k n k0
0 / n
jk r
Spherical wave:
e jkR
E
R
k 0 r 0 0 r k0
n r
P S PANDEY THDC IHET
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33
For most purposes, a travelling light wave can be presented as a
one-dimensional, scalar wave provided it has a direction of
propagation.
Such a wave is usually described in terms of the electric field E.
Wavelength
A plane wave propagating
in the direction of z is:
Eo
z
E( z, t ) Eo sin(t z)
Phase
The propagation constant (or wave number)
Phase velocity v p
c/n
vp
n = Propagation medium ref. index
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34
A pure single frequency EM wave propagate through a wave guide
at a
Phase velocity v p c / n
However, non-monochromatic waves travelling together will
vg c / ng
have a velocity known as Group Velocity:
dn
ng n
d
1.49
Ref. index
Where the fibre
group index is:
ng
1.46
n
1.44
500
(nm)1700 1900
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35
Electric and
magnetic fields are
orthogonal to each
other and to the
direction of
propagation Z
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36
Wave fronts
(constant phase surfaces)
Wave fronts
rays
Wave fronts
E
r
z
A perfect plane wave
A perfect spherical wave
A divergent beam
(a)
(b)
(c)
Examples of possible EM waves
S.O.Kasap, optoelectronics and Photonics Principles and Practices, prentice hall, 2001
P S PANDEY THDC IHET
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37
If the electric field is oscillating along a straight
line, it is called a linearly polarized (LP) or plane
polarized wave
If the E field rotates in a circle (constant magnitude)
or in an ellipse then it is called a circular or
elliptically polarized wave
Natural light has random polarization
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45
Snells Law: n1 Sin 1 = n2 Sin 2
Critical Angle:
Sin c=n2/n1
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46
Refractive index, n defined by:
1 1
Speed of light,
c
V
n
n1 sin 1 n2 sin 2
n1
Here n1 < n2
n2
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47
Transmitted
(refracted) light
kt
n2
n 1 > n2
ki
Incident
light
sin c
n2
n1
kr
Reflected
light
(a)
2 90
c
Critical angle
(b)
Evanescent wave
1
1 c TIR
(c)
Light wave travelling in a more dense medium strikes a less dense medium. Depending on
the incidence angle with respectcto
, which is determined by the ratio of the refractive
1 (a)
c (b) 1 c (c)
indices, the wave may be transmitted (refracted) or reflected.
1 c and total internal reflection (TIR).
n2
sin c
n1
[2-19]
P S PANDEY THDC IHET
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48
Propagation mechanism in an ideal step-index optical waveguide.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
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September 14
49
Calculate the angle of refraction at the air/core interface
Solution - use Snells law: n1sin1 = n2sin2
1sin(30) = 1.47sin(refraction)
refraction = sin-1(sin(30)/1.47)
refraction = 19.89
nair = 1
ncore = 1.47
ncladding = 1.45
incident = 30
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50
The angle of incidence that produces an angle of
refraction of 90 is the critical angle
n1sin(c) = n2sin(90)
n1 = Refractive index of the core
n1sin(c) = n2
n2 = Refractive index of the cladding
c = sin-1(n2 /n1)
Light at incident angles
greater than the critical
angle will reflect back
into the core
Critical Angle, c
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51
Core - Thin glass center of the fiber where the light travels
Cladding - Outer optical material surrounding the core that reflects the light back into the core
Buffer coating - Plastic coating that protects the fiber from damage and moisture
Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The
bundles are protected by the cable's outer covering, called a jacket.
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Three important angles
The reflection angle always equals the incident angle
Refraction Angle
Incident Angles
Reflection Angle
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The angle of light entering a fiber which follows the critical
angle is called the acceptance angle, a
a = sin-1[(n12-n22)1/2]
Numerical Aperture (NA)
describes the light- gathering
ability of a fiber
n1 = Refractive index of the core
n2 = Refractive index of the cladding
Acceptance Angle, a
NA = sina
Critical Angle, c
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54
> c, a > amax
2
1
Core n1
Air (no =1) Cladding n2
From Snells Law: n0 sin a = n1 sin (90 - )
a = amax when = c
Thus, n0 sin amax = n1 Cos c
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55
n0 sin amax = n1 (1 - sin2 c)0.5
Or
1 n2
Since c sin
n1
n0 sin a max
Then
n12
2 0.5
n2
n 2
n1 1 2
n1
0.5
n12
2 0.5
n2
Numerical Aperture ( NA)
NA determines the light gathering capabilities of the fibre.
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56
Therefore
n0 sin amax = NA
Fibre acceptance angle
Note n1 n2
n
Thus
NA
amax sin 1
n0
Relative refractive index difference
NA n1 (2)0.5
0.14< NA < 1
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Acceptance / Emission Cone
NA
= sin
n2core - n2cladding
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Index of Refraction
Snells Law
Critical Angle
Acceptance Angle
Numerical Aperture
c
v
n1 sin 1 n2 sin 2
n2
c sin
n1
1
a sin 1 n12 n22
NA sin a n12 n22
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Most common designs: 100/140 or 200/280 m
Plastic optical fiber (POF):
0.1 - 3 mm
Cladding
Core
Refractive
Index (n)
1.480
Primary coating
(e.g., soft plastic)
100 m
1.460
140 m
Diameter (r)
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No of modes
A fiber can support:
many modes (multi-mode fibre).
a single mode (single mode fiber).
The number of modes supported in a fiber is determined by
parameter V (Normalized Frequency).
a n n
2
1
2
2
or
2a
V
NA
There are two main fibre types:
(1) Step index:
Multi-mode
Single mode
(2) Graded index multi-mode
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61
SM step index
MM step index
MM graded index
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Input
pulse120-400m
50-200 m
Output pulse
n1 =1.48-1.5
n2 =1.46
Advantages:
dn=0.04,100 ns/km
Allows the use of non-coherent optical light source, e.g. LED's
Facilitates connecting together similar fibres
Imposes lower tolerance requirements on fibre connectors.
Cost effective
Disadvantages:
Suffer from dispersion (i.e. low bandwidth (a few MHz)
High power loss
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50-100 m
Input
pulse120-140m
n2 n1
Output
pulse
dn = 0.04,1ns/km
Advantages:
Allows the use of non-coherent optical light source, e.g. LED's
Facilitates connecting together similar fibres
Imposes lower tolerance requirements on fibre connectors.
Reduced dispersion compared with STMMF
Disadvantages:
Lower bandwidth compared with STSMF
High power loss compared with the STSMF
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65
n2
n1
3
2
1
(a) Multimode step
index fiber. Ray paths
are different so that
rays arrive at different
times.
(b) Graded index fiber.
Ray paths are different
but so are the velocities
along the paths so that
all the rays arrive at the
same time.
n2
O'
O''
3
2
1
2
3
n1
n2
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Representative Fiber Parameter Values
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67
Along the fiber
1
1, 3
(a) A meridional
ray always
crosses the fiber
axis.
Meridional ray
Fiber axis
Skew ray
Fiber axis
2
3
(b) A skew ray
does not have
to cross the
fiber axis. It
zigzags around
the fiber axis.
Ray path projected
on to a plane normal
to fiber axis
Ray path along the fiber
Illustration of the difference between a meridional ray and a skew ray.
Numbers represent reflections of the ray.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Wave Equation
If e.m.w. are to propagate in z- axis,
they will have functional dependence :
is the z- component of propagation vector & depends on boundary conditions at coreclad interface
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Determined by solving Maxwells equations in cylindrical
coordinates
2 E z 1 E z 1 2 E z
2
q
Ez 0
2
2
2
r r r
r
2 H z 1 H z 1 2 H z
2
q
Hz 0
2
2
2
r r
r
r
- These eqn. contains either Ez or Hz .Therefore longitudinal components are
uncoupled & chosen arbitrary. However coupling of Ez & Hz are required by
boundary conditions.
-If boundary conditions do not lead coupling, either Ez =0 (TE) or Hz = 0 (TM).
- Hybrid modes exists HE or EH
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q2 is equal to
2-2 = k2 2.
It is sometimes called u2.
is the z component of the wave prop.
constant k ;
k = 2/. The equations can be solved
only for certain values of , so only certain
modes may exist.
A mode may be guided if lies between n2k
and n1k.
V = ka(NA) , a is the radius of the fiber core.
This normalized frequency determines
how many different guided modes a fiber
can support.
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The solutions are separable in r, , and z.
The and z functions are exponentials of the form
ei. The z function oscillates in space,
The function must have the same value at
(+2) & .
The r function is a combination of Bessel functions
of the first and second kinds. The separate
solutions for the core and cladding regions must
match at the boundary.
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Either the electric field component (E) or the
magnetic field component (H) can be completely
aligned in the transverse direction: TE and TM
modes.
The two fields can both have components in the
transverse direction: HE and EH modes.
For weakly guiding fibers (small delta), the types of
modes listed above become degenerate, and can be
combined into linearly polarized LP modes.
Each mode has a subscript of two numbers, where
the first is the order of the Bessel function and the
second identifies which of the various roots meets
the boundary condition. If the first subscript is 0,
the mode is meridional. Otherwise, it is skew.
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Where u2 = k12 - & w2 = -k22
2p
2
2
V = k0a(n1 - n2 )
an1 2D
l
Normalized Propagation constant b defined as
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Each mode has a specific
Propagation constant
Spatial field distribution
Polarization
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Straight lines of d/d correspond to the group velocity of the different modes
The group velocities of the guided modes all lie between the phase velocities
for plane waves in the core or cladding c/n1 and c/n2
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For each mode, there is some value of V
below which it will not be guided because the
cladding part of the solution does not go to
zero with increasing r.
Below V=2.405, only one mode (HE11) can be
guided; fiber is single-mode.
Based on the definition of V, the number of
modes is reduced by decreasing the core
radius and by decreasing .
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The order of the mode is equal to the # of field zeros across the guide.
The order of the mode is [Also related to the angle of ray congruence
makes with axis].
The steeper the angle, the higher the order of the mode.
Higher order of mode: Fields are distributed more toward the edges
of the guide and penetrate further into the cladding region.
Radiation modes : Still solutions of the Maxwell eqs. with the same
boundary conditions. These infinite continuum of the modes results
from the optical power that is outside the fiber acceptance angle
being refracted out of the core.
Leaky modes : Partially confined to the core & attenuated by
continuously radiating this power out of the core as they traverse
along the fiber .A mode remains guided as long as
n2 k n1k
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Guided Modes:
TIR
(t z) = 2n
Radiation / Clad Modes:
TIR
(t z) 2n
Leaky Modes:
NO TIR
(t z) = 2n
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- Exact analysis for the modes of a fiber is mathematically very complex.
- A simpler but highly accurate approximation can be used, based on n1 n2
1 , is the basis of weakly guided mode approximation
- In this approximation, electromagnetic field pattern & propagation constant
of the mode pairs HE+1,m & Eh-1,m are very similar.
- This holds likewise for three modes, TE0m, TM0m and HE2m .
- With (v,m) = (0,1) and (2,1) , mode groupings {HE11} ,{TE01,TM01,HE21} ,
{HE31,EH11}, {HE12}, {HE41,EH21} and {TE02,TM02,HE22}.
- This results Four field components instead of Six.
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The electric field vector lies in transverse plane.
TE lm modes :
TM lm modes : The magnetic field vector lies in transverse plane.
Hybrid HE lm modes : TE component is larger than TM component.
Hybrid EH lm modes : TM component is larger than TE component.
y
l= # of variation cycles or zeros in direction.
m= # of variation cycles or zeros in r direction.
Linearly Polarized (LP) modes in weakly-guided fibers ( n1
r
x
n2 1 )
LP0m (HE1m ), LP1m (TE0m TM0m HE 0m )
Fundamental Mode:
LP01 (HE11 )
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(a) The electric field (b) The intensity in (c) The intensity (d) The intensity
of the fundamental the fundamental
in LP 11
in LP 21
mode
mode LP01
Core
Cladding
E01
The electric field distribution of the fundamental mode
in the transverse plane to the fiber axis z. The light
intensity is greatest at the center of the fiber. Intensity
patterns in LP 01, LP11 and LP 21 modes.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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b
1
LP 01
0.8
LP 11
0.6
LP 21
0.4
LP 02
0.2
0
V
0
2
3
2.405
Normalized propagation constant b vs. V-number
for a step index fiber for various LP modes.
1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Propagation constant of the lowest
mode vs. V number
V=k0a(n1 -n2 )=2p
a
NA
0
an 2
1
Graphical Construction
to estimate the
total number of Modes
Fundamentals of Photonics - Saleh and Teich
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Fiber Optics Communication Technology-Mynbaev & Scheiner
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At
low V, M4V2/2+2
At
higher V, MV2/2
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1-d Mirror Guide
d
M= 2
l0
1-d Dielectric Guide
M 2
2-d Mirror Guide
2-d Dielectric Guide
d
NA
l0
2
p
2
d
4 l 0
2
p
2d
M NA
4 l 0
2-d Cylindrical Dielectric Guide
The V parameter
characterizes the number of
wavelengths that can fit across
the core guiding region in a fiber.
For the mirror guide the number of
modes is just the number of
wavelengths that can fit.
For dielectric guides it is the number
that can fit but now limited by the
angular cutoff characterized by the
NA of the guide
2
d
4 2
M 2 V = 16 NA
p
l 0
V=2p
a
NA
0
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For each mode, the shape of the Bessel
functions determines how much of the optical
power propagates along the core, with the
rest going down the cladding.
The effective index of the fiber is the
weighted average of the core and cladding
indices, based on how much power
propagates in each area.
For multimode fiber, each mode has a
different effective index. This is another way
of understanding the different speed that
optical signals have in different modes.
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The total average power propagating in the
cladding is approximately equal to
Pclad
4
P
3 M
September 14
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This shows the fraction of the power
that is propagating in the cladding
vs the V number for different modes.
V, for constant wavelength, and material
indices of refraction is proportional to
the core diameter a
As the core diameter is dereased more
and more of each mode propagates in
the cladding. Eventually it all propagates
in the cladding and the mode is no longer
Guided
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0
