SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY

(Approved by A.I.C.T.E., New Delhi & affiliated to J.N.T.U. Anantapur)

(Accredited by NAAC with ‘A’ Grade)
(An ISO 9001:2008 certified institute)
Siddharth nagar, Narayanavanam road, Puttur- 517583
Chittoor Dist., AP., INDIA

(20EC0433) OPTICAL
FIBRE COMMUNICATION

1
 UNIT-1
Introduction to optical fibers

2
 SYLLABUS
UNIT-I
Introduction: The general Optical Communication System, Advantages &
disadvantages of Optical fiber communication, Ray Theory transmission: Optical
Fiber Structure, Total internal reflection, Angle of incidence, Refractive Index,
Numerical Aperture, Skew Rays, Single mode & multimode fibers, Step index &
graded index fibers,
Transmission Characteristics of Optical Fibers: Attenuation, Absorption losses,
scattering losses, Bending Losses, Core and Cladding losses, Signal Distortion in
Optical Wave Guides-Information Capacity determination, Group Delay, Intermodal
dispersion.
UNIT-II
Fiber Optical Sources and Coupling: Direct and indirect Band gap materials, LED
structures, Light source materials, Quantum efficiency and LED power, Modulation
of a LED, lasers Diodes-Modes and Threshold condition, Rate equations, External
Quantum efficiency, Resonant frequencies, Temperature effects.

3
 SYLLABUS
UNIT-III
Fiber Optical Receivers: PIN and APD diodes, Photo detector noise, SNR, Detector
Response time, Avalanche Multiplication Noise, Comparison of Photo detectors.
Fundamental Receiver Operation, pre-amplifiers, Error Sources, Receiver
Configuration
UNIT- IV
Optical Fiber System Design & Technology: System specification, Point-to- links,
link power budget, Rise Time Budget, Bandwidth Budget, Power Budget (Adaptors,
Attenuators and its effects must be explained) and Receiver Sensitivity, Link Budget
calculations, Optical Multiplexing & Demultiplexing techniques, Optical Amplifiers
and its Applications.
UNIT- V
Optical Networks: Basic networks, Broadcast-and-select WDM networks,
Wavelength-routed networks, Performance of WDM+EDFA systems, Optical
CDMA, Ultra high capacity networks.
4
 Course objectives
The objectives of this course:
1. To understand Optical Fiber Communications.
2. To understand the Ray Theory, single& amplitude;
multimode fibers, fiber materials,
losses, dispersion in OFC.
3. To understand the connectors, splices, couplers,
LASER, LED sources.

5
 Course outcomes
On successful completion of this course, the student will be able to
1. Learn the basic elements of optical fiber transmission link, fiber modes
configurations and structures.
2. Understand the different kind of losses, signal distortion in optical wave
guides and other signal degradation factors.
3. Learn the various optical source materials and optical receivers such as LED
structures,
quantum efficiency, Laser diodes, PIN, APD diodes, noise performance in photo
detector,
receiver operation and configuration.
4. Analyze the use of analog and digital links such as the various criteria like
power loss
wavelength to be considered for point-to-point link in digital link system.
5. Learn the fiber optical network components, variety of networking aspects,
and operational principles WDM.
6. Analyze the different techniques to improve the capacity of the system.
6
 Evolution of fiber optic system
First generation
Uses GaAs semiconductor laser
Operating region was near 0.8 μm.
 Bit rate : 45 Mb/s
 Repeater spacing : 10 km
Second generation
Bit rate: 100 Mb/s to 1.7 Gb/s
Repeater spacing: 50 km
Operation wavelength: 1.3 μm
Semiconductor: InGaAsP
7
 Continued…..
Third generation
Bit rate : 10 Gb/s
Repeater spacing: 100 km
Operating wavelength: 1.55 μm
Fourth generation
Fourth generation uses WDM technique
Bit rate: 10 Tb/s
Repeater spacing: > 10,000 km
Operating wavelength: 1.45 to 1.62 μm
Fifth generation
Uses Roman amplification technique and optical
solitons
8
Bit rate: 40 - 160 Gb/s
 Elements of an Optical Fiber
Transmission link

9
Advantages of Optical Fibre

 Thinner
 Less Expensive
 Higher Carrying Capacity
 Less Signal Degradation&
Digital Signals
 Light Signals
 Non-Flammable
 Light Weight
 Advantages of fiber optics

 Much Higher Bandwidth (Gbps) - Thousands of channels

can be multiplexed together over one strand of fiber
 Immunity to Noise - Immune to electromagnetic
interference (EMI).
 Safety - Doesn’t transmit electrical signals, making it
safe in environments like a gas pipeline.
 High Security - Impossible to “tap into.”
 Advantages of fiber optics
 Less Loss - Repeaters can be spaced 75 miles apart (fibers can
be made to have only 0.2 dB/km of attenuation)
 Reliability - More resilient than copper in extreme
environmental conditions.
 Size - Lighter and more compact than copper.
 Flexibility - Unlike impure, brittle glass, fiber is physically
very flexible.
 Fiber Optic Advantages

 greater capacity (bandwidth up to 2

Gbps, or more)

 smaller size and lighter weight

 lower attenuation

 immunity to environmental
interference

 highly secure due to tap difficulty and

lack of signal radiation

13
 Disadvantages of fiber optics

 Disadvantages include the cost

of interfacing equipment
necessary to convert electrical
signals to optical signals. (optical
transmitters, receivers) Splicing
fiber optic cable is also more
difficult.
Areas of Application

 Telecommunications
 Local Area Networks
 Cable TV
 CCTV
 Optical Fiber Sensors
 RAY OPTICS
Refraction and Total Internal Reflection
 Optical fibers work on the principle of total
internal reflection
 The angle of refraction at the interface between
two media is governed by Snell’s law:

16
n1 sin 1  n2 sin  2
17
 Numerical Aperture
 Light gathering and acceptance capability of
fiber .
 The angle of acceptance is twice that given by
the numerical aperture

N . A.  n12  n22

18
Snell’s Law
Total Internal Reflection in Fiber
21
 REFRACTIVE INDEX
 Refractive index is denoted by ‘n’

Refractive index(n)=velocity of light in first substance/velocity

of light in second substance

22
 Optical Fiber
 An optical fiber is cylindrical transparent
waveguide that conveys electromagnetic waves
at Optical frequency.
It consists of
 Core :
Carries light
 Made up of glass
 Refractive Index n1
 Cladding :
Surrounds the core and refractive
index is n2
 Avoids scattering loss
23
 Provide mechanical strength
  The index of refraction of the cladding is less than that of the core,
causing rays of light leaving the core to be refracted back into the core
n1>n2
 A light-emitting diode (LED) or laser diode (LD) can be used for
the source
 Jacket:
 Advantages of optical fiber include:
Greater bandwidth than copper
Lower loss
Immunity to crosstalk
No electrical hazard

24
Optical Fiber Modes and Configurations
The optical fiber is a dielectric waveguide that
operates at optical frequency.
 The propagation of light along a waveguide can be
described in terms of a set of guided
electromagnetic waves called the modes.
These guided modes are referred to as bound or
trapped modes.
Only certain discrete number of modes can
propagate along fiber.
Modes satisfies the homogeneous equation in the
25 fiber and boundary conditions a the surface.
 modes and configurations

26
 Single mode and multimode

27
 Single mode Step index Fiber:
 Core size is small. Typical core sizes are 2 to 15 μm.
 Only one mode can propagate through the cable.
 Single mode fiber is also known as fundamental or mono mode fiber.
 Does not suffer from mode delay differences(Dispersion)

Multimode step Index Fiber:

 Core size is small. Typical core sizes are 50 to 1000 μm.
 Multiple modes can propagate through the cable.
 Suffer from mode delay differences(Dispersion).Txn BW is low

Graded-Index Multimode Fiber:

 Core refractive index diminishes gradually from the center axis out toward the
cladding.
 The core size is varying from 50 to 100 μm.
 The light ray is propagated through the refraction
 The light ray enters the fiber at many different angles
 Minimizing dispersion losses.

28
Based on the index profile the optical fibers are two types

 Step-index fibers
 Graded-index
 Step-index fibers: Index of refraction changes radically
between the core and the cladding.

 Graded-index fibers: The index of refraction gradually

decreases away from the center of the core.
 Graded-index fiber has less dispersion than a multimode step-
index fiber

29
 Single mode fiber structure
Single mode fibers can be constructed by
 Core diameter be a few wavelengths(usually 8-12)
 Small index difference
 Large variations in values of the physical size of core a and
index difference ∆.
 V-Number< 2.4
 Example: For typical single mode fiber a=3μm, NA=0.1 and λ=0.8μm
Yields V=2.356

30
 Continued……
Mode Field Diameter
 In single mode fibers geometric distribution of light is important to
predict the performance of fiber.
 The mode filed diameter is fundamental parameter of a single mode
fiber.
 This parameter is determined from mode field distributions of
fundamental LP01 mode.
 The method is how to approximate electric field distribution.
 For a Gaussian distribution, the MFD is given by the 1/e2 width of
the optical power
 The Gaussian distribution
E ( r ) = E0 exp(- r /W0)
E0=Field at zero radius W0=Width of electric field distribution
31
 Continued…..

32
 Continued…..
 The spot size W0 is gives as –
MFD = 2 W0
Propagation modes in single mode fiber:
 In single mode amplifier, there are two independent degenerate
modes.
 Horizontal mode
 Vertical mode
 These modes very similar , but their polarization planes are
orthogonal
 Constitute fundamental HE11 mode
 Modes propagate with equal propagation constants
(Kx=Ky)
33
 Continued…..

The modes propagating with different phase

velocities and the difference between their
effective refractive indices is called the fiber
birefringence.
Bf=ny-nx
 Similarly ,the birefringence may define as
β=k0(ny-nx)
34 k0=2π/λ is the free space propagation
 Graded –Index fiber Structure
 The index of refraction gradually decreases with increasing radial
distance r from center, but constant in the cladding.
 Refractive index variation in core

α Indicates shape of index profile.

 Index difference

 The total numerical aperture is

 Axial numerical aperture is define as

35
 TRANSMISSION CHARACTERISTICS
OF OPTICAL FIBERS

There are 2 main characteristics of optical fiber

Signal attenuation
Signal distortion

36
 Comparison of NA for fibers having various α profiles.
37
 Signal Attenuation
It determines the maximum unamplified or repeaterless
distance between transmitter and receiver.

Signal Distortion
•Causes optical pulses broaden.
•Overlapping with neighboring pulses, creating errors in the
receiver output.
•It limits the information carrying capacity of a fiber.
Attenuation
Attenuation is a measure of decay of signal strength or loss
of light power that occurs as light pulses propagate through the
length of the fiber.
Attenuation Units: As light propagates through the fiber, its power
decreases with distance. Let the couples optical power is p(0) i.e. at origin
(z = 0). Then the power at distance z is given by,
Z=0
P(0) mW Z= l

Where, αp is fiber attenuation constant (per km).  p l

P(l )  P(0)e mw

This parameter is known fiber loss or fiber attenuation.

 Attenuation as a function of Wavelength

Optical fiber attenuation as a function of wavelength yields nominal values of 0.5 dB/km
at 1310 nm and 0.3 dB/km at 1550 nm for standard single mode fiber. Absorption by the
water molecules causes the attenuation peak around 1400nm for standard fiber. The
dashed curve is the attenuation for low water peak fiber.
 Signal Degradation
in the Optical Fiber

Signal Distortion/ Attenuation

Dispersion

Scattering Radiative
Absorption Losses
Intermodal
Intramodal
Polarization
losses
Dispersion/
Delay/ -mode
Chromatic
Modal Delay Dispersion Extrinsic
Intrinsic Atomic
Dispersion (Impurity
Absorption Defects
atoms)

Material Waveguide
Absorption Absorption Inhomogeneities Compositional
Dispersion Dispersion in in Microscopic Macroscopic
or defects fluctuations
Infrared Ultraviolet bends bends
in fiber in material
region region
 Attenuation
The Basic attenuation mechanisms in a fiber:

1. Absorption:
It is related to the fiber material.

2. Scattering:
It is associated both with the fiber material
and with the structural imperfections in the
optical waveguide.

3. Radiative losses/ Bending losses:

It originates from perturbation (both
microscopic and macroscopic) of the fiber
geometry.
Absorption

Absorption is caused by three different mechanisms:

1. Absorption by atomic defects
2. Extrinsic Absorption
3. Intrinsic absorption
1. Absorption by atomic defects
Atomic defects are imperfections in the atomic structure of the fiber
material.
Examples:
•Missing molecules
•High density clusters of atom groups
•Oxygen defects in the glass structure.

•Absorption losses arising from these defects are negligible compared

with intrinsic and impurity absorption.
•Can be significant if the fiber is exposed to ionization radiations.
1 rad(Si) = 0.01 J/Kg
Absorption
2. Extrinsic absorption by impurity atoms
The dominant absorption factor in silica fibers is the
presence of minute quantities of impurities in the fiber
material.
•These impurities include
•OH- (water) ions dissolved in the glass.
•Transition metal ions, such as iron, copper,
chromium and vanadium
 Absorption
3. Intrinsic absorption by the basic constituent atoms
Intrinsic absorption is associated with the basic fiber
material (e.g pure SiO2).
Intrinsic absorption results from:
1. Electronic absorption bands in the ultraviolet region
2. Atomic vibration bands in the near infrared region

Electronic absorption (EA) occurs when a photon interacts with an electron

in the valance band and excites it to a higher energy level.
The electronic absorption is associated with the band gap of the material.

where, x is mole fraction of GeO2, λ is operating wavelength.

The infrared absorption is associated with the vibration frequency of
chemical bond between the atoms of which the fiber is composed.
**Optical fiber attenuation characteristics and their limiting mechanisms
for a GeO2 doped low loss water content silica fiber.
Scattering Losses
Scattering losses in glass arise due to
1. Compositional fluctuations
2. Inhomogeneities or defects occurring during fiber manufacture
These two effects give rise to refractive index variation, within the glass
over distances.
These index variation case Rayleigh-type scattering of the light and
inversely proportional to wavelength.
It decreases dramatically with increasing wavelength.
Scattering loss for single component glass is given by,

where, n = Refractive index, KB = Boltzmann‘s constant

βT = Isothermal compressibility of material
Tf = Temperature at which density fluctuations are frozen into the glass as it
solidifies
 Rayleigh scattering in an optical fiber

Combining the infrared, ultraviolet, and scattering losses for single mode fiber.
Radiative losses / Bending Losses
Radiative losses occur whenever an optical fiber undergoes a bend of
finite radius of curvature.
Fiber can be subject to two types of bends:
1.Macroscopic bends
2.Microscopic bends
1. Macrobending losses or bending loss:
 Losses due to curvature and an abrupt change in radius of curvature.
Ex: Fiber turning edge of the room.

 Radiation losses depend on the value radius of curvature R

 As the lower order modes remain close to the core axis and the higher
modes are closer to the cladding so the higher modes will radiate out
of the fiber first
Macro bending
Radiative losses / Bending Losses
Microbending losses:
 Microbending is a loss due to small bending or distortions
 Microbends are repetitive small scale fluctuations in radius
of curvature of the fiber axis.
 Microbends causes repetitive coupling of energy between
the guided modes and the leaky or nonguided modes in
the fiber.
Caused by:
• Nonuniformities in the manufacturing of the fiber
• Nonuniform lateral pressures during cabling
• High pressures
Microbending losses
Minimizing microbending losses:

A compressible jacket extruded over a fiber reduces microbending resulting from

external forces.

Bends are shown full size — and may have caused damage to the fiber
 Core and Cladding Losses
 Since the core and cladding have different indices of refraction hence
they have different attenuation coefficients α1 and α2 respectively
 For step index fiber, the loss for a mode order (v, m) is given by,

 For low-order modes, the expression reduced to

 For graded index fiber, loss at radial distance is expressed as,

 The loss for a given mode is expressed by,

 Where, P(r) is power density of that model at radial distance r.

 Signal Distortion in Fibers
Optical signal weakens from attenuation mechanisms and broadens due
to distortion effects.
 The pulse gets distorted as it travels along the fiber lengths as
consequence of pulse spreading.
 Pulse spreading in fiber is referred as dispersion
 Dispersion is caused by difference in the propagation times of light rays
that takes different paths during the propagation.
 Dispersion limits the information bandwidth
Pulse Broadening And Attenuation
Information Capacity Determination
Information capacity of an optical fiber is specified by the bit rate-
distance product BL.
 Pulse spread should be less than the width of a bit period
∆T < 1 /B General requirement
∆T ≤ 0.1 /B For high performance link
Bit rate distance product BL < n2 c/ n12 ∆

Group Delay: The group delay in an optical device is the time delay for a
pulse to pass it. Group delay per unit length can be defined as
1/Vg=

Where vg is the group velocity at which energy in pulse travels in fiber.

The total delay difference δτ over a distance L is: 2
d g L  d d 
      2  2 
2 
d 2c  d d 
d d  L   d 2 
     L 
d d  V g   d
2

 is called GVD parameter, and shows how much a light pulse
broadens as it travels along an optical fiber.

The more common parameter is called Dispersion, and can be defined as

the delay difference per unit length per unit wavelength as follows
1 d g d  1 
   2c  2
D 
L d d  V g 
  2

The pulse spreading σg of fiber length of L, can be well approximated by:

d g
g     DL 
d
D has a typical unit of [ps/(nm.km)].
Dispersion

 Dispersion distorts both pulse and analog modulation

signals.
 In a pulse modulated system, this causes the received pulse
to be spread out over a longer period.
 It is noted that actually no power is lost to dispersion, the
spreading effect reduces the peak power.

 Pulse dispersion is usually specified in terms of

“Nanoseconds-per-kilometer”.
Dispersion
Dispersion occurs due to following mechanisms:
 Intermodal Delay or Modal Delay
 Intramodal Dispertion or Chromatic Dispersion
 Material Dispertion
 Waveguide Dispertion
 Polarization –Mode Dispersion

1.Intermodal delay/ modal delay:

 Intermodal distortion or modal delay appears only in multimode fibers.
result of each mode having a different value of the group velocity at a single
frequency.
 The amount of pulse spreading is a function of the number of modes
and length of the fiber
 Broadening of pulse is simply obtained from ray tracing for a fiber of
length L:
∆T= Tmax – Tmin = (Ln1∆/c)
Light rays with steep incident angles have longer path lengths than lower angle rays.

How to minimize the effect of modal dispersion?

1. Graded index fiber 2. Single mode fiber
 We could decrease the number of modes by increasing the
wavelength of the light
V = 2πa / λ x (n12 – n22)1/2 = 2πa / λ x (NA)
 Change in the numerical aperture can help but it only makes a
marginal improvement.
 The smaller the core, the fewer the modes.
Step Index Multi-mode

Graded Index Multi-mode

 Intramodal Dispersion or Chromatic Dispersion

 This takes place within a single mode.

 Intramodal dispersion depends on the wavelength, its effect on signal
distortion increases with the spectral width of the light source.

Two main causes of intramodal dispersion are as:

1. Material Dispersion
2. Waveguide Dispersion
1.Material Dispersion:
 Occurs due to refractive index of the material varies as a function of
wavelength. n ( )
 Material-induced dispersion for a plane wave propagation in
homogeneous medium of refractive index n:
d 2 d 2 d  2 
 mat  L  L  L n (  )
dω 2c d 2c d   
L dn 
 n   
c d 
 The pulse spread due to material dispersion is therefore:
d mat L  d 2 n
g     2  L  Dmat ( )
d c d
Dmat ( ) is material dispersion
Material dispersion can be reduced:
• Either by choosing sources with narrower spectral output widths OR
• By operating at longer wavelengths.

Waveguide Dispersion:
 Waveguide dispersion is due to the dependency of the group velocity of
the fundamental mode as well as other modes on the V number.
 Let consider that n is not dependent on wavelength.
 Defining the normalized propagation constant b as:

 / k  n2
2 2 2
 / k  n2
b 
n1  n2
2 2
n1  n2
 solving for propagation constant:
  n2k (1  b)
 Using V number:
V  ka(n1  n2 )1/ 2  kan2 2
2 2

 Delay time due to waveguide dispersion can then be expressed as:

L d (Vb) 
 wg  n2  n2 
c dV 
Material dispersion as a function of optical
wavelength for pure silica and 13.5
percent GeO2/ 86.5 percent SiO2.
Total Dispersion, zero Dispersion

Fact 1) Minimum distortion at wavelength about 1300 nm for single mode silica fiber.
Fact 2) Minimum attenuation is at 1550 nm for sinlge mode silica fiber.
Strategy: shifting the zero-dispersion to longer wavelength for minimum attenuation and dispersion.

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

 References
 https://www.thefoa.org/tech/ref/OSP/fiber.html
 http://en.wikipedia.org/wiki/Optical_communication
 http://www.journals.elsevier.com/optics-communications/

69
 SIDDARTHA INSTITUTE OF SCIENCE AND
TECHNOLOGY
(Approved by AICTE, New Delhi & Affiliated to JNTUA, Ananthapuramu)
(Accrediated NBA and Accrediated NAAC with “A” Grade )
Siddharth Nagar, Naravanavanam Road, Puttur-517583, AP, India

Department of Electronics and Communication Engineering

OPTICAL FIBER COMMUNICATION

Unit-II
Fiber Optical Sources and coupling
 Contents

Fiber Optical Sources and Coupling :

• Direct and indirect Band gap materials
• LED structures
• Light source materials
• Quantum efficiency and LED power,
• Modulation of a LED,
• lasers Diodes
• Modes and Threshold condition
• Rate equations
• External Quantum efficiency
• Resonant frequencies
• Temperature effects
 Direct Band Gap Semiconductors
The E-k Diagram The Energy Band Diagram
Ek

CB
Conduction
Band (CB) Empty ψk e-
e-
Ec Ec
Eg hυ hυ
Valence
Ev Ev
Band (VB) h+ Occupied h+
k
VB

–š /a š /a

The E-k diagram of a direct bandgap semiconductor

such as GaAs. The E-k curve consists of many discrete
points with each point corresponding to a possible
state, wavefunction ψk(x), that is allowed to exist in the
crystal.
Indirect Band Gap Semiconductors

E E E

CB
Indirect Bandgap, Eg
Ec CB
Direct Bandgap Eg Photon CB Ec Er Ec
Ev kcb Phonon
Ev Ev
VB
VB kvb VB
–k k –k k –k k
(a) GaAs (b) Si (c) Si with a recombinationcenter
LED
 Light-Emitting Diodes (LEDs)
A light-emitting diode (LED) is a semiconductor device
that emits light when an electric current flows through it.
When current passes through an LED, the electrons
recombine with holes emitting light in the process.
LEDs allow the current to flow in the forward direction
and blocks the current in the reverse direction.
The LED symbol is the standard symbol for a diode, with
the addition of two small arrows denoting the emission
of light.
 ……………….CONTINUED

The two main types of LEDs presently used for

lighting systems are aluminum gallium indium
phosphide (AlGaInP, sometimes rearranged as
AlInGaP) alloys for red, orange and yellow LEDs; and
indium gallium nitride (InGaN) alloys for green, blue
and white LEDs
……………….CONTINUED
 LED STRUCTURES
• For photonic communications requiring data rate 100-
200 Mb/s with multimode fiber with tens of
microwatts, LEDs are usually the best choice.
• LED configurations being used in photonic
communications:
1- Surface Emitters (Front Emitters)
2- Edge Emitters
 Double heterojunction structures
 As shown it is GaAs/AlGaAs based Double Heterojunction
LED.
 As shown thin layer of GaAs is sandwiched between two
layers of AlGaAs. GaAs is lightly doped and has narrower
bandgap (Eg1) of about 1.43 eV. AlGaAs layers have wider
bandgap (Eg2) of about 2.1 eV.
 When forward bias is applied through its top and bottom
contacts as shown in the figure, electrons are injected from
highly doped (n+) AlGaAs layer to central active (p-) GaAs
layer.
• The injected electrons are trapped within the middle layer
due to double heterojunction potential barriers (Eg2 > Eg1)
existing on both the sides of the middle layer.
 The figure depicts energy band diagram when it is forward
biased. Electrons are forced to recombine with the holes
without too much diffusion from interfaces.
 They recombine radiatively with energy equal to the band
gap of GaAs.
 As recombination between electrons and holes is limited to
narrower central part, internal quantum efficiency of such
LED is higher compare to single junction LED.
Surface-Emitting LED
Benefits or advantages of Surface Emitting LED
➨LED offers high optical coupling efficiency.
➨Optical loss (due to internal absorption) is very low. This
is because of carrier recombination near its top
heterojunction.
➨InP/InGaAsP based LED is used for long wavelength
applications.
➨It offers higher efficiency with low to high radiance.
Drawbacks ➨The surface emitting LED can transmit data
rate less than 20 Mbps than edge emitting LED.
➨It contains short optical link with large NA (Numerical
Aperture).
Edge-Emitting LED
Benefits or advantages ➨It offers higher efficiency with low to
high radiance. ➨It offers better modulation bandwidth and
more directional emission pattern.
➨It offers 5-6 times more coupled power into NA (Numerical
Aperture) of step/graded index fibers. This is due to small beam
divergence.
➨It offers high data rates (> 20 Mbps) than surface emitting
LED.
Drawbacks ➨Its structure is complex.
➨It is difficult to design heat sink.
➨It is expensive compare to other LED types.
➨There are many issues to be handled during mechanical
mounting and installation.
 Light Source Materials
active region material of an optical source must have
direct band gap.
In direct band gap materials,radiative recombination
is sufficiently high to produce adequete optical
emission.
These materials are compound of group III elements(
Al, Ga or In) and of group IV elements(P,As).
These materials determine the wave length of light
emitted.
Quantum Efficiency & LED power

• When there is no external carrier injection, the

excess density decays exponentially due to
electron-hole recombination.
n(t) =n0 e−t /τ
n0 : initial injected excess electron density
τ: carrier lifetime.
• Bulk recombination rate R:
dn n
R =− =
dt τ
• Bulk recombination rate (R)=Radiative
recombination
0 rate + nonradiative recombination
rate
• n is the excess carrier density,
Internal Quantum Efficiency & Optical Power
η int: internal quantum efficiency in the active region
Rr τnr τ
ηint = = =
Rr +R nr τr +τnr τr
Optical power generated internally in the active region in the
LED is:

I hcI
Pint =η hν =η
qλ
int int
q
Pint : Internal optical power,
I : Injected current to active region
 External Quantum Eficiency

No.of photons emitted from LED

η =
ext No.of LED internally generated photons

• In order to calculate the external quantum efficiency, we

need to consider the reflection effects at the surface of the
LED. If we consider the LED structure as a simple 2D slab
waveguide, only light falling within a cone defined by
critical angle will be emitted from an LED.
 φc
η = 1 ∫T(φ)(2π sin φ)dφ
4π 0
ext

4n1n2
T (φ) : Fresnel Transmission
1
Coefficient ≈T (0) =
If n2 =1 ⇒ η ≈ (n1 +n )22
ext
n1 (n1 +1)2
Pint
LED emitted optical power, P =ηext Pint ≈
n1 (n1 +1) 2
 Modulation of LED
• The frequency response of an LED depends on:
1- Doping level in the active region
2- Injected carrier lifetime in the recombination
region,τ .
3- Parasitic capacitance of the LED
• If the drive current of an LED is modulated at a
frequency of  the output optical power of the device
will vary as:
P( ) = P0
2
1+(ωτ)i
 Advantages

• LEDs consume less power, and they require low

operational voltage
• The emitted light is monochromatic.
• In expensive
• Reliable
• Easy to handle
• Less temparature dependance
Dis advantages
• Low output power
• Short distance communication
• More harmonic distortion
 APLLICATIONS
• Leds are used at 850nm and 13510 nm
• Lan & wan
• CCTV
• Used for TV back-lighting
• Used in displays
• Used in Automotives
• LEDs used in the dimming of lights
Spectral width of LED types
 LASER

• LASER means light amplification by stimulated

emission of radiation.
• It is widely used optical source for optical
communication.
• It is working on the principle of stimulated emission.
• It has coherent light.
• Laser diode suffers from 3 problems when used as
optical source :
• Temperature sensitivity
• Back reflections
• Susceptible to optical interference
 Pumped active medium
• Three main process for laser action:
1- Photon absorption
2 Spontaneous emission
3 Stimulated emission

Energy
Random Coherent
absorbed from
the incoming release of release of
photon energy energy
 Lasing in a pumped active medium
• In thermal equilibrium the stimulated emission is
essentially negligible, since the density of electrons
in the excited state is very small, and optical
emission is mainly because of the spontaneous
emission. Stimulated emission will exceed
absorption only if the population of the excited states
is greater than that of the ground state. This
condition is known as Population Inversion.
Population inversion is achieved by various
pumping techniques.
Howling Dog Analogy
 In Stimulated Emission incident and stimulated
photons will have

• Identical energy € Identical wavelength

€ Narrow linewidth
• Identical direction € Narrow beam width
• Identical phase € Coherence and
• Identical polarization
Stimulated Emission
 Fabry-Perot Resonator
Relative intensity
M1 M2 m =1
A 1 υf R ~ 0.8
m =2 R ~ 0.4
δυm
B
L m =8 υ
υm - 1 υm υm + 1
(a) (b) (c)
Resonant modes : kL =m m =1,2,3,..
Schematic illustration of the Fabry-Perot optical cavity and its properties. (a) Reflected
waves interfere. (b) Only standing EM waves, modes, of certain wavelengths are allowed
in the cavity. (c) Intensity vs. frequency for various modes. R is mirror reflectance and
lower R means higher loss from the cavity.

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

(1−R) 2
Itrans =Iinc [4-18]
(1−R) 2 +4R sin 2 (kL)
R: reflectance of the optical intensity, k: optical wavenumber
Mirror Reflections
How a Laser Works
 Laser Diode
• Laser diode is an improved LED, in the sense that uses
stimulated emission in semiconductor from optical
transitions between distribution energy states of the
valence and conduction bands with optical resonator
structure such as Fabry-Perot resonator with both
optical and carrier confinements.
 Laser Diode Modes
• Nanosecond & even picosecond response time (GHz
BW)
• Spectral width of the order of nm or less
• High output power (tens of mW)
• Narrow beam (good coupling to single mode fibers)
• Laser diodes have three distinct radiation modes
namely, longitudinal, lateral and transverse
modes.
• In laser diodes, end mirrors provide strong optical
feedback in longitudinal direction, so by roughening
the edges and cleaving the facets, the radiation can
be achieved in longitudinal direction rather than
lateral direction.
 DFB(Distributed FeedBack)
Lasers

The optical feedback is provided by fiber Bragg

Gratings € Only one wavelength get positive feedback
 Threshold Condition
• To determine the lasing condition and resonant
frequencies, we should focus on the optical wave
propagation along the longitudinal direction, z-axis.
The optical field intensity, I, can be written as:
I ( z, t ) =I ( z)e j (ωt −βz )
• Lasing is the condition at which light amplification
becomes possible by virtue of population inversion.
Then, stimulated emission rate into a given EM
mode is proportional to the intensity of the optical
radiation in that mode.
g th =βJth
Optical output vs. drive current

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

 Rate equations
Rate equations relate the optical output power, or # of
photons per unit volume, Φ , to the diode drive current
or # of injected electrons per unit volume, n. For
active (carrier confinement) region of depth d, the
rate equations are:
dΦ
=Cn Φ +R Φ
−
dt
sp
τ ph [4-25]

dn J n
= − −CnΦ
dt qd τsp

Photon rate =stimulated emission + spontaneou s emission +

photon loss
 Threshold current Density & excess electron density

• At the threshold of lasing: Φ ≈0, dΦ/ dt ≥0, Rsp ≈0

1
from eq.[4 - 25] ⇒ CnΦ −Φ /τ ph ≥0 ⇒ n ≥ =nth [4-26]
Cτ ph

• The threshold current needed to maintain a steady state threshold

concentration of the excess electron, is found from electron rate
equation under steady state condition dn/dt=0 when the laser is just
about to lase:

J n
0 = th − th ⇒ J th =qd nth [4-27]

qd τ sp τ sp
 Laser operation beyond the
threshold
J >Jth

• the steady state photon density, resulting

from stimulated emission and spontaneous
emission as follows:

τ ph
Φs = (J −J th ) +τ ph Rsp
qd
 External quantum efficiency

• Number of photons emitted per radiative electron-hole pair

recombination above threshold, gives us the external quantum
efficiency.

η (gth −α)
η ext =i
g th
[4-29]
 Resonant Frequencies
• Lasing condition:

exp(− j2βL) =1 ⇒ 2βL =2mπ , m =1,2,3,...

• Assuming β =
2πn the resonant frequency of the mth
mode is:

ν = mc m =1,2,3,... [4-30]
m
2Ln

−ν
c
⇔ ∆λ =
λ2
∆ν =ν m m−1
= [4-31]

2Ln 2Ln
Spectrum from a Laser Diode
 REFERENCES
•https://en.wikipedia.org/wiki/Direct_and_indirect_b
and_gaps
•https://www.sciencedirect.com/topics/engineering/i
nternal-quantum-efficiency
•https://prezi.com/ewkesi281r3w/laser-diode-
modes-and-threshold-conditions/
•https://www.ques10.com/p/36411/illustrate-elastic-
tube-splicing-with-neat-diagram/
•FibertoFiberJointLosses TypesofFibertoFiberJointL
osses #Lossesduetodifferenceindiameter
 SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY
(Approved by AICTE, New Delhi & Affiliated to JNTUA,
Ananthapuramu)
(Accrediated NBA and Accrediated NAAC with “A” Grade )
Siddharth Nagar, Naravanavanam Road, Puttur-517583, AP, India

Department of Electronics and Communication Engineering

OPTICAL FIBER COMMUNICATION

UNIT-III

FIBER OPTICAL RECEIVERS

 contents
• PIN and APD diodes
• Photo detector noise
• SNR
• Detector response time
• Avalanche Multiplication noise
• Comparison of photo detectors
• Fundamental Receiver Operation
• Preamplifiers
• Error sources
• Receiver configuration
 Photo Detectors
• Optical detectors convert optical signal (light)
to electrical signal (current/voltage)
– Hence referred ‘O/E Converter’
• Photodetector is the fundamental element of
optical receiver, followed by amplifiers and
signal conditioning circuitry
• There are several photodetector types:
– Photodiodes, Phototransistors, Photon
multipliers, Photo-resistors etc.
 requirements of photo detectors
• Compatible Physical Dimensions (small size)

• High Response or Selectivity at desired

wavelength.

• Low Noise added to the system and high Gain

• High Bandwidth Fast response time

• Stable performance

• Long Operating Life and low Cost

 Principle of photo detector
• Working principle is optical absorption
• The main purpose ofis its fast reponse
• For foc most suited photo detectors are
PIN(P type intrinsic N type) and (Avalancha
photo diode)
 Performance of photodetector
• Quantum efficiency: ratio of no of electron-
hole carrier generated to no of incident
photons
• Responsivity:output current to incident
optical power
• Wavelength
• Dark current: electrical current under total
darkness condituon.
 Photodiodes
• Photodiodes meet most of the requirements,
hence widely used as photo detectors.
• Positive-Intrinsic-Negative (pin) photodiode
– No internal gain, robust detector
• Avalanche Photo Diode (APD)
– Advanced version with internal gain M due
to self multiplication process
• Photodiodes are sufficiently reverse biased
during normal operation  no current flow
without illumination, the intrinsic region is fully
depleted of carriers
 PIN DIODE

• A diode with a wide and undoped intrinsic

semiconductor region between a p-type and
an n-type semiconductor region.
• It was even used for microwave
applications and as a photo detector as it is
said to be a good light absorber.
 PiN Photodiode
Semiconductor positive-negative structure
with an intrinsic region sandwiched between
the other two regions.
Normally operated by applying a reverse-bias
voltage.
Dark current can also be produced which is a
leakage current that flows when a reverse bias
is applied without incident light.
pin energy-band diagram
 Structure and Working of a Pin Diode
• The PIN diode comprises a semiconductor
diode having three layers naming the P-type
layer, Intrinsic layer and N-type layer, as
shown in the figure below.
• The P and N regions are there, and the region
between them consists of the intrinsic
material, and the doping level is said to be
very low in this region.
•
 The thickness of the intrinsic layer is very narrow, which
ranges from 10 – 200 microns.
 The P region and the N-type regions are known to be
heavily doped.
 The changes in the properties of the diode are known from
the intrinsic material.
 These diodes are made of silicon.
 The intrinsic region of the PIN diode acts like an inferior
rectifier which is used in various devices such as
attenuators, photodetectors, fast switches, high voltage
power circuits, etc.
 Advantage of PIN photodiodes
The output electrical current is linearly
proportional to the input optical power making it a
highly linear device.
Low bias voltage(<4v).
Low noise
Low dark current
High-speed response
 Quantum Efficiency

Ip is the photocurrent generated by a steady-state

optical power Pin incident on the photodetector.

The performance of photo diode is characterized

by responsivity R
R=Ip/P0=nq/hv (A/W)
 Avalanche Photodiode (APD)
• APD has an internal gain M, which is obtained by
having a high electric field that energizes photo-
generated electrons.
• These electrons ionize bound electrons in the
valence band upon colliding with them which is
known as impact ionization
• The newly generated electrons and holes are also
accelerated by the high electric field and gain energy
to cause further impact ionization
• This phenomena is the avalanche effect
RAPD (Reach Through APD ): P+πPN+
 Responsivity ()

APD’s have an internal gain M, hence

 APD  PIN M

where, M = IM/Ip
IM : Mean multiplied current

M = 1 for PIN diodes

 advantages
• Low level light can be detected
• Increase in sensitivity of receiver
• SNR is high
• Excellent linearity
 disadvantages
• Complex structure
• High reverse bias voltage is required
• Additional noise
 Photodetector Noise
• photodiode is generally required to detect very
weak optical signals.
• requires that the photodetector and its amplification
circuitry be optimized to maintain a given signal-to-
noise ratio.

SNR Can NOT be improved by amplification

 Notation: Detector Current
• The direct current value is denoted by, IP (capitol
main entry and capital suffix).
• The time varying (either randomly or periodically)
current with a zero mean is denoted by, ip (small
main entry and small suffix).
• Therefore, the total current Ip is the sum of the DC
component IP and the AC component ip .
I P  I p  ip
T /2
1
 Lim T  
2 2
i p i p (t )dt
T T / 2
 Quantum (Shot Noise)
Quantum noise arises due optical power fluctuation
because light is made up of discrete number of
photons

i2
Q  2qI p BM F ( M )
2
 Dark/Leakage Current Noise
There will be some (dark and leakage ) current
without any incident light. This current generates
two types of noise
2
Bulk Dark Current Noise iDB  2qI D BM F ( M )
2

ID: Dark Current

Surface Leakage Current Noise

2
iDS  2qI L B

(not multiplied by M) IL: Leakage Current

 Thermal Noise
The photodetector load resistor RL contributes
to thermal (Johnson) noise current

iT2  4 K BTB / RL

KB: Boltzmann’s constant = 1.38054 X 10(-23) J/K

T is the absolute Temperature
• Quantum and Thermal are the significant noise
mechanisms in all optical receivers
• RIN (Relative Intensity Noise) will also appear
in analog links
 SNR
Detected current = AC (ip) + DC (Ip)
Signal Power = <ip2>M2
i p2 M 2
SNR 
2q( I p  I D ) M 2 F ( M ) B  2qI L B  4k BTB / RL

Typically not all the noise terms will have equal

weight.
Often thermal and quantum noise are the most
significant.
Noise Calculation Example
 Limiting Cases for SNR
• When the optical signal power is relatively high, then the shot
noise power is much greater than the thermal noise power. In
this case the SNR is called shot-noise or quantum noise
limited.
• When the optical signal power is low, then thermal noise
usually dominates over the shot noise. In this case the SNR is
referred to as being thermal-noise limited.
 Limiting Cases of SNR
In the shot current limited case the SNR is:
i p2
SNR 
2q ( I p ) F ( M ) B

For analog links, there will be RIN (Relative

Intensity Noise) as well
i p2 M 2
SNR 
 2q( I p  I D ) M 2 F ( M )  4k BT / RL  ( RIN ) I p2  B
 Detector Response Time

It is defined as time required by generated

photo carriers to travel across the depletion
region.
It depends mainly on
1.Transit time
2.Diffusion time
3.RC time constant
td  w / vd
For a high speed Si PD, td = 0.1 ns
 Transit time
• It depends on carrier drift velocity and
depletion layer.it is given by

td  w / vd
Diffusion time
Fundamental Receiver operation
 Fundamental Receiver Operation
• The first receiver element is a pin or an avalanche
photodiode, which produces an electric current
proportional to the received power level.
• Since this electric current typically is very weak, a
front-end amplifier boosts it to a level that can be
used by the following electronics.
• After being amplified, the signal passes through a
low-pass filter to reduce the noise that is outside
of the signal bandwidth.
• Also filter can reshape (equalize) the pulses that
have become distorted as they traveled through
the fiber.
 Preamplifiers
• Optical amplifier being used as a front-end
preamplifier for an optical receiver.
• A weak optical signal is amplified before
photo-detection so that signal to noise ratio
degradation due to noise can be suppressed
in the receiver.
• It provides a larger gain factor and BW.
• Three types: semiconductor optical
amplifiers, Raman Amplifiers and Erbium
doped fibre amplifiers.
 Error Sources
The term noise describes unwanted components of
an electric signal that tend to disturb the
transmission and processing of the signal
• The random arrival rate of signal photons produces
quantum (shot) noise
• Dark current comes from thermally generated e-h
pairs in the pn junction
• Additional shot noise arises from the statistical
nature of the APD process
• Thermal noises arise from the random motion of
electrons in the detector load resistor and in the
amplifier electronics
 Receiver configuration

Bandwidth of the front end: B  1 2 RT CT

CT: Total Capacitance = Cd+Ca
RT: Total Resistance = Rb // Ra
 Receiver Sensitivity
• A specific minimum average optical power
level must arrive at the photodetector to achieve
a desired BER at a given data rate. The value of
this minimum power level is called the receiver
sensitivity.
• Assuming there is no optical power in a
received zero pulse, then the receiver sensitivity
is
 Receiver Sensitivity Calculation
The receiver sensitivity as a function of bit
rate will change for a given photodiode
depending on values of parameters such as
wavelength, APD gain, and noise figure.
 The Quantum Limit

• The minimum received optical power required

for a specific bit-error rate performance in a
digital system.
• This power level is called the quantum limit,
since all system parameters are assumed ideal
and the performance is limited only by the
detection statistics.
 REFERENCES
• https://electronics360.globalspec.com/articl
e/10397/pin-vs-apd-different-sensitivity-
different-applications
• https://www.slideshare.net/Surajduvey/phot
o-detector-noise
• https://en.wikipedia.org/wiki/Quantum_limi
t
• https://www.brainkart.com/article/Fundame
ntal-Receiver-Operation_13633/
 SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY
(Approved by AICTE, New Delhi & Affiliated to JNTUA, Ananthapuramu)
(Accrediated NBA and Accrediated NAAC with “A” Grade )
Siddharth Nagar, Naravanavanam Road, Puttur-517583, AP, India

Department of Electronics and Communication Engineering

OPTICAL FIBER COMMUNICATION

Unit-IV

OPTICAL FIBER SYSTEM DESIGN AND

TECHNOLOGY
Contents
• System specification
• Point-to- links
• link power budget
• Rise Time Budget
• Bandwidth Budget
• Power Budget and Receiver Sensitivity
• Link Budget calculations
• Optical Multiplexing & Demultiplexing techniques
• Optical Amplifiers and its Applications.
System Specifications:
Photodetector, Optical Source, Fiber
• Photodetectors: Compared to APD, PINs are less expensive
and more stable with temperature. However PINs have
lower sensitivity.
• Optical Sources:
1- LEDs: 150 (Mb/s).km @ 800-900 nm and larger than 1.5
(Gb/s).km @ 1330 nm
2- InGaAsP lasers: 25 (Gb/s).km @ 1330 nm and ideally
around 500 (Gb/s).km @ 1550 nm. 10-15 dB more power.
However more costly and more complex circuitry.
• Fiber:
1- Single-mode fibers are often used with lasers or edge-
emitting LEDs.
2- Multi-mode fibers are normally used with LEDs. NA and 
should be optimized for any particular application.
Point-to-Point Link
• The components must be carefully chosen to ensure the
desired performance level and can be maintained for the
expected system life time.

Figure represents the block diagram of a simplex point-to-point

link. The three major optical links building blocks are,
Transmitter, Receiver and Optical fiber.
The key system requirements are needed in analyzing a
link.
• Signal dispersion
• Data rate
• Transmission distance and cost.
Optical sources (such as LED or LASER) are used based on
the following characteristics.
• Emission wavelength
• Spectral line width
• Output power
• Effective radiating area
• Emission pattern
• Number of emitting modes
.
The characteristics of photo detector such as,
• Responsivity
• Operating wavelength
• Speed and
• Sensitivity
The choice of optical fiber
• Single mode and multimode (step or graded index)
• Core size
• Core refractive index profile
• Band width or dispersion
• Attenuation
• Numerical aperture or modefield diameter
Link Budget Considerations
(1) Power Budget: determines the power margin between
the optical transmitter output and the minimum
receiver sensitivity needed to establish a specific Bit
Error Rate (BER).
(2) Bandwidth Budget: Determines dispersion limitation
of optical fiber link
Link Power/Loss Analysis
 Rise-Time Budget
t sys  [ttx  t mod  tGVD  t rx ]
2 2 2 2 1/ 2

2 1/ 2
 2  440 Lq   350 
2

 ttx     D   L    
2 2 2

  B0   Brx  

t tx [ ns ] : transmitter rise time t rx [ ns ] : receiver rise time t mod [ n ] : modal dispersion

Brx [ MHz ]:3dB Electrical BW L[ km ]:Length of the fiber B0 [ MHz ]:BW of the 1 km of the fiber;

q  0.7 tGVD [ns]: rise- time due to group velocitydispersion

D[ ns /( km .nm )]:Dispersion   [nm]: Spectral width of the source

 Total Rise time, Tsys:
Tsys=1.1(TTX2+TRX2+Tfiber2)1/2
 What is a good Rise time?
For a good reception of signal
Tsys < 0.7 x Pulse Width (PW)

PW = 1/BitRate for NRZ

1/2BitRate for RZ
 Example:
Rise Time Budget Measurement for
Long Haul Application

Tx rise time, TTX = 0.1 ns

Rx rise time, TRX= 0.5 ns
Linewidth() = 0.15 nm

Dispersion Coefficient, D = 18 ps/nm-km

Fiber length = 150km
Bit Rate = 622Mbps
Format = RZ
 Simple Calculation….

Fiber rise time, TF =Length x D x Linewidth()

= 150 km x 18 x 0.15 nm
= 0.4 ns
Total Rise time, TSYS = 1.1 TLS2 + TPD2 + TF2
= 1.1 0.01 + 0.25 + 0.16

TSYS = 0.77 ns
Let say,
Bit Rate = STM 4 = 622 Mbps
Format = RZ
Tsys < 0.7 x Pulse Width (PW)

Pulse Width (PW) = 1/(622x106)

= 1.6 ns
0.77 ns < 0.7 x 1.6 ns
0.77 ns < 1.1 ns !!
Good Rise Time Budget!!
Let say,
Bit Rate = STM 16 = 2.5 Gbps
Format = RZ
Tsys < 0.7 x Pulse Width (PW)

Pulse Width (PW) = 1/(2.5x109)

= 0.4 ns
0.77 ns < 0.7 x 0.4 ns
0.77 ns ≥ 0.28 ns !!
Bad Rise Time Budget!!
Power Budget
PRX > PMIN
PRX = Received Power
PMIN = Minimum Power at a certain BER
PRX = PTX – Total Losses - PMARGIN
PTX = Transmitted Power
PMARGIN ≈ 6 dB
•Total optical loss = Connector loss + (Splicing loss + Fiber
attenuation) + System margin (Pm)
PT = 2Lc + αfL + Lsp+System margin (Pm)
Requirements Cont’d:
• Loss,L = LIL + Lfiber + Lconn. + Lnon-linear
LIL = Insertion Loss
Lfiber = Fiber Loss
Lconn.= Connector Loss
Lnon-linear= Non-linear Loss

• Gain,G = Gainamp + Gnon-linear

Gainamp = Amplifier Gain

Gnon-linear = Non-linear Gain
 Example:
Power Budget Measurement for Long Haul
Transmission
185 km

Splice Connector

PTx = 0 dBm PSEN = -28 dBm

Attenuation Coefficient,  = 0.25 dB/km
Dispersion Coefficient, D = 18 ps/nm-km
Number of Splice = 46
Splice Loss = 0.1 dB
Connector Loss = 0.2 dB
PMargin = 6 dB
 Simple Calculation….
Fiber Loss = 0.25 dB/km X 185 km
= 46.3 dB
Splice Loss = 0.1 dB X 46

CONCLUSION: = 4.6 dB
Connector Loss = 0.2 dB X 2
= 0.4 dB

BAD SYSTEM!!
Total Losses = 46.3 + 4.6 + 0.4
= 51.3 dB
PMargin = 6 dB
PRX = PTX – Total Losses – PMargin
= 0 – 51.3 – 6
PRX = -57.3 dB
Power Budget, PRX < PSEN !!
 First we calculate the amplifier’s gain..

Gain  PSEN - PRX

Gain  -28 – (-57.3)
Gain  29.3 dB
To make it easy, Gain  30 dB
Now…Where to put the
amplifier?
 Receiver sensitivity
•Performance can be measured as a low bit error rate
(BER).
•A measure of a good receiver is to have the same
performance with the lowest level of incident
optical power.
•BER ÷ probability of an incorrect identification of
a bit by the decision circuit of a receiver.
•Receiver Sensitivity ÷ Receiver sensitivity is the
minimum power level at which the receiving node
is able to clearly receive the bits being transmitted.
 Optical
Multiplexing & Demultiplexing techniques
• Normally, there are three main different techniques
in multiplexing light signals onto a single optical
fiber link: optical time division multiplexing (OTDM),
code division multiplexing (CDM), and wavelength
division multiplexing (WDM).
• WDM is one of the most common way using
wavelengths to increase bandwidth by multiplexing
various optical carrier signals onto a single optical
fiber.
• What Is multiplexing?
• Multiplexing (Muxing) is a term used in the field of
communications and computer networking. It generally
refers to the process and technique of transmitting multiple
analog or digital input signals or data streams over a single
channel. Since multiplexing can integrate multiple low-speed
channels into one high-speed channel for transmission, the
high-speed channel is effectively utilized.
• What Is demultiplexing?
• Demultiplexing (Demuxing) is a term relative to multiplexing.
It is the reverse of the multiplexing process. Demultiplex is a
process reconverting a signal containing multiple analog or
digital signal streams back into the original separate and
unrelated signals.
 Optical Amplifiers and its Applications
•However, when the length of the optical fiber
is a distance as long as 10 km or 100 km, that
transmission loss cannot be ignored. When the
light (signal) propagating a long-distance
optical fiber becomes extremely weak, it is
necessary to amplify the light using an optical
amplifier.
•An optical amplifier amplifies light as it is
without converting the optical signal to an
electrical signal, and is an extremely important
device that supports the long-distance optical
communication networks of today.
 Applications of Optical Amplifiers

• Applications of Optical Amplifiers

• Typical applications of optical amplifiers are:
• An amplifier can boost the (average) power of a laser output to
higher levels (→ master oscillator power amplifier = MOPA).
• It can generate extremely high peak powers, particularly
in ultrashort pulses, if the stored energy is extracted within a short
time.
• It can amplify weak signals before photodetection, and thus reduce
the detection noise, unless the added amplifier noise is large.
 Design of Digital systems:
System specifications:
•Photodetectors: Compared to APD, PINs are less
expensive and more stable with temperature.
However PINs have lower sensitivity.
•Optical Sources:
1- LEDs: 150 (Mb/s).km @ 800-900 nm and larger
than 1.5 (Gb/s).km @ 1330 nm
2- InGaAsP lasers: 25 (Gb/s).km @ 1330 nm and
ideally around 500 (Gb/s).km @ 1550 nm. 10-15 dB
more power. However more costly and more
complex circuitry.
•Fiber:
System Rise Time
• Calculate the total rise times
Tx, Fiber, Rx

• Calculate Fiber rise time, TFiber

Tfiber = D x  x L
D = Dispersion Coefficient
 = Linewidth
L = Fiber Length

Tx Rise Time, TTX = normally given by manufacturer

Rx Rise Time, TRX = normally given by manufacturer
 REFERENCES

• https://en.wikipedia.org/wiki/System_requirements_specification
• https://www.ques10.com/p/29974/what-is-rise-time-budget-
analysis-derive-an-expr-1/
• https://www.researchgate.net/publication/320264322_Bandwidth_B
udget_Analysis_for_Visible_Light_Communication_Systems_utilizing
_Available_Components
• https://www.youtube.com/hashtag/linkpowerbudgetanalysisofoptica
lfibercommunicationsystem
• https://study.com/academy/lesson/point-to-point-link-based-
systems-definition-uses.html
 SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY
(Approved by AICTE, New Delhi & Affiliated to JNTUA, Ananthapuramu)
(Accrediated NBA and Accrediated NAAC with “A” Grade )
Siddharth Nagar, Naravanavanam Road, Puttur-517583, AP, India

Department of Electronics and Communication Engineering

OPTICAL FIBER COMMUNICATION

UNIT-V
Optical Networks
Optical Networks
•Definition: An Optical Network is basically a
communication network used for the exchange
of information through an optical fiber
cable between one end to another. It is one of the
quickest networks used for data communication.
CONTENTS

Basic Networks

Broadcast-and-select WDM Networks

Wavelength routed Networks

Performance of WDM+EDFA Networks

Ultra high capacity networks

 Basics Of Networks
Station : Stations in an optical network serves as the source and destination of the
information being transmitted and received.
Examples: computers, terminals, telephones or other equipment for
communicating.
Network:
The pattern of contacts or flow of information between the stations is called a
network.
Node:
Node is nothing but acts as a hub for multiple transmission lines inside the network. In case
of a single transmission line, an optical network does not require nodes, as in this case
stations at both the ends can be directly connected to the fiber cables.
Trunk: A trunk is basically a transmission line i.e., optical fiber cable in order to transmit the
optical signal.
Topology:
When multiple fiber cables are employed in an optical network, then
these are connected through nodes. But the way in which the multiple
nodes are connected together denotes the topology of the network.
Router : A router is basically placed inside an optical network that provides a
suitable path for signal transmission.
Networks classification:
LANs :
LANs means Local area networks. It is a interconnect users in a localized area such
as a department, a building, an office or factory complex, or a university campus .
MANs :
MANs means Metropolitan area networks. which provides user connection
with in a city or in the metropolitan area surrounding a city.
WANs :
WANs means wide area network. it covers a large geographical area ranging
from connection between near by cities to connection of users across a country.
Network Topologies :
 The popular protocol used in optical LANs is the Fiber Distributed Data
Interface (FDDI).

SONET and SDH are two protocols which are widely used on a ring
network with active nodes in MANs and WANs.
• Bus Topology: In a bus topology, the various nodes are connected
through a single trunk line with the help of optical couplers. This allows
a convenient as well as a cost-effective method to transmit the signal.
However, in a bus topology, it is difficult to determine the faulted node
as well as it also takes time to restore the transmitted signal from that
particular node.
• Ring Topology: In a ring topology, one single node is joined to its
neighbouring node thereby forming a closed path. So, the transmitted
information in the form of light is sent from one node to another.
• Star Topology: In star connection, the various nodes of the network are
connected together with a single central hub. This central hub can be
active or passive network. This central hub then controls and directs the
transmitted optical signal inside the optical network.
• Mesh Topology: In a mesh topology, an arbitrary connection is formed
between the nodes in the network. This point to point connection can
Basically, in mesh connection, failure of any link or node is generated
then firstly that particular failure is detected and then the signal traffic is
diverted from failed node to another link inside the connection. be
changed according to the application.
Broadcast-and-Select Network
Broadcast-and-select networks are based on a passive star coupler
device connected to several stations in a star topology.
Broadcast-and-Select WDM Network
All-optical WDM networks have full potential of optical transmission capacity and
versatility of communication networks beyond SONET architectures.
 These networks can be classified as
(1) Broadcast-and-select techniques
(2) Wavelength-routing networks.
 Broadcast-and select techniques employing passive optical stars, buses and
wavelength routers are used for local networks can be classified as
(1) Single-hop networks
(2) Multi-hop networks
 Single hop refers to network where information transmitted in the form of light
reaches its destination without being converted to an electrical form at any
intermediate point. In a multi hop network, intermediate electro-optical conversion
can occurred.
Broadcast and Select Signal Hop Network
• Two alternate physical architectures for a WDM-based local network have n sets
of transmitters and receivers are attached to either a star coupler or a passive bus.
Each transmitter sends its information at a fixed wavelength.
• All the transmissions from the various nodes are combined in a pasive star. Coupler or
coupled onto a bus and sent out to all receivers.
• An interesting point to note is that the WDM setup is protocol transparent.
Protocol transparent means that different sets of communicating nodes can use different
information exchange rules (protocols) without affecting the other nodes in the network.
 The architectures of single-hop broadcast-and-select networks are fairly simple, there
needs to be careful dynamic coordination between the nodes.
 A transmitter sends its selective filter to that wavelength.
 Two sending stations need to coordinate their transmission so the collisions of information
streams at the some wavelength do not occur.
 Broadcast and Select Multi hop Network

. Broadcast and Select Multihop Network

Drawback of single-hop networks is the need for rapidly unable lasers or receiver optical fibers.

 This drawback can be overcome by the designs of multi hop networks.

 Multihop networks do not have direct paths between each node pair.
 Each node has a small number of fixed tuned optical transmitter and receivers.

An example, a four node broadcast and select multi hop network where
each node transmits on one set of two fixed wavelengths and receives on another
set of two fixed wavelengths.
Information destined for other nodes will have to be routed through intermediate
stations.
 Considering the operation, a simplified transmission scheme in which message
are sent as packets with a data field and an address header containing source and
destination identifiers (i.e,. routing information) with control bits.
 At intermediate node, the optical signal is converted to an
electrical format.
 The address header is decoded to examine the routing
information field, which will indicate where the packet should go.
 Routing information is used to send the electronic packets
from optical transmitter to the next node in the logical path toward
its final destination.
 Advantage: There are no destination conflicts or packet
collisions
. Broadcast in Multihop
and Select the network.
Network
 For H hops between nodes, there is a network throughput
penalty of at least 1/H.
The Shuffle Net Multihop Network
various topologies for multi hop light wave networks are
(1) The shuffle net graph
(2) The de Bruijin graph
(3) He toroidal Manhattan street network
· A scheme called the perfect shuffle is widely used to form processor
interconnect patterns in multiprocessors.
· For optical networks, the logical configuration consists of a cylindrical
arrangement of k column, each having p nodes. Where P is the number of fixed
transceiver pairs per node.
The total number of nodes is then
a (p,k)=(2,2) shuffle net, where the (k+1)th column represents the
completion of a trip around the cylinder back to the first column.
 Performance parameter for the shuffle net is the average number of hops
between any randomly chosen nodes.
 Since, all nodes have p output wavelength, p nodes can be reached from
any node in one hop, p2 additional nodes can be reached in two hope, until
all the (pk-1) other nodes are visited.
 The maximum number of hops is
Consider figure above, the connections between nodes 1 and 5 and nodes 1 and 7. In first
case, the hop number is one.
 In second case three hops are needed with routes 1- 6 – 7 or 1 – 5 – 2 -7.?
 The average of hops Bar H of a shuffle net is
 In multi hopping, part of the capacity of a particular link directly connecting two nodes is
actually utilized for carrying between them.
 The rest of the link capacity is used to forward messages from other nodes.
 The system has Np=kpK+1 links, the total network capacity C is

The per-user throughput δ is

Different (p,k) combination result in different throughputs, to get a better network performance.
•
Wavelength Routed Networks
Two problems arise in broadcast and select networks,
 More wavelengths are needs as the number of nodes in the network grows.
 Without the widespread are use of optical booster amplifier, due to this splitting losses
is high.

Wavelength routed networks overcome these limitations through wavelength reuse,

wavelength conversion, and optical switching.
The physical topology of a wavelength routed network consists of optical wavelength
routers interconnected by pair of point-to-point fiber link in an arbitrary mesh
configuration.
Each link can carry a certain number off wavelength which can be directed
independently to differently output paths at a node.
Each node may have logical connections with several other nodes in the
network, where each connection uses a particular wavelength.
The paths taken by any two connections do not overlap, they can use the same
wavelength.
Optical CDMA
The simplest configuration , CDMA achieves multiple access by assigning a
unique code to each user.
 To communicate with another node, user imprint their agreed upon code onto the data. The
receiver can then decode the bit stream by locking onto the code sequence.
 The principle of optical CDMA is based on spread-spectrum techniques.
 The concept is to spread the energy of the optical signal over a frequency band that is
much wider than the minimum bandwidth required to send the information.
 Spreading is done by a code that is independent of the signal itself.

 On optical encoder is used to map each bit of information into the high-rate (longer-

code-length) optical sequence. The symbols is the spreading code are called chips.

 The energy density of the transmitted waveform is distributed more or less uniformly

over the entire spread-spectrum bandwidth.

 The set of optical sequences becomes a set of unique ‘address codes or signature

sequences’ the individual network users.

 The signature sequence contains six chips. When the data signal contains 1 data bit, the
six-chip sequence is transmitted, no chips are sent for a 0 data bit.
 Time-domain optical CDMA allows a number of users to access a network
simultaneously, through the use of a common wavelength.
 Both asynchronous and synchronous optical CDMA techniques. In synchronous

accessing schemes follow rigorous transmission schedules, the produce more

successful transmission (higher throughputs) than asynchronous methods where

network access is random and collisions between users can occurs.

 An optical CDMA network is based on the use of a coded sequence of pulses.

 The setup consists of N transmitter and receiver pairs interconnected in a star

Basics of EDFA
The key feature of EDFA technology is the Erbium Doped Fiber (EDF),
which is a conventional silica fiber doped with erbium. Basically, EDFA consists of a
length of EDF, a pump laser, and a WDM combiner. The WDM combiner is for
combining the signal and pump wavelength so that they can propagate
simultaneously through the EDF. EDFAs can be designed that pump energy
propagates in the same direction as the signal (forward pumping), the opposite
direction to the signal (backward pumping), or both direction together. The pump
energy may either by 980nm pump energy or 1480nm pump energy, or a
combination of both. The most common configuration is the forward pumping
configuration using 980nm pump energy. Because this configuration takes
advantage of the 980nm semiconductor pump laser diodes, which feature effective
cost, reliability and low power consumption. Thus providing the best overall design
in regard to performance and cost trade-offs.
Why EDFA Is Essential to WDM Systems?
We know that when transmitting over a long distance, the signal
is highly attenuated. Therefore it is essential to implement an optical signal
amplification to restore the optical power budget. This is what EDFA commonly
used for: it is designed to directly amplify an input optical signal, which hence
eliminates the need to first transform it to an electronic signal. It simply can
amplify all WDM channels together. Nowadays, EDFA rises as a preferable
option for signal amplification method for WDM systems, owing to its low-noise
and insensitive to signal polarization. Besides, EDFA deployment is relatively
easier to realize compared with other signal amplification methods.
Channel WDM System With or Without EDFA:
What Is the Difference?
Two basic configurations of WDM systems come in two forms: WDM
system with or without EDFA. Let’s first see the configuration of a WDM system
without using it. At the transmitter end, channels are combined in an optical
combiner. And these combined multiple channels are transmitted over a single fiber.
Then splitters are used to split the signal into two parts, one passes through the
optical spectrum analyzer for signal’s analysis. And other passes through the
photodetector to convert the optical signal into electrical. Then filter and electrical
scope are used to observe the characteristics of a signal. In this configuration signals
at a long distance get attenuated. While this problem can be overcome by using
erbium-doped fiber amplifier.
As for WDM system which uses EDFA, things are a little bit different.
Although the configuration is almost the same as a WDM system without it,
some additional components are used. These components are EDFAs which
are used as a booster and pre-amplifier, and another additional component
is an optical filter. With the adoption of an optical amplifier, this system
doesn’t suffer from losses and attenuation. Hence, it is possible to build
broadband WDM EDFAs which offer flat gain over a large dynamic gain
range, low noise, high saturation output power and stable operation with
excellent transient suppression. The combination provides
reliable performance and relatively low cost, which makes EDFAs preferable
in most applications of modern optical networks.
 Among the various technologies available for optical amplifiers, EDFA

technology proves to be the most advanced one that holds the dominant position in

the market. In the future, the WDM system integrated with high-performance

EDFA, as well as the demand for more bandwidth at lower costs have made optical

networking an attractive solution for advanced networks.

Performance of WDM+EFDA
An optical network that involves WDM (wavelength division
multiplexing) currently gains in much popularity in existing telecom infrastructure.
Which is expected to play a significant role in next-generation networks to support
various services with a very different requirement. WDM technology, together with
EDFA (Erbium Doped Fiber Amplifier), allowing the transmission of multiple
channels over the same fiber, that makes it possible to transmit many terabits of data
over distances from a few hundred kilometers to transoceanic distances, which
satisfy the data capacity required for current and future communication networks.
This article explains how can WDM system benefit from this technology.
 To send information from node j to node k, the address code for node k is impressed
upon the data by the encoder at node j.
 At the destination, the receiver differentiates between codes by means of correlation
detection.
 Each receiver correlates its own address f(n) with the received signal s(n). The receiver
output r(n) is
If the received signal arrives at the correct destination, then s(n)=f(n).

Equation (5.57) represents an autocorrelation function, if s(n) not equal to f(n) the
equation (5.57) represents a cross-correlation function.

For a receiver to be able to distinguish the proper address correctly, it is necessary to

maximize the autocorrelation function and minimize the cross-correlation function.

Prime-sequence codes and optical orthogonal codes (OOCs) are the commonly used
spreading sequences in optical CDMA systems.

An OOC systems the number of simultaneous user an is bounded by

ULTRA HIGH CAPACITY NETWORKS
Advance of optical communication systems has provide
channels with enormous bandwidth at least 25THz and dense WDM technology,
ultrafast optical TDM.
To using dense WDM techniques to increase the capacity of long-haul
transmission link and ultrafast optical TDM schemes.
These are particularly attractive in LAN or MANs

TDM Schemes To Shared-Media Local Neteorks Have Two Methods:

(1) Bit-interleaved TDM.
(2) Time-slotted TDM.
1. Ultra High Capacity WDM Networks
Two popular approaches are used to achieve increased capacity.
(a) to widen the spectral bandwidth of EDFAs from 30 to 80 nm, by using
broadening techniques.
(b) Increasing the capacity of a WDM link is to improve the spectral efficiency of
the WDM signals.
Most of the demonstrations use a rate of 20 Gb/s for each individual wavelength to
avoid non-linear effects.
Examples are,
(1) A 50-channel WDM system operating at an aggregated 1-Tb/s rate over a 600 km link.
(2) A 132-channel WDM system operating at an aggregated 2.6 Tb/s rate over a 120-
km/link.
2. Bit-Interleaved Optical TDM
 Repetition rate typically ranges from 2.5 to 10 Gb/S, which corresponds to the
bit rate of the electric data tributaries feeding the system.
 An optical splitter divides the pulse train into N separate streams.
 The pulse streams is 10 Gb/S and N=4, each of these channels is then
individually modulated by an electrical tributary data source at a bit rate B.
 The modulated outputs are delayed individually by different fractions of the
clock period, and are then interleaved through an optical combiner to produce an
aggregate bitrate of NXB.
 Optical post amplifier and preamplifier are generally included in the link to
compenstate for splitting and attenuation loss.
 At the receiving end, the aggregate pulse stream is demultiplexed into the
original N independent data channels for further signal processing.
 A clock-recovery mechanism operating at the base bit rate B is required at the
receiver to drive and synchronize the demultiplexer.