Concepts of optical fiber communication

OPTICAL FIBER
COMMUNICATION
BY NAVEEN JAKHAR, ITS

IN THIS PRESENTATION…
 GENERAL: History of Transmission Systems
 OFC
• History of OFC
• Advantages
• Applications
• ITU-T Recommendations
• Fiber optic principle
• Windows of operation
• Trends in OF Communication
• Fiber classification
• OF Cable Types
• Optical Fiber transmission impairments
• Optical Sources and Detectors
• Optical Link Characterization and Design
13 September 2016 2

HISTORY OF TRANSMISSION
SYSTEMS
The developments…
• Open Wire Systems
• Coaxial Cables
• UHF Systems
• Microwave Systems
• Digital Transmission Systems
• Satellite Communication Systems
• Optical Fiber Cable
13 September 2016 3

OPEN WIRE SYSTEMS
• Till 1950s, the long distance voice
communication was almost entirely
transported over Open Wire Carrier
system.
• The voice signals for these systems
were multiplexed using FDM to a
higher frequency carrier and carried
through open wire systems.
• These open wire systems were
capable of carrying traffic of three
to twelve subscribers at a time.
13 September 2016 4

COAXIAL CABLES
• With the introduction of U/G symmetrical
pair cable carrier system which was
followed by the Coaxial Cable system,
greatly enhanced, by the decade end, the
simultaneous voice channel carrying
capacity to 960 voice channels.
• The first Coaxial Cable System was
commissioned between Agra and Delhi in
the year 1959.
• Over the years, this system was improved
and developed to carry 2700
simultaneous voice channels.
13 September 2016 5

UHF SYSTEMS
• Close on the heel of coaxial systems, in the
mid-60s wireless microwave systems
were developed and inducted in the
network.
• The first Microwave system was installed in
1965 between Calcutta and Asansole.
Microwave systems with 60, 300 and 1800
voice channels capacity were inducted
into the telecom network subsequently.
• These systems were mostly indigenous
(developed and manufactured within the
country).
13 September 2016 6

DIGITAL TRANSMISSION SYSTEMS
• By mid-1980s Digital TAX exchanges were introduced in the network with the aim
to improve STD services.
• Till 1989, Coaxial cable and UHF transmission medias were used to provide
connectivity.
• Induction of Digital Transmission Systems which were mainly Digital UHF, Digital
Microwave, Digital Coaxial and Optical Fiber Systems, started during 1989-90.
U/G coaxial cable was initially used for the connectivity of large and medium
cities and however, later on, it was also used for connecting small towns.
• Media diversity was provided through Radio Relay (UHF and Microwave)
Systems. These Radio relay systems were very reliable and beneficial particularly
for connecting hilly and backward areas where laying and maintenance of
underground cable is extremely difficult. 13 September 2016 7

SATELLITE SYSTEMS
• Work for connecting far flung, inaccessible area, and island community started
in late 70s by DoT.
• The first Domestic Satellite Network was established by connecting Port-Blair and
Car-Nicobar in Andaman & Nicobar islands, Kavaratti in Lakshadweep islands, Leh
in Ladakh region and Aizwal in North Eastern region. These station were
simultaneously linked to the gateway at Delhi and Chennai. This satellite network
was commissioned in November 1980 through International Telecommunication
Satellite.
• Satellite Communication capacity increased with launch of INSAT-1 and INSAT-2
series satellites. MCPC - VSAT (Multi Channel per Carrier - Very Small Aperture
Terminals) systems were developed and deployed in remote and inaccessible
areas of Garhwal region of (then) Uttar Pradesh, Himachal Pradesh, Arunachal
Pradesh, J&K, Orissa, Sikkim etc. for providing STD facilities. These terminals were
linked to an Earth Station generally co-located with the TAX.
13 September 2016 8

OPTICAL FIBER CABLE
• Introduction of Optical Fiber Cable Systems in the country started in 1989-
90.
• These systems are capable of carrying large no. of voice channels
compared to the existing technologies that were available at that time and
offer the circuit at low cost per kilometer of circuit. The DoT deployed these
OFC systems in a big way for connectivity right upto the level of Tehsils.
• By the year 2000, a huge network of optical fiber cable was in place and a
large number of PDH & SDH technology OFC systems were deployed for
providing backbone connectivity to switching network.
• From 2002-03, DWDM technology systems were inducted over the OFC
backbone.
13 September 2016 9

• 1790: Optical Semaphore invented by Claude Chappe of France.
• 1880: Photophone invented by A.G. Bell at Washington.
• 1940: Optical guides reflective coated to carry visible light.
• 1960: LASER invented by Theodore Maiman.
• 1963: Unguided communications with LASER.
• 1966: OPTICAL FIBER invented by Corning Glass researchers:
ROBERT MAURER
DONALD KECK &
PETER SCHULTZ
13 September 2016 12
Optical Communications
- Historical Perspective

HISTORICAL PERSPECTIVE
(CONTD…)
• Material for fiber was fused silica with special properties like:
• Extreme purity
• A high melting point
• Low refractive index.
Initially very high loss fiber was developed.
Typical loss of ~17db/km [at λ =820 nm]
• 1970: low loss fiber developed.
OFC systems became practical.
• Currently :
fiber losses=<0.2-0.35 db/km

The Developments

ADVANTAGES OF FIBER COMMUNICATIONS
• High information carrying capacity
• Low attenuation
• Plentiful Resource
• Greater safety
• Immunity to RFI
• Immunity to EMI
• No cross-talk
• Higher Security
• Small size and light weight
• Less Corrosion
• Less temperature sensitive

ADVANTAGES OF FIBER COMMUNICATIONS
• High information carrying capacity:
A valid comparison would be on the basis of cost per meter
per telephone channel, rather than just cost per meter.
• Resource plentiful:
The basic materials are either silicon dioxide for glass fibers
or transparent plastic which are plentiful.
• Less attenuation:
A typical fiber attenuation is 0.3 dB/km. Whereas a coaxial
cable (RG-19/U) will attenuate a 100MHz signal by 22.6
dB/km.
• Greater safety:
Optic fibers glass/plastic, are insulators. No electric current
flows through them.

ADVANTAGES OF FIBER
COMMUNICATIONS (2)
• Immunity to RFI:
Fibers have excellent rejection of radio-frequency
interference (RFI) caused by radio and television stations,
radar, and other electronics equipment.
• Immunity to EMI:
Fibers have excellent rejection of electromagnetic
interference (EMI caused by natural phenomena such as
lightning, sparking, etc).
• No cross-talk:
The optic wave within the fiber is trapped; none leaks out
during transmission to interfere with signals in other fibers.
• Higher Security:
fibers offer higher degree of security and privacy.

ADVANTAGES OF FIBER
COMMUNICATIONS (3)
• Small size and light weight:
typical optical cable fiber dia 125m, cable dia 2.5 mm and
weight 6 kg/km. A coaxial cable (RG-19/U), outer dia 28.4 mm,
and weight 1110 kg/km.
• Less Corrosion:
Corrosion caused by water/chemicals is less severe for glass
than for copper.
• Less temperature sensitive:
Glass fibers can withstand extreme temperatures before
deteriorating. Temperatures up to 800 ̊C leave glass fiber
unaffected.

APPLICATIONS OF OF COMMUNICATIONS
• Telecommunications
Long-Distance Transmission
Inter-exchange junction
Fiber in the loop (FITL) -- FTTC, FTTB, FTTH
• Video Transmission
Television broadcast, cable television (CATV), remote monitoring, etc.
• Broadband Services
provisioning of broadband services, such as video request service,
home study courses, medical facilities, etc.
• High EMI areas
Along railway track, through power substations can be suspended
directly from power line towers, or poles.
• Military applications
• Non-communication fiber optics: e.g. fiber sensors.13 September 2016 19

BSNL
• Dark fiber is optical fiber infrastructure that’s in place, but not
being used
• Its like an unplugged electrical extension cord
• It can be ‘lit’ with active telecom equipment, when required by
TSPs or other end-users
• Provides significant cost savings and substantial time-
efficiencies to end users
• In India, companies registered as IP-I can provide assets such
as Dark Fiber.
The ‘Dark Fiber’ Concept

Series of Recommendations by the ITU-T, A to Z
G series: Transmission systems and media, digital
systems and networks
Some of the G series:
G.600-G.699: Transmission media and optical systems
characteristics
G.700-G.799: Digital terminal equipments
G.800-G.899: Digital networks
G.900-G.999: Digital sections and digital line system
ITU-T Recommendations

BSNL
G.600-G.699: Transmission media and optical systems
characteristics
G.600-G.609: General
G.610-G.619: Symmetric cable pairs
G.620-G.629: Land coaxial cable pairs
G.630-G.639: Submarine cables
G.640-G.649: Free space optical systems
G.650-G.659: Optical fibre cables
G.660-G.679: Characteristics of optical
components and subsystems
G.680-G.699: Characteristics of optical systems
ITU-T Recommendations

BSNL
Fiber optic principle

Ray Theory:
• A number of optic phenomena are adequately explained by considering light
as narrow rays.
• The theory based on this approach is called geometrical optics.
• These rays obey following rules:
1. In a vacuum, rays travel at a velocity of c = 3x108m/s. In any other medium,
rays travel at a slower speed, given by
v = c/n n = refractive index of the medium.
2. Rays travel straight paths, unless deflected by some change in medium.
3. If any power crosses a medium-boundary, the ray direction is given by
Snell’s law:
n1 sin θi = n2 sin θr
THEORY OF FIBER OPTICS

INCIDENT RAYS
REFLECTED RAYS
REFRACTED RAYS
1
1
3
2
2
3
n2
θr
θi
Principle of Total Internal Reflection
n1 = 1.465
n2 = 1.461
n1

THE OPTICAL FIBER
Cladding (n2)
125 mCore (n1)6-10 m
Refractive index n1 > n2

3
2
1
3
2
1
LIGHT PROPAGATION IN FIBER
Core (n1)
Cladding (n2)

Air 1.0
Carbon dioxide 1.0
Water 1.33
Ethyl alcohol 1.36
Magnesium fluoride 1.38
Fused silica 1.46
Polymethyl methacrylate polymer 1.5
Glass 1.54
Sodium chloride 1.59
Zinc sulfide 2.3
Gallium arsenide 3.35 Silicon
3.5
Indium gallium arsenide phosphoide 3.51
Aluminium gallium arsenide 3.6
Germanium 4.0
Index of Refraction in
different materials

DUAL NATURE OF LIGHT
Wave Nature of Light :
• Many light phenomena can be explained by realizing that light is an
electromagnetic wave having very high oscillation frequencies.
• The wavelength of light beam:
 = v/f
{where, v = velocity of light , f = frequency}
Particle Nature of light :
• Sometimes light behaves as though it were made up of very small particles
called photons. The energy of a single photon in Joules is:
Wp = hf
{where, h = 6.626 x 10-34 js [Planck’s constant], f = frequency}

RELATION BETWEEN Λ AND REFRACTIVE
INDEX
WHEN LIGHT WAVES ENTER A MEDIUM, THEIR
WAVELENGTH IS REDUCED BY A FACTOR EQUAL
TO THE REFRACTIVE INDEX N OF THE MEDIUM BUT
THE FREQUENCY OF THE WAVE IS UNCHANGED
if λ0 is the vacuum wavelength of the wave the
wavelength of the wave in the medium, λ' is
given by

1015
1014
1013
1012
1011
1010
109
108
107
106
105
104
103
102
101
RADIO
POWER
MICROWAVE
ULTRAVIOLET
INFRARED
Electromagnetic Spectrum
V I S I B L E L I G H T
UHF
VHF
HF
MF
LF
VLF
Hz

OPTICAL SPECTRUM
• Light
• Ultraviolet (UV)
• Visible
• Infrared (IR)
• Communication wavelengths
• 850, 1310, 1550 nm
• Low-loss wavelengths
• Specialty wavelengths
• 980, 1480, 1625 nm
UV IR
Visible
850 nm
980 nm
1310 nm
1480 nm
1550 nm
1625 nm

125 GHz/nm

• Visible light has a wavelength range of 0.4-0.7 m
• Silica glass fiber attenuates light heavily in visible &
UV regions.
• Glass fiber is relatively efficient in wavelength ranges
upto and in the infrared region.
• Three windows of operation are at 850, 1310 and
1550 nm.
Window Concept in OFC Spectrum

WINDOW CONCEPT IN
OFC SPECTRUM

First Window
This is the band around 800-900 nm. This was the first
band used for optical fiber communication in the 1970s
and early 1980s.
The fiber losses are relatively high in this region,
Therefore, the first telecom window is suitable only for
short-distance transmission.
This window was relevant only for the initial silica fiber,
which had different attenuation characteristics
compared to low loss fiber developed later on.

Second Window
This is the window around 1310 nm which came into use
in the mid 1980s. This band had the property of zero
dispersion of light waves(on single-mode fiber).
The fiber attenuation in this window is about 0.35-0.4
dB/km.
This is the band in which the majority of long distance
communications systems were designed.

Third Window
The window from around 1510 nm to 1625 nm has the
lowest attenuation available on current optical fiber
(about 0.26 dB/km). In addition optical amplifiers are
available which operate in this band.
Almost all new communications systems, from the late
1990s operate in this window.
The loss peaks at 1250 and 1400 nm are due to traces
of water in the glass.

WAVELENGTH BANDS USED IN OFC
BAND DESCRIPTION WAVELENGTH RANGE
nm
O Band Original 1260-1360
E Band Extended 1360-1460
S Band Short wavelength 1460-1530
C Band Conventional 1530-1565
L Band Long wavelength 1565-1625
U Band Ultra long wavelength 1625-1675

• The potential transmission capacity of optical fiber is
enormous.
• The second window is about 100 nm wide and ranges from
1260 nm to 1360 nm (loss of about 0.4dB/ km). The third &
fourth window is around 100 nm wide and ranges from
1530 nm to 1625 nm (loss of about 0.26 dB/km).
• The useful range is therefore around 200 nm.
• A λ-range of 100nm will correspond to a frequency
bandwidth of 30 THz (on a centre wavelength of 1000nm).
• Assuming that a modulation technique resulting in 1 bit/Hz
of analog bandwidth is available, then we can expect a
digital bandwidth of 3 ×1013 bits per second (30Tbps)!

BSNL
G.655 standard OF cable
•Single mode
•1550 nm
•Carries up to 200 λ
DWDM
•10 Gbps to 40 Gbps per λ- commercially deployed
•100G and beyond 100G products are under
development.
Example:
Bharti-Singtel Chennai-Singapore Submarine
OFC link is 104λ x STM-64 ! (as in 2004-05)
working on G.655 NZDSF
Current trends

BSNL
Bell Labs in Sep’2009 announced ultra-high speed
transmission of more than 100 Petabits per
second.kilometer, shown over a distance of 7000kms.
155 λ x 100G DWDM was used for the experiment.
Employs Advanced DSP with Coherent Detection.
Corning Inc. has developed a new multi-core fiber
design. In Jan’2013, NEC Labs and Corning announced
transmission speeds upto 1.05 Pb/s over 52.4km of a
single 12-core fiber.
Latest trends

BSNL
Classification of
Optical Fibers

CONSTRUCTION OF OPTICAL FIBER
• Basic fiber has a core
with refractive index n1
surrounded by cladding
layer with refractive
index n2, n1 > n2
• Change in RI is achieved
by selectively doping the
core (like with GeO2).
• The difference between
n1 and n2 is less than
0.5%.
• The cladding layer is
surrounded by one or
more protective coating.
CORE
CLADDING
n2
n2
n1 > n2n1 > n2

CLASSIFICATION OF OPTICAL FIBER
Material Classification:
• Liquid core fiber.
• Fused-silica-glass fiber: have silica-core and silica-cladding.
• Plastic-clad-silica (PCS) fiber: have silica core and plastic cladding.
• All-plastic fiber : have both core and cladding made up of plastic.
• Compound glass fiber such as fluoride glass fiber.
Modal classification :
• Fibers can be classified based on number of modes available for
propagation - Single-mode (SM) fiber
- Multi-mode (MM) fiber
Classification based on refractive index profile :
• Step index (SI) fiber
• Graded index (GRIN) fiber

BSNL
CLASSIFICATION OF OPTICAL FIBER
Single-mode (SM) fiber
• one mode of light at a time through the core
• modal dispersion is greatly reduced
• a higher bandwidth capacity
 Multi-mode (MM) fiber
• has larger core, than the SM fiber
• numerous modes or light paths, can be carried simultaneously
through the waveguide.
oStep Index (SI)- there is a step in the refractive index at the
core and cladding interface
oGraded-index (GRIN) refers to the fact that the refractive index of
the core is graded- it gradually decreases from the center to
outward of the core; reducing modal dispersion.

BSNL
CLASSIFICATION OF OPTICAL
FIBER
8-12 m 125m
50 - 100m 125m
50 m 125m
c) Multi mode GRIN fiber (Graded-Index)
b) Multi mode step-index fiber
a) Single mode step-index fiber
Index profile
Index profile
Index profile

BSNL
13 September 2016
50
Output of Single mode fiber
Output of Multi-mode fiber

BSNL
OPTICAL FIBER CABLE
TYPES

CABLING OF OPTICAL
FIBER
• Cabling is needed to protect the fiber from mechanical
damage and environmental degradation.
• OF Cables have following common parts-

BSNL
OF CABLE CROSS SECTION
1.Optical fibre
2.Central
strength
member
3.Filling
compound
4.Loose tube
5.Filler
6.Wrapping
tape
7.Optional
aramid or
glass strength
members
8.Sheath

BSNL
CABLE COMPONENTS
Component Function Material
Buffer/ loose tube
buffer
Protect fibre From Outside Nylon, Mylar, Plastic
Central Member
Facilitate Stranding,
Temperature Stability, Anti-
Buckling
Steel, Fibre glass
Primary Strength
Member
Tensile Strength (pulling,
shearing, and bending)
Aramid Yarn, Steel
Cable Jacket
Contain and Protect Cable Core
Abrasion Resistance
polyethylene, polyurethane,
polyvinyl chloride or teflon.
Cable Filling
Compound
Prevent Moisture intrusion and
Migration
Water Blocking Compound
Armoring
Rodent Protection, Crush
Resistance
Steel Tape

• Centre Strengthening
Member – GRP(glass
reinforced plastic),
FRP(fiber reinforced plastic)
• Loose Tube Buffers – 2.4
mm dia, Fibres are
placed inside along with
jelly.
• Primary Strength Member –
Aramid Yarn
• Inner Sheath – Black
• Outer Nylon Sheath -
Orange
OF Cable Construction

BSNL
Loose Tube Buffers
•The Fibers are loosely drawn inside the Buffer
Tubes to take care of Temp. variations
•The OF Cable which is used outside is known as
Loose Tube Buffers
•The Correction Factor is 0.98/0.985
 980 meters of OFC will contain 1000 meters
Fiber inside (Cable length is less by 1.5 to 2%)

OPTICAL FIBER CABLE
TYPES
• Conventional Loose-tube OFC
• Armoured OFC (Underground Installation - Directly Buried)
• Aerial Optical Fibre Cables
• Ribbon OFC – high/very high fiber-count, for OAN
• Micro-duct OFC- high fiber count in same duct
• ADSS(All-Dielectric Self-Supporting) – aerial installations
• OPGW (Optical Ground Wire)- power line installations

BSNL
Construction Of Cable

BSNL
FIBER COUNT IN CABLE
•6 fiber
•12 fiber
•24 fiber
•48 fiber
•96 fiber
Standard OFC length on drum is
2000M (2Km). Other drum lengths
like 4km are also available.

BSNL
RIBBON-FIBER CABLE

BSNL
MICRODUCT OFC

BSNL
Armored Cable
Aerial Cable/Self-
Supporting

BSNL
ADSS CABLE
(33 KV)

BSNL
OPGW
(400 KV)

BSNL
These are tight Buffered cable
•Has only one fibre per cable
•Connector ended
•Used in the indoor
applications
•Connecting equipment to
outside OFC cable
•Connecting meters to the
equipment
micro
meter
Pig Tail Cable

BSNL
Specification Of OFC
Fibre -
Core - 8-10 Microns (Single Mode)
50 - 100Microns (Multimode)
Cladding - 125 Microns (overall Dia)
Attenuation - better than 0.5 db /KM
Primary Coating 250 Microns UV cured Acrylate
Secondary Coating –2.4 mm nylon PE Jelly filled tube
Central Strength Member – Fibre Reinforced Plastic (FRP)
Moisture Barrier- non metallic polythylene sheet free from
pinholes and other defects
Polythene sheath Polythene free from pin holes

BSNL
Nylon Outer Sheath (0.7mm thickness)- Protective sheath against
termite & partially against rodent
Strength to withstand a load - 3X9.8 W Newtons, where W is
weight of O/F cable per KM in Kg
MAX Strain allowed in fibre - 0.25%
MAX attenuation variation - Permissible + 0.02 dB from
normal 20 degree centigrade to 60 degree centigrade
Flexibility – Maximum bending radius allowed 24d, d is the
diameter of OF cable
Cable drum lengths - 2 KM +10%
Cable ends - one end fitted with grip
Other end sealed with cap
Specification Of OFC

BSNL
OF Cable jointing

OF CABLE JOINTING
Jointing of optical fiber is imperative in fiber communication.
For this the following are used-
CONNECTORS
COUPLERS
SPLICES

OPTICAL FIBER CONNECTORS
CONNECTORS USED FOR ARRANGING TRANSFER
OF OPTICAL ENERGY FROM ONE FIBER OPTIC
COMPONENT TO ANOTHER IN AN OPTICAL FIBER
SYSTEM
COMPONENTS INCLUDE FIBER, FILTER, COUPLER,
OPTO ELECTRONIC DEVICES ETC.

BSNL
FIBER
CONNECTO
RS

COUPLERS
FIBER OPTIC COUPLERS EITHER SPLIT OPTICAL SIGNALS
INTO MULTIPLE PATHS OR COMBINE MULTIPLE
SIGNALS ON ONE PATH.
. THE NUMBER OF INPUT AND OUTPUT PORTS,
EXPRESSED AS AN N X M CONFIGURATION,
CHARACTERIZES A COUPLER
. FUSED COUPLERS CAN BE MADE IN ANY
CONFIGURATION, BUT THEY COMMONLY USE
MULTIPLES OF TWO (2 X 2, 4 X 4, 8 X 8, ETC.).

SPLITTERS
THE SIMPLEST COUPLERS ARE FIBER OPTIC
SPLITTERS
. THESE DEVICES POSSESS AT LEAST THREE PORTS
.
A TYPICAL
‘T’COUPLER

SPLICES
SPLICE IS A PERMANENT INTERCONNECTION BETWEEN TWO
FIBERS
TWO TYPES OF SPLICES –
•Mechanical splice
•Fusion splice

FIVE GENERAL STEPS TO COMPLETE
FUSION SPLICE
1. STRIP, CLEAN & CLEAVE
2. LOAD SPLICER
3. SPLICE FIBERS
4. DIAGNOSE AND CORRECT IF ERRORS OCCUR
5. REMOVE AND PROTECT SPLICE

BSNL
CLEAVER

BSNL
LOAD SPLICER

BSNL
RIBBON FUSION SPLICER

LAYING OF CABLE
• Optic fiber cables are laid underground as well as
overhead.
• Underground laying is much frequent practice.
• Over ground laying is used in special cases
• A large collection of accessories are required to make a
strong and reliable overhead OFC alignment.
• Sometimes ordinary overhead alignments are erected for
emergent situations

PROPERTIES OF OPTICAL
FIBER AND
TRANSMISSION
IMPAIRMENTS

LOSSES IN OPTICAL FIBERS
• There are several points in an optic system where
losses occur.
• These are:
•couplers
•splices
•Connectors
•Fiber itself

CLASSIFICATION OF FIBER
LOSSES
• Losses due to absorption.
• Even the purest glass will absorb heavily within specific
wavelength regions. Other major source of loss is impurities
like, metal ions and OH ions.
• Losses due to scattering:
• caused due to localized variations in density, called Rayleigh
scattering and the loss is:
L = 1.7(0.85/)4 dB/km  is in micrometers
• Losses due to geometric effects:
• micro-bending
• macro-bending
• Losses are also termed as
Attenuation in a fiber

BSNL
LOSSES DUE TO MICRO BENDING

BSNL
LOSSES DUE TO MACRO BENDING

DISPERSION IN FIBER
• Dispersion is spreading of the optical pulse as it travels down
the length.
• Dispersion limits the information carrying capacity of fiber
• Dispersion is classified as : Chromatic Dispersion , Modal
Dispersion, and PMD
• Chromatic dispersion consists of:
• Material Dispersion
• Waveguide Dispersion
• Modal Dispersion:
• pulse spreading caused by various modes (only for MM
fiber).
• For visible light, refraction indices n of most transparent
materials (e.g., air, glasses) decrease with increasing wavelength
λ

BSNL
CONSEQUENCES OF DISPERSION

BSNL
MATERIAL DISPERSION
• Pulse spreading caused due to variation of velocity with
wavelength
• Every laser source has a range of optical wavelengths;
figure shows examples for LD and LED laser sources

BSNL
MATERIAL DISPERSION
FIBER
LOGIC 1 LOGIC 1
λ1
λ
λ
λ
λ
λ
0
2
1
0
2
λλ λ λ1 0 2
1.0
0.5
Light source
spectrum

BSNL
HOW TO REDUCE MATERIAL
DISPERSION?
• By using sources with smaller band width or spectral
width
LED 20-100 nm
LD(semiconductor) 1-5 nm
YAG laser 0.1 nm
He Ne laser 0.002nm

BSNL
WAVEGUIDE DISPERSION
• The figure below shows the light distribution inside the
fiber (in the core and cladding) for different wavelengths
• Dispersion directly proportional to wavelength
.

BSNL
CHROMATIC DISPERSION
13 September 2016 93For G.652 fiber, CD is nil at 1310nm

BSNL
POLARIZATION MODE DISPERSION
(PMD)
Most single-mode fibers support two perpendicular
polarization modes, a vertical one and a horizontal one.
Because these polarization states are not maintained,
there occurs an interaction between the pulses that results
is a smearing of the signal.
PMD has more impact on higher bit-rates, more than
10Gbps.

BSNL
DISPERSION COEFFICIENTS
• CD Coefficient
- CD Coefficient, indicated as D, is expressed in ps/(nm.km).
- It specifies the arrival time delay in picoseconds, that would be
included per 1km of the transmission fiber if the wavelength
deviates by 1nm.
• PMD Coefficient
- It is indicated by PMDQ and the unit is ps /(km)-1/2

BSNL
OPTICAL SOURCES
AND
DETECTORS

OPTICAL SOURCES
• The basic elements in transmitters: Light source, Electronic interfaces,
Electronics processing circuitry, Drive circuitry, optical interfaces, output
sensing and stabilization, Temperature sensing and control.
• Most common light sources (the device which actually converts electrical
signals to its optical equipment) :
• LEDs
• LASER diodes.
• Laser power is very sensitive to temperature. Hence temperature sensing
and control is required
• Operating characteristics of a laser are notably, threshold current, output
power, and wavelength change with temperature 13 September 2016 97

BSNL
LED VS LASER DIODE
LED - LIGHT EMITTING DIODE
- Shorthaul and medium haul
communication systems where
- Power requirements are small
- Low bit rate optical communication
- broad spectral width is not a problem
LD - LASER (Light Amplification by Stimulated
Emission of Radiation) Diode
- Used for long distance and high bit-rates
-very narrow spectral width (0.1 to 2nm)
Cooled DFB Lasers are available in precisely selected s
(for DWDM applications)

BSNL
LASERS
• Active Transmit device—Converts electrical signal into light
pulse.
• Conversion, or modulation is normally done by externally
modulating a continuous wave of light or by using a device
that can generate modulated light directly.
• Light source used in the design of a system is an important
consideration because it can be one of the most costly
elements.
• Its characteristics are often a strong limiting factor in the
final performance of the optical link
• Light emitting devices used in optical transmission must be
compact, monochromatic, stable, and long-lasting.

BSNL
SEMICONDUCTOR LASERS
• Two type
• Febry Perot- Normally used in SONET/SDH systems
• Distributed Feedback- well suited for DWDM applications, as it
emits a nearly monochromatic light, is capable of high speeds, has
a favorable signal-to-noise ratio.
• The ITU draft standard G.692 defines a laser grid for
point-to-point WDM systems based on 100-GHz
wavelength spacing with a center wavelength of 1553.52
nm

BSNL
EXTERNAL MODULATION IN DFB LASER

DETECTORS
• The basic elements in an optical receiver: Detector,
Amplifier, Decision circuits
• The detectors used in fiber optic communications are
semiconductor photodiodes or photo detectors.
• It converts the received optical signal into electrical form.
• PiN photodiode: cheaper, less temperature sensitive,
and requires lower reverse bias voltage.
• Avalanche PhotoDiode (APD): used where high
receive sensitivity and accuracy is required.
• But APDs are expensive and more temp sensitive

BASIC FIBER OPTIC
COMMUNICATIONS SYSTEM
An Optical Fiber System consists of :
a transmitter to convert electrical signals to optical
a receiver to convert optical signal to electrical
a medium - optical fiber cable.

BSNL
• Decibels (dB): unit of level (relative measure)
• X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501
• Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power
and represents loss or gain.
• Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt
X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW
• Wavelength (): length of a wave in a particular medium. Common
unit: nanometers, 10-9m (nm)
• 390nm (violet) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm
• Frequency (): the number of times that a wave is produced within a
particular time period. Common unit: TeraHertz, 1012 cycles per
second (Thz)
• Wavelength x frequency = Speed of light   x  = C
Some terminology

BSNL
• Attenuation = Loss of power in dB/km
• The extent to which optical power from the source is diminished as it passes
through a given length of fiber-optic (FO) cable, tubing or light pipe. This
specification determines how well a product transmits light and how much cable
can be properly illuminated by a given light source.
• Optical Signal to Noise Ratio (OSNR) = Ratio of optical signal power
to noise power for the receiver. (OSNR = 10log10(Ps/Pn)).
Some more terminology

BSNL
DB VERSUS DBM
• dBm used for output power and receive sensitivity (Absolute Value)
A dBm is a specific measurement referenced to 10-3 watts or 1
milliwatt (mW). The calculation, where X is the measured power in watts,
for laser output measured in dBm:
Examples
10dBm 10 mW
0 dBM 1 mW
-3 dBm 500 uW
-10 dBm 100 uW
-30 dBm 1 uW

BSNL
DB VERSUS DBM
• dB used for power gain or loss (Relative Value)
For example, output power in Watts (A) compared to input
power in Watts (B) used to represent attenuation of a fiber
related to the Common (base 10) logarithm value:

BSNL
BIT ERROR RATE (BER)
• BER is a key objective of the Optical
System Design
• Goal is to get from Tx to Rx with a BER <
BER threshold of the Rx
• BER thresholds are on Data sheets
• Typical minimum acceptable rate is 10 -12

BSNL
OPTICAL BUDGET
Optical Budget is affected by:
• Fiber attenuation
• Splices
• Patch Panels/Connectors
• Optical components (filters, amplifiers, etc)
• Bends in fiber
• Contamination (dirt/oil on connectors)
Basic Optical Budget = Output Power – Input Sensitivity
Pout = +6 dBm R = -30 dBm
Budget = 36 dB

BSNL
OPTICAL LINK BUDGET
Pt - (Lcp+ Lct+ Lsp+ Lfb+ Msys)  Srec
where
Pt = light source transmitting power, in dBm
Lcp =coupling loss source to fiber, in dB
Lct =connector’s losses (2nos, source to fiber & fiber
to detector), in dB
Lsp =splicing losses, in dB
Lfb =fiber loss, in dB
Msys =system loss margin requirement, in dB
Srec =required PD receiver sensitivity, in dBm

BSNL
Transmitter Receiver
Fiber Fiber
Splice
Receiver Sensitivity
Margin
LINK POWER BUDGET
P
O
W
E
R

BSNL
An Optical Link is required to be commissioned between two Stations A & B.
Do the Power Budgeting. Check its feasibility. What is the Total Link Loss?
Data is given below :-
• Distance between two stations = 69 km.
• Splice Loss. = 0.1 dB / Splice.
• Connector Loss. = 1 dB / Connector.
• Coupling Loss (Source to fiber). = 3 dB.
• Laser Output. = 0 dBm.
• Receiver Sensitivity. = -37 dBm.
• fiber Loss. = 0.4 dB/km.
• System Margin. = 3 dB.
• Extra Cable to be kept at Joint = 20 m / Joint.
• fiber Length to be taken. = 102% of Cable Length.
• Shrinkage. = 1 %.
• Extra Cable at Terminals = 100m each
• Cable Length on drum = 2km /cable drum
EXERCISE

BSNL
Distance Between Station A & B = 69 km.
Cable Length after taking Shrinkage = 69x101% = 69.69 km.
Number of Cable Drums required = 69.69/2 = 35.
Total number of Splices in the Cable Route = 35- 1= 34.
Extra Cable to be kept at Joints = 20x34 = 680 m.
Leading-in Cable at Both Ends = 100+100=200m.
As the Cable Length exceeds 70 km, there will be one more Joint in the Route and
we need to provide additional 20 meter of cable at Joint Location, Hence :
Total number of Splices in the Link = 34+1= 35
Cable Length after keeping provision for Joint = 69.69+0.70+0.20
=70.59 km
Fiber Length. =70.59 x 102%
= 72.00 km.
SOLUTION
Contd…

BSNL
Link Loss:
Source to Fiber Coupling Loss = 03.00 dB.
Connectors Losses = 1 x 2 = 02.00 dB.
Fiber Loss = 0.4 x 72.0 = 28.80 dB.
Splicing Loss = 0.1 x35 = 03.50 dB.
Total link loss = 37.30 dB.
Laser Output – Link Loss = 0 – 37.30 = -37.30dBm.
Projected loss by including 3dB Margin = -40.30 dBm
Which is beyond Receiver Sensitivity level of -37 dBm.
Hence Link is NOT Feasible! 13 September 2016 115
SOLUTION

THANK YOU

Concepts of optical fiber communication

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (20)

Similar to Concepts of optical fiber communication

Similar to Concepts of optical fiber communication (20)

More from Naveen Jakhar, I.T.S

More from Naveen Jakhar, I.T.S (20)

Recently uploaded

Recently uploaded (20)

Concepts of optical fiber communication