Over view ofTransmission

Technologies &Optical Fiber

Communication by Naveen Jakhar, ITS

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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

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History of Transmission Systems

The developments…

• Open Wire Systems

• Coaxial Cables

• UHF Systems

• Microwave Systems

• Digital Transmission Systems

• Satellite Communication Systems

• Optical Fiber Cable

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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.

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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.

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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).

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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.

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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.

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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.

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Optical Fiber Communication

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• 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 MAURERDONALD KECK &PETER SCHULTZ

Optical Communications- Historical Perspective

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• 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

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HISTORICAL PERSPECTIVE (contd…)

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The Developments

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• 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

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• 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

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• 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 (2)

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• Small size and light weight:typical optical cable fiber dia 125m, 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.

Advantages of Fiber communications (3)

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Applications of OF Communications

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• TelecommunicationsLong-Distance TransmissionInter-exchange junctionFiber in the loop (FITL) -- FTTC, FTTB, FTTH

• Video TransmissionTelevision broadcast, cable television (CATV), remote monitoring, etc.

• Broadband Servicesprovisioning of broadband services, such as video request service, home study courses, medical facilities, etc.

• High EMI areasAlong 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.

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• 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

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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 equipmentsG.800-G.899: Digital networksG.900-G.999: Digital sections and digital line system

ITU-T Recommendations

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G.600-G.699: Transmission media and optical systems characteristics

G.600-G.609: GeneralG.610-G.619: Symmetric cable pairsG.620-G.629: Land coaxial cable pairsG.630-G.639: Submarine cablesG.640-G.649: Free space optical systemsG.650-G.659: Optical fibre cablesG.660-G.679: Characteristics of optical

components and subsystemsG.680-G.699: Characteristics of optical systems

ITU-T Recommendations

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Fiber optic principle

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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 byv = 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

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INCIDENT RAYS

REFLECTED RAYS

REFRACTED RAYS

1

1

3

2

2

3

n2θr

θi

Principle of Total Internal Reflection

n1 = 1.465n2 = 1.461

n1

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The Optical Fiber

Cladding (n2)

125 mCore (n1) 6-10 m

Refractive index n1 > n2

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3

2

1

Light Propagation in fiber

Core (n1)

Cladding (n2)

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Air 1.0Carbon dioxide 1.0Water 1.33Ethyl alcohol 1.36Magnesium fluoride 1.38Fused silica 1.46Polymethyl methacrylate polymer 1.5Glass 1.54Sodium chloride1.59Zinc sulfide 2.3Gallium arsenide 3.35 Silicon3.5Indium gallium arsenide phosphoide 3.51Aluminium gallium arsenide 3.6Germanium 4.0

Index of Refraction in different materials

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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}

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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

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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

UHFVHFHF

MF

LFVLF

Hz

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• 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 nm980 nm

1310 nm1480 nm

1550 nm1625 nm

125 GHz/nm

Optical Spectrum

• 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.

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Window Concept in OFC Spectrum

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Window Concept in OFC Spectrum

Window Concept in OFC Spectrum

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Window Concept in OFC Spectrum

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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.

 

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Window Concept in OFC Spectrum

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.

 

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Window Concept in OFC SpectrumThird 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.

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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

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Window Concept in OFC Spectrum

• 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)!

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G.655 standard OF cable

•Single mode•1550 nm•Carries up to 200 λDWDM•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

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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

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Classification ofOptical Fibers

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• 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.

Construction of Optical Fiber

CORE

CLADDING

n2

n2

n1 > n2n1 > n2

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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

CLASSIFICATION OF OPTICAL FIBER

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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.o Step Index (SI)- there is a step in the refractive index at the core

and cladding interfaceo Graded-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.

Classification of Optical Fiber

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8-12 m 125m

50 - 100m 125m

50 m 125m

c) Multi mode GRIN fiber (Graded-Index)

b) Multi mode step-index fiber

a) Single mode step-index fiber

Classification of Optical fiber

Index profile

Index profile

Index profile

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Output of Single mode fiber

Output of Multi-mode fiber

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Optical Fiber CableTypes

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• Cabling is needed to protect the fiber from mechanical damage and environmental degradation.

• OF Cables have following common parts-

Cabling of Optical Fiber

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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

                    

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Cable Components

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Component Function Material

Buffer/ loose tube

bufferProtect fibre From Outside Nylon, Mylar, Plastic

Central MemberFacilitate 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

ArmoringRodent Protection, Crush

ResistanceSteel 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

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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%)

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• 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

Optical Fiber Cable Types

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Construction Of Cable

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Fiber Count in Cable

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•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.

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Ribbon-fiber Cable

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Microduct OFC

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Armored Cable

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Aerial Cable/Self-Supporting

ADSS Cable(33 KV)

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OPGW (400 KV)

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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

micrometer

Pig Tail Cable

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Specification Of OFCFibre - 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

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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

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OF Cable jointing

OF Cable jointing

Jointing of optical fiber is imperative in fiber communication.For this the following are used-

CONNECTORSCOUPLERSSPLICES

OPTICAL FIBER CONNECTORS

Connectors used for arranging transfer of optical energy from one fiber optic component to another in an optical fiber systemComponents include fiber, filter, coupler, opto electronic devices etc.

Fiber connectors

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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

Mechanical splices

                                   

                                                                                                                                                            

FIVE GENERAL STEPS TO COMPLETE FUSION SPLICE

1. Strip, clean & Cleave2. Load Splicer3. Splice Fibers4. Diagnose and Correct If Errors Occur5. Remove and Protect Splice

CLEAVER

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LOAD SPLICER

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Ribbon fusion splicer

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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

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Properties of Optical Fiber and

Transmission Impairments

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Losses in Optical Fibers

• There are several points in an optic system where losses occur.

• These are: – couplers– splices– Connectors– Fiber itself

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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

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Losses due to micro bending

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Losses due to macro bending

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• 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

DISPERSION IN FIBER

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Consequences of Dispersion

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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

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MATERIAL DISPERSION

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FIBERLOGIC 1 LOGIC 1

λ 1

λ

λ

λ

λ

λ

0

2

1

0

2

λλ λ λ1 0 2

1.0

0.5

Light source spectrum

How to reduce material dispersion?

• By using sources with smaller band width or spectral width

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LED 20-100 nm

LD(semiconductor) 1-5 nm

YAG laser 0.1 nm

He Ne laser 0.002nm

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

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.

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Chromatic Dispersion

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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.

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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

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Optical Sources and

Detectors

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• 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

Optical Sources

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LED vs LASER Diode

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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)May 1, 2023

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.

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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

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External Modulation in DFB Laser

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• 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

DETECTORS

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Optical Link Design

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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.

Basic Fiber Optic Communications system

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• 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 10log10(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

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• 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

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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:

Examples10dBm 10 mW

0 dBM 1 mW

-3 dBm 500 uW

-10 dBm 100 uW-30 dBm 1 uW

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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:

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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

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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

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Optical Link Budget

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Pt - (Lcp+ Lct+ Lsp+ Lfb+ Msys) Srec

where Pt = light source transmitting power, in dBmLcp =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

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Transmitter ReceiverFiber Fiber

Splice

Receiver Sensitivity

Margin

LINK POWER BUDGET

POWER

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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

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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…

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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!

SOLUTION

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Thank You

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