OPTICAL FIBRE COMMUNICATION

2011 AUGUST

Pcm equipment

Pcm equipment(2) contd

Global Capacity TrendWORLD INTERNET USAGE AND POPULATION STATISTICS

World RegionsPopulation (Est 2007) Million

Population % of the World

Internet Usage, Latest Data (Million)

% Population ( Penetration )

Usage % of World

Usage Growth 2000-2007

Africa 933 14.2% 34 3.6% 2.9% 643.1%

Asia 3,713 56.5% 437 11.8% 37.2% 282.1%

Europe 810 12.3% 322 39.8% 27.4% 206.2%

Middle East 193 2.9% 20 10.1% 1.7% 494.8%

North America 335 5.1% 233 69.5% 19.8% 115.2%

Latin America/Caribbean 557 8.5% 110 19.8% 9.4% 508.6%

Oceania / Australia 34 0.5% 19 54.5% 1.6% 146.7%

WORLD TOTAL 6,575 100.00% 1,173 17.84% 100.00% 225.00%

Source: www.internetworldstats.com

Current Estimated Capacities between Continents

North America335 Million

Latin America/Caribbean557 Million

Africa933 Million

Europe810 Million

Asia3.7 Billion

Oceania/Australia34 Million

Middle East193 Million

8Tbs

12Tbps6Tbps 5Tbps

6Tbps

9Tbps

2-3Gbps

Internet Usage by World Region

EXPLOSIVE GROWTH OF INTERNET

SEA-ME-WE 4 Cable System Configuration Diagram

FTTH, PON(passive Optical Network)

Power Line Communication System

LIGHT

Light is an is electromagnetic radiation of a wavelength that is visible to the human eye

CHARACTERISTICS OF LIGHT• Light is made up of , either electromagnetic

wave or particles called photons. Light can be considered as rays that follows straight line between or within optical elements, bending only at surfaces.

• Light is composed of electrical & magnetic fields, which vary in amplitude as they move through space together at the speed of the light. The two fields are perpendicular to each other and to the direction on which the light travels.

MEDIUM

Medium is a material substance which can transmit energy waves.

E.g. Air, Liquid, Solid

Liquid

Solid

Air

What is Optical Fibre?An optical fibre consists 3 different parts.

– Core– Cladding– Buffer Coating

Basic Theory :Light has to be confined to the core so that the digital signal can be transmitted from one place to another through light.

Basic Elements of the Optical Fibre System :

Light Source

Glass Fibre

LIGHT

Transmitter

ELECTRICITY

LED/LaserLightSource

Receiver

ELECTRICITY

Avalanche Photo Diode

• Refractive IndexLight changes its speed when it travels from one material to another, such as from air into glass. This cause an effect called refraction. Hence bending of the light at the surface of a material is expected. The speed of the light in the vacuum is highest.

Basic Theorems behind Optical Fibre (a)

• Snell’s LawSnell's law states that the ratio of the sines (Sin) of the angles of incidence and refraction is equivalent to the ratio of velocities in the two media, or equivalent to the opposite ratio of the indices of refraction.

Sin Ө 1 n 2 Sin Ө 2 n 1

Sin Ө 1 n 2

Sin Ө 2 n 1

n1 SinӨ1 = n2 SinӨ2

n1< n2

Ө1

Ө2

Medium 1n1

Medium 2n2

• When the refraction angle reaches 90°, there is no refraction and the incident angle reaches its critical angle (ΦC)

• When incident angle reaches the critical angle, there is no refraction.• Beyond the critical angle, light ray becomes totally internally reflected .

• Critical Angle & Total Internal Reflection

Basic Theorems behind Optical Fibre (b)

Φ1ΦC

Medium 2n2

Medium 1n1

Φ2

Total Internal Reflection

• According to the increase of the angle of incidence, the angle of refraction increases.

Special points to be considered in optical fibre (a)

1. Numerical Aperture (NA)– NA is defined as the sine of half the angle of a fibre’s light

acceptance cone (see Figure). – All modes of light entering the fiber at angles less than that which

correspond to the NA, will be bound or confined to the core of the fiber.

– The larger the NA of a fiber, the larger the light acceptance cone.

Air n0 = 1 Core n1

Cladding n2

αmax

ΦC

90 - ΦC

n0Sinαmax = n1Sin(90 - ΦC) n1Sin ΦC = n2Sin90

Numerical Aperture consideration

From Snell’s Law :

n1Sin ΦC = n2Sin90

Numerical Aperture Calculation

n0Sinαmax = n1Sin(90 - ΦC)

Therefore, nosinαmax = NA

NA determines the light gathering

capabilities of the fibre

What is Acceptance Angle?

Fibre accepting angle:

Question 1

A silica optical fibre with a core diameter large enough to be considered by ray theory analysis has a core refractive index of 1.50 and a cladding refractive index of 1.47

Determine: (a) the critical angle at the core cladding interface; (b) the NA for the fibre; (c) the acceptance angle in air for the fibre

Solution: (a) The critical angle at the core-cladding interface

(b) The numerical aperture is:

(c) The acceptance angle in air θa is:

E.g. The speed of light in water V(l) is 2.24 x 10 8 ms -1 and the speed of light in a vacuum V(V) is 2.99 x 10 8 ms -1. What is the Refractive index of Water(n w)?

V(l) = 2.24 x 10 8 ms -1

V(V) = 2.99 x 10 8 ms -1

Refractive index of water = V(V) V(l)

= 2.99 x 10 8 ms -1

2.24 x 10 8 ms -1

= 1.33

So The Refractive Index of water is 1.33 (No units).

What is Snell’s Law?• This describes the bending of light rays when it

travels from one medium to another.

Glass

Air

Air

Water

Snell's law states that the ratio of the sines (Sin) of the angles of incidence and refraction is equivalent to the ratio of velocities in the two media, or equivalent to the opposite ratio of the indices of refraction.

Sin Ө 1 n 2 =Sin Ө 2 n 1

Sin Ө 1 n 2 =Sin Ө 2 n 1

n 1 Sin Ө 1 = n 2 Sin Ө 2

PO - Ray of Incidence n 1 - RI for medium 1 OQ - Ray of Refraction n 2 - RI for medium 2Ө 1 - Angle of IncidenceӨ 2 - Angle of Refraction

TOTAL INTERNAL REFLECTIONn 1 Sin Ө 1 = n 2 Sin Ө 2

With the increase of the angle of incidence, the angle ofrefraction increases accordingly.

When reaches φ2 90°, there is no refraction and φ1 reaches a critical angle (φc )

Beyond the critical angle, light ray becomes totally internally reflected

EQUATIONS ASSOCIATED WITH RAY PROPAGATION

• CRITICAL ANGLE

• NUMERICAL APERTURE

LIGHT GATHERING CAPACITY• An optical fibre will pick up light from any source. However collecting

light for small core fibres will be a important step towards communication fibres. This means collecting light from one source and transferring that light to the optical fibre. This demands COUPLING light from core to the fibre, in a efficient way. Larger light sources are generally easy to align with fibres, but their lower intensity generally delivers less light. Transferring light between fibres requires careful alignment and tight tolerance. When two fibres’ are permanently joined are called SPLICING. Temporary joints made by two fibres are called as CONNECTORS. Special device named COUPLERS are needed to join 3 or more fibres. Losses in transferring signals in copper can be neglected but it is not so with fibres. Hence it should account for the losses deriving from coupling , from connectors, splices and the efficiency of the light source into the fibre.

NUMERICAL APPERATURE

• LIGHT GATHERING CAPACITY OF A OPTICAL FIBRE CABLE IS DEFINED AS THE NUMERICAL APPERTURE

• THE ACCEPTANCE ANGLE IS THE ANGLE WHERE THE OF SOURCE ISINTRODUCED TO THE FIBRE

• ACCEPTANCE ANGLE

Optical Fibre

• An optical fibre consists of two parts the core and the cladding• The core is a narrow cylindrical strand of glass and the cladding

is a tubular jacket surrounding it• The core has a (slightly) higher refractive index than the claddingTherefore, total Reflection of light ncore > ncladding

OPTICAL FIBRE FREQUENCIES

• Question 1: A silica optical fibre with a core diameter large enough to be considered by ray theory analysis has a core refractive index of 1.50 and a cladding refractive index of 1.47

• Determine: (a) The critical angle at the core cladding

interface; (b) The NA for the fibre(c) The acceptance angle in air for the fibre

This means the angle over which the light rays entering the fibre

, to be guided along it’s core. Acceptance angle is measured in air outside the fibre ,it defers from confinement angle of the

MODES OF FIBRE

• There are 2 main type of modes in optical Fibre

• SINGLE MODE STEP INDEX

• MULTIMODE STEP INDEX GRADED INDEX

TYPES OF FIBRE

Elements of Optical Transmission System

Optical Fibre Transmission System

Major components1. Modulator2. Light source3. Connectors (Couplers)4. Optical glass Fibre5. Light sensor/Detector6. Optical amplifiers/Repeaters7. Optical fibre joints (splices)

LIMITATIONS TO TRANSMISSION

ATTENUATION

Example

• To calculate the ratio of 1 kW (one kilowatt, or 1000 watts) to 1 W in decibels, use the formula

• Similarly for amplitude, current or voltage (power is proportional to the square of the above 3 quantities. )

Laser Output Power, Receiver Sensitivity and dBm

Example 1

Answer (Example 1)

• Connector loss= 8*1dB= 8dB• Cable loss= (4*100)/1000=0.4dB• System margin = 5dB• Sensitivity= -30 dB • Transmitter Power = connector loss+cable

loss+system margin+sensitivity • Therefore, 8 + 0.4 + 5 – 30 = -16.6dB

ReceiverTransmitter 8 Connectors

Example 2

Answer (Example 2)

• Connector loss= 2*1.5dB = 3 dB• Cable loss= 0.4dB * 50 = 20 dB• System margin = 8 dB• Sensitivity= -34 dB • Transmitter Power = connector loss+ cable loss + system

margin + sensitivity • = 3+20+8-34= -3 dB• No: of splices= 3/ 0.15 = 20 splices

Transmitter Receiver2 Connectors

Answer (Example 2)

• Connector loss= 2*1.5dB = 3 dB• Cable loss= 0.4dB * 50 = 20 dB• System margin = 8 dB• Sensitivity= -34 dB • Transmitter Power = connector loss+ cable loss + system

margin + sensitivity • = 3+20+8-34= -3 dB• No: of splices= 3/ 0.15 = 20 splices

Transmitter Receiver2 Connectors

Question

If the System Margin is -10dB, calculate the receiver sensitivity.

Give your overall observation?

Answer

Question on Practical system – 1

Long haul telephone optical fibre system consist with the following components

Part One Question Contd

Calculate the Power Budget and estimate whether the system will Function?

Part 1 Answer

Part Two Question – Use of Optical Amplifiers

The Previous system has been re engineered with two optical amplifiers as detail out below

Verify the Power budget for correct operation?

Part Two Answer – Use of Optical Amplifiers

Part Two Answer Contd – Use of Optical Amplifiers

Example 3 (f)Part Two Answer Contd– Use of Optical Amplifiers

Part Three Questions – Onsite Re engineering Problems

Due to site and optical amplifier operational problem, the above system was again re engineered as follows

By calculating the Power Budget verify whether the system can be operational or not?

Then the Calculation for segments 2 and 3 are given below;

Part Three Answer – Onsite Re engineering Problems

Part Three Answer Contd– Onsite Re engineering Problems

DISPERSION• Data carried in an optical fibre consists of pulses of light energy composed of a large number of frequencies travelling at a

given rate.• There is a limit to the highest data rate (frequency) that can be sent down a fibre and be expected to emerge intact at the

output.• This is because of a phenomenon known as Dispersion (pulse spreading), which limits the "Bandwidth” of the fibre.

Consequences of Dispersion• Frequency Limitation

• Distance : A given length of fibre, has a maximum frequency(bandwidth) which can be sent along it. To increase the bandwidth for the same type of fibre one needs to decrease the length of the fibre.

TYPES OF DISPERSION

DISPERSION

CHROMATICDISPERSION

MODALDISPERSION

MATERIALDISPERSION

WAVEGUIDE DISPERSION

CHROMATIC DISPERSION• It is a result of group velocity being a function of

wavelength. In any given mode different spectral components of a pulse travelling through the fibre at different speed.

• It depends on the light source spectral characteristics.

• May occur in all fibre, but is the dominant in single mode fibre

CHROSMATIC DISPERSION

Material dispersion - different wavelengths => different speeds

Waveguide dispersion – different wavelengths => different angles

MATERIAL DISPERSION

• Refractive index of silica is frequency dependent. Thus different frequency (wavelength) components travel at different speed

M is the material dispersion parameter. It characterizes the amount of pulse broadening by material dispersion per unit length of fibre and per unit of spectral width.

WAVEGUIDE DISPERSION• This results from variation of the group velocity with

wavelength for a particular mode. Depends on the size of the fibre.

• The angle between the ray and the fibre axis varying with wavelength which subsequently leads to a variation in the transmission times for the rays, and hence dispersion.

• This can usually be ignored in multimode fibres, since it is very small compared with material dispersion. It is significant in monomode fibres.

• Changing the design of the core-cladding interface can alter waveguide dispersion.

MODAL DISPERSION - SIMMF

• Lower order modes travel travelling almost parallel to the centre line of the fibre cover the shortest distance, thus reaching the end of fibre sooner.

• The higher order modes (more zig-zag rays) take a longer route as they pass along the fibre and so reach the end of the fibre later.

• Mainly in multimode fibres

The time taken for ray 1 to propagate a length of fibre L gives the minimum delay time:

The time taken for the ray to propagate a length of fibre L gives the maximum delay time:

Since The delay difference :

MODAL DISPERSION - GIMMF• The rays follow smooth curves rather than the

zig-zags of step-index fibres

Ray paths in a graded-index fibre

(a) a central ray;

(b) a meridional ray

(c) a helical ray avoiding the centre

MODAL DISPERSION - GIMMF• The intermodal dispersion is smaller than in

step-index fibres.• A helical ray, for example, although traversing

a much longer path than the central ray, does so in a region where the refractive index is less and hence the velocity greater

• To a certain extent the effects of these two factors can be made to cancel out, resulting in very similar propagation velocities down the fibres for the two types of ray

EQUATIONS FOR SINGLE MODE

• Number of modes

• Diameter

• Cut off wavelength

• For single-mode transmission: λ > λc• If λ < λc, two or more modes propagate

(multimode fibers)

EQUATIONS FOR SINGLE MODE

• Number of modes

• Diameter

• Cut off wavelength

• For single-mode transmission: λ > λc• If λ < λc, two or more modes propagate

(multimode fibers)

Wave division multiplexing

• 1. Concepts• 2.How to multiply the capacity in a given

optical fibre core by adding electronics at the terminal equipement without installation of new optical fibre systems

• Hence cost saving is evident in WDM or dense WDM (DWDM)

• Let’s study !!

Attenuation in Fibreoptical fibre behaves differently for different wavelength of light. The following diagram shows that. The three windows of wavelengths where the attenuation is lower is given

below. Hence these 3 windows are mostly used for practical purposes.

1. General Observation on Attenuation and the Present Day Technology

• Attenuation is low between 1500nm-1700nm in wavelength.• This gives rise to operate 24Tbps speed • How?

C=fλ where C=3*108

• And f1-f2=[c/(1500nm)]-[c/1700nm]=24Tbps• The present day technology goes up to 10Gbps or 40Gbps.

• STM1 STM4 STM16 STM64…… STM256 155.52Mbps 620Mbps 2.5Gbps 10Gbps 40Gbps

6.4ns 1.6ns 400ps 100ps 25ps

Present day technology adapting to the optical fibre

The following 2 major factors play a vital role in designing the maximum capacity of an optical fibre• How far the digital multiplexing can be achieved

• As at present , 488ns micro information of a bit pertaining to 2Mbps PCM stream will be reduced to 25ps when it goes through STM64 (10Gbps). If the technology improves to shrink less than 25ps , then the number of bits in the higher order PCM will be more than 10Gbps.

•To transmit 10Gbps, the optical fibre requires a bandwidth of around 0.078ns = 78ps ( for 1 wavelength)•If the available bandwidth in the optical fibre is 200ns , the number of wavelengths that can be produced is around 2400 , which will result in producing a total of 24Tbps.•Hence both Time Division Multiplexing and Dense Wave Division Multiplexing can further improve the traffic carrying capacity of an optical fibre up to a total of 24Tbps.

Optical Fibre

Optical Fibre

96

Overview of WDMDigital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Single Pair of Fibers

Single Pair of Fibers

Single Pair of Fibers

Single Pair of Fibers

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

WDM MUX WDM MUX

Single Pair of Fibers

Traditional Digital Fiber Optic Transport

Digital Fiber Optic Transport using WDM

Number of wavelengths = ( 24 * 103 Gb ) / 10 Gb = 2400 wavelengths

Future ScenariosTheoretical Maximum of an Optical Fibre Cable

Only 1 core is needed

Transponders

Optical Fibre

1 λ1

2 λ2

2399 λ2399

2400λ2400

TDM

10Gbps2Mbps

488ns 100 ps

Optical tools for maintanance

• OTDR• Splicing machine

Optical Time Domain ReflectometryPrinciple (OTDR)

Fusion splicing • It is the process of fusing or welding two fibers together usually by an

electric arc. Fusion splicing is the most widely used method of splicing as it provides for the lowest loss and least reflectance, as well as providing the strongest and most reliable joint between two fibers.

• Virtually all singlemode splices are fusion.• Fusion splicing may be done one fiber at a time or a complete fiber ribbon

from ribbon cable at one time. First we'll look at single fiber splicing and then ribbon splicing.