optical fiber communications. outline history types of fiber light propagation losses in optical...

Optical Fiber Communications

Outline

HistoryTypes of fiberLight propagationLosses in optical fiberOptical fiber classificationSourcesDetectorsOptical fiber system link budget

Introduction

EM waves are guided through media composed of transparent material

Without using electrical current flow

Uses glass or plastic cable to contain the light wave and guided them

Infinite bandwidth – carry much more information

History

Photophone Alexander Graham Bell Mirrors and detectors transmit sound wave via

beam light Awkward, unreliable, no practical application

Smoke signals and mirrorsUncoated fiber cables

1930, J.L. Baird and C.W. Hansell scanning and transmitting TV image

History

1951 – light transmission via bundles of fibers – leads to fiberscope – medical field

1958 – light amplification – stimulated emission 1960 – laser invention 1967 – fiber cable with clad 1970 – low loss optical cable. < 2 dB/km 1980 – optical cable refined – affordable optical

communication system 1990 – 0.16dB/km loss

History

1988 – long haul transmission system1988 – SONET 1990 – optical voice and data network are

common

Advantages

Wider bandwidth Better than metallic cables Up to several thousand GHz Speed up to several Gbps

Immunity to crosstalk glass fiber/plastic are non-conductor to electrical

current immune to adjacent cables

Immunity to static interference immune to static noise – EMI, lightning etc.

Advantages

Environmental Immunity more resistant to environment, weather variations wider temperature range operation less affected by corrosive liquids and gases

Safety and convenience safer and easier to install and maintain no current and voltage associated no worry about explosion and fire caused lighter and compact, flexible, lesser space required

Advantages Lower transmission loss

lesser loss compared to metallic cables 0.19 dB/km loss @ 1550 nm amplifiers can be spaced more farther apart

Security virtually impossible to tap into a fiber cable

Durability and reliability last longer, higher tolerance to changes in environment and

immune to corrosion Economics

Approximately the same cost as metallic cables less loss between repeaters. Lower installation and overall

system’s cost

Disadvantages

Interfacing cost Optical cable – transmission medium Needs to be connected to standards electronics

facilities – often to be expensive Strength

lower tensile strength can be improved with kevlar and protective jacket

glass – fragile – less required for portability Remote electrical power

need to be include electrical line within fiber cable for interfacing and signal regeneration

Disadvantages

Loss due to bending bending causes irregularities in cable

dimension – the light escapes from fiber core – loss of signal power

prone to manufacturing defect

Specialized tools, equipment and training tools to splice, repair cable test equipment for measurements skilled technicians

Optical Spectrum

Optical Communication systems

Types of fiber

Optical fiber construction

Types of fiber

Optical fiber construction special lacquer, silicone, or acrylate coating –

outside of cladding – to seal and preserve the fiber’s strength, protects from moisture

Buffer jacket – additional cable strength against shocks

Strength members – increase a tensile strength

Outer polyurethane jacket

Types of fiber

fiber cables – either glass, plastic or both Plastic core and cladding (PCP) Glass core – plastic cladding (PCS) Glass core – glass cladding (SCS)

Plastic core – more flexible - easier to install but higher attenuation than glass fiber – not as good

as glass Glass core – lesser attenuation – best propagation

characteristics but least rugged

Selection of fiber depends on its application – trade off between economics and logistics of particular application

Physics of light

Physics of light Einstein and Planck – light behaves like EM wave

and particles – photon – posses energy proportional to its frequency

energy of the photons

Planck constant

light frequency

pE

h

f

E hfp

p

p

E hf

hcE

light propagation

the lowest energy state – grounds state energy level above ground state – excited state if energy level decays to a lower level – loss of

energy is emitted as a photons of light The process of decaying from one level to another

– spontaneous decay or spontaneous emission Atoms can absorbs light energy and change its

level to higher level – absorption

2 1pE E E

light propagation

Optical power flow of light energy past a given point in a specified

time

= optical power

= instanteneous charge

= instanteneous change in time

(energy)(time)

P

dQ

dt

dPtdQdt

light propagation

Optical power generally stated in decibel to define power level (dBm)

Question 10 mW in dBm?

10log1

PdBm

mW

light propagation

Velocity of Propagation in vacuum – 3 x 108 m/s but slower in a more dense material than free space when it passes through different medium or from one

medium to another denser material – the ray changes its direction due to the change of speed

light propagation

from less dense to more denser material – the ray refracted closer to the normal

from more denser material to less denser material – the ray refracted away from the normal

light propagation

Refraction Occurs when the light travels between two

different material density and changes it speed based on the light frequency

Refractive Index the ratio of the velocity of propagation of a light

ray in a given material

light propagation

n = refractive index

c = speed of light

v = speed of light in a given material

cn v

light propagation

Snell’s Law how a light ray reacts when it meets the

interface of two transmissive materials that have different indexes of refraction

light propagation

Snell’s Law angle of incidence

angle at which the propagating ray strike the interface with respect to the normal

angle of refraction the angle formed

between the propagating ray and the normal after the ray entered the 2nd medium

light propagation

Snell’s Law

1

2

1

2

1 1 2 2

n = refractive index material 1

n = refractive index material 2

= angle of incidence

= angle of refraction

sin sinn n

light propagation

Question medium 1 – glass = 1.5 medium 2 – ethyl alcohol = 1.36 angle of incident – 30o

determine the angle of refraction

light propagation

Critical Angle the angle of incident ray in which the refracted

ray is 90o and refracted along the interface

light propagation

Critical Angle the minimum angle

of incident at which the refracted angle is 90o or greater

the light must travel from higher refractive index to a lesser refractive index material

light propagation

Critical Angle

21 2

1

2

2

1

1 2

1

sin sin

90

sin (1)

sin

c

c

n

n

n

n

n

n

light propagation

Acceptance Angle the maximum

angle in which external light rays may strike the air/glass interface and still propagate down the fiber

light propagation

Acceptance Angle

(max)

0

1

2

= acceptance angle

= refractive index of air

= refractive index of fiber core

= refractive index of fiber cladding

2 21 21sin(max)

0

in

n

n

n

n n

in n

1 2 2sin 1 2(max) n nin

light propagation

Numerical Aperture - NA to measure the magnitude of the acceptance

angle describe the light gathering or light-collecting

ability of an optical fiber the larger the magnitude of NA, the greater the

amount of external light the fiber will accept

light propagation

Numerical Aperture - NA

2 21 2

in

1

1

sin

θ = acceptance angle

NA = numerical aperture

n = refractive index fiber core

n = refractive index fiber cladding

1sin

inNA

NA n n

NAin

Optical Fiber Configurations

Mode of propagation single mode

only one path for light rays down the fiber

multimode many higher order

path rays down the fiber


Index Profile graphical presentation of the magnitude of the

refractive index across the fiber refractive index – horizontal axis radial distance from core – vertical axis


Index Profile

step index – single mode

step index – multimode

graded index - multimode

optical fiber classification

Single Mode Step Index dominant – widely used in telecommunications

and data networking industries the core is significantly smaller in diameter

than multimode cables


Multimode Step Index similar to single mode – step index fiber but the core diameter is much larger light enters the fiber follows many paths as it

propagate down the fiber results in different time arrival for each of the

path


Multimode Mode Graded Index non uniform refractive index – decreases

toward the outer edge the light is guided back gradually to the center

of the fiber


Comparison Single mode step index

(+) minimum dispersion – same path propagation – same time of arrival

(+) wider bandwidth and higher information txn. rate (-) small core – hard to couple light into the fiber (-) small line width of laser required (-) expensive – difficult to manufacture


Comparison Multimode step index

(+) relatively inexpensive, simple to manufacture (+) easier to couple light into the fiber (-) different path of rays – different time arrival (-) less bandwidth and transfer rate

Multimode graded index intermediate characteristic between step index

single and multimode

losses in optical fiber

Attenuation power loss – reduction in the power of light

wave as it travels down the cable effect on system’s performance by reducing:

system’s bandwidth information tx rate efficiency overall system’s capacity


Attenuation

(dB)

out

in

A = total reduction in power level

P = cable output power

P = cable input power

10log( )PoutA dB Pin


Attenuation depends on signal’s wavelength generally expressed as decibel loss per km dB/km

losses in optical fiber Attenuation

optical power in decibel units is

P(dBm)= Pin(dBm)-A(dB)P= measured power level (dBm)

Pin =transmit power (dBm)A= cable power loss, attenuation (dB)

t

P = measured power level

P = transmitted power level

A = cable power loss

l = cable length

/1010 AlP Pt


Question Single-mode optical cable input power 0.1 mW light source 0.25 dB/km cable loss determine

optical power 100 km from the transmitter side


Absorption Loss absorption due to impurities – absorb lights and

convert it into heat contributors:

Ultraviolet – ionized valence electron in the silica material. infrared – photons of light absorbed by glass’s atom –

converted into random mechanical vibrations - heating ion resonance – caused by OH- in in the material. OH-

trapped in the glass during manufacturing process


Absorption Loss


Material – Rayleigh, Scattering Losses permanent submicroscopic irregularities during

fiber drawing process when the light propagates and strike one of the

impurities, they are diffracted – causes the light to disperse and spread out

some continues down the fiber, some escapes via cladding – power loss


Chromatic – Wavelength, Dispersion Loss many wavelengths being txn. from LED each wavelength travels at different velocity arrives at end of fiber at different time resulting in chromatic distortion solution: using monochromatic light source


Radiation Losses loss due to small bends and kinks in the fiber two types of bend:

microbend – difference in the thermal contraction rates between core and cladding. Geometric imperfection along the axis.

constant radius bend – excessive pressure and tension during handling and installation


Modal Dispersion Losses pulse spreading difference in the propagation times of light rays

that take different path occur only in multimode fiber solution: use graded index fiber or single mode

step index fiber


Coupling Losses imperfect physical connection

three types of optical junctions:• Light source to fiber connection• Fiber to fiber connection• Fiber to photodetector connection

Caused by:• Lateral displacement• Gap dispalcement• Angular displacement• Imperfect surface


Coupling Losses Lateral Displacement

axis displacement between 2 pieces of adjoining fiber cable

amount of loss – couple tenth to several decibels Gap displacement – miss alignment

end separation the farther apart, the greater the light loss if the two fiber is spliced, no gap between fiber if the two fiber is joined with a connector, the ends

should not touch each other


Coupling Losses angular displacement (misalignment)

less than 2o, the loss will typically less than 0.5 dB imperfect surface finish

end fiber should be polished and fit together squarely


coupling loss

Light Sources

Light source for optical communication system

efficiently propagated by optical fiber sufficient power to allow light to propagate constructed so that their output can be

efficiently coupled into and out of optical fiber

Light Sources

Light Sources

LED p-n junction diode made from a semiconductor (AlGaAs) emits light by spontaneous emission

Sources Homojunction LED

p-n junction two different mixture of the same type of atoms

Heterojunction LED made from p type semiconductor material from one set

of atom and n type semiconductor material from another set

Burrus Etched well surface emitting LED for higher data rate the well helps concentrate the emitted light ray allow more power to be coupled into the fiber

ILD Injection Laser Diode

Sources

Light Detectors

PIN diodes light doped material between two heavily

doped n and p type semiconductor most common as light detector

APD avalanche photo diode more sensitive than PIN diode require less additional amplification

Detectors Characteristic of Light detectors

responsivity a measure of conversion efficiency of photodetector ratio of output current to the input optical power

dark current the leakage current that flows through photodiode when

there is no light input transit time

time of light induced carrier to travel across the depletion region of semiconductor

spectral response the range of wavelength values that a given photodiode

will respond light sensitivity

the minimum optical power a light detector can receive and still produce a usable electrical output signal

Lasers LASER-Light amplification stimulated by the emission of radiation --laser technology deals with the concentration of light into a very small,

powerful beam --there are 4 types of lasers 1)Gas lasers: Helium and Neon enclosed in a glass tube laser, CO2 lasers --Output is continuous mono chromatic (one colour) 2)Liquid lasers: organic dye enclosed in a glass tube for an active medium --A powerful pulse of light excites the organic dye 3)Solid lasers: solid, cylindrical crystal such as ruby, for the active medium.

Ruby is excited by a tungsten lamp tied to an alternating-current power supply.

--Output is continuous 4)Semiconductor lasers: Made from semiconductor p-n junctions and are

commonly called Injection laser diodes (ILD’s). -- a direct-current power supply controls the amount of current to the active

medium

Laser characteristics

All lasers use an active material to convert energy into laser light a pumping source to provide power or energy optics to direct the beam through the active material to

be amplified optics to direct the beam into a narrow powerful cone of

divergence a feedback mechanism to provide continuous operation an output coupler to transmit power out of the laser

optical fiber communications. outline history types of fiber light propagation losses in optical...

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