01 Optical Concepts

7/31/2019 01 Optical Concepts

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Optical Concepts


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2. 28 Optical Concepts

Milestones in Optics

1960 - Laser development

1966 - Fiber optics development1970 - First fiber production with attenuation of 20 dB/km

1972 - Production of fiber with attenuation of 4 dB/km

1976 - Mass production of fiber with attenuation

of 0.5 dB/km for 1310nm

1977 - First field trial using fiber optics in Chicago

1979 - Production of Single Mode fiber with attenuation

of 0.2dB/km for 1550nm

1985 - Mass production of Single Mode Fiber

1991 - Development of optical amplifiers


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Why Fiber Optics?

Capacity, Distance, Reliability

Microwave

Networks

FiberOptics

A communications network is one that conveys / transportsinformation (audio, video, and data) over substantialdistances between customer and network sites

Twisted Pair

Coax


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Network Providers combine the signals from different usersand send them over a single

transmission

Basic Network Information Rates

Examples of information rates for some typical voice,

video and data services:Video on demand/interactive TV 1.5-6 Mbps

Video games 1-2 Mbps

Remote education 1.5-3 Mbps

Electronic shopping 1.5-6 MbpsData transfer 1-3 Mbps

Video-conferencing 0.4-2 Mbps

Voice 64 Kbps


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The Optical Fiber

Very thin strands of pure silica

glass through which laser light

travels in optical networks


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The Optical Fiber


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The Light Theory

The Quantum nature

The wavelengthnature

Vs.

Of light



Energy levels: E1, E2,E3

E1 E3E2

Photon

The Quantum Nature of Light

When a photon insides on an atom, it transfers its

energy to an electron within this atom, exciting itto a higher energy level



Spontaneous emission

Absorption

Energy

E1

E2

E3

Ground Level

Excited State Level

The energy of the photon must be exactly equal to thatrequired to excite the electron to a higher energy level, to be

absorbedConversely, an electron in an excited state can drop to a lowerenergy state by emitting a photon, with exactly the sameenergy

This energy equals h, h: Plank constant, : photon frequency

E3-E2 h32

Photon Absorption and Emission



Spontaneous emission Stimulated emission

In spontaneous emission:thehigh energy state lifetime is betweennanoseconds to milliseconds

Instimulated emission:the emitted photon is identical in wavelength,

phase and direction to the incident photon

E1 E3E2

Photon

E1 E3E2

Photon

Photons Behavior



Population Inversion : E3 > E2Stimulated emission amplification

Amplification

Energy

E1

E2

E3

The amount of electrons in stimulated level must be higher than

those in ground level

Optical Amplification



The Wavelength Nature

Wavelength is the distance between identical points in theadjacent cycles of a waveform signal propagated in space or

along a wire

Wavelength is specified in nanometers (units of 10-9 meter)



Electromagnetic Spectrum



Wavelength Vs Frequency

The higher the frequency of the signal, the shorter its

wavelength:= c / f

c= 299,792,458 m/s

The standard unit of frequency is Hertz

If a wave completes one cycle per second, then

f = 1 Hz

1 THz = 10 cycles per second



Optical Parameters

Attenuation refers to any reduction in the strength of a signal

Attenuation occurs with any type of signal, sometimes called

loss

A natural consequence of signal transmission over long

distances

The extent of attenuation is usually expressed in dB:

dB = a common unit of measurement for the relative

difference between two power levels

dB = 10 log(Pout / Pin ) = 10 log Pout - 10 log Pin

dBm = a measure of absolute power

dBm = 10 log P(mw)

Characteristics of Different Wavelengths



Beginningof the 80s

Beginning

of the 90s Today

Characteristics of Different Wavelengthsin Pure Silica Fiber


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Optical Parameters

Dispersion refers to the expansion or widening of the

signal by the time it reaches the receiving end

Dispersion is due to the fact that different

wavelengths propagate in different velocities

Dispersion does not alter the wavelength (frequency)

but it directly affects the bit rate

Measured as the amount of delay in picoseconds

(10 - seconds) per km of fiber per nm change in the

wavelength


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Dispersion

Input pulses

Output pattern

Inter-symbol interference

Puls

eShapeandAmplitudes

Distance along fiber

1 10

1 0 1

1 0 1

111


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Dispersion Types

Modal dispersion

Material dispersionWaveguide dispersion

Polarization dispersion

Chromatic Dispersion

t

Before After

t


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Modal Dispersion

Depends on the diameter of the core and the critical

angleSignificant in Multi-mode fibers and not in Single-mode

fibers

Input Surface Refraction

CladdingCore

Jacket

t

1 1

t

Pulse entering

the fiber

t

11

Pulse exiting

the fiber

t


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Material dispersion

Effect of the fiber material on the propagation velocityof the wave

Waveguide dispersion relates to the ratio betweencore radius and wavelength

Input Surface Refraction

CladdingCore

Jacket


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Material dispersion and Waveguide dispersion

may act in opposite waysFiber is engineered in order to give a resultant

chromatic dispersion near to zero


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1

0

1 0 1 1 0 1 1 0 1 1 1 1

2 dB per 100 kmfor 2.5Gps

Dispersion Penalty

The dispersion of the

signal causes attenuation

Dispersion Penalty


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Polarization Dispersion

This kind of dispersion is significant in higher bitrates,

from 10 GbpsDue to manufacturing imperfections, the non-circular

core of the fiber may contribute to cause PD

Caused by several sources: core shape, external

stress, material properties, older fibers etc.


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G.653

G.655

G.652 /

G.654

Fiber ITU-T

Standard

l

nm

Dispersion

ps/(nm km)

Attenuation

dB/ km

Zero-Dispersion G.652 1310 0 < 0.5

Non-Zero-

Dispersion

G.655 1550 3 < 0.35

G.652 1550 18 < 0.4Zero-Dispersion

Wavelength Vs Dispersion

O


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1 0 1 0 1

Dispersion - Limits How Fast: ps/(nm.km)

1 0 1

Attenuation- Limits How Far: dB/km

1 0 0 0 0

1 1 1

Optical Parameters

The How far and How fast are not only fiber-dependent

LASER - Light Amplification by Stimulated


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28 28 O ti l C t

Typical Transmittance Profile of a Laser Diode

f1 f1 f1 f1

Transmittance

Thefrequencys lasers output is uniform

LASER- Light Amplification by StimulatedEmission of Radiation

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01 Optical Concepts