ECE 455 1

ECE 445 – Optical Fiber Communications

Lecture 01 - Introduction

• ECE 445

• Lecture 01

• Fall Semester 2016

Stavros IezekielDepartment of Electrical and

Computer Engineering

University of Cyprus

iezekiel@ucy.ac.cy

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Why is this course useful to you?

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• There are many ways we can answer this question.

• Perhaps the most obvious answer is to consider the social impact of optical

communications.

• In 1966, two engineers at Standard Telecommunication Laboratories in England wrote a

paper which in essence started the race to develop optical fibre for communications:

• One year later, the Philco-Ford company tried to predict the future; they were in many

ways very close to what we have.

https://www.youtube.com/watch?v=rpq5ZmANp0k

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All of this is made possible by a global infrastructure of optical cables

http://www.submarinecablemap.com/#/

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The aim of this course is to study optical fibre technology and its application to optical

communication links and systems

The basic questions we will seek to answer in the first few lectures include:

1. What are the advantages of using photonics for communications?

2. What is optical fibre?

3. What is an optical communications link? (Basic architecture)

4. How does light propagate in an optical fibre?

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WHAT IS LIGHT?

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"Light is waves on Mondays, Wednesdays, and

Fridays; it's particles on Tuesdays, Thursdays,

and Saturdays, and on Sundays, we think about

it!"

W.H. Bragg, 1930.

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Optics is one of the oldest branches of science.

It is concerned with the generation, propagation, manipulation and detection of light. For

many centuries, the development of optical sources and optical detectors was very slow,

hence progress was strongest in studies of light propagation and light manipulation, e.g.:

Refraction (ray optics) Interference

(wave optics)

Polarisation (electromagnetic optics)

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By the late 19th century, the theoretical work of Maxwell and the experiments of Hertz had

resulted in the electromagnetic view of light, in which it holds that light consists of

coupled time-varying electric and magnetic fields that satisfy a wave equation (which itself

can be derived from Maxwell’s equations):

c = speed of light = 2.998 × 10-8 ms-1 in vacuo

Solution is a travelling-wave:

λfc =

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However, the development of modern physics (and especially the work of Planck and

Einstein) led to the photon view of light.

Energy of a photon:

λ

hchfE ==

h = Planck’s constant = 6.626 × 10-34 J·s

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Light as photons: Photoelectric effect

• Increasing the intensity of the light increases the number of photoelectrons, but not their

maximum kinetic energy.

• Red light will not cause the ejection of electrons from potassium, no matter what the

intensity.

• Weak violet light will eject only a few electrons, but their maximum kinetic energies are

greater than those for intense light of longer wavelengths!

Explained by Planck relationship:λ

hchfE ==

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Electronic techniques Optics

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GapMicrowaves Photonics

• Nowadays, most electronic communication (e.g. wireless) is in the microwave region.

• Typically there is a three orders of magnitude difference between microwaves and photonics

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OPTICAL COMMUNICATIONS

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The most basic optical communication link:

Optical

sourceModulation Channel

Optical

detector

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Optical communications has a long history, having been used by many civilizations. One

example is the friktories of ancient Greece:

This was a very early

example of digital optical

communications.

Το σύστημα με τις Φρυκτωρίες παρουσιάζεται στον ειδικό

χάρτη όπου εμφανίζει με φωτεινές ενδείξεις τους πυρσούς

να ανάβουν και να μεταδίδουν το μήνυμα από την Τροία

στις Μυκήνες....

/

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• Claude Chappe’s Optical Telegraph (France)

• Based on a semaphore system

• Repeater spacing ≈ 6 miles

• Message could cover 100 miles in 30 minutes

• Bit rate < 1 bit/s

Digital optics, 1793-1852:

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© Alexander Graham Bell Foundation

Transmitter

• Light modulated by vibrating mirror

(i.e. opto-mechanical)

Bell’s photophone 1880

- Analogue optical link

Receiver

• Light is photodetected using selenium

(resistance decreases with increasing light

intensity)

• First example of optoelectronic receiver

https://www.youtube.com/watch?v=Kc9Mjzfowcs

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For some applications, such as satellite-

to-satellite free space optical links, this is

not a problem.

But for terrestrial free space

optical communications,

weather conditions have to be

considered:

One of the problems with these early systems was the fact that there was no guided

channel between the transmitter and receiver, in other words the channel was free-space

optics.

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Sunlight

Building motion

Alignment

WindowAttenuation

Fog

Each of these factors can attenuate the signal. However, there are ways to mitigate each environmental factor.

Scintillation

Obstructions

Low Cloud

Σπινθηροβολία

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An efficient way of guiding light is useful for modern long distance optical

communications….

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Kao and Hockham proposed the use of optical fibres for communications - 1966

More specifically, they showed that a target figure of 20 dB/km for fibre

attenuation would make optical communications viable. At this stage, losses were

way too high (1000 dB/km for glass, as opposed to tens of dB/km at most for

coaxial cable), but they showed this was mainly due to impurities.

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Optical

Loss

(dB/km)

Egyptian

Venetian

Optical glass

Optical fibre

Work at Corning in the early 1970’s eventually led to fiber losses of 20 dB/km, and

over time these have been reduced to as low as 0.2 dB/km (at 1550 nm).

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Fibre offers a lot of bandwidth!

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Optical Fibres: Basic Structure

•A circular core of refractive index n1 is surrounded by cladding with a slightly lower

value of refractive index (n2 < n1). The fibre is encapsulated by the buffer and

additional layers as appropriate.

• Light is confined to the core of the fibre by total internal reflection – TIR at the

core-cladding interface.

CORE CLADDING BUFFER COATING

Not to scale!

• dielectric waveguides that operate at optical wavelengths; mostly made from silica glass, but plastic versions (for multimode) also available

• confine electromagnetic energy in the form of light within core and guide the light parallel to the longitudinal axis:

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Advantages of optical fibre

• Very wide bandwidth compared to metallic transmission lines, i.e. potentially

thousands of GHz

• Very low loss (as low as 0.2 dB/km)

• Can achieve low dispersion (depends on wavelength of source and fibre type)

• Small size and weight

• Electrical isolation (glass and plastic)

A fiber-optic cable (right) containing 144 tiny glass

fibers is compared with a cross section of a

conventional copper cable.

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However, even though fibre itself is small in cross-section, in some applications the overall

cable is not so small or light:

A lot of optical fibre is installed in

undersea (submarine) systems,

and must be well protected.

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The basic ingredients of a classical communications link include:

• coherent oscillator (i.e. laser)

• mixer (e.g. directly modulated laser or modulator)

• envelope detector (e.g. photodiode)

However, there are other components (analogous to electronic

components) that are also used in optical communications:

• amplifiers (Erbium-doped fibre)

• couplers, combiners and splitters

• wavelength selective components – filters, multiplexers

• isolators and circulators

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Basic architecture of an optical fibre link

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Most fibre links are digital, and consequently we worry about bit rate – distance

products and bit error rates:

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Figures of merit

• The designers of a long distance high-bit rate fibre link have a number of

objectives.

• One is to achieve as high a bit rate as possible.

• However, it is also important to maximise the distance between optical amplifiers

or repeaters (i.e. the repeater spacing).

• The two figures are multiplied to give a key figure of merit used to assess link

performance:

Bit-rate - repeater spacing product (bits/s - km)

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Bit rate – distance product improvements