Optical Fibre Communications
- Birth and Destiny of a SuperHighway
November 2000
Optical Fibre Now Connects the World
• An incredibly successful innovation
What the Wheel did for Transport,
Optical Fibre has done for
Communications
• 60 million kilometres of optical fibre are
being installed around the Globe this year
So where did the story begin
-1
In the Beginning
• Existing copper technology offered insufficient
bandwidth
• 40 years ago work on guided optical communication
started in the Harlow laboratories in the UK
• Several potential guiding mechanisms were investigated
• By late 1965, Charles Kao and his team believed optical
fibre held most promise, and so they pioneered it’s use
for communication
• In 1966 they published the paper that started it all:
— “Dielectric-fibre surface waveguides for optical frequencies”; K.C.Kao & G.A.Hockham,
Proceedings of the IEE, Volume 113, No.7, July 1966
So how did we communicate before Optical Fibre?
-2
Early Optical Communication
Fire Heliograph
Aldis Lamp
-3
Copper Conductors - The Telegraph
• Low loss and
• No longer constrained by line of sight
-4
Copper Conductors - The Telephone
• End customers had direct access to a network
• Convenient human interface
-5
Copper Conductors
Coaxial Cable
• Lower loss at higher electrical carrier frequencies
• Carries many more speech channels than twisted-pair
cables
Multiple coax cable in 1970s:
Rate = 120 Megabit/second
Distance = 2 km
But:
• Bulky
• Performance Limited by Quantity & Conductivity of Copper
• Insufficient Capacity to meet expected demand
-6
Long Haul Microwave Waveguide
• Circular microwave waveguide
Propagates single HE01 Mode
Operated in the 30-110 GHz band
10 cm diameter
• Low loss:
2dB/km
• Anticipated to carry 100 x 500 Mbit/sec channels
(a total of 50 Gigabit/second)
But:
• Rigid
• Expensive to manufacture
• Difficult to install
Especially when compared to optical fibre
-7
Why Optical Communication?
Light is a very high frequency electromagnetic wave
— 600,000 Giga-Hertz “looks” Green
Why use Light instead of Electrical signals?
• Loss of Copper increases with frequency
• Dielectric guiding can be Lossless
• Laser just been invented
-8
Laser invention
Equivalent to a radio-wave oscillator but at Optical frequencies
• 1958
Schawlow and Townes first propose Concept
• 1960
First working Laser (Ruby)
• 1961
First Gas Laser (Helium Neon)
• 1962
First Semiconductor Laser (GalliumOptical
Arsenide)
Early Semiconductor Laser
Plus Human Hair
• The invention of the Laser stimulated research into
Optical Communication:
— High Brightness
— High Spectral purity
-9
Diversions Which Led Nowhere
1 - Optical Pipes
• 2.5 cm Steel pipes with silvered bore
• Theory predicted low loss at shallow angles
Roof pipe
to trybeside
to
optical
Harlow
stop solar heating
labs
But:
• High Loss in Practice, 133 dB/km
• Rigid
• Very Temperature sensitive
- 10
2 - Confocal Lens System
Multiple Lenses Spaced by Focal Length
Idea was to steer the
beam by deflecting
the previous lens
But:
• Alignment problems
• Large Bend Radius
• Very Temperature sensitive
- 11
Refraction and Total Internal Reflection
The Confocal lens system steered the
beam by Refraction at the lens surface
Total Internal Reflection can be Lossless
- 12
Fibres made of Solid Dielectric (Glass)
• Logie Baird is reported to have made bundles of bare
glass fibres in 1927
• Well known that a solid glass fibre would guide light, but
only if the surface is not touched
- 13
Large Core Glass Fibre with Cladding
• 1951 Idea first proposed for Light Guides, not for
Communications
— by Brian O’Brien, Van Heel, Moller Hansen, Lawrence Curtiss,
Poor glass caused
high loss in 1960s
- 14
Clad Optical Fibre
First Invented for Optical Fibre Bundles
This is an incoherent fibre
bundle (of many fibres),
used for illuminating
inaccessible locations
Coherent fibre
bundles are used in
fibre Endoscopes to
transfer images over
a metre or so
These fibre bundles use fibres with Large
Cores so they can collect more light
- 15
“Single Mode” Optical Guide
• If a guide is much narrower than wavelength of light l,
then light only propagates along a Single Mode or path
— The core is so small that the light spot is diffraction limited
• This enables much shorter pulses to be transmitted
Single Mode operation enables Higher Information Rates
- 16
Air-Clad Guide (Single Mode)
Guide thickness must be ~0.1 microns
•But
• Fragile
• High Optical Loss at Supports
- 17
Thin Film Waveguides
Supports
Thin dielectric film ~0.1
microns thick
1 cm wide
Lab experiments (1965), Prism coupling into,
and out of bent Thin-Film Waveguide
But:
• Loss
• No Guidance in plane of the film
• Only bends in one axis
- 18
The Visionaries
Their 1966 paper
that started it all
Charles Kao, measuring light
propagation over short fibre sample
Alec Reeves (in 1937
invented the concept of
digitising signals). He
initially directed the
team at Harlow, UK.
George Hockham
verified the concept
using microwave
theory and experiments
- 19
What the Paper Says
• Charles Kao and George Hockham became confident of the potential
of optical fibre as a future trunk transmission medium after detailed
study. They published their findings in the definitive paper in 1966*
• The waveguide structure:
— "Theoretical and experimental studies indicate that a fibre of glassy
material ... cladded structure with a core ....and an overall diameter of
about 100 l represents a practical waveguide with important potential as a
new form of communication medium.”
• The Loss:
— "The realisation of a successful fibre waveguide depends, ..on the
availability of suitable low-loss dielectric material. The crucial material
problem appears ...difficult but not impossible. Certainly, the required
loss figure of around 20 dB/km is much higher than the lower limit of loss
imposed by fundamental mechanisms.”
* "Dielectric-fibre surface waveguides for optical frequencies" Proc. IEE July 1966
- 20
Clad Single-Mode Fibre
• Very Small Glass Core surrounded by Glass Cladding of
lower refractive index
— Cladding Diameter = 125 microns
— Refractive index n1 = ~ 1.54, (n1 - n2)/n1 = ~ 1%
— Core Diameter = 6 microns
• Difficult to work with in the early days due to Small Core
- 21
Others believed it impossible in 1966
• Incredibly high Absorption Loss > 1000 dB/km
• High Scattering Loss
• Difficult to join fibre ends
Red light scattering out of the side of
the fibre due to imperfections
- 22
Major Breakthrough in Fibre Loss
• Early fibres made by Rod-in-Tube process
— Core rod assembled inside Cladding tube (like seaside rock)
• Could not push loss below ~200 dB/km
— Molten glass contaminated by crucible etc.
• In 1970 Corning Glass company invented a New Process
— They created ultra-pure glass in situ, by Oxidation of Silicon
TetraChloride and Germanium TetraChloride vapour
— These chemicals were already being produced in high purity for the
semiconductor industry
Low loss made fibre communications possible
- 23
Fibre Preform Manufacture
Deposition and Collapse into Preform
Deposition
— Reactant gases are passed through a
rotating Silica tube
Deposition
— The tube is heated by an Oxy-Hydrogen
burner that traverses its length
Collapse
— Further heating at a higher temperature
during rotation of the tube causes it to
collapse uniformly
Preform
— The short thick preform rod has a
refractive index profile identical to that
of the desired fibre, but many times
larger diameter
- 24
Fibre Drawing
• The Short Thick Preform is Drawn into a Long Thin fibre
• The bare fibre is immediately coated with Acrylate resin
to protect the surface
- 25
Fibre Loss Tumbled over the Years
• Absorption loss was reduced by Purifying the materials
— Eliminated Fe, Cu, Cr, Co, Water
• Eventually, Two Fundamental Limits were reached:
1 Infra-Red absorption of Silica at much longer wavelengths
2 Rayleigh scattering, ~ 4th power of optical frequency
— Pushed operation to longer wavelengths (1.5 microns)
— Same mechanism gives us Blue Sky and Red Sunsets
• Lowest loss fibre ever, was ~ 0.16 decibel per km in 1979
0.2 decibel per kilometre is typical today
- 26
Evolution of Fibre Loss v. Wavelength
In one decade the optimum operating
wavelength moved from 0.85 to 1.55 microns
- 27
Fibre Cables
• One or several fibres are then incorporated into a cable
• Provides further protection appropriate to the application
- 28
Splicing and Connectors
• Fibres are joined by butting the cleaved ends together
• Then fusing them together using an electric arc
cladding
core
fusion splice
Fibre Connector
- 29
Why use Optical Fibre?
• First Applications
— Immune to Electrical Interference
— No Radiated Signals
• Next:
— Occupied Less Duct Space
• Next
Fibre cable compared alongside
coax and multi-copper pair cables
— Longer Distances
• Now:
— Very Low Cost
— Very High Information capacity
- 30
Simple Fibre Transmission System
• Transmitter
— Converts electrical signal into modulated optical signal
• Fibre to Guide the Light
• Receiver
— Converts modulated optical signal back into electrical signal
- 31
Packaged Components
Photodiode and Low Noise
electrical pre-amplifier
Laser mounted in package
- 32
Fibre Transmission System 1967
STL/ Nortel Networks
Range 20 metres
Fibre Bundle of 27 fibres
Loss = 1200 dB/km at wavelength of 0.85 microns
Gallium Arsenide Light Emitting Diode, Silicon PIN Photo-Detector
Purpose:
Transmission of Video in high electrical interference environments
- 33
Fibre Transmission System 1973
Range: 2 kilometres
STL/ Nortel Networks
Single Fibre (Multimode)
Loss = 25 dB/km at wavelength of 0.85 microns
Gallium Arsenide Sawn-Cavity Laser, Silicon Avalanche Photodiode
Purpose:
Transmission of Video to show viability of Single fibre transmission
- 34
Fibre Transmission System 1977
Hitchin-Stevenage Field Experiment
Range 3 to 6 kilometres between Regenerators
Remotely powered Regenerators in roadside foot-way boxes
4 Fibre Cable (Multimode)
Loss = 5 dB/km at wavelength of 0.85 microns
6 Gallium Arsenide Narrow Stripe Lasers
STL/STC/Nortel Networks
6 Silicon Avalanche Photodiodes
Purpose:
To demonstrate practical operation in UK Telephone network
- 35
Optical Fibre Across the Ocean Floor
Signal regularly boosted by:
Regenerators ~ 100km
OR
Optical Amplifiers ~35 km
1980 First deep optical cable trial,
1988 First Transoceanic Optical fibre
Loch Fyne Scotland
system, North America - UK
- 36
Progress through the 1990s
• Total Capacity per fibre has increased Nearly 1000 Times
in the last decade:
— 1989
560 Megabits per second
— 1999
320 Gigabits per second (= 10 Gbit/sec x 32 Colours)
Two Key Innovations made it possible:
1
Optical Amplifiers
2
Wavelength Division Multiplexing
- 37
Optical Amplifiers
Few metres of special
optical fibre doped with
Erbium ions
Optical Fibre
Weak Light
Signal In
Pump Laser at
shorter wavelength
splices
Pump
Amplified Light
Signal Out
Optical Amplifier Module
- 38
Wavelength Division Multiplexing
Multi-Coloured transmission
• Several different coloured signals are combined into a
single light beam by means of a dispersive element such
as a prism or a grating
Single output to be
Modulated
Lasers
coupled into optical
transmission fibre
data
data
data
data
• We use as many as 160 different Infra-Red “Colours”
• These are passed through a Single fibre Simultaneously
• Signals separated the same way at the far end
- 39
Wavelength Division Multiplexing
• Frequency Division Multiplexing at Optical frequencies
- 40
Time Division Multiplexing
• We pass Many signals simultaneously through a Single
optical fibre (or copper conductor), by sampling each
sequentially in Time
- 41
State-of-the-Art Commercial system is:
10 Gbit/sec per Colour
Carries > 75% of US Internet Backbone Traffic
Jacksonville
- 42
10 Gbit/sec Being Deployed in Europe
Pari
s
UTA
- 43
Irish Fibre Network
New Esat Network - 10 Gigabit/s per Wavelength
Route
Distance Optical Amplifiers
• Fenian St –
Galway
205 km
3
• Galway
Limerick
118 km
1
• Limerick –
Cork
130 km
1
• Cork
–
Waterford 180 km
2
• Waterford–
Dundrum 250 km
3
• Dundrum –
Fenian St. 20 km
0
–
Control room
- 44
Fibre Ring Network - Irish Republic
• 4 Wavelengths used now
• Expandable up to 32 wavelengths (320 Gbit/sec)
Fenian St
4l
Galway
4l
Optical
Amplifier
Dundrum
4l
4l
4l
Optical
Amplifier
Optical
Amplifier
4l
• Optical signals travel both ways round the ring
Optical
Amplifier
4l
• This gives protection against Cable Cuts
Optical
Amplifier
4l
Optical
Amplifier
4l
Waterford
4l
Limerick
Cork
4l
Optical
Amplifier
Optical
Amplifier
4l
Optical
Amplifier
Optical
Amplifier
4l
4l
4l
Multiwavelength Optical Amplifier
Optical Wavelength Multiplexer
- 45
Optical Economics
1.6Tb/s
Capacity per Fibre
Cost per
Gigabit - Kilometre
320Gb/s
50Mb/s
1984
2.5b/s
1994
1998 2000
Growing Much faster
than Moore’s law
1993
1998
2002
And Benefiting
Customers
High Speed Transport R&D
• Continued Research to achieve still higher Bit rates/Colour
• We have Laboratory systems running at 40, 80 Gigabit/sec
And even 100 Gigabit/second
Light travels less than a millimetre
during the light pulse duration
I used to be impressed by the Speed of Light!
- 47
From One Colour to Many
• We have gone from a Single Track:
• To a Multi-Lane Super-Highway
— Product today uses 32 Colours (x10Gbit/sec)
— Next generation will carry 160 Colours
• And there are still demands to reduce congestion further
How will these demands be met?
- 48
How to get More through a Fibre Cable
• Higher Bit Rates per Colour
• More Colours
• More Optical Bandwidth & Higher Spectral Efficiency
If growth in demand continues at the present rate,
we will eventually run out of Optical bandwidth
• More Fibres in Parallel
— Multi-Fibre cables
— Typical cables in North America today contain ~50 fibres
— State-of-the-art multifibre cable contains 864 fibres within a 1inch diam.
But:
— Difficult to repair if someone accidentally cuts the cable!
- 49
What about the Future?
Research and Development continues to:
• Increase the information capacity per fibre
• Decrease the cost/bit of using this great capacity
• Increase the convenience of using Optical fibre
• Increase the (already high) reliability even further
- 50
To Conclude
I have described:
• The Invention of Optical Fibre Communications
• How the technology has grown up since its Birth
• Where it is heading now
- 51
Optical Fibre Communications
The Key Innovations:
• The Laser - a source of coherent optical waves
• Singlemode clad fibre as a communications waveguide
• Tiny semiconductor lasers operating at room temperature
• Low loss glass by vapour deposition from high purity
chemicals
• Fusion splicing of fibres
• Low pulse dispersion fibre designs
• Optical Amplifiers
• Wavelength Division Multiplexing
- 52
Thankyou
for Making it Happen
Richard Epworth