The Miracle of Optical Communication · 2| © 2016 Infinera Confidential & Proprietary The Number 1...
Transcript of The Miracle of Optical Communication · 2| © 2016 Infinera Confidential & Proprietary The Number 1...
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1 | © 2016 Infinera Confidential & Proprietary
The Miracle of Optical CommunicationGeoff BennettDirector, Solutions and Technology
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The Number 1 Rule
Know Your Audience
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?Fiber(Multi-Wavelength)
How do we carry all of this data?
Copper
Fiber(Single Wavelength)
Wireless(3G, 4G, 5G, WiFi, etc.)
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The Worldwide Web of FiberSubsea Cable Systems
Level(3)’s Network
North America
Interoute’s Network
Europe
Virgin Media Network
UK
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Light is a great way to communicate
Nothing travels faster…
Light can carry enormous
amounts of data
Light can travel enormous distances
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How fast does light travel?
300,000km per second*(So fast that for many centuries it wasn’t clear that it “travelled” at all!)
To see light travel we have two options…
Option 1: Slow Down Time
* in a vacuum
MIT Media Lab's Camera Culture Grouphttps://www.youtube.com/watch?v=-fSqFWcb4rE
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How fast does light travel?
To see light travel we have two options…
Option 1: Slow Down Time
Option2: Use very long distances
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You are here (Earth) The Moon
370,000km (1.3 light seconds)
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So light travels fast…now we need a good light source
The Sun?
A TorchA Laser!
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How about a semiconductor laser?
This is it! Ì Laser• Intense• Monochromatic• Coherent
Ì Semiconductor Laser• Small• Efficient• Mass produced• Totally reliable
Ì “Right kind of semiconductor”SiliconGa As
In P
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Let’s take a look at “color” The Electromagnetic Spectrum
← Increasing frequency (and energy)
Increasing wavelength →
1 2 3390-700nm
405nm 532nm 650nm
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Let’s take a look at “color” The Electromagnetic Spectrum
← Increasing frequency (and energy)
Increasing wavelength →
1 2 Infrared3390-700nm
405nm 532nm 650nm
1550nm
Long haul telecoms lasers
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How can I see “invisible light”?
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If I could have brought a real “death ray”…
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Why not send light through the air?
Ì We do…it’s called “Free Space Optics”
Problem: light is scatteredby “stuff” in the atmosphere
Let’s see another consequence of scattering
• Why is space “black”?• Why is the sun “yellow”?• Why is the sky “blue”?• Why are sunsets “red”?
Sunlight is white, so…
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Sky is “blue”
A critical piece of information…
Ì Shorter visible light wavelengths are scattered preferentially
Increasing wavelength →
Pollen grain
“White” light from the sun
Sun is “yellow”
Shorter visible wavelengths
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c
c
“White” light from the sun…
Preferential scattering of
blue/violet light by atmosphere
• The sun looks “yellow” at noon on Earth
• The sky looks “blue”
• Space looks “black” to this astronaut
Light has to pass through “more atmosphere”
Much more blue light is scattered
• Sunset looks “red”
c…in space, no particles
to scatter the light
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Let’s see a demo
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Another obvious freespace problem…the Earth is curved!
Even if there was no beam divergence or scatteringthe “line of sight” distance is very limited
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If only we could “guide light” then…
We could deliberately choose a medium that was extremely pure, and “transparent” to light
We could overcome distance limits of free space caused by the curvature of the Earth
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What properties of light could we use to create a
“waveguide”?
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Let me tell you (briefly) about refraction
Speed of light in air 300,000km/s
Speed of light in water 225,000km/s
We say that the light is refracted here
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Ì The refractive index for a given medium is the ratio of the speed of light in vacuum to the speed of light in that medium• Refractive index should be greater than 1
Let me tell you (briefly) about refraction
Material Refractive Index Speed of Light(km/s)
Vacuum 1 (by definition) 299,792Air 1.0003 299,702Water 1.33 255,408Glass 1.5 199,862Diamond 2.42 123,881
This is why diamonds sparkle!
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A Couple of Demonstrations on Refraction
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The “levitating” coin
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The “reversing” arrow
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Optical Fiber
This is an optical fiber
This is a human hair!
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How do we tell people that optical fiber works?
Total internal reflection
Try this next time you’re in a swimming pool
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How Does Optical Fibre Work?
Cladding
Core
Refractive
Index
Cladding is made of pure SiO2
Core is doped with GeO2 to raise refractive index by about 1%
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How Does Optical Fibre Work?
Cladding
Core
Refractive
Index
There is a refractive index boundary between the core
and the cladding
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How Does Optical Fibre Work?
Cladding
Core
Refractive
Index
Light that hits the boundary is reflected back into the fibre core
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Find a Perspex block, and a laser pointer (green works best)
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How optical fiber really works!
http://www.youtube.com/watch?v=zTx7UoPXvr4
HIGHER REFRACTIVE INDEXLOWER REFRACTIVE INDEX
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A quick reminder…
Light travels in straight lines
If we send light through the atmosphere:• Absorbed or scattered by “stuff” in the air• The Earth is curved
We can use refraction, and the properties of glass to create an optical “waveguide” that solves all of these problems
These will limit how far, or how fast we can send data
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Putting the ones and zeros onto the light
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A simple example of modulation…
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Ì For most of the history of optical communication an On/Off Keying modulation was perfectly adequate
Optical Modulation: The First 30 Years!
“Square law” detector – which means it’s hard to use digital signal processing to correct linear impairments
Many km of fiber
OK for up to 10Gb/s per wavelength
Transmitter
Laser Shutter“Modulator”
Receiver
PD
Photodetector
1100101000…Impairments!!!
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But things gets harder as we transmit faster
Fast Ethernet
Gigabit Ethernet
10GbE
100GbE
How long is one bit on a fiber?
2 metres (7 feet)
20 cm (8 inches)
2 cm (<1 inch)
2 mm (1/12 inch)
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Why is “bit length” important?
Ì Let me tell you about a particular impairment in optical fiber…
Chromatic Dispersion
Key point: Longer wavelength light travels faster than shorter wavelength light in most transparent media
(normal dispersion vs anomalous dispersion)
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Chromatic Dispersion:Different colors of light travel at different speeds down the fiber
Tx Rx
When we modulate a laser the single color will “spread out”
It’s not very much…but it’s enough to cause problems!
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Dispersion gets worse as we increase the transmission rate
Ì If you transmit at a slow rate…
Ì Increasing the transmission rate means the pulses are closer
After dispersion, it’s no longer possible to distinguish individual 1s and 0s
The dispersed pulses do not “overlap”
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Can we send more than one color of light along the fiber?
And if we can, why don’t the multiple colors get scrambled up?
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Let’s go back to the idea of laser light
Ì I told you it is monochromatic (one color)
Wavelength
As long as these channels don’t “overlap” we can keep the different signals separate
But there’s a problem…
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As we modulate faster, the signal gets “wider”
Wavelength
No modulation10Gb/s20Gb/s50Gb/s100Gb/s
The closer we space the channels, or the faster we modulate, then the sooner
we see interference
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Key Question
How do we get multiple optical wavelengthsalong the same fiber?
And…how do we separate them at the other end?
Answer: We use refraction (again)
This is the basis of Dense Wavelength Division Multiplexing
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Sharing a Single Fiber Using Multiple Colors of Light
SeparateLaser Transmitters
SeparateReceivers
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This is how fiber capacity scales up…
Total capacity on a single fiber pair
=Data rate
per wavelength
xNumber
of wavelengths
Sometimes it’s easier to scale this
And sometimes it’s easier to scale this
But being able to scale both is very useful
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How do we push the limits of fiber capacity?
Ì Remember…As bits get “shorter” fiber impairments are more serious
As we modulate faster, the signals get broader
And these phenomena can
interact!
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Let’s apply some brain power to the problem!
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How do we get to higher data rates?
Is there a “better” modulation technique
than NRZ?
Can we build a“smarter” receiver than a
direct detector?
Phase modulation
• Coherent detection• Signal processing
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Coherent means “related to phase”
These two waves are “out of phase”
These two waves are“in phase”, or “coherent”
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Let’s give a real world example of phase…
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The Move to Coherent Transmission
Wavelength
Phase Modulation…• More tolerant to fiber impairments
Ì Light is a wave……waves have a speed, wavelength and frequency
Coherent Detection…• Very low noise amplification
• Allows sophisticated signal processing
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ADC DSP
Coherent WDM Detection
PDLO
We could take a mixed signal that uses a phase-basedmodulation technique
Use a local oscillator to choose the “color” we want to “detect”
This means there’s a “reference laser” in the coherent detector
Coherent Detector
Other“clever stuff”
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How can a coherent detector be more sensitive to high data rate signals than a direct detector?
vs PD
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Let’s see how sensitive our ears are:
Note A: 1,000Hz
Note B: 1,100Hz
1
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Let’s see how sensitive our ears are:
Note A: 1,000Hz
Note B: 1,100Hz
1
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Let’s see how sensitive our ears are:
2
Note A
Note B
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Let’s see how sensitive our ears are:
2
Note A: 1,000Hz
Note B: 1,010Hz
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Let’s see how sensitive our ears are:
3
Note A:
Note B
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Let’s see how sensitive our ears are:
3
Note A: 1,000Hz
Note B: 1,001Hz
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Example 2 again…10Hz beats
2
Note A
Note B
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What about higher frequencies?
4
Note A: 1,000Hz
Note B: 1,010Hz
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ADC DSP
Coherent WDM Detection: What does the “clever stuff” do?
PDLO …11010110…
Convert the photons to electrons
Convert the “analog electrons” into “digital electrons”
• Deal with “any amount” of chromatic dispersion
• Deal with “any amount” of PMD
Reliable, high data rate signal
Coherent detectors are linear…
…now we have a linear detector, we can do “clever stuff”
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What is this?• Two concentric cylinders with a small gap
between them• The gap is filled with glycerin
What does he do?He puts three colored blobs into the glycerin
Stage 1: He turns the crank clockwiseThe pulses are “dispersed” until they are barely recognizable.
Stage 2: He turns the crank the same number of turns counter-clockwise
…let’s see what happens.
How does Digital Dispersion Compensation Work?
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Where we are going: Fiber Capacity Evolution
0
10
20
30
40
50
60
70
Pre-2010 2010 2016 2020
Fiber Capacity (Tb/s)
Fiber Capacity (Tb/s)
The technology exists to deliver 60Tb/s with 1,000km reach on a fiber by 2020
8Tb/s
24Tb/s
60Tb/s
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Wow! That was a lot to take in ☺
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What have we learned?
Ì Optical fiber is the asset that keeps on giving!• Millions of times more capacity over the same fiber
Ì It is very important that we can use fiber as a waveguide
Ì We use refraction to make fiber and DWDM workÌ Coherent transmission and detection allows us to break the
10Gb/s “barrier” and open up terabit scale fiber capacity
Ì So what does a 100Gb/s coherent optical circuit look like?
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Laser
ModulatorsDemodulators
LocalOscillator
Laser
ConnectingFibers
8 x photodetectors
ConnectingFibers
Many km of optical fiber
Transmitter
Receiver
Infinera: What do we make?
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Many km of optical fiber
Infinera: What do we make?
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Many km of optical fiber
WavelengthMux/Demux
600 Optical Functions>250 fiber connections
Integrated onto two chips
Infinera: What do we make?
High DensityLow Power
High Reliability
ArchitecturalReengineering
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Infinera: What do we make?
An Infinite Pool of Intelligent Bandwidth
Data CenterResidential B’BandMobile Access
What do we enable?
Etc.