The Miracle of Optical Communication · 2| © 2016 Infinera Confidential & Proprietary The Number 1 Rule Know Your Audience

1 | © 2016 Infinera Confidential & Proprietary

The Miracle of Optical CommunicationGeoff BennettDirector, Solutions and Technology


The Number 1 Rule

Know Your Audience


?Fiber(Multi-Wavelength)

How do we carry all of this data?

Copper

Fiber(Single Wavelength)

Wireless(3G, 4G, 5G, WiFi, etc.)


The Worldwide Web of FiberSubsea Cable Systems

Level(3)’s Network

North America

Interoute’s Network

Europe

Virgin Media Network

UK


Light is a great way to communicate

Nothing travels faster…

Light can carry enormous

amounts of data

Light can travel enormous distances


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


How fast does light travel?

To see light travel we have two options…

Option 1: Slow Down Time

Option2: Use very long distances


You are here (Earth) The Moon

370,000km (1.3 light seconds)


So light travels fast…now we need a good light source

The Sun?

A TorchA Laser!


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


Let’s take a look at “color” The Electromagnetic Spectrum

← Increasing frequency (and energy)

Increasing wavelength →

1 2 3390-700nm

405nm 532nm 650nm


Let’s take a look at “color” The Electromagnetic Spectrum

← Increasing frequency (and energy)


1 2 Infrared3390-700nm

405nm 532nm 650nm

1550nm

Long haul telecoms lasers


How can I see “invisible light”?


If I could have brought a real “death ray”…


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…


Sky is “blue”

A critical piece of information…

Ì Shorter visible light wavelengths are scattered preferentially


Pollen grain

“White” light from the sun

Sun is “yellow”

Shorter visible wavelengths


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


Let’s see a demo


Another obvious freespace problem…the Earth is curved!

Even if there was no beam divergence or scatteringthe “line of sight” distance is very limited


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


What properties of light could we use to create a

“waveguide”?


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


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!


A Couple of Demonstrations on Refraction


The “levitating” coin


The “reversing” arrow


Optical Fiber

This is an optical fiber

This is a human hair!


How do we tell people that optical fiber works?

Total internal reflection

Try this next time you’re in a swimming pool


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%



Cladding

Core

Refractive

Index

There is a refractive index boundary between the core

and the cladding



Cladding

Core

Refractive

Index

Light that hits the boundary is reflected back into the fibre core


Find a Perspex block, and a laser pointer (green works best)


How optical fiber really works!

http://www.youtube.com/watch?v=zTx7UoPXvr4

HIGHER REFRACTIVE INDEXLOWER REFRACTIVE INDEX


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


Putting the ones and zeros onto the light


A simple example of modulation…


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!!!


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)


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)


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!


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”


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?


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…


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


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


Sharing a Single Fiber Using Multiple Colors of Light

SeparateLaser Transmitters

SeparateReceivers


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


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!


Let’s apply some brain power to the problem!


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


Coherent means “related to phase”

These two waves are “out of phase”

These two waves are“in phase”, or “coherent”


Let’s give a real world example of phase…


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


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”


How can a coherent detector be more sensitive to high data rate signals than a direct detector?

vs PD


Let’s see how sensitive our ears are:

Note A: 1,000Hz

Note B: 1,100Hz

1



Note A: 1,000Hz

Note B: 1,100Hz

1



2

Note A

Note B



2

Note A: 1,000Hz

Note B: 1,010Hz



3

Note A:

Note B



3

Note A: 1,000Hz

Note B: 1,001Hz


Example 2 again…10Hz beats

2

Note A

Note B


What about higher frequencies?

4

Note A: 1,000Hz

Note B: 1,010Hz


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”


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?


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


Wow! That was a lot to take in ☺


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?


Laser

ModulatorsDemodulators

LocalOscillator

Laser

ConnectingFibers

8 x photodetectors

ConnectingFibers

Many km of optical fiber

Transmitter

Receiver

Infinera: What do we make?






WavelengthMux/Demux

600 Optical Functions>250 fiber connections

Integrated onto two chips


High DensityLow Power

High Reliability

ArchitecturalReengineering



An Infinite Pool of Intelligent Bandwidth

Data CenterResidential B’BandMobile Access

What do we enable?

Etc.


Thank You!Geoff [email protected]

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The Miracle of Optical Communication · 2| © 2016 Infinera Confidential & Proprietary The Number 1 Rule Know Your Audience