12 Nov 2018 0

Optical Transmission Fundamentals

Context

Technology

HEP Specifics

F. Vasey, CERN-EP-ESE

12 Nov 2018 1

Context: Bandwidth DemandInternet traffic is growing at ~Moore’s lawGlobal interconnection bandwidth estimated to reach 8’000 Tb/s in 202110x increase compared to 2017

Frankfurt DE-CIX internet exchange point

12 Nov 2018 2

Context: Bandwidth availability

- People and objects are increasingly interconnected, ubiquitously - Bandwidth demand is spiraling- Access Bandwidth has grown x200’000 in 30yrs

12 Nov 2018 3

Context: … but who is feeding the network ?In 2016:- 1’200’000 petabytes have been exchanged worldwide- 49 petabytes have been produced by LHC experiments

12 Nov 2018 4

Context: Capacity is what matters

TDM

WDMSDM

Bits/Symbol

- Technology is developed at a pace thatmatches the needs for capacity in the networks

- Developers of electronics for physicsexperiments surf this wave but don’t drive it

- A good understanding of the datacomenvironment and its evolution is thus essential to assess the potentials of optoelectronics and its future benefits

12 Nov 2018 5

Technology

Context

Technology

Networks

Hardware Toolbox

Modulation Formats

Capacity

Standards

HEP Specifics

12 Nov 2018 6

Technology: Networks

Context

Technology

Networks

Hardware Toolbox

Modulation Formats

Capacity

Standards

HEP Specifics

12 Nov 2018 7

Core, Regional-Metro and Access Networks

Access

- Different technologies are developed for the different network types- But network specificities tend to fade- Technologies tend to migrate from one type to the other

12 Nov 2018 8

Core Network

100 - 400Gb/s Fully proprietary

technology WDM: Up to 80

wavelengths per fibre (50/100GHz spacing)

EDFA every ~100km

Dispersion compensation

Forward Error Correction

Performance driven

12 Nov 2018 9

Access Network

10-100Gb/s

Standardized technology

ISDN

xDSL

Ethernet

GPRS, Edge, 3G, 4G

PONs…

No wavelength multiplexing (so far)

Cost/regulation sensitive

WDM mesh

Backbone Network

Metropolitan Network

DSLAM

xDSL

POTS

ISDNEthernet, SONET etc

OLT

PON

CO

CO

Access Network

5G

Mobile

Datacenter

Access Network: Fiber penetration

12 Nov 2018 10

4G

Penetration still far from 50%

12 Nov 2018 11

Summary: of direct interest to DAQ systems developers:

10-100Gb/s

Standardized technology

ISDN

xDSL

Ethernet

GPRS, Edge, 3G, 4G

PONs…

No wavelength multiplexing (so far)

Cost/regulation sensitive

WDM mesh

Backbone Network

Metropolitan Network

DSLAM

xDSL

POTS

ISDNEthernet, SONET etc

OLT

PON

CO

CO

Access Network

5G

Mobile

Datacenter

12 Nov 2018 12

Technology

Context

Technology

Networks

Hardware Toolbox

Modulation Formats

Capacity

Standards

HEP Specifics

12 Nov 2018 13

• EM spectrum usage:

• 1840s Telegraph

• 1880s Telephone

• 1890s Radio

• 1940s Microwaves

• 1950s bipolar transistors

• 1960s Lasers

• 1970s Optical fibres

• 1990s Er-doped fibre

Reproduced from [2]

EM Spectrum

12 Nov 2018 14

Data Transmission Model

Emission

Absorption

RefractionAttenuationDispersion

• Optical communication systems are not new• Greek fire chains with relay stations existed 1000years BC• Missing for a long time was the perfect match of bandwidth,

distance and availability

12 Nov 2018 15

1. Emission in semiconductor cristal

Reproduced from [4]

Injection luminescence Competition with non-radiative transitions External quantum efficiency must be maximised Coupling efficiency problematic: horizontal, vertical

12 Nov 2018 16

Semiconductor heterostructure

Reproduced from [2]

Modulation by direct injection

Electrical confinement

Optical confinement

Temperature dependence

12 Nov 2018 17

Ternary Material System

Reproduced from [1]

(quasi) lattice matched structures

Epitaxial growth

Tight defect control

InGaAs

12 Nov 2018 18

Semiconductor Laser Structure: horizontal cavity

Reproduced from [2]

Narrow Spectrum Multiple longitudinal modes High modulation bandwidth Divergent beam Cleaved facets

12 Nov 2018 19

Semiconductor Laser Structure: vertical cavity

Single longitudinal mode

On wafer testing

Direct coupling to fibre

Complex epi growth

Difficult to realize in InP material system

12 Nov 2018 20

2. Absorption in semiconductors

Reproduced from [4]

Electron-holes separated by bias field

Absorption depth optimisedwrt recombination time

Capacitance wrt light collection efficiency

12 Nov 2018 21

Detector Material Responsivities

12 Nov 201822

Pin diode

Reproduced from [4]

Vertical access

Back or front illuminated (transparent substrate)

Excellent coupling

Short wavelength response dominated by surface absorption

12 Nov 2018 23

3. Optical Fibres A discrete set of guided modes propagate

Most energy in core

Launch from edge only

Leakage in bends

Subject to modal dispersion

a. Step Index Single-mode fibre

Small core diameter (typ. 9mm)

Difficult coupling

Two polarizations

12 Nov 2018 24

b. Graded Index Multi-mode fibre

Large core diameter (typ. 50mm)

Equalized phase velocities limit modal dispersion

Difficult index profile realization

Easy coupling

3. Optical Fibres

12 Nov 2018 25

Optical Fibres: material engineering

Ge-doped core or F-doped cladding

Waveguide dispersion engineering

12 Nov 2018 26

Silica transmission windows

12 Nov 2018 27

Optical Fibres: Dispersion

12 Nov 2018 28

Optical Systems: Speed and Range

OS1/OS2

• Dispersion (and attenuation) will limitMM systems to short distances

• Low cost will dictatethe use of VCSELs at 850nm (1st window)

• As speed increasesnew MM fiber must bedeveloped to maintaindistance)

Optical Fibers: Capacity [Datarate×distance]

12 Nov 2018 29

Single Mode FibreEdge Emitting Laser, InGaAsP1310-1550nm

Multi Mode FibreVCSEL, AlGaAs850nm

12 Nov 2018 30

4. Light Amplification

1) Semiconductor Optical Amplifiers(SOA)

2) Erbium doped fiber amplifiers (EDFA)

http://upload.wikimedia.org/wikipedia/commons/6/6d/Doped_fibre_amplifier.svg

Extending reach with optical amplifiers

12 Nov 2018 31

12 Nov 2018 32

5. Light in/out coupling

Edge coupling

Grating coupling

6. Packaging: Optical Subassemblies (OSA)

12 Nov 2018 33

Packaging: Transceiver form factors

12 Nov 2018 34

12 Nov 2018 35

7. Light Splitting and Processing

Bulk optics

Bi-conic fused silica splitter

Photonic lightwave circuit (PLC)

Glass

Silicon Nitride

Si photonics

splice

12 Nov 2018 36

Wavelength Multiplexers/Demultiplexers

Bulk optics (thin films)

Planar Lightwave Circuit (PLC)

Fiber Bragg Gratings

Arrayed Waveguide Gratings (AWG)λ1 λ2 λ3 λ4 λ5

λ1 λ2 λ3 λ4 λ5

λ1 λ2 λ3 λ4 λ5

λ1 λ2 λ3 λ4 λ5

λ1 λ2 λ3 λ4 λ5

1

2

3

4

5

a

b

c

d

e

λ5

λ4

λ3

λ2

λ1

λ4

λ3

λ2

λ1

λ5

λ3

λ2

λ1

λ5

λ4

λ1

λ5

λ4

λ3

λ2

λ2

λ1

λ5

λ4

λ3

Cyclic property of a 5X5 AWG

λ1λ2λ3

λ1

λ2

λ3

1440 nm Rx

1550 nm Rx

1310 nm Tx

Dichroic mirrors

lens

fiber

Adding wavelength selectivity to a Laser

12 Nov 2018 37

Distributed Feedback and Distributed Bragg laser

12 Nov 2018 38

Technology: Modulation Formats

Context

Technology

Networks

Hardware Toolbox

Modulation Formats

Capacity

Standards

HEP Specifics

12 Nov 2018 39

Modulation

Direct Modulation

Limited by:

Carrier lifetime

Wavelength chirp

Indirect Modulation

Phase modulation

EO effect in crystals (LiNbO3)

Free carrier injection in pin (Si)

Intensity modulation

PM + Interferometer

Electro absorption

Mach Zehnder Modulator

12 Nov 2018 40

Common modulation techniques

QAM SNRDetection Complexity

ASK-PAM8

8-PSK

16-QAM

12 Nov 2018 41

Implementation example 1: QAM16

Non linearityPre/post comp

Dispersion equalization

A D

Implementation example 2: PAM4

12 Nov 2018 42

12 Nov 2018 43

Technology: Capacity

Context

Technology

Networks

Hardware Toolbox

Modulation Formats

Capacity

Standards

HEP Specifics

TDM

WDM

Bits/Symbol

12 Nov 2018 44

Capacity=Bit Rate * Distance

80x40Gbpsx10’000km=32’000Tb/s.km

12 Nov 2018 45

Capacity increase is possible with advanced modulation techniques

QAM and Coherent detection

Polarization multiplexing

High speed DSP, D/A and A/D

And to extend the limits:

Amplifier waveband extension

Multi-core fiber

In-line electrical processing

…

Turukhin et al (TE subcom), 2017, demonstrated 105.1Tb/s transmission in a 12-core fiber over 14,352km using a power-efficient 8D-APSK modulation format and 82 wavelength channels. Also demonstrated a potential capacity of 4.59Eb/s.km

12 Nov 2018 46

Technology: Standards

Context

Technology

Networks

Hardware Toolbox

Modulation Formats

Capacity

Standards

HEP Specifics

12 Nov 2018 47

Bit Rates vs Standards

X10 in 4 yrs

X10 in >10 yrs

12 Nov 2018 48

IEEE802.3ba Ethernet Roadmaphttps://ethernetalliance.org/the-2018-ethernet-roadmap/

Ethernet Interfaces and Nomenclature

12 Nov 2018 49

Beyond 40/100GbE

12 Nov 2018 50

Outlook

12 Nov 2018 51