WDM Piotr Turowicz Poznan Supercomputing and Networking Center

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WDM Piotr Turowicz Poznan Supercomputing and Networking Center piotrek@man. poznan. pl 9 -10 October

WDM Piotr Turowicz Poznan Supercomputing and Networking Center [email protected] poznan. pl 9 -10 October 2006 1

Agenda Dense Wavelength Division Multiplexing – – The traditional and emerging challenges How does

Agenda Dense Wavelength Division Multiplexing – – The traditional and emerging challenges How does DWDM work? What are the enabling technologies? The evolution of optical fibres 2

Optical Networking Challenges Traditional Challenges Faster Further More Wavelengths 3

Optical Networking Challenges Traditional Challenges Faster Further More Wavelengths 3

Optical Networking Challenges Traditional Challenges Emerging Challenges Faster Access (FTTN, FTTC, FTTH) Further Switching

Optical Networking Challenges Traditional Challenges Emerging Challenges Faster Access (FTTN, FTTC, FTTH) Further Switching More Wavelengths Muxing 4

What is a Wavelength Mux? Tributaries are sent in their own timeslots Time Division

What is a Wavelength Mux? Tributaries are sent in their own timeslots Time Division Mux 5

What is a Wavelength Mux? Tributaries are sent in their own timeslots Time Division

What is a Wavelength Mux? Tributaries are sent in their own timeslots Time Division Mux Tributaries are buffered and sent when capacity is available Statistic al Mux 6

What is a Wavelength Mux? Electrical inputs Tributaries are sent in their own timeslots

What is a Wavelength Mux? Electrical inputs Tributaries are sent in their own timeslots Time Division Mux Tributaries are buffered and sent when capacity is available Statistic al Mux Tributaries are sent over the same fibre, but at different wavelengths Wavelength Division Mux Tributaries may arrive on different fibres, and at "grey" wavelengths 7

Early WDM Deployment • Two transmission wavelengths, most common. . . Ø 1310 nm

Early WDM Deployment • Two transmission wavelengths, most common. . . Ø 1310 nm Ø 1550 nm • Coupler used to combine streams into the fibre • Splitter (another coupler) and filters used to separate and detect specific streams 8

Dense WDM How many channels? • Many more than 2 channels! • Initial ITU

Dense WDM How many channels? • Many more than 2 channels! • Initial ITU Grid allows 32 channels with 100 GHz Spacing • Proprietary systems with up to 160 channels are currently available as slideware Be very, very careful regarding manufacturer claims! (c. f. Never ask a barber if he thinks you need a haircut) 9

Question. . . Why don't the streams on different wavelengths get "mixed up"? 10

Question. . . Why don't the streams on different wavelengths get "mixed up"? 10

Dense WDM: ITU Channel Spacing 1565 1560 1555 1550 1545 1540 1535 1530 1525

Dense WDM: ITU Channel Spacing 1565 1560 1555 1550 1545 1540 1535 1530 1525 0. 6 Attenuation (d. B/km) 0. 5 0. 4 ITU Channel Spacing 100 GHz (Currently) 0. 3 0. 2 0. 1 1200 1300 1400 1500 Wavelength (nm) 1600 1700 11

A Basic Answer • Light is sent into the fibre on a very narrow

A Basic Answer • Light is sent into the fibre on a very narrow range of wavelengths… Ø A typical DFB laser peak width is ~10 MHz (~1 pm at 1500 nm) • Different channels are spaced so that they don't "overlap" In this context, "overlap" implies a power coupling (ie. interference) between one channel and its neighbours Ø Typical spacing "rule of thumb"…take the transmission rate in Gbps, multiply by 2. 5, and you have the minimum channel spacing in GHz (eg. 100 GHz at 40 Gbps) Ø Another "rule of thumb": each time you double the transmission rate or the number of channels, an additional 3 d. B of transmission budget is needed Ø • Need to know the range of available wavelengths in the fibre 12

DWDM Channel Spacing • Must have enough channel spacing to prevent interaction at a

DWDM Channel Spacing • Must have enough channel spacing to prevent interaction at a given transmission rate… 40 Gbps 100 GHz Ø 10 Gbps 25 GHz Ø 2. 5 Gbps 6 GHz Ø • Must test lasers from large batch, ensure temperature stability, and include margins for component ageing • Total range of wavelengths must be able to be consistently and reliably amplified by EDFA Ø "Accepted" EDFA range is 1530 to 1565 nm (C-band) • Must be aware of fibre limitations (see later) 13

Why (and Where) DWDM? • DWDM increases capacity on a given point to point

Why (and Where) DWDM? • DWDM increases capacity on a given point to point link Bandwidth is multiplied by factor of 4, 8, 16 etc. • Typical 1 st generation DWDM is deployed in point topologies, over long-haul distances • In Metro installations, there is an active debate between mesh and ring-based topologies • Economics of Metro DWDM are not clear-cut Often is cheaper to deploy more fibre These markets are… Changing rapidly h Are sensitive to nature of installed fibre h Are very sensitive to disruptive technologies h …more later! 14

DWDM Enabling Technologies • • • The notion of "Service Transparency" Laser sources Receivers

DWDM Enabling Technologies • • • The notion of "Service Transparency" Laser sources Receivers Tuneable filters Fibre gratings Modulation and Modulators Wavelength couplers and demuxers Optical amplifiers Points of flexibility Optical Cross-Connect (OXC) Optical Add-Drop Mux (OADM) 15

Service Transparency • Each Lambda can carry any serial digital service for which it

Service Transparency • Each Lambda can carry any serial digital service for which it has an appropriate physical interface SONET/SDH Which can be carrying ATM, Po. S and other services Ø ESCON Ø c. f. SCSI, which is a parallel communication channel (parallel to serial converters are available for SCSI) Ø Fast/Gigabit Ethernet Ø • Each channel can be transmitting at different rates 16

Why Lasers? • Lasers in general. . . High power output (compared to beam

Why Lasers? • Lasers in general. . . High power output (compared to beam diameter) Ø Narrow transmission spectrum Ø High spatial quality beam (diffraction limited) Ø Well-defined polarisation state Ø • Semiconductor lasers Small Size To improve efficiency with fibre coupling To allow high density port counts Ø Industrial scale production Needs lots of them! Ø 17

A Basic Semiconductor Laser Reflective coating P N Partially reflective coating 18

A Basic Semiconductor Laser Reflective coating P N Partially reflective coating 18

How Do Lasers Work? "High" energy level Energy absorbed (pump) Electrons exist in a

How Do Lasers Work? "High" energy level Energy absorbed (pump) Electrons exist in a stable "low" energy state until we pump in energy to promote them to a higher state "Low" energy level "High" energy level Energy emitted High energy state is unstable and electron will soon decay back to the low energy state, giving out a characteristic level of energy in the process "Low" energy level Electron Characteristic energy 19

A Laser Cavity Reflective Surface Containment Layer Electrodes Atom in "high" energy state Photon

A Laser Cavity Reflective Surface Containment Layer Electrodes Atom in "high" energy state Photon of characteristic energy Atom in "low" energy state Gain Medium Reflective Surface Atom will emit photon and return to "low" energy state. The emitted photon has exactly the right energy to stimulate emission in the other high energy atoms Photons that travel parallel to sides of resonant cavity are returned to stimulate further emissions 20

Tuneable Lasers What and Why? • The ability to select the output wavelength of

Tuneable Lasers What and Why? • The ability to select the output wavelength of the laser… Ø The primary sources are fixed wavelength • What happens if one of these lasers fails? How many backup lasers would we need? Ø What is the range of wavelengths over which we need to operate? Ø • We could use one tuneable laser to back up all of the primary sources 21

Tuneable Lasers What and Why? • There are three parameters that we trade-off in

Tuneable Lasers What and Why? • There are three parameters that we trade-off in a tuneable laser… Tuning range (goal 35 nm) Ø Power output (goal 10 m. W) Ø Settling latency (app. specific) Ø • Tunable lasers with a "slow" settling speed can be used in service restoration applications • Tunable laser with a "fast" settling speed can also be used in next generation optical switching designs 22

Signal Modulation • Notion of imposing a digital signal on a carrier wave Amplitude

Signal Modulation • Notion of imposing a digital signal on a carrier wave Amplitude Modulation Ø Frequency Modulation Ø Phase Modulation Ø • In Optical Communications, typically Amplitude Modulation Ø NRZ and RZ encoding • Directly modulated lasers • Externally modulated lasers 23

Modulation Schemes • NRZ: non-return to zero Ø Most common modulation scheme for short-mediumlong

Modulation Schemes • NRZ: non-return to zero Ø Most common modulation scheme for short-mediumlong haul • RZ: return to zero Ø Signal 1 1 1 0 Signal 1 Ultra-long haul 0 1 0 0 1 24

A Traditional Optical Repeater • High speed electrical components ØHigh cost, lower reliability •

A Traditional Optical Repeater • High speed electrical components ØHigh cost, lower reliability • Single wavelength operation • Regenerator will make amplifier rate-specific This system is not Service-Transparent! 25

OEO Amps in a DWDM System ~40 km TX Amp RX RX Amp TX

OEO Amps in a DWDM System ~40 km TX Amp RX RX Amp TX 26

Solution: Broadband, All-Optical Amplifier • Single amplifier for multiple wavelengths • No electrical components

Solution: Broadband, All-Optical Amplifier • Single amplifier for multiple wavelengths • No electrical components Cheaper, more reliable, not rate-dependent Gain element 27

The EDFA What is "Erbium Doped"? • Fibre is "doped" with the element Erbium

The EDFA What is "Erbium Doped"? • Fibre is "doped" with the element Erbium Ø Controlled level of Erbium introduced into silica core and cladding Core 28

The EDFA How Does It Work? • Energy is "pumped" into the fibre using

The EDFA How Does It Work? • Energy is "pumped" into the fibre using a pump laser operating at 980 nm • Erbium acts as lasing medium, energy transferred to signal • Not specific to wavelength (operates in the EDFA Window) • Not specific to transmission rate 29

The EDFA How Does It Work? 30

The EDFA How Does It Work? 30

The EDFA Window Region of "flat gain" 5 EDFA Window: 1530 -1565 nm Attenuation

The EDFA Window Region of "flat gain" 5 EDFA Window: 1530 -1565 nm Attenuation (d. B/km) 4 3 OH 2 - OH OH 1 - - 0 700 800 900 1000 1100 1200 1300 Wavelength (nm) 1400 1500 1600 First window Second window Third window Fourth window Fifth window 1700 31

CWDM 32

CWDM 32

CWDM Coarse wavelength division multiplexing (CWDM) is a method of combining multiple signals on

CWDM Coarse wavelength division multiplexing (CWDM) is a method of combining multiple signals on laser beams at various wavelenghts for transmission along fiber optic cables, such that the number of chanels is fewer than in dense wavelength division multiplexing (DWDM) but more than in standard wavelength division multiplexing (WDM). 33

CWDM systems have channels at wavelengths spaced 20 nanometers apart, compared with 0. 4

CWDM systems have channels at wavelengths spaced 20 nanometers apart, compared with 0. 4 nm spacing for DWDM. This allows the use of low -cost, uncooled lasers for CWDM. In a typical CWDM system, laser emissions occur on eight channels at eight defined wavelengths: 1610 nm, 1590 nm, 1570 nm, 1550 nm, 1530 nm, 1510 nm, 1490 nm, 1470 nm. But up to 18 different channels are allowed, with wavelengths ranging down to 1270 nm 34

CWDM 35

CWDM 35

CWDM 36

CWDM 36

CWDM System CWDM Coarse Wavelength Division Multiplexing 37

CWDM System CWDM Coarse Wavelength Division Multiplexing 37

CWDM System CWDM Coarse Wavelength Division Multiplexing 38

CWDM System CWDM Coarse Wavelength Division Multiplexing 38

The Evolution of Fibre • Fibre properties Attenuation Ø Dispersion Ø Non-linearlity Ø •

The Evolution of Fibre • Fibre properties Attenuation Ø Dispersion Ø Non-linearlity Ø • Fibre Evolution Dispersion-Unshifted Fibre (USF) Ø Dispersion-Shifted Fibre (DSF) Ø Non-Zero Dispersion-Shifted Fibre (NZDF) Ø Emerging fibre types Ø • Soliton Dispersion Management 39

Optical Fibre Properties Traditional Challenges Faster Fibre Properties Ø Ø Ø Further Ø Ø

Optical Fibre Properties Traditional Challenges Faster Fibre Properties Ø Ø Ø Further Ø Ø More Wavelengths Attenuation Modal Dispersion Chromatic Dispersion Polarisation Mode Dispersion Non-linearity » Self-phase modulation » Cross-phase modulation » 4 -wave mixing 40

Fibre Optic Properties Signal Attenuation 5 ~190 THz Attenuation (d. B/km) 4 1 3

Fibre Optic Properties Signal Attenuation 5 ~190 THz Attenuation (d. B/km) 4 1 3 ~50 THz OH 2 - 2 OH 1 OH - 5 3 4 - 0 700 800 900 1000 1100 1200 1300 Wavelength (nm) 1400 1500 1600 First window Second window Third window Fourth window Fifth window 1700 41

Fibre Optic Properties Modal Dispersion • In multimode cable, different modes travel at different

Fibre Optic Properties Modal Dispersion • In multimode cable, different modes travel at different speeds down the fibre Result: signal is "smeared" Ø Solution: single mode fibre Ø Signal in Signal out 42

Fibre Optic Properties Chromatic Dispersion Different wavelengths travel at different speeds down the cable

Fibre Optic Properties Chromatic Dispersion Different wavelengths travel at different speeds down the cable Result: signal is "smeared" Ø Solution: narrow spectrum lasers Ø Solution: avoid modulation chirp Ø Solution: dispersion management Ø Signal in Signal out 43

Fibre Optic Properties Polarisation Mode Dispersion Different polarisation components travel at different speeds down

Fibre Optic Properties Polarisation Mode Dispersion Different polarisation components travel at different speeds down the cable Result: signal is "smeared" Ø Solution: design and installation experience, good test equipment Ø Pulses start journey in phase Fast Slow PMD delay time After travelling down fibre, pulses are now out of phase 44

Fibre Optic Properties Non-Linear Effects • Self Phase Modulation • Cross Phase Modulation •

Fibre Optic Properties Non-Linear Effects • Self Phase Modulation • Cross Phase Modulation • 4 -Wave Mixing Effects are "triggered" when power level of signal exceeds a certain threshold 45

Self Phase Modulation (SPM) • Non-linear effect • Occurs in single and multi wavelength

Self Phase Modulation (SPM) • Non-linear effect • Occurs in single and multi wavelength systems Ø Spectral broadening In DWDM system, SPM will occur within a single wavelength • Two main effects… Spectral broadening Ø Pulse compression • Solution is positive dispersion in signal path Intensity Ø Time 46

Cross-Phase Modulation (XPM) • Pulses in adjacent WDM channels exchange power Ø ie. only

Cross-Phase Modulation (XPM) • Pulses in adjacent WDM channels exchange power Ø ie. only happens in multichannel systems • Primary effect is spectral broadening • Combined with high dispersion, will produce temporal broadening • Low levels of positive dispersion will help prevent inter-channel coupling 47

Four Wave Mixing Case 1: Intensity modulation between two primary channels at beat frequency

Four Wave Mixing Case 1: Intensity modulation between two primary channels at beat frequency Result is two "phantom" wavelengths fp 2 f 1 -f 2 f 1 f 2 2 f 2 -f 1 fq fr f. F Case 2: Interaction of three primary frequencies Result is a "phantom" fourth wavelength f. F = f p + f q - f r 48

Fibre Evolution 1 st Generation: USF 20 1310 nm 1550 nm 10 0. 4

Fibre Evolution 1 st Generation: USF 20 1310 nm 1550 nm 10 0. 4 0 0. 3 -10 Dispersion (ps/nm-km) Attenuation (d. B/km) 0. 5 Dispersion 0. 2 -20 1300 1400 1500 Wavelength (nm) Attenuation 1600 USF 49

Fibre Evolution 2 nd Generation: DSF 20 1310 nm 1550 nm 10 0. 4

Fibre Evolution 2 nd Generation: DSF 20 1310 nm 1550 nm 10 0. 4 0 0. 3 -10 Dispersion (ps/nm-km) Attenuation (d. B/km) 0. 5 Dispersion 0. 2 -20 1300 1400 1500 Wavelength (nm) Attenuation 1600 USF DSF 50

Fibre Evolution 3 nd Generation: NZDSF 20 1310 nm 1550 nm 10 0. 4

Fibre Evolution 3 nd Generation: NZDSF 20 1310 nm 1550 nm 10 0. 4 0 0. 3 -10 Dispersion (ps/nm-km) Attenuation (d. B/km) 0. 5 Dispersion 0. 2 -20 1300 1400 1500 Wavelength (nm) 1600 USF DSF NZDF Attenuation 51

Next Generation Fibres. . . • Remove OH- interaction to open 5 th window

Next Generation Fibres. . . • Remove OH- interaction to open 5 th window Ø Example: Lucent "All Wave" Fibre • Minimise intrinsic PMD during manufacture PMD is the "2. 5 Gbps speed bump" Ø Example: Corning LEAF Ø PMD is very dependent on installation stresses Ø • Reduce loss at higher wavelengths (>1600 nm) Selctive doping using chalcogenides (Group VI elements) Ø Fibre bend radius becomes significant Ø 52

References Reichle & De-Massari http: //www. porta-optica. org 53

References Reichle & De-Massari http: //www. porta-optica. org 53