WDM

1. Piotr Turowicz Poznan Supercomputing and Networking Center piotrek@man.poznan.pl 9-10 October 2006 WDM

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

3. Traditional Challenges Faster Optical Networking Challenges Further More Wavelengths

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

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

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

7. 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 Statistical 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

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

9. Dense WDM How many channels? • Many more than 2 channels! • Initial ITU Grid allows 32 channels with 100GHz 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)

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

11. 0.6 1535 1550 1560 1530 1540 1545 1555 1565 1525 0.5 ITU Channel Spacing 100GHz (Currently) 0.4 Attenuation (dB/km) 0.3 0.2 0.1 1500 1600 1700 1400 1300 1200 Wavelength (nm) Dense WDM:ITU Channel Spacing

12. A Basic Answer • Light is sent into the fibre on a very narrow range of wavelengths… • A typical DFB laser peak width is ~10MHz (~1pm at 1500nm) • 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 • 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. 100GHz at 40Gbps) • Another "rule of thumb": each time you double the transmission rate or the number of channels, an additional 3dB of transmission budget is needed • Need to know the range of available wavelengths in the fibre

13. DWDM Channel Spacing • Must have enough channel spacing to prevent interaction at a given transmission rate… • 40Gbps 100GHz • 10Gbps 25GHz • 2.5Gbps 6GHz • 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 • "Accepted" EDFA range is 1530 to 1565 nm (C-band) • Must be aware of fibre limitations (see later)

14. 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 1st generation DWDM is deployed in point to 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 • Are sensitive to nature of installed fibre • Are very sensitive to disruptive technologies …more later!

15. 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)

16. 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, PoS and other services • ESCON • c.f. SCSI, which is a parallel communication channel (parallel to serial converters are available for SCSI) • Fast/Gigabit Ethernet • Each channel can be transmitting at different rates

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

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

19. 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 Electron "High" energy level Energy emitted "Low" energy level 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 Electron Characteristic energy

20. A Laser Cavity Containment Layer Reflective Surface 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 Electrodes Atom in "high" energy state Photon of characteristic energy Photons that travel parallel to sides of resonant cavity are returned to stimulate further emissions Atom in "low" energy state

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

22. Tuneable LasersWhat and Why? • There are three parameters that we trade-off in a tuneable laser… • Tuning range (goal 35nm) • Power output (goal 10mW) • Settling latency (app. specific) • 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

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

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

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

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

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

28. The EDFAWhat is "Erbium Doped"? • Fibre is "doped" with the element Erbium • Controlled level of Erbium introduced into silica core and cladding Cladding Core

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

30. The EDFAHow Does It Work?

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

32. CWDM

33. 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).

34. CWDM 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

35. CWDM

36. CWDM

37. CWDM System CWDMCoarse Wavelength Division Multiplexing

38. CWDM System CWDMCoarse Wavelength Division Multiplexing

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

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

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

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

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

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

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

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

47. Cross-Phase Modulation (XPM) • Pulses in adjacent WDM channels exchange power • ie. only happens in multi-channel 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 

48. fp fq fr • fF 2f1-f2 f1 f2 2f2-f1 Four Wave Mixing • Case 1: Intensity modulation between two primary channels at beat frequency • Result is two "phantom" wavelengths • Case 2: Interaction of three primary frequencies • Result is a "phantom" fourth wavelength • fF = fp + fq - fr

49. Fibre Evolution1st Generation: USF 20 0.5 1310nm 1550nm 10 0.4 0 Attenuation (dB/km) Dispersion (ps/nm-km) 0.3 -10 Dispersion 0.2 -20 USF 1300 1400 1500 1600 Wavelength (nm) Attenuation

50. Fibre Evolution2nd Generation: DSF 20 0.5 1310nm 1550nm 10 0.4 0 Attenuation (dB/km) Dispersion (ps/nm-km) 0.3 -10 Dispersion 0.2 -20 USF 1300 1400 1500 1600 DSF Wavelength (nm) Attenuation