© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Agenda Introduction Digital Transmission Dispersion in optical Networks. Dispersion challenges for 40G OSA challenges for 40G/ROADM’s

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Dispersion & 40G October 2007 Mike Harrop

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. 40 Gb/s drivers: - Bandwidth in the core must be larger than any single constituent signal from the edge - Must be less expensive than 4x more wavelengths - Must allow re-use of current fiber layout - Today, all tier-1 providers are looking into 40 Gb/s 40 Gb/s vs. 10 Gb/s

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. 10Gbps uses NRZ-OOK (non-return-to-zero, On-Off Keying) 40 Gb/s vs. 10 Gb/s

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Main issues on NRZ (non-return-to-zero) 40 Gb/s: - Pulse is 4x smaller: which means 6 dB less OSNR sensitivity - CD is 16 times less - PMD is 4 times less - 2 nd order PMD becomes a factor - Non-linearity is more probable - No re-use of current fiber networks Obviously NRZ CANNOT support 40 Gb/s. How will 40 Gb/s be implemented??? 40 Gb/s vs. 10 Gb/s

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Viable 40 Gb/s requires: - Advanced modulation techniques - Sophisticated CD compensation - Low-noise amplifiers - Powerful FEC - Great characterization tools 40 Gb/s vs. 10 Gb/s

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. NRZ-OOK (on/off keying) vs. RZ-OOK (on/off keying) More resistant to dispersion Higher Peak power (Non-linear effects danger) Lower duty cycle (33% typical) But not enough… Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. RZ-OOK: RZ with CD: = To avoid CD, Duty Cycle is LOW (33%) Consequence: to have long distance, Peak Power is HIGH Meaning: High risks of Non-Linear… Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Introducing CS-RZ-OOK (Carrier Suppressed) - Cheap - No huge gain Dispersion Robustness - 100GHz max spacing Each bit has a phase shift, no constant « carrying » wave  Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Introducing RZ-DuoBinary A « 1 » isif ODD amount of zeros before A « 1 » isif EVEN amount of zeros before - Narrow Spectral Width (2.5 less) - Allows 50GHz - Bad sensitivity (Low duty cycle) - Dispersion Robust (2.5 more) Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Introducing RZ-DPSK (Differential Phase Shift Keying): Most promising modulation scheme: - 3 dB higher Average Power (always light) - Extremely robust CD/PMD - Complex and expensive A « 1 » and a « 0 » have light A « 1 » is represented by a phase change  Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. OOK: With CD: CD effects are reduced, Duty Cycle is High (50-67%) Consequence: to have long distance, Peak Power remains low Meaning: Low risks of Non-Linear… Controlling CD

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Each vendor will offer a semi-proprietary solution - CD values may be system dependant - PMD values may be system dependant - No standard values DuoBinary System example (Cisco): Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Table of comparison between most popular CD-PMD Robustness OSNR Spectral eff. NRZ-OOKLow Bad Bad CS-RZ-OOKMedium Bad Bad DuoBinaryMedium Very Bad Good DPSKVery High Good (3dB) Good Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Table of comparison between most popular 40Gbps Application NRZ-OOK CS-RZ-OOK DuoBinary DPSK No Go Regional Metro LongHaul Modulation Techniques

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Controlling CD Compensation Schemes CD can be a true killer: - Granularity of compensation must match ~5 km for G.652 fibers - Must be adapted to fiber TYPE and BRAND!

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Controlling CD Typical Compensation Schemes A: - Dispersion-compensating fibers are used for rough compensation -Tunable compensators at the end (in feedback) to maintain the CD within acceptable limits B: - Tunable compensation at each site

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Controlling CD

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Controlling CD Negative Pre-Chirp - Red-shifting high frequencies, blue-shifting low ones - Only limited amount can be applied 10 Gb/s: 40 Gb/s: LaunchAfter Fiber Pre-Amp DCFAmp Low-chirp Neg-chirp

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Controlling CD Typical Compensation Schemes Must take SLOPE into account:

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. PMD Issues Source: John Peters, Ariel Dori, and Felix Kapron, Bellcore 10G 40G

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. PMD changes within DWDM channel… Same units as CD Reduces further the CD tolerance of systems Wavelength PMD Issues: 2 nd Order PMD

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Conclusions As systems move towards 40G; CD is system-dependent, but critical. FTB-5800 is the only analyzer on the market with the required accuracy. PMD and 2 nd Order PMD are critical. The FTB-5500B is the most accurate analyzer on the market.

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Questions?