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Agenda

Agenda. Introduction Digital Transmission Dispersion in optical Networks. Dispersion challenges for 40G OSA challenges for 40G/ROADM’s. Mike Harrop mike.harrop@exfo.com. Dispersion & 40G. October 2007. 40 Gb/s vs. 10 Gb/s. 40 Gb/s drivers:

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Agenda

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  1. Agenda Introduction Digital Transmission Dispersion in optical Networks. Dispersion challenges for 40G OSA challenges for 40G/ROADM’s

  2. Mike Harrop mike.harrop@exfo.com Dispersion & 40G October 2007

  3. 40 Gb/s vs. 10 Gb/s • 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

  4. 40 Gb/s vs. 10 Gb/s 10Gbps uses NRZ-OOK (non-return-to-zero, On-Off Keying)

  5. 40 Gb/s vs. 10 Gb/s 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 - 2nd 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???

  6. 40 Gb/s vs. 10 Gb/s Viable 40 Gb/s requires: - Advanced modulation techniques - Sophisticated CD compensation - Low-noise amplifiers - Powerful FEC - Great characterization tools

  7. Modulation Techniques 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…

  8. Modulation Techniques 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…

  9. Modulation Techniques 0 p 0 p 0 p 0 1 0 1 1 1 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

  10. Modulation Techniques Introducing RZ-DuoBinary A « 1 » is if ODD amount of zeros before A « 1 » is if EVEN amount of zeros before - Narrow Spectral Width (2.5 less) - Allows 50GHz - Bad sensitivity (Low duty cycle) - Dispersion Robust (2.5 more)

  11. Modulation Techniques 0 p 0 0 0 p 0 1 1 0 0 1 • 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

  12. Controlling CD 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…

  13. Modulation Techniques 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):

  14. Modulation Techniques Table of comparison between most popular CD-PMD Robustness OSNR Spectral eff. NRZ-OOK Low Bad Bad CS-RZ-OOK Medium Bad Bad DuoBinary Medium Very Bad Good DPSK Very High Good (3dB) Good

  15. Modulation Techniques Table of comparison between most popular 40Gbps Application NRZ-OOK CS-RZ-OOK DuoBinary DPSK No Go Regional Metro LongHaul

  16. 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!

  17. 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

  18. Controlling CD

  19. Controlling CD • Negative Pre-Chirp • Red-shifting high frequencies, blue-shifting low ones • Only limited amount can be applied • Launch After Fiber Pre-Amp DCF Amp • Low-chirp • 10 Gb/s: • Neg-chirp • 40 Gb/s:

  20. Controlling CD • Typical Compensation Schemes • Must take SLOPE into account:

  21. PMD Issues • Source: John Peters, Ariel Dori, and Felix Kapron, Bellcore 10G 40G

  22. PMD Issues: 2nd Order PMD PMD changes within DWDM channel… Same units as CD Reduces further the CD tolerance of systems Wavelength

  23. 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 2nd Order PMD are critical. The FTB-5500B is the most accurate analyzer on the market.

  24. Questions?

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