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Agenda. Introduction Digital Transmission Dispersion in optical Networks. Dispersion challenges for 40G OSA challenges for 40G/ROADM’s. Mike Harrop [email protected] Dispersion & 40G. October 2007. 40 Gb/s vs. 10 Gb/s. 40 Gb/s drivers:

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Agenda
Agenda

Introduction Digital Transmission

Dispersion in optical Networks.

Dispersion challenges for 40G

OSA challenges for 40G/ROADM’s


Dispersion 40g

Mike Harrop

[email protected]

Dispersion & 40G

October 2007


40 gb s vs 10 gb s
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


40 gb s vs 10 gb s1
40 Gb/s vs. 10 Gb/s

10Gbps uses NRZ-OOK (non-return-to-zero, On-Off Keying)


40 gb s vs 10 gb s2
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???


40 gb s vs 10 gb s3
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


Modulation techniques
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…


Modulation techniques1
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…


Modulation techniques2
Modulation Techniques

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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 techniques3
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)


Modulation techniques4
Modulation Techniques

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  • 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


Controlling cd
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…


Modulation techniques5
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):


Modulation techniques6
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


Modulation techniques7
Modulation Techniques

Table of comparison between most popular

40Gbps Application

NRZ-OOK

CS-RZ-OOK

DuoBinary

DPSK

No Go

Regional

Metro

LongHaul


Controlling cd1
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!


Controlling cd2
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



  • Controlling cd4
    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:


    Controlling cd5
    Controlling CD

    • Typical Compensation Schemes

    • Must take SLOPE into account:


    PMD Issues

    • Source: John Peters, Ariel Dori, and Felix Kapron, Bellcore

    10G

    40G


    Pmd issues 2 nd order pmd
    PMD Issues: 2nd Order PMD

    PMD changes within DWDM channel…

    Same units as CD

    Reduces further the CD tolerance of systems

    Wavelength


    Conclusions
    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.



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