© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Agenda Introduction Digital Transmission Dispersion in optical Networks. Dispersion challenges

© 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


Dispersion & 40G

October 2007

Mike [email protected]


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


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

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/s


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


NRZ-OOK (on/off keying) vs.

RZ-OOK (on/off keying)

More resistant to dispersionHigher Peak power (Non-linear effects danger)Lower duty cycle (33% typical)

But not enough…

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…



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

1 0 10 1 1



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)



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

1 1 10 0 0



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


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):



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



Table of comparison between most popular

40Gbps Application

NRZ-OOK

CS-RZ-OOK

DuoBinary

DPSK

No Go

Regional

Metro

LongHaul



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 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 CD


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:

Launch After Fiber Pre-Amp DCF Amp

Low-chirp

Neg-chirp


Controlling CD

Typical Compensation Schemes

Must take SLOPE into account:


PMD Issues

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

10G

40G


PMD changes within DWDM

channel…

Same units as CD

Reduces further the CD tolerance of

systemsWavelength

PMD Issues: 2nd Order PMD


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.


Questions?

Documents

© 2006 EXFO Electro-Optical Engineering Inc. All rights reserved. Agenda Introduction Digital Transmission Dispersion in optical Networks. Dispersion challenges