Optical Layer Concatenation of Long- Span Amplified Systems Span - - PowerPoint PPT Presentation

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Optical Layer Concatenation of Long- Span Amplified Systems

J Schwartz¹, S Ediridinghe¹, W Wong¹ Rodolpho Acebo Jr², Gregg Palinski³

¹Xtera Communications, ²Pacnet, ³Global Crossing

Span Amplified Systems

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Presenter Profile

Since joining the former Azea Networks, now Xtera, Joerg has managed the NXT systems definition and developed the company’s systems engineering competency, providing network solution design, field and lab trials, sales support, design, field and lab trials, sales support, and systems research. Previous experience includes System Design for Ericsson, submarine terminal development for Alcatel, and founding optical components supplier Quarterwave. Joerg Schwartz VP System Engineering Email: Joerg.Schwartz@xtera.com Tel: +44 1708 335408 Mobile Tel: +44 7817 394626

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Why link concatenations?

• Avoids terminals at intermediate

stations needed where no traffic is dropped

• Allows extension of submarine

line to Point of Presence

• This means simpler, more
• Long Island

USA

• New York

POP

reliable networks

• Easiest on a per fibre pair basis
• Possible also for single/bands of

channels

• Can be re-configured if demand

changes (unlike Branching Units)

• Sylt CLS

Germany Holland CLS Amsterdam POP

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Concatenation Options

• Type of links to be combined

– Repeatered submarine with repeatered submarine – Repeatered submarine with unrepeatered submarine – Repeatered submarine with terrestrial – Unrepeatered submarine with unrepeatered submarine – Unrepeatered submarine with unrepeatered submarine → see Poster by Philippe Perrier – Combinations of these

• Location of interconnect

– At SLTE client interface – trivial but costly (only saves SDH equipment) – At cable station interface – most common – At beach manhole

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Potential Issues and Challenges

• Implementation

– Compatibility of different links – Compatibility of line currents (if PFE is shared)

• Network management

– Removal of network elements requires re-configuration – Removal of network elements requires re-configuration – Line performance monitoring – Management access to remote pass-through amplifiers

• Transmission

– Impact on operating margins – Impact on maximum link capacity

• A range of transmission issues are possible and require

a closer look…

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Loss and Link Attenuation

• If an existing segment is extended by a unrepeatered terrestrial or subsea

link, this reduces the power at the input of the receiver, or the first repeater

• Linking amplified segments a high-loss amplification span at the

interconnection has to be avoided– this would reduce the Optical Signal to Noise Ratio (OSNR) and performance

11.00 12.00 Amplifier

• Insertion of a optical ‘pass-through’ amplifier at the interconnection point

fixes the problem

6.00 7.00 8.00 9.00 10.00 11.00 1545 1550 1555 1560 1565

OSNR (dB/nm) Wavelength (nm)

no Amplifier

Concatenation of two similar 1800 km segments with long shore end spans

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Optical Signal to Noise Ratio

• As for single span line design, adequate receive OSNR

performance is first priority

• Most submarine systems – independent of length – are designed to

meet a target OSNR of ~7 dB (in 1 nm)

– Cascading two segments reduces this by 3 dB, i.e. to ~4 dB/nm 20

Original Link

• Reduced OSNR leads to degraded Q and/or less channels being

supported

5 10 15 2 4 6 8 10 12 14 OSNR (dB/nm) Distance (1000 km)

Original Link Extension with same characteristics Extension with different characteristics

Performance Limit

• 3 dB

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Capacity Impact

• Reduced OSNR means that margins are reduced by ∆Q
• To recover this and maintain the capacity, SLTE with improved

performance (improved FEC gain, less photons per bit) is needed

• Alternatively, the number of wavelengths sharing the (fixed) repeater

power has to be reduced by a factor of ~10(∆Q/10), e.g. 0.8 for -1 dB

5

• If Q penalty is higher than OSNR degradation in dB, this suggests

that additional degradations apply, such as nonlinearities

1 2 3 4 5 1 2 3 4 5

Observed Q Penalty (dB) Distance Imposed OSNR Degradation (dB)

6200+6200 km (10G) 2000+2000 km (40G) 2000+1000 km (10G) 2300+2300 km (20G) 7000+500+100 km (20G)

Nonlinear Region

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Nonlinearities

• Nonlinear propagation impairments increase with transmission

distance, hence they are more pronounced for extended links

• The lower nonlinear threshold reduces the achieve able Q

13 14 15

Original link (7000 km) Extended link (7600 km)

• Detailed computer simulations help understanding and mitigating

the impact of nonlinearities, e.g. via dispersion management

8 9 10 11 12 13 1 2 3 4 5 6 7 8 Q(dB) OSNR (dB/nm)

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Dispersion

• Repeatered links are usually fully dispersion managed

– However links can be differently designed, e.g. different dispersion slope

• r zero accumulated dispersion wavelength
• Terrestrial/unrepeatered links use G.652/G.654 fibres. If added to a

link, these off-set the dispersion pre-/post-compensation

2000

S1 S2 S3

• Dispersion management needs to checked for entire bandwidth and

adjusted at intermediate location as required, e.g. via DCF+amplifiers

• 2000
• 1500
• 1000
• 500

500 1000 1500 2000 1000 2000 3000 4000 5000 6000 7000 8000 Dispersion (ps/nm) Distance (km)

S1 S2 S3

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Gain Shape

• Combining different generation wetplant or links made by different
• riginal suppliers often means mixing very different gain

characteristics

• Mismatch leads to distorted dispersion maps and different gain

characteristics in the two different directions, reducing the bandwidth

Segment 5+6 Segment 5 only

• Intermediate addition of filters or loading waves reduce the impact

Segment 5+6 Segment 6+5 Segment 6 only Segment 5 only

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Pass-Through Optimisation

• The impact of the gain mismatch can be mitigated by intermediate

elements

• Gain shaping filters can be used to re-adjust the channel powers

before re-launch of the signal into the next segment

• In the example below loading waves have been added at the

concatenation location

• This avoids build-up of 1538 nm gain peak and optimises OSNR

1530 1540 1550 1560 1570 1580

Segment A Segment B Segment A+B, loaders added Segment A+B

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Conclusions

• Serial concatenation of several subsea links, or

extending them with unrepeatered/terrestrial spans helps

• ptimising submarine network topologies
• The capacity impact of increasing the transmission

distance can usually be minimised by using advanced distance can usually be minimised by using advanced terminal equipment

• If the links to be combined are very dissimilar,

intermediate elements such as dispersion compensation, amplification, filtering, or loading must be considered

• Careful system design, validated by computer

simulations or practical tests are important for successful implementation