Coherent Optical Networking

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New Flexible Architectures

Dispersion CompensationCapacity EvolutionWhy Coherent?Significant technical challenges moving to 40G/100G using Direct Detection

Modulation Formats

DBPSK40Gbaud

On/Off Keying40 Gbaud

DQPSK20 Gbaud

COH-DP-8-PSK6.7 Gbaud

COH-DP-16QAM5 Gbaud

0

0 1 2 3 4 5 6 7 8 9

1

2

3

4

5

6

Bits per Symbol

ASE

Tol

eran

ce (d

B)

COH-BPSK40Gbaud

COH-DP-QPSK10 Gbaud

These modulation formats with their constellation diagrams represent the same bit rate (in this example 40 Gb/s) and their tolerance to amplified spontaneous emission (ASE) noise. Both coherent BPSK and coherent DP-QPSK offer superior performance and cost advantages.

Polarization Multiplexing or Dual Polarization: The channel bit rate can be doubled without affecting bandwidth by combining orthogonal polarizations.

With coherent technology, system capacity is the product of three dimensions, an increase of: 1) symbol rate, 2) constellation multiplicity, and 3) subcarrier multiplicity.

All three dimensions must be exploited in order to optimize system spectral efficiency, performance, cost, and reliability, and become critical elements as the network evolves to 400G/1T. A software-programmable modem can alter baud rates and modulation formats to suit link performance.

Spectral Efficiency

Gb

/s in

50

GH

z

Spec

tral

Eff

icie

ncy

103 20

6

2

0.8

0.2

102

101

100

-10 -5 0 5 10 15 20 25 30 35 40

OSNR in 0.1 nm [dB]

40 Gb/s

100 Gb/s

10 Gb/s

In practice, spectral efficiency is limited by non-linearities. A spectral efficiency of 12 bits/s/Hz is conceivable with ~1600 km reach by employing compensation for intra-channel nonlinearities. The maximum C-Band capacity can exceed 50 Tb/s.

Coherent optical processors enable new, more flexible architectures.

Conventional 10G Optical Link with DCMs

1 span DCM DCM DCM DCM

1 span Fiber Effects: CD, PMD, PDL

DP QPSK Signal

Tx

Tx

Rx

Rx

DSP

Coherent 40G/100G Optical Links with Electronic Dispersion Compensation

With the removal of DCMs and their associated amplifiers from the network, electronic dispersion compensation provides benefits of: 1) lower CAPEX, 2) lower power consumption 3) higher system availability, and 4) reduced latency (20-25% in NDSF-based networks).

Uncompensated systems provide an optimal performance environment for coherent transponders, and offer the easiest migration path to coherent 40G/100G systems. Compensating for CD, PMD, etc. in the DSP allows for path-independent performance within the transparent reach of the transponder and opens the door to simple re-routing of wavelengths across the network.

Uncompensated networks remove all the link engineering associated with compensating for CD and PMD, and operate with better performance than 10G over PMD-challenged fiber.

QAM, M-ARY

freq

“Super Channels”

Df

Single manageableoptical entity

Symbol rate (symbols/sec)

Bit

s p

er s

ymb

ol

Number of s

ubcarri

ers

-8-8 -6 -4 -2 0 2 4 6 8

-6

-4

-2

0

2

4

6

8

Real part of signal

Imag

inar

y p

art

of

sig

nal

Coherent Optics

Simple Power Splitter & Combiner

LO

LO

LO

GridlessUp to 30% spectrum recovery

Optical BroadcastLow-cost components replacinghigh-cost components

ADC Analog-Digital ConverterASE Amplified Spontaneous EmissionBPSK Binary Phase Shift KeyingCD Chromatic DispersionCMOS Complimentary Metal Oxide SemiconductorDAC Digital-Analog ConverterDBPSK Differential Binary Phase Shift KeyingDCM Dispersion Compensation ModuleDD Direct DetectionDQPSK Differential Quadrature Phase Shift KeyingDP-QPSK Dual Polarization Quadrature Phase Shift KeyingDSP Digital Signal ProcessingDWDM Dense Wavelength Division MultiplexingFEC Forward Error Correction

PMD Polarization Mode Dispersionps picosecondPSK Phase Shift KeyingQAM Quadrature Amplitude ModulationQPSK Quadrature Phase Shift KeyingROADM Reconfigurable Optical Add-Drop MultiplexerSTM Synchronous Transmission Mode

GigE Gigabit EthernetIEEE Institute of Electrical and Electronics EngineersIMDD Intensity Modulated Direct DetectionITU-T International Telecommunications Union, Telecommunication Standardization SectorMSA Multi-Source Agreement NDSF Non Dispersion-shifted Fibernm nanometerOIF Optical Internetworking ForumODU Optical channel Data UnitOTU Optical Transport Unit OTN Optical Transport NetworkPDL Polarization Dependent LossPM QPSK Polarization-Multiplexed Quadrature Phase Shift Keying (= DP-QPSK)

Acronyms Products

Standards

OTN Network

IEEE IEEE

OIF OIF

ITU-T ITU-T

Included in G.Sup43

ODU1 (L) OTU1

ODU2 (L) ODU2 (H)

OTU2

ODU2e (L)

ODU3 (L) ODU3 (H)

OTU3

ODU4 (L) ODU4 (H)

OTU4

ODU0 (L)

8 x

32 x

80 x

1GbE

2.5G STM-16/OC-48

10G STM-64/OC-192

STM-256/OC-768,40GbE

FC1200, 10GbE

100GbE

ODU1 (H)2 x

3 x 10 x

2 x

Included in revised G.709

10 x

40 x ODUflex Various

OTU3e1 ODU3e1 (H)

4 x

G.Sup43

OTU3e2 ODU3e2 (H)

4 x

G.Sup43

OTU2e G.Sup43

ITU-T OTN Hierarchy

ActivSpan 4200

AdvancedServices Platform

Coherent Optical Technology

Phase Shift

X-pol I

X-pol Q

Y-pol I

Y-pol Q

Laser Frequency Control

PolarizationBeam Splitter

Analog to Digital

Converter

DigitalSignal

Processing

Clock andData

Recovery

Complexity buried in cost-effective CMOS

Coherent +DWDM

Coherent technology scalesfor capacity growth

C-bandCapacity

Pb/s

Tb/s

Gb/s

DD +DWDM

DirectDetection (DD)

Coherent technology, with a local oscillator approximately centered on the signal’s frequency band, provides a 4.3dB improvement in noise tolerance over traditional direct detection.

The most important characteristic of a coherent receiver, access to the optical electrical field (E-field), provides access to amplitude, phase and polarization information; one can now increase transmission rates via advanced modulation techniques which use multiple bits per symbol. Digital signal processing techniques can now also be used to fully compensate for linear degradations from optical filtering, chromatic dispersion, polarization mode dispersion, and polarization dependent loss.

The spectral selectivity characteristic of a coherent receiver allows further capacity scaling using coherent demultiplexing of multiple carriers. It also enables new, more flexible photonic architectures such as an elegant colorless architecture with minimal optical filtering.

With coherent technology, one can overlay 40G/100G on 10+ year-old systems that had been engineering for 10G IMDD, and reap the benefits of increased spectral efficiency with minimal network investment.

There are four key components that comprise state-of-the-art coherent designs: 1) high-speed, high-resolution DACs for channel pre-equalization and offering modulation format freedom (BPSK, QPSK, QAM), 2) high-speed, high-resolution ADCs performing channel post-equalization and polarization recovery, 3) ultra-high-performance FEC for maximum reach, and 4) advanced DSP algorithms compensating for optical filtering effects, CD, PMD, and PDL.

Phase Shift

ActivFlex 6500 14-slotActivFlex 6500 7-slot ActivFlex 6500 32-slot

Packet-Optical Transport

The Shannon limit (orange line), is the upper limit for channel capacity for a fixed amount of noise.

Inter-Metro & £Long-Haul

Packet-Optical Network

Inter-Metro & £Long-Haul

Packet-Optical Network

Undersea Network

Unified Network Management

Packet-Optical Platforms

ActivFlex 5430ActiFlex 5410

ReconfigurableSwitching Systems

Packet-Optical Switching