coherent dwdm

8/10/2019 Coherent DWDM

1/7

Invited Papers

Coherent DWDM technology for high speed optical communications

Ross Saunders

Opnext Subsytems Inc., 151 Albright Way, Los Gatos, CA 95032, USA

a r t i c l e i n f o

Article history:

Available online 7 September 2011

Keywords:

DWDM

Coherent

Optical

DSP

QPSK

QAM

a b s t r a c t

The introduction of coherent digital optical transmission enables a new generation of high speed optical

data transport and fiber impairment mitigation. An initial implementation of 40 Gb/s coherent systems

using Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) is already being installed in carrier

networks. New systems running at 100 Gb/s DP-QPSK data rate are in development and early technologylab and field trial phase. Significant investment in the 100 Gb/s ecosystem (optical components, ASICs,

transponders and systems) bodes well for commercial application in 2012 and beyond. Following in

the footsteps of other telecommunications fields such as wireless and DSL, we can expect coherent optical

transmission to evolve from QPSK to higher order modulations schemes such as Mary PSK and/or QAM.

This will be an interesting area of research in coming years and poses significant challenges in terms of

electro-optic, DSP, ADC/DAC design and fiber nonlinearity mitigation to reach practical implementation

ready for real network deployments.

2011 Published by Elsevier Inc.

1. Introduction

To keep pace with the rapidly growing volumes of data network

traffic drivenby the growthof the internet, service providers are al-

ways looking to increase thefiber capacity and wavelength spectral

efficiency in their networks [1]. Typical Dense Wavelength DivisionMultiplexing (DWDM) networks of today employ a 50 GHz channel

spacing, as per the international standard[2]. At 10 Gb/s data rate

spectral efficiency was not a major concern and simple On Off

Keying (OOK) modulation format was adequate for operation on

the 50 GHz DWDM grid. At 40 Gb/s, the spectral width of the signal

is 4x largerfor OOK, yielding a signalspectralwidththatiss toowide

to fit through 50 GHz channel spacing optical filters without induc-

ing excessive penalties. So at 40 Gb/s data rate system and tran-

sponder developers investigated alternate modulations schemes

to enable 40 Gb/s propagation over the same 50 GHz DWDM grid,

such as Phase Shaped Binary Transmission, PSBT [1], Differential

Phase Shift Keying (DPSK) [3] and DP-QPSK [4]. PSBT and DPSK

offered increased spectral efficiency over OOK, whilst still coding

1 bit per symbol. DP-QPSK, on theotherhand,codes 4 bits per sym-bol (in-phase and quadrature phase components of each polariza-

tion tributary). Coding more bits/symbol, enabled by the advent of

digital coherent transmission [5], reduces the spectral width of

the signal (to 1st order proportional to the baud rate). In fact,

DP-QPSK is so spectrally efficient that it canpropagate a higher data

rate of 127 Gb/s through many cascaded 50 GHzoptical filters, such

as Reconfigurable Optical Add/Drop Multiplexers (ROADMs) [6].

This higher 127 Gb/s data rate not only allows payload transport

of 100GE traffic[7], but also OTU4 link management overhead [8]

and 20% overhead, Soft Decision Forward Error Correction

(SD-FEC) [911] for high performance applications. Therefore,

100 Gb/s transmission using DP-QPSK offers the promise of a good

modulation format fit for DWDM networks operating on a 50 GHzgrid[12]. This was observed several years ago and was the reason

why this formatwas adopted by theOptical Internetworking Forum

(OIF) as a recommended modulation format for 100 Gb/s line

systems[13]. This industry forum has helped to focus investment

and multi-source agreements at the optical component and module

level to help foster an ecosystem that should accelerate network

adoption of 100G DP-QPSK transmission.

Looking to the future, as the internet growth continues with

expanding services such as High Definition (HD) video, mobile

broadband and telecommuting, the question is how will optical

transmission technology keep pace? Learning from other telecom-

munications fields such as wireless, satellite, radio and Digital Sub-

scriber Line (DSL) broadband access, we can say that all these

mediums utilize coherent transmission and all increase transmis-sion rates and spectral efficiency by coding more bits per symbol.

For optical fiber technology development we should surely follow

the lead from these other telecommunications industries but we

have some fundamental and unique challenges that makes our life

difficult. Challenges such as:(i) operation at thebleedingedge high-

est electronics speed of >100 Gb/s for the key technologies such as

ADC/DAC/DSP/FEC/RF electronics/electro-optics; (ii) optical signal-

to-noise ratio (OSNR) requirements become tough as Shannons

Limit dictates that as we increase spectral efficiency via higher-or-

der modulation we need more OSNR and (iii) fiber nonlinearity

poses a major obstacle as high density signal constellations such

1068-5200/$ - see front matter 2011 Published by Elsevier Inc.

doi:10.1016/j.yofte.2011.06.016

Fax: +1 613 678 6707.

E-mail address: [email protected]

Optical Fiber Technology 17 (2011) 445451

Contents lists available at ScienceDirect

Optical Fiber Technology

www.elsev ier .com/locate /yofte


2/7

as M-ary Quadrature Amplitude Modulation are very sensitive to

phase errors due to nonlinear phase noise and Cross Phase Modula-

tion (XPM). Although these challenges appear daunting and formi-

dable it would be unwise to bet against theses problems being

solved by human engineering ingenuity given time and money, as

history has proven. This will be an extremely fertile area of optical

communication research over the next decade and beyond.

2. 100 Gb/s DP-QPSK implementation

2.1. Technology

The basic functional block diagrams for an optical coherent

detection modulation scheme, with control of the amplitude of

both in-phase, I and quadrature phase, Q, components of the mod-

ulated signal is shown in Fig. 1. Note that although the data

throughput is 100 Gb/s, extra overhead bytes are required for

64B/66B Ethernet PCS encoding, OTU4 framing overhead, training

sequence and FEC, which in combination adds 28% to the line rate.

This modulation format codes 4 bits per symbol (for the

In-phase, I and Quadrature-phase, Q components of the each polar-

ization multiplexed tributary), yielding a symbol rate of 32 Gbaud.

The transmit side consists of nested Mach Zehnder Modulators(MZM) structures. The coherent receiver requires mixing the

received signal light with a tunable laser local oscillator. Polariza-

tion Beam Splitters (PBS) and optical phase hybrids are included in

the receive structure to provide polarization and phase diversity. A

key advantage is that the Carrier Phase Estimator (CPE), polariza-

tion and I&Q demultiplexing is all achieved in the electronic do-

main using very fast Analog-to-Digital Converter (ADC) and

Digital Signal Processing (DSP). This alleviates the traditional prob-

lem with optical coherent technology in that a highly stable optical

Phase Locked Loop (PLL) is not required in this design.

Thecriticalenablingtechnologyin thisdesign is thedigitalcoher-

entreceiver,as shown in Fig.2. Thedistorted signalcoming from the

four balanced photodiodes is first quantized using quad 6 bit ADCs.

The adaptive equalizer in the DSP then provides the equalization of

CD, PMD, ROADM filtering distortion and unwanted S21 transferfunction imperfections in the Tx/Rx electro-optic drive chains.

Another critical enabling technology is next-generation soft

decision FEC, enabling up to 3 dB higher coding gain than current

state-of-the-art FEC. A FEC algorithm called Low Density Parity

Check (LDPC) is used, with increased overhead [9] and uses soft

decision decoding of the input. This is in contrast to all optical

transport systems today which use hard decision decoding. As the

digital coherent receiver already generates a quantized version of

the analog signal, this quantized signal can then be passed to a soft

decision FEC decoder.Fig. 3shows the LDPC FEC coding gain as a

function of soft decision bit resolution and overhead rate for an ide-

alized case with no implementation penalty. FEC design optimiza-

tion is a tradeoff between decoder complexity/chip size vs

performance in selection of bit resolution and electro-optic band-width/ROADM tolerance vs performance in selection of the over-

head rate.

Another important property of any metro, regional or long haul

technology is the capability to express through multiple cascaded

optical 50 GHz channel spacing ROADMs, without incurring a large

penalty. DP-QPSK offers excellent tolerance to narrowband optical

filtering due to the low baud rate, which reduces the spectral width

of the signal, and the adaptive equalizer in the Rx, which cleans-up

filtering distortion. Results comparing DP-QPSK with alternative

100G modulation formats are shown in Fig. 4. As can be seen,

DP-QPSK modulation format shows excellent tolerance to extre-

mely narrowband optical filtering. Depending on the ROADM band-

width and profile, 100G DP-QPSK is capable of expressing through

up to 10 cascaded ROADMs at 50 GHz channel spacing[6].

For 100G deployments to be cost-effective in Long Haul DWDM

networks, it is critical that the optical reach is sufficiently large that

the requirement for OEO regenerators is limited, ideally non-exis-

tent. The optical reach is maximized using two critical technologies:

(i) coherent detection (buys 2.5 dBQ) and(ii) LDPC soft decision FEC

(buys up to 3 dBQ). Optical reach up to 2000 km with EDFA only,

5000 km with EDFA + Raman and 7000 km for submarine links is

possible using 100G DP-QPSK technology, with deployable margin.

The propagation performance depends on fiber type and whether

in-line DCMs are deployed or not. Nonlinearity is mitigated by not

deploying in-line DCMsand performing all CD compensation within

the Opnext 100G module. Fig. 5 shows propagation results over dif-

ferent fiber types, with and without in-line DCMs. Note that the

optimum fiber type is SMF-28 fiber without in-line dispersion

compensation. This is also the lowest cost fiber plant for thecarrier.

2.2. Photonic and electronic integration

A 100 Gb/s DP-QPSK offers the ultimate in optical performance

and meets all the key marketing requirements dictated by large

CW laser

Tx Block Diagram

Rx Block DiagramBalanced

Photodiode

Iout

MZII

MZIQ

4x32Gb/s

[32Gbaud]inputs

Gray/2

PBS

TE

TM

MZII

MZIQ

/2

Gray

Gray

Gray

Balanced

Photodiode

BalancedPhotodiode

Balanced

Photodiode

4x32Gb/s

[32Gbaud]

outputsADC/DSP

Iin

Local

Oscillator

90 hybrid

(phase/polarizationdiversity)

Fig. 1. A 128 Gb/s DP-QPSK Tx/Rx implementation.

446 R. Saunders/ Optical Fiber Technology 17 (2011) 445451


3/7

carriers. The challenge is that this is a much more complex modu-

lation scheme than previous generations of optical transport

equipment. Both the electronic and photonic complexity has been

increased substantially. This creates a design challenge in terms of

cost, manufacturability, reliability and footprint. The industry

approach to tackling this significant challenge has been to develop

components with a high level of electronic and photonic integra-

tion. On the electronic side, SiGe MUX and CMOS modem chips

with large gate count and integrated system functionality. On the

photonics transmit side, a single optical assembly contains the

nested MZMs, PBS and splitters, as per OIF implementation

agreements [14]. On the photonics receive side, a single optical

assembly houses the PBS, phase hybrids, balanced photodetectors

and Trans-Impedance Amplifiers (TIAs). This integration is shown

in Figs. 6a and b. This high level of integration greatly eases the

manufacturability of the module.

2.3. Dynamic optical networking

Looking to the future it can be envisaged that future optical net-

works will utilize some level of all-optical switching to facilitate

fast restoration and protection of optical circuits. One limitation

of early 40 Gb/s systems is that Chromatic Dispersion (CD) tuningcan take a relatively long time, on the order of seconds or even

minutes. This is true for both direct detection systems that typi-

cally use a thermally tuned optical tunable dispersion compensator

and for 1st generation coherent systems where the CD equalizer

hunts for the optimum CD value of the link.

By designing coherent transmission systems with in-built train-

ing sequences, it is possible for the CD equalizer to know virtually

instantaneously the optimum tap weight coefficients in the equal-

izer to compensate for an unknown link CD, without requiring this

hunting algorithm. This enables very fast (ms) timeframe CD

acquisition and equalization. The Jones matrix equalizer inside

the MODEM can also rapidly adapt to the optimum State of

Polarization (SOP) and DGD compensation of the link. This rapid

Adaptive Equalizer

(n tap Digital FIR filter)A>D converters

Decision

Circuit

+

Adaptive

Algorithm

Input

distorted

data

Recovered

output

data

Error signal

ITE

QTE

ITM

QTM

Fig. 2. Digital coherent receiver.

hard

decision

soft

decision

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

0.0% 10.0% 20.0% 30.0% 40.0% 50.0% 60.0%

FEC Overhead

NECG(

dB) 6 bits

4 bits

3 bits

2 bits

1 bit

Current gen EFEC

3to

4dBgain

Fig. 3. LDPC soft decision FEC performance.

Fig. 4. A 128 Gb/s DP-QPSK optical filtering tolerance.

6

7

8

9

10

11

12

13

-4 -3 -2 -1 0 1 2 3 4

Launch Power (dBm)

Q(

dB)

LEAF (no DCM)

SMF-28 (no DCM)

LEAF (with DCM)

SMF-28 (with DCM)

LDPC

FEC cliff

Min Q

w/ margin

Fig. 5. A 128 Gb/s DP-QPSK propagation over a 1520 km link.

R. Saunders/ Optical Fiber Technology 17 (2011) 445451 447


4/7

MODEM equalization of CD and PMD enables fast optical path res-

toration/protection, a key requirement for dynamic optical net-working architectures of the future.

Another advantage of coherent transmission is that the MODEM

itself provides real-time link parameter performance monitoring.

This allows in-skin monitoring of CD, PMD, SOP and SNR for link

troubleshooting and link quality metrics (e.g. could be used for

pre-emptive switching criteria) without the use of intrusive exter-

nal test equipment.

As next-generation DWDM line systems often advocate color-

less, directionless ROADMs to allow versatile optical wavelength

routing, another advantage of coherent is that a colorless receiver

can be used, such as optical splitters. The correct DWDM wave-

length is then selected by tuning the laser Local Oscillator (LO)

in the receive transponder. This eliminates the use of colored

DWDM demultiplexers (fixed single wavelength per fiber output)

and allows more flexible dynamic optical networkingarchitectures.

3. Mary QAM for Beyond 100Gb/s

3.1. Introduction

A high data rate can be achieved by coding multiple bits/symbol

and using coherent detection and Mary Quadrature Amplitude

Modulation (M-QAM) modulation format. Present day optical

transport speeds are limited by electronics speed, whereas this

technique allows data rates many multiples higher than the elec-

tronics speed. Although the use of QAM is well known in other

industries, such as satellite and wireless communications, it has

not been implemented to date for optical transmission.

A novel advantage of using M-QAM in DSP is that by enabling

programmable modulation (e.g. from QPSK to 256-QAM) the bit

rate transmitted can be traded for optical reach. This technique

will maximize the data rate for any given link length and distortion

properties of the channel. This capability is analogous to rate adap-tive DSL modems that maximize the data rate over local copper

90 deg

Hybrid

Mixer

XIADC

XI

XQADC

XQ

YIADC

YI

YQADC

YQ

I

Q

90 deg

Hybrid

Mixer

I

Q

SIGNAL

LOCAL

OSCILLATOR

DSP

PBS

PS

PBCPSSIGNAL

LASER

RZ PULSE CARVER

(OPTIONAL)

QPSK MODULATOR

QPSK MODULATOR

~CLOCK

MUX

I

Q

MUX

I

Q

Drive = 2V

BW > R

Fig. 6. A 128 Gb/s DP-QPSK photonic and electronic integration. (a) Transmit. (b) Receive.

CW laser

Tx Block Diagram

Rx Block Diagram

Balanced

Photodiode

Iout

Iin

MZII

MZIQQuad-phaseNRZ data

In-phase

NRZ data

/2

Balanced

Photodiode

Local

Oscillator

90 hybrid

(phase diversity)N.B. Needs

polarization

diversity ortracking for Iinand LO mixing

ADC/DSP

DSP/DAC

Baud rate, bn

Baud rate, bn

One polarization tributary shown for simplicity

Fig. 7. M-QAM Tx/Rx implementation.



5/7

connections from local office to customer premise, using a training

sequence at installation. The increased data throughput without

any significant increase in the number of active electro-optic com-

ponents should yield a reduction in field failure rate and improved

reliability vs todays optical transponder technology.

3.2. Mary QAM Implementation

The transmit/receive block diagrams are shown in Fig. 7. Thisshows the basic functional block diagrams for an optical coherent

detection modulation scheme, with control of the amplitude of

both in-phase, I and quadrature phase, Q, components of the mod-

ulated signal. In this case, the nominal baud rate, bn, is constant,

but the baud rate could also be rate adaptive to offer more contin-

uous rate adaption, rather than the discrete steps from moving

between M-QAM symbol spaces. Note that 2 polarizations can be

utilized (only 1 shown inFig 7) to double the traffic carrying capac-

ity, using a polarization beam combiner and separate quadrature

modulators for each orthogonal polarization state. The Rx consists

of a synchronous coherent detection scheme. The method shown in

Fig. 7 uses a free-running optical Local Oscillator (LO) and feed

forward carrier recovery, polarization demultiplexing, CD/PMD

compensation using analogue to digital conversion followed by

an adaptive digital Finite Impulse Response (FIR) filter.

As an example, the baud rate could be set to 25Gbaud (the de-

sign could also support variable baud rate to maximize capacity for

a given optical channel filter) and both polarizations modulated. To

maximize reach, the transponder could be configured to transmit

DP-BPSK, as shown inFig. 8. This would give the maximum OSNR

sensitivity and maximum launch power possible, therefore maxi-

mizing the distance that can be transmitted between signal 3R

regeneration points in the network. The capacity in this example

would be 50 Gb/s (1 bits/s/Hz). If the specific channel has excess

performance margin, the transponder could reconfigure to

PM-QPSK as shown inFig 8(b). Note that in this case the OSNR sen-

sitivity is the same (I and Q component noise is independent) butthat the launch power would be slightly lower than DP-PSK as it is

more sensitive to nonlinear phase noise (90 between symbol

states for QPSK, whereas 180 for PSK). This doubles the capacity

to 100 Gb/s. In a similar fashion, if the channel has still more mar-

gin (i.e. typically if it operates over a shorter reach) the transpon-

der can re-configure as DP-8QAM (Fig. 8c), DP-16QAM (Fig. 8d),

DP-32QAM (Fig. 8e), DP-64QAM (Fig. 8f), DP-128QAM (Fig. 8g)

and DP-256QAM (Fig. 8h). As can be seen from the constellation

diagrams inFig. 8, each time the M-QAM bits/symbol rate is incre-

mented, the channel carrying capacity increases, at the expense of

an increase in the required OSNR. This is all done in DSP/software.

The OSNR increase is due to the reduction in the minimum Euclid-

ean distance from symbol to symbol.

For denser M-QAM constellations, more SNR is required per

symbol for a given BER. This is shown in Fig. 9.

An analytic estimation of required OSNR sensitivity (assumes

ideal implementation) for different M-QAM bit rates in shown in

Fig. 10.

Fig. 8. Different M-ary QAM constellations, required DAC/ADC bit resolutions.



6/7

In addition to the challenging OSNR levels required for M-QAM

optical transmission, M-QAM is more sensitive to nonlinear phasenoise and distortion, so fiber nonlinearity provides a major obstacle

to transmission distance. This is an active area for further study.

Reducing nonlinearity by using more distributed optical amplifica-

tion, such as Raman amplification or more frequent Erbium Doped

Fiber Amplifiers (EDFAs) will help to reduce peak power and hence

nonlinear distortion. The key optical component to help reduce the

nonlinearity is indeed theoptical fiber itself. New optical fibers with

reduced attenuation, reduced nonlinear coefficient n2 and higher

effective core area would all help to reduce nonlinearity and enable

higher optical launch powers and hence increased reach. SuchSuper

Large Effective Area (SLA) fibers already utilized in submarine net-

work deployments are already a good step in the right direction

[14,15].

3.3. Advanced coding techniques

The two front battle between diminishing OSNR sensitivity and

increasing tolerance to nonlinear distortion for higher order modu-

lation schemes has some other weapons at its disposal. Inevitably

these techniques come at the expense of increased implementation

complexity so they areboundedby what is practicalto fit into MOD-

EM ASICs andthermal management limitations. CMOS migration to

lower geometries such as 28 nm will help open-up the possibilities

here, both in terms of transistor speed and gate count density.

Some potential areas of future study:

1. Soft Decision Forward Error Correction (SD-FEC). Need further

study here to improve net coding gain and reduce implementa-

tion penalty through more efficient algorithms, more decoder

iterations/bit resolution, code puncturing, etc.

Fig. 10. M-QAM bit rate vs OSNR sensitivity tradeoff.

Fig. 9. M-QAM symbol error rate probability.



7/7

2. Trellis/Block Coded Modulation (TCM/BCM). With coded modula-

tion additional coded bits can be used to provide redundancy

rather than send extra symbols (e.g. mapping raw 16-QAM into

Trellis-coded 32-QAM, 1 extra bit coding redundancy). This

effectively expands the signal constellation and increases the

minimum Euclidean distance between adjacent symbols, relax-

ing the OSNR sensitivity requirements[16].

3. Optimized constellation geometry. Rectangular QAM constella-

tions as shown inFig. 8are not the most efficient but are easierto realize. A more optimized M-QAM constellation, such as cir-

cular, increases the minimum Euclidean distance and OSNR

sensitivity. In addition, nonlinear phase noise is intensity

dependent and should be considered in the constellation

design. This optimization of the M-QAM constellation will come

at the expense of added complexity, likely higher DAC resolu-

tion and RF drive chain linearity/S-parameter performance.

4. Optimized symbol mapping. The M-QAM symbol mapping should

be carefully designed and optimized holistically combined with

the SD-FEC and TCM/BCM code designs. Symbol mapping diver-

sity minimizes bit errors and optimum combinations of M-QAM

symbol mapping with SD-FEC/TCM design should be sought.

5. Nonlinear compensation/mitigation. Coherent MODEMs can be

designed with some level of nonlinear compensation using

techniques such as digital backpropagation [17]. In addition

the carrier phase estimation filter shape and bandwidth profile

can be optimized to mitigate against effects such as XPM, pos-

sibly in a dynamic or programmable manner. These techniques

should allow increased optical launch power and hence higher

received OSNR but once again at the expense of increased elec-

tronic DSP complexity.

6. Use of multiple carrier Orthogonal Frequency Division Multiplexing

(OFDM).It has been claimed that the use of OFDM can reduce

nonlinearity vs single carrier transmission, at least in certain

applications such as highly periodic dispersion managed sys-

tems [18]. This needs further study to weigh-up the pros and

cons. Whether single carrier or multiple carriers are used in

coherent systems, if spectral efficiency is to increase then

higher-order modulation such as M-QAM is needed in eithercase.

4. Conclusions

The advent of coherent DWDM technology is enabling 100GE

optical transport over backbone optical networks with link engi-

neering rules similar to 10 Gb/s OOK channels. This enables a

10scaling of network/fiber capacity and is possible without any

change in DWDM channel spacing or DWDM common equipment

design. The formation of a 100G DWDM ecosystem in the OIF in the

infancy of this technology has helped focus R&D capital investment

and should act as a catalyst driving early technology adoption by

system vendors and service providers. Standardization by the IEEE

on 100GE and ITU on OTU4 encapsulation has also been critical in

laying the foundation for this technology. Moreover, the collabora-tion between IEEE and ITU on 100GE encapsulation into OTU4

frame format and commonality in such things as the electrical

interface PMD has really helped to focus engineers and minimize

time wasted reinventing the wheel. The stage is now set for

service providers to start certification testing and initial field appli-

cations of 100 Gb/s DWDM wavelengths. As always for new tech-

nology introduction there will be a period of frantic bug-fixing,

ASIC re-spins and hardware design fixes as system vendors and

service providers run thorough verification lab testing and robust-

ness is built into 100G transponder designs. First office field roll-

outs are expected in 2012 timeframe and as volume begins to ramp

optical vendors will already be working on cost-reduced, footprint-

reduced, performance enhanced next-gen 100G solutions to meet

the very high volumes expected in 2013/14 as volumes ramp [19].

Migrating to data rates beyond 100 Gb/s faces some real chal-

lenges in terms of OSNR sensitivity and nonlinearity. Perhaps wewill just utilize more wavelengths and fibers without increasing

spectral efficiency but that method will also run into scaling issues

as fibers run-out and managing too many DWDM overbuilds be-

comes unwieldy for carriers. Coherent transmission certainly

opens-up the capability of moving to higher-order modulation for-

mats and increased spectral efficiency but to meet the optical reach

requirements we may need a fundamental improvement in optical

fiber and/or optical amplification technology. This will be a fertile

area of optical research in coming years as engineers tackle how to

scale optical transport data-carrying capability whilst staying

within the fundamental constraints of Shannons Limit[20].

References

[1] M. Birk et al., Field trial of a 40 Gbit/s PSBT channel upgrade to an installed1700 km 10 Gbit/s system, in: Proc. OFC 2005, Paper OTuH3, Los Angeles, CA.

[2] ITU-T Rec. G.694.1, Spectral Grids for WDM Applications: DWDM FrequencyGrid, 06/2002.

[3] A.H. Gnauck et al., IEEE Photon. Technol. Lett. 15 (3) (2003).[4] C. Laperle et al., Wavelength Division Multiplexing (WDM) and Polarization

Mode Dispersion (PMD) Performance of a Coherent 40 Gbit/s Dual PolarizationQuadrature Phase Shift Keying (DP-QPSK) Transceiver, Paper PDP16, NFOEC2007.

[5] M. Taylor, Coherent detection method using DSP for demodulation of signaland subsequent equalization of propagation impairments, IEEE Photon.Technol. Lett. 16 (2) (2004) 674676.

[6] B. Zhang et al., Penalty Free Transmission of 127 Gb/s Coherent PM-QPSK over1500 km of NDSF with 10 Cascaded 50 GHz ROADMs, Paper NTuC5, NFOEC/OFC 2010.

[7] IEEE P802.3ba, 40 Gb/s and 100 Gb/s Ethernet Task Force. .

[8] ITU-T Rec. G.709, Interfaces for the Optical Transport Network, Edition 3.0, 12/2009.

[9] I. Djordjevic et al., Generalized low-density parity-check codes for opticalcommunication systems, J. Lightw. Technol. 23 (5) (2005) 19391946.

[10] M. Scholten et al., Enhanced FEC for 40G/100G, ECOC 2009, WorkshopPresentation WS1-06.

[11] F. Chang, K. Onohara, T. Mizuochi, Forward error correction for 100G transportnetworks, IEEE Commun. Mag. 48 (3) (2010) S48S55.

[12] M. Birk et al., Field trial of a real-time, single wavelength, coherent 100 Gbit/sPM-QPSK channel upgrade of an installed 1800 km link, in: Proc. OFC/NFOEC,2010, Paper PDPD1.

[13] OIF, 100G Ultra Long Haul DWDM Framework Document. .

[14] Vascade Optical Fiber Data Sheet, Corning Inc. .

[15] UltraWaveOcean Fibers, OFSFitel Inc..

[16] D. Lin, D.J. Costello, Error Control Coding: Fundamentals and Applications,second ed., ISBN:0-13-042672-5, 2004.

[17] E. Ip, J.M. Kahn, Compensation of dispersion and nonlinear impairments usingdigital backpropagation, J. Lightw. Technol. 26 (20) (2008) 34163425.

[18] L. Du, A. Lowery, Fiber Nonlinearity Compensation for CO-OFDM Systems withPeriodic Dispersion Maps, Paper OTu01, OFC/NFOEC 2009.

[19] D. Innis et al., OVUM Market Research: The 40G and 100G Optical Modules,Components, IC, Actives and Passives Forecast: Revenues, Unit Volumes, andASPs, November 29, 2010.

[20] C.E. Shannon, A mathematical theory of communication, Bell Syst. Tech. J. 27(July, October) (1948). 379423, 623656. .


coherent dwdm

Documents