1 advanced modulation formats for high-capacity optical transmission pierluigi poggiolini...

Advanced Modulation Formats for High-Capacity Optical Transmission

Pierluigi PoggioliniPolitecnico di Torino

Winter School Winter School 20052005

www.optcom.polito.it

special thanks to:

Roberto Gaudino, Andrea Carena, Vittorio Curri, Gabriella

Preface: a grim situation...

The optical transmission system market situation has been dismal since 2001

Both component and system sales have fallen to 25% of their peak 2000 values!

Capex by carriers has not substantially recovered yet

R&D has been brutally cut worldwide Lit capacity still exceeds required capacity System and device manufacturers are still in

trouble everywhere and many have not survived

Global Components & Networks Market (Actual and Forecast)

$1.6 $1.5 $1.9 $2.2 $2.5

$10.5$8.3 $9.0

$10.2$11.2

2000 2001 2002 2003 2004 2005 2006

Optical Components Optical Networks

www.nren.nasa.gov/workshop7/pps/03.birkner.keynote.ppt

...but there’s hope

Internet traffic has been consistently growing at a fast pace even in 2001-2004

Forecasts claim it will keep growing strong

Capacity in deployed long-haul links of core networks is expected to saturate between 2005 and 2006, perhaps quicker than expected

Internet Edge Traffic

0.23 0.470.89

0.460.73

0.110.050.20

0.230.22 0.31

1999 2000 2001 2002 2003 2004 2005 2006

IP Traffic Non-IP Traffic

Hitting the capacity lid

5 7 13 21

2000 2001 2002E 2000 2001 2002E 2003E 2004E 2005E 2006EEnd of Year

Range of Estimate

Lit capacity does not intersect with required

capacity until 2005installed lit

capacity

required capacity

U.S. Long-haul capacity (Terabps)U.S. Long-haul capacity (Terabps)

Capacity utilization in the US, early 2003

41.8% of total capacity

already lit

Is recovery round the corner?

If Internet growth data proves correct, already in early 2005 we should see signs of capex recovery

Someone claims they are in fact visible Biq question:

will carrier be willing to deploy more cables?

will they be willing to light up more fibers? or will they prefer to upgrade existing WDM

systems to a higher capacity?

A foreseeable scenario It seems to be more likely that carriers will

rather upgrade than dig or light up more fibers Upgrade means that the installed EDFA bandwidth

will be the constraint (typically 20-30 nm)

DWDM systems capable of delivering the highest capacity within the installed amplifier bandwidth will be of interest

Those who will be able to sell the most bandwidth-efficient systems at the lower cost, will win

Fiber attenuation vs

S C L0.2 dB/km

wavelength [nm]

it is difficult to use thisband because RAMANamplification is needed

Another scenario

Assume carriers are willing to light up more fibers Even so, they will want to make sure they need to

light up the minimum amount

Again, bandwidth efficiency is the key This presentation concentrates on new modulation formats

with a specific slant: increased bandwidth efficiency … but not only…

Resilience to Impairments

Key to the new wave of system installation is cost reduction

Cost reduction means: using the minimum amount of optics come up with systems that are robust and

tolerant to transmission impairments such as:

fiber dispersion non-linearity crosstalk and cascaded filtering

This talk

Preface The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and

DPSK Future prospects: multilevel systems Conclusion

This talk

The system context

The majority of links, even in core networks are medium-long haul: 500-1500 km (EU)

Thanks to FECs (Forward Error Correcting codes) the launched power has been dramatically reduced

In this scenario, non-linear fiber propagation effects are often rather mild

The system context - II

So, we primarily concentrate on linear effects, and specifically on: inter-channel crosstalk TX/RX filter distortion fiber chromatic dispersion

However, we will also touch on non-linearity tolerance to get a feel of the potential of the new formats for ultra-long-haul in core networks

Bit rate per channel In 2000, 40 Gbit/s was on the brink of being

commercially deployed It is not clear whether in 2004 there is any

manufacturer that sells 40 Gbit/s systems Nonetheless, it is still believed that 40 Gbit/s

will follow suit when the recovery takes place So we assume 40 Gbit/s However, most results are easily scalable to

10 Gbit/s since we assume negligible non-linear effects

The OSNR A key parameter affecting the error probability of

an optical system is the OSNR The Optical Signal to Noise Ratio at the

receiver is defined in this talk as:

noise is typically EDFA or RAMAN ASE noise noise is typically white and the noise

bandwidth must always be specified

signal power at RX

noise power at RX in 0.5 nm SIG

Signal and noise

dB [mW/THz]

noise must beassociated to a

bandwidth

OSNR operating level

We assume an OSNR at the receiver due to optical ASE noise of 11 dB over 0.5 nm, typically yielding, in an actual (good) system:

uncorrected BERs on the order of 10-5 10-6

We assume FECs are used, with 7% overhead

FEC corrected BERs better than 10-15 42.65 Gbit/s per channel

What is a FEC?

The bit stream is transformed into another bit stream, with more bits in it

The extra bits (called “redundancy”) make it possible to correct for reception errors

The Reed-Solomon (RS) (255,239) code is now commonly used and standardized in ITU-T G.975 and G.709

This code provides 3.3 dB coding gain at 10-6 corrected bit error rate and 5.8 dB coding gain at 10-13 corrected bit error rate with a redundancy of 7%.

More are being standardized

5.8 dB

3.3 dB uncoded

RS(255,239)

FEC Performance

Optical Signal to Noise Ratio OSNR

BitErrorRate

This talk

Bandwidth efficiency

How many channels can we put in any given amount of optical bandwidth?

It depends on the system BANDWIDTH EFFICIENCY:

[bit/(s×Hz)]bit rate

channel spacing

Frequency spacing f

Power Spectrum of a WDM signal

frequency

Examples

RB =10 Gbit/s, f =100 GHz B=0.1

RB =10 Gbit/s, f =50 GHz B =0.2

RB =40 Gbit/s, f =100 GHz B =0.4

Where do we stand with B?

40 Gbit/s is not commercial 10 Gbit/s is currently offered at a

minimum channel spacing of 50 GHz B

=0.2 Ciena is claiming to be shipping 10 Gbit/s

at 25 GHz spacing B =0.4

(but that was announced back in 2001 and it is not clear if today you can really buy the system)

Current record Many laboratory experiments have been

performing better, one of them up to B =2 !!!

They have used advanced modulation formats and a number of additional tricks

We will try to work our way up to that result

It is going to be a long trip so buckle up, sit back and enjoy the ride!

Next generation commercial systems

First, we will try to beat the current state of the art for commercial systems

To do so it is necessary to do better than 0.2

A challenging goal could be:0.8B

done either:RB =10 Gbit/s, f =12.5 GHz

RB =40 Gbit/s, f =50 GHz, B =0.8RB =42.7 Gbit/s, f =50 GHz, B

Assessing B

How do people assess the system B ?

First, they set a tolerable level of system penalty (say, 1 dB on the OSNR)

Then, they find the maximum B compatible

with such penalty

This is done either experimentally or through simulations

The simulations In the following we will look at a series of simulation

results that we obtained at Politecnico

These results have been confirmed in many cases by several experiments performed by various groups

In the simulations we assumed: a “center” channel two adjacent interfering channels ASE noise-limited performance no non-linearity

The penalty was evaluated on the center channel

f=75 GHz

An example of 3-channel IM/DD NRZ spectrum

f=75 GHzwith ASE

An example of 3-channel IM/DD NRZ spectrum

Pierluigi Poggiolini:

perché non 50 GHz????

Evaluating the penalty

We estimated the center channel BER using appropriate techniques

We computed the OSNR penalty with respect to the best possible BER performance

What is the “best possible performance”?

Best possible performance

The best possible performancefor IM/DD is obtained with: optimum (matched filter) receiver single channel transmission all TX and RX components ideal no propagation effects

dB 11OSNR -8102BER(over 0.5 nm)

What is “penalty”?

PENALTY is defined as:

how much the OSNR must be increased from 11 dB to get back to BER=2 10-8

Example

Optimum system: OSNR=11 dB BER=2 10-8

Actual multi-channel system with impairments OSNR=11 dB BER=5 10-5

OSNR=14 dB BER=2 10-8

The resulting penalty is 3dB

This talk

Despite all the hype regarding alternative modulation formats, IM/DD is still used in 100% of installed systems

To date, there is no single installed commercial system using anything else (any evidence of the contrary?)

The situation is bound to change, starting with submarine systems

Tyco will probably deploy DPSK in its next generation of trans-oceanic links

However, IM/DD is and will remain the leader for a long, long time and so let’s see what it can do for us

The IM/DD spectrum

1 01 00 11 1 0100

optical power at the TX output

frequency

optical spectrum at the TX output

IM/DD - NRZ FORMAT

The IM/DD spectrum

frequency [THz]

optical power spectral density [dB (mW/THz)]

optical spectrum at the TX output

Reducing the side lobes

Bandwidth efficiency is directly related to how compact the spectrum can be made

With high sidelobes, when channels are pulled together they interfere and penalty occurs:

channelselection

filter

Reducing the side lobes

Bandwidth efficiency is directly related to how compact the spectrum can be made

With high sidelobes, when channels are pulled together they interfere and penalty occurs:

0.83 1.2 Bf R

Are there ways to reduce the side lobes?

it is the sharp edges in the time pulses that generate high side-lobes

one possibility is to smooth out the pulses

are there easy ways to smooth out the pulses??

Practical ways of smoothing out pulses

electrical filter after the driver smooths out pulses

a simple and practical way to smooth out pulses is to insert an electrical filter before the modulator

sharp electrical pulses at driver sharp optical pulses

FILTER

Electrical TX filter

Typically, 3-6 pole Bessel filters are used

One can then tune the filter bandwidth to decrease the spectrum side lobes

Eye distorsion must be kept under control

5-Pole Bessel TX filter

-3dB bandwidth equal to 1.0*RB

timefrequency

Filtering tradeoffs

very narrow filter

very broad filter too little filtering means not enoughcrosstalk reduction between WDM channels

too much filtering induces excessive eye closure

the “right” filtering reduces crosstalkand causes little or no eye closure

Real systems are complex Reducing the sidelobes is effective only if the RX

optical filter is tight enough The optical RX filter is necessary also to perform

ASE noise-limiting Typically, an electrical filter follows the

photodiode, too So, there are an electrical TX filter, an electrical RX

filter and an optical RX filter Their cascaded bandwidth must be large enough

not to distort the eye

SO WE HAVE TO OPTIMIZE THEM ALL TOGETHER!!!

Need tight filter

not OKbetterbest

Filters in an actual system

optical RX filter

electrical TX filter electrical RX filter

How we went about it

Thorough simulations were run with three WDM channels

All three filters were taken into account Electrical TX -> Bessel (3-pole) Electrical RX -> Bessel RX (5-pole) Optical RX -> Supergaussian order 2 (ideal)

Noise was injected into the system between TX and RX

EDFA noise

RX for center channel

center channel

right channel

left channel

Many tens of thousands of runs Tens of thousands of run were executed, varying

the channel spacing f

Many values of bandwidth of the three filters were sampled

The optima were found as combinations of filter bandwidths yielding the minimum penalty

Minimum penalties were collected as a function of channel spacing f

Optimized IM/DD NRZ

30 40 50 60 70 80 90 100 110 120

Channel spacing [GHz]

TX electrical BW: 50 GHxRX optical BW: 50 GHz

RX electrical BW: 40 GHz

Comments on IM/DD NRZ results a “baseline” penalty of 1 dB is incurred because

the filter shapes (not just bandwidths)are not the communication theory optimum ones

50 GHz is out of reach (4 dB penalty) a comfortable spacing is 100 GHz theoretically 75 GHz is possible however, frequency stability tolerances may

make it critical to implement max “theoretical” B is 0.53 max “comfortable” B is only 0.4

What else can we do?

The problem to fight is channel crosstalk Perhaps we can better shrink the TX

signal spectrum using an optical TX filter There are then 4 filters:

Electrical TX -> Bessel (3-pole) Electrical RX -> Bessel RX (5-pole) Optical TX -> Supergaussian order 2 Optical RX -> Supergaussian order 2

center channel

right channel

left channel

EDFA noise

RX for center channelTX filter

TX filter

RX filter

Optimizations

There are then 4 parameters to span too many!!!!

To reduce the number of parameters we set for the optical filters:

BWTX = BWRX

In certain cases this may approach a communications theory optimum

Optimized NRZ with optical TX filter

30 40 50 60 70 80 90 100 110 120

NRZ with TX filter

Comments

50 GHz is still out of reach (3 dB penalty)

60 GHz theoretically possible max “theoretical” B is 0.66 now 75 GHz is comfortable max “comfortable” B is 0.53

still, results are rather unsatisfactory

Return-to-Zero

1 1 1 0 1 1 0 0 1 0 1 0 0 1 0 0 1 1 0 1 0 1 0 0

Is RZ presumably better?RZ

transmitted pulse transmitted power spectrum

frequency

bit time slot

This talk

Alternative formats

Several new formats have recently been proposed for bandwidth-efficient transmission

Among them: Carrier-suppressed RZ (CS-RZ) Duobinary DPSK single-sideband (SSB) versions of the

above still others…

The forerunners

In principle, Single Side-Band formats are very effective at increasing bandwidth efficiency, but implementation and other problems greatly hinder them virtually abandoned

CS-RZ is a variation on RZ that has some advantages but is outperformed by other formats (especially DPSK) on all counts

Other exotic formats have failed so far to prove themselves

We will see that both DPSK and duobinary offer a combination of advantages that makes them the forerunners for practical adoption

DPSK highlights

DPSK requires a phase modulator at the TX

The RX compares the signal with a copy of itself, delayed by one bit (differential detection)

DPSK transmitter

DPSK transmission can be done either at constant power or going

through the origin of the complex phasor plane:

animare

requires a phase modulator

TX opticalfield phasor 0

can be done with a conventionalMZ modulator, biased at zero

power transmission

TX opticalfield phasor

DPSK: pulse-shape and spectrum

Optical Spectrum

Optical Phase vs. t

Using a Phase Modulator

Optical Power vs. t

Using a MZ Amplitude Modulator

MZ modulator bias and swing

driving voltage V

bias for DPSK

swing for DPSK

bias for NRZ swing for IM/DD

DPSK Receiver

1 bit delay line 0.5 cm at 40 Gbit

3 dB coupler

Balanced receiver

optical amplifie

RX filter

delay is equalto 1 bit

Asymmetric MZ interferometer

The MZ interferometer

the asymmetric MZ interferometer can be viewed as a filter that converts phase modulation into amplitude modulation

amplitude modulation is then detected by the photodiodes

the two MZ outputs act as follows:

delayedininout EEE _2 delayedininout EEE _1

where the delay is equal to one bit.

How does it work?

The RX therefore performs differential demodulation

A differential encoder at the TX is then needed

E E E E

( ) 2out

E E E E

When two subsequent bits have the same phase, we get:

When they have opposite phases, we get:

Differential Encoder at the TX

The coder is relatively simple to implementeven at 42.65 Gbit/s

DPSK sensitivityDPSK sensitivity

6 7 8 9 10 11 12 13

OSNR (over RB) [dB]

1.e-10

7.e-13

2.7 dB

The two MZ ports are different!

In principle, DPSK can be detected using only one photodiode, on either Out1 or Out2

The two ports form slightly different signals which give rise to different “eyes”

The BPD sums the eyes reinforcing each other

However, using detection on only one port, the 2.7 dB sensitivity advantage over IM/DD is lost

Eye asimmetry

Optimization

We did for DPSK the same as for IM/DD We used the four-filter set-up, including

the optical TX filter Optimization was again run for every

channel spacing and the OSNR penalty evaluated

Problem: the baseline performance is different !

Optimization results: optical TX filter

30 40 50 60 70 80 90 100 110 120

NRZ with TX filter DPSK with TX filter

Optimization results: optical TX filter

30 40 50 60 70 80 90 100 110 120

NRZ with TX filter

DPSK with TX filter + 2.5 dB

Comments on DPSK

50 GHz seems feasible (0.9 dB penalty)

however, due to tolerances, it is not really “comfortable”

60 GHz is certainly “comfortable”

max “comfortable” B is then 0.66

DPSK implementation impairments

DPSK is more complex than IM/DD Certain problems are likely to occur:

imbalance in the balanced detector incorrect summing phase of the two

MZ branches (“detuning”) It is important to evaluate the impact of

such impairments to determine whether the potential DPSK advantages are for real

The AMZ as a Filter The AMZ is a linear component It can be characterized using a pair of transfer

functions from In to Out1 from In to Out2

Both are of the form:but have a 90 degree phase shift between them

Asymmetric MZ interferometerIn Out1

0( ) cos / BH f f f R

Mach-Zehnder Transfer Functions

Green: Out1Red: Out2

The bit rateconcides withthe lobe width,or “FSR”

The alternate portpassband spacing(interleaving)is half the bit rate

carrier

42.65 GHz

( )H f

Balanced Photo-Detector

The BPD must ideally be perfectly balanced

The responsivity of the two photodiodes and subsequent electronics should be identical

Tolerances

We investigated the matter:G. Bosco, P. Poggiolini, “The effect of receiver imperfections on the performance of direct-detection optical

systems using DPSK modulation,” Proceedings of OFC 2003, paper ThE6

JLT paper coming out shortly

The results indicate: up to 3 dB imbalance in the detector causes less

than 0.5 dB penalty the MZ interferometer must be “tuned” to better

than 4 GHz for 3 dB penalty 2 GHz for 1 dB penalty 1.3 GHz for 0.5 dB penalty

and this is difficult at 193 THz!

System Robustness

What about fiber chromatic dispersion...?

What about dispersion…

40 Gbit/s systems are 16 times more sensitive to fiber dispersion than 10 Gbit/s

Dispersion can be compensated for, but perfect compensation across all channels is difficult and expensive

A key feature at 40 Gbit/s is therefore dispersion tolerance

How do IM/DD and DPSK do as for dispersion tolerance?

NRZ with TX filter optimized for 50 GHz

Dispersion [ps/nm]

3dBtolerance:

+- 65 ps/nm

DPSK with optical TX filter optimized for 50 GHz

Dispersion [ps/nm]

3dBtolerance:

+- 140 ps/nm

It is this goodonly withbalanceddetector.Otherwisesame as NRZ/RZ!!!

Intermission II

What about non-linearities...?

DPSK and fiber non-linearity For a long time DPSK had been thought to be very

vulnerable to fiber non-linear effects This is mostly due to a paper by Mollenauer et al.

(1990) that claimed DPSK to be completely killed by Kerr effectsJ. P. Gordon and L. F. Mollenauer, “Phase noise in photonics communications systems using linear amplifier,” Opt. Lett., vol. 15, pp. 1351–1355, 1990.

The idea stuck in the minds of the researchers and it took almost ten years before the matter was investigated anew

When it was done, it was found that DPSK was instead much more tolerant than IM/DD to the actually limiting effects of modern long-haul systems:

XPM Leibrich, Wree, Rosenkranz “CF-RZ-DPSK for suppression of XPM on dispersion-managed long-haul optical WDM transmission on standard single-mode fiber”, IEEE PTL, February 2002

Intra-channel FWM (IFWM)X. Wei and X.Liu “Analysis of intrachannel four-wave mixing in differential-phase-shift keyed-transmission with large dispersion”, Optics Letters, vol. 28, n. 23, pp. 2300-2302, 2003.

DPSK is the new research focus Note that Mollenauer’s paper was not wrong

Simply, his system assumptions were good for 1990, but absolutely outdated by 2000 standards!

Since 2000, all new transmission records have been obtained using DPSK

Both at OFC 2004 and ECOC 2004 all postdeadline papers used DPSK and IM/DD or solitons were completely abandoned

Solitons finally defeated The suppression of XPM made it possible to

attain new incredible records, such as: 10,000 km, 1.6 Tbit/s with:

40 channels 40 Gbit/s per channel, RZ-DPSK Raman, FEC

Rasmussen et al. “DWMD 40 G transmission over trans-Pacific distance (10000 km) using CSRZ-DPSK, enhanced FEC and all-Raman amplified 100 km UltraWave fiber spans”, postdeadline PD18-1, OFC 2003

Even more impressive, the “father” of solitons, Linn Mollenauer, concedes “defeat”:

Xu, Liu, Mollenauer “Comparison of RZ-DPSK and On-Off Keying in Long-Haul Dispersion Managed Solitons”, IEEE PTL, April 2003

Why more non-linearity tolerance? the better sensitivity allows to operate at lower launched power

(by 2.5 dB approx.) XPM is chirp on a channel generated by amplitude modulation

on the other channels if power is constant on all channels no XPM

however, power is not really constant when using a MZ modulator – it goes through zero at every bit change

also, power fluctuations are generated due to dispersion finally, many record experiments use RZ-DPSK so power goes

to zero every bit time So things are not so simple but indeed XPM can be shown to be a

“second-order” effect with DPSK When doing RZ-DPSK XPM takes on a deterministic, bit-periodic

shape that can be “filtered out” at the RX

Leibrich, Wree, Rosenkranz “CF-RZ-DPSK for suppression of XPM on dispersion-managed long-haul optical WDM transmission on standard single-mode fiber”, IEEE PTL, February 2002

Final comments on DPSK 60 GHz is confortably reached but our (initial) goal of

50 GHz is not convincingly achieved

However: dispersion tolerance is extremely good sensitivity is higher by 2.7 dB the RZ version has outstanding non-linearity

resilience over the long-haul (but people now claim also NRZ does: see OFC 2004 post-deadline )

DPSK might be not the absolute best pick for ultra-high bandwidth efficiency, but it will be very successful in some system scenarios (like ultra-long-haul and submarine)

Duobinary

Duobinary Duobinary is a member of a large family of

systems that is called “partial response”

The signal that is transmitted at a certain time depends on both the bit at that time and on one or more of the previous bits

These systems can be very complex and duobinary is one of the simplest versions

Some have very good spectral efficiency and duobinary is one of them

It was first proposed in the ‘60s for radio communications, specifically due to its bandwidth efficiency

Duobinary signaling

There are several ways to introduce duobinary signaling

A non-standard, but highly elucidating way is to put it in relation to DPSK

Duobinary can be seen as a DPSK system where a certain amount of “bit interference” or “bit correlation” is introduced at the TX

Structure of the signal, DPSK At the optical field level, the DPSK signal can be

written as:

where an can be either 1 or –1 and r(t) is a typical transmission pulse, for instance:

( ) ( )DPSK nn

E t a r t nT

1( )r t

Bit sequence, DPSK

For a sequence of bits we then get:

1 0 1 1 0 1 0 0 1 1 1 0

( )DPSKE t

Structure of the signal, duobinary At the optical field level, the duobinary signal can be

written as:

where an can be either 1 or –1 and r’(t) is a transmission pulse stretching over two bits, for instance:

( ) ( )DUO nn

E t a r t nT

1( )r t

bit “interference”

So when two bits with opposite sign are emitted in sequence, they “destructively interfere” at least in part:

( )r t

( )r t T

( )DUOE t

A three-level signal As a result of the “interference”, the resulting

signal has in fact not two, but three possible levels:

2T 3T 4T

3 levels

Bit sequence, duobinary

The overall signal takes on a different look than for DPSK:

( )DUOE t

However, this signal is exactly what we get at Out1 of the outputs of the RX Mach-Zehnder in a DPSK system!

This is because the RX MZ inserts a one-bit correlation which turns the DPSK signal into a duobinary signal

DPSK and duobinary

1 bit delay line 0.5 cm at 40 Gbit

3 dB coupler

Balanced receiver

optical amplifier RX filter

delay is equalto 1 bit

Asymmetric MZ interferometer

the signal here is a DUOBINARY signal

DPSK vs. duobinary

Therefore one can think as follows duobinary and DPSK are close relatives in DPSK “bit interference” is done at the RX by

the MZ in duobinary “bit interference” is set up at the TX

There’s however one subtle difference: with DPSK we have two MZ output ports to look

at, and this improves the sensitivity with duobinary we only have a single, standard

photodetector as RX

Duobinary RX The duobinary RX is a conventional single-

photodiode RX that looks at power By looking at power, the photodetected signal has

again just two levels:

In order for the bit sequence to be correct, differential encoding is necessary in duobinary too (as in DPSK)

2( )DUOE t

Field and power eyes

( )RXE t2

( )RXE t

three levels two levels

Experimental eye

Differential Encoder at the TX

This confirms the very close similarity with DPSK

Duobinary TX structures: DPSK-like

One theoretically possible implementation: a DPSKTX with the first section of a DPSK RX transported here

Asymmetric MZinterferometer filter

(1 bit delay)

DPSK TX

Alternative duobinary TXs

The bit “interference” or, more correctly, the bit correlation can be introduced using: a very narrow electrical filter a very narrow optical filter a combination of the two

In turns out that the best (but still practical) TXs use a combination of the two

They perform better than the DPSK-like TX

Alternative Duobinary TX structure

CODER:identical to

DPSK part of the bit “correlation”is introduced here

modulator is biasedat max extinction as

for DPSK

CW laser

(one of the many possible implementations)

se c’e’ il filtro, allora il delay-and-add logico non serve più

TX filter

part of the bit “correlation” is introduced here

Bandwidth efficiency

We elected to use a “mixed” electrical-optical bit correlation structure

For every channel spacing we carefully optimized all the filters (as for the other formats)

The results are very interesting!

Comparison, with TX optical filters

30 40 50 60 70 80 90 100 110 120

NRZ with TX optical filter

Duobinary with TX filter

Baseline beaten

Duobinary beats the sensitivity baseline (the “0 dB penalty”) !!!

Does this mean that the optimum receiver performance of duobinary is better than that of IM/DD, too?

Duobinary “beats” IM/DD limit

We have investigated the fundamental performance of duobinary

We found that the fundamental limit of direct-detection duobinary is 0.91 dB better than that of IM/DD

Bosco, G.; Carena, A.; Curri, V.; Gaudino, R.; Poggiolini, P.; “Quantum limit of direct-detection receivers using duobinary transmission,” IEEE Photonics Technology Letters , Volume: 15 Issue: 1 , Jan 2003. Page(s): 102-104

duobinary sensitivity

6 7 8 9 10 11 12 13

OSNR (over RB) [dB]

1.e-10

7.e-13

0.91 dB

duobinary

A word of caution on sensitivity Duobinary may easily loose 2-3 dB sensitivity if

filters are not carefully optimized Specifically, if correlation is introduced only

through electrical TX filtering (no optical TX filter), there is a typical 3 dB penalty

Theory shows that the best sensitivity is reached using the minimum bandwidth pulse together with matched RX optical filtering

Work is in progress on theory for best duobinary filtering and pulse-shaping in practical RXs

Using the 4-filter set-up, the sensitivity limit is approached to within 0.5 dB

The minimum BW pulse

“ripples” decayas 1/t2

Theory says that for IM/DDit also is 1/T.

However, duobinary is physicallyrealizable whereas IM/DD is not

IM/DD NRZ

The corresponding spectrum

the minimum total BW is1/T, equal to the bit rate

Comparing more realistic pulses

NRZ ISI-freepulse

f-1/2T

-1/2T 1/2T

Duobinary ISI-freepulse

raisedcosineroll-off 0.5

The bottom line...

30 40 50 60 70 80 90 100 110 120

NRZ with TX filter

DPSK with TX filter

Duobinary vs. NRZ spectra

TX spectra withfilters optimized

for 50 GHz spacing

IM/DD NRZ

duobinary

The spectrum isnoticeablynarrower!

Eye diagrams, filters optimized for f=50 GHz

Duobinary

IM/DD NRZ

Comments

duobinary beats IM/DD NRZ and DPSK 50 GHz is “comfortably” reached!!!

B = 0.8

The optimum filters at f=50 GHz are: Electrical TX -> 16 GHz Electrical RX -> greater than 40 Optical TX and RX -> 35 GHz

Duobinary Robustness

What about fiber chromatic dispersion...?

What about dispersion…

Being related to DPSK, there is hope that duobinary too has a good dispersion tolerance

The results show that indeed duobinary performs equally well

NRZ with TX filter optimized for 50 GHz

Dispersion [ps/nm]

3dBtolerance:

+- 65 ps/nm

DPSK with optical TX filter optimized for 50 GHz

Dispersion [ps/nm]

3dBtolerance:

+- 140 ps/nm

It is so goodonly withbalanceddetector.Otherwisesame as NRZ/RZ!!!

Duobinary with TX filter optimized for 50 GHz

Dispersion [ps/nm]

3dBtolerance:

+- 140 ps/nm!!!!

Optimized duobinary

The best result for dispersion with duobinary is obtained using all-electrical filtering

The system sensitivity is suboptimum with no dispersion but actually improves for increasing dispersion

The system is not optimum as for bandwidth efficiency but is still fairly good

Results for optimized duobinary

Chromatic Dispersion, [ps/nm]

230140

The reason why duobinary (and DPSK) are so tolerant to dispersion was investigated in:

Pennickx et al.: “The Phase-Shaped Binary Transmission: a new technique to transmit far beyond the chromatic dispersion limit”, IEEE PTL, February 1997

it is shown that the phase shift among adjacent bits and their “interference”, preserves the eye “zero level”

this seems to be key to a good dispersion tolerance

Zero forcing on zero level

2T 3T 4T

zero level crossed on every transition

Zero forcing on zero level

2T 3T 4T

zero level crossed on every transitioneven when pulses spread out

NRZ Duobinary

no dispersion

65 ps/nm

no dispersion

65 ps/nm

There’s more to it

the zero-forcing effect is not the whole story

the pulse shape is also important, as the results of TX electrical filter optimization show

not everything has been fully understood yet

Intermission IV

What about non-linearities...?

Duobinary and non-linear effects

Duobinary is less tolerant than DPSK to non-linear effects

However, the current literature does not warn of any particular signs of pathological sensitivity to non-linearity either

Its behaviour may in fact be somewhat better than conventional NRZ/RZ IM/DD

Final comments on duobinary 50 GHz is confortably reached!! Also:

dispersion tolerance is extremely good sensitivity is good (up to 0.91 dB better than NRZ/RZ) the bandwidth of the TX/RX electro-optical circuitry can be

reduced by 30-40% with respect to NRZ/RZ However:

it does not have the outstanding non-linearity resilience of DPSK

This makes duobinary and DPSK complementary: DPSK: great for long-haul in core networks duobinary: great for metro and extended metro,

with dense packing of channels and no optical dispersioncompensation

This talk

How did we get this far? The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and

Can we reach ?

given the results presented so far, no studied format can attain even just = 1

more complex formats are needed multilevel transmission is a promising solution With multilevel TX, more than one bit is sent with

each pulse (but pulses are more complex) 4-DPSK (or DQPSK) seems to be the forerunner

4-level Phase Modulation

Four “signal points” are used

This way, each “phase”, or “signal point”, carries two bits

23 bits “00”

bit “10”

bits “01”bits “11”

Signaling rate decreases

With a “conventional” system, the TX has to emit as many pulses per second as the bit rate: 40 Gbit/s 40 Gpulses per second

With a 4-signal system, the ratio is halved: 40 Gbit/s 20 Gpulses/s

This, by itself, halves the spectral width and therefore bandwidth efficiency greatly increases

RX General Structure

2-DPSK

RX General Structure

4-DPSK

How does it work?

when a new symbol arrivesit has a “component”with respect to both

which is detected by theBPD

How does it work?

when a new symbol arrivesit has a “component”with respect to both

which is detected by theBPD

DQPSK sensitivity

6 7 8 9 10 11 12 13

OSNR over RB [dB]14 15

1.e-10

7.e-13

2-DPSK

4-DPSK

DQPSK sensitivity The ultimate sensitivity of DQPSK is:

1 dB better than IM/DD 1.7 dB worse than DPSK

This is a very good performance! However, as for DPSK, impairments can kill! Our recent findings:

G. Bosco and P. Poggiolini, "Analysis of impact of receiver imperfections on performance of optical DQPSK systems", IEE Electronics Letters, vol. 40, no. 18, Sep. 2004, pp. 1147-1149.

up to 2.5 dB imbalance in the detector is less than 0.5 dB penalty (DPSK was 3 dB imbalance)

the MZ interferometer must be “tuned” to better than 360 MHz for 1 dB penalty (vs. 2 GHz DPSK) 240 MHz for 0.5 dB penalty (vs. 1.3 GHz DPSK)

and this is extremely difficult!!!!

So DQPSK is very challenging from a practical viewpoint.

Dispersion An immediate benefit of multilevel transmission is

that the pulse duration increases

For DQPSK, pulses are twice as long for the same bit rate

As a consequence, dispersion tolerance may be expected to greatly increase

The most comprehensive paper to date is:

Jin Wang and Joseph M. Kahn, “Impact of Chromatic and Polarization-Mode Dispersions on DPSK Systems Using Interferometric Demodulation and Direct Detection”, IEEE JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 2, FEBRUARY 2004

Still an open issue Our findings are that DQPSK dispersion tolerance is approximately

twice DPSK for the same bit rate, in line with Wang and Kahn

Wang and Kahn also claim that applying pulse carving (sort of RZ-DQPSK) dispersion tolerance can become 4 times that of DPSK: this needs double-checking

Literature is not univoquely settled on this matter and more “definitive” studies are needed (to some extent on DPSK as well)

What is clear is that pulse shape and filtering optimization makes a lot of difference

Also, the use of a TX optical filter seems to allow reaching the best possible tolerance, for DQPSK up to 5-6 times that of IM/DD for the same bit rate

no narrow optical TX filter

Filters wereoptimized foreach value ofdispersion !

Still DQPSKfalls shortof duobinary(230 ps/nm)

55110 175

Results for optimized duobinary

230140

Is DQPSK for real ? DQPSK experiments have started to be performed

only very recently (last 2 years)

Yet, the results have been very impressive

T.Tokle, et al. “6500 km transmission of RZ-DQPSK WDM signals,'' Electron. Lett. , vol. 40, no. 7, Apr. 2004, pp. 444-445.

K. Ishida, et al. “Transmission of 20 x 20 Gb/s RZ-DQPSK signals over 5090 km with 0.53 b/s/Hz spectral efficiency”, in Proc. of OFC 2004, paper FM2.

Back to bandwidth efficiency

We performed again all the optimisation that we did for IM/DD and DPSK

We used the 4-filter set-up as for the other systems

DQPSK OSNR penalty

20 30 40 50 60 70 80 90 100 110 120

NRZ with TX filter

DPSK with TX filter

DQPSK with TX filter

= 1.23

Orthogonal launch

vertical polarization

horizontal polarization

Power Spectrum of WDM signal

frequency

TX center channel

TX right channel

TX left channel

MOST IMPORTANTCROSSTALK TERMUSEFUL CROSSTALK

RX filter

All channels on the same polarization

same polarization

TX center channel

TX right channel

TX left channel

MOST IMPORTANTCROSSTALK TERM

USEFUL CROSSTALK

RX filter

Left and Right channels on same, center on orthogonal polarization

orthogonal polarizations

The final showdown…

20 30 40 50 60 70 80 90 100 110 120

NRZ with TX filter

DPSK with TX filter

DQPSK with TX filter

DQPSK with TX filter orth. launch

Is this just “speculative”? Check this out:

Noboru Yoshikane and Itsuro MoritaKDDI R&D Laboratories Japan

“160% spectrally efficient [=1.6] 5.12 Tbit/s (64x85.4 Gbit/s RZ DQPSK) transmission without polarisation demultiplexing”ECOC 2004 post-deadline Th4.4.3

Title misleading – they do: optical TX filtering effectively convert RZ pulses into NRZ orthogonal launch total 320 km they accept a brutal (6-7 dB) penalty due to

various imperfections but…

More extreme polarization tricks The absolute record for bandwidth efficiency has

been recently obtained using again DQPSK but with more extreme polarization tricks

Pak S. Cho et al., Photonics Technology Letters, IEEE , February 2004, Page(s): 656-658

They indeed reached an appalling:

Polarization multiplexing

vertical polarization

horizontal polarization

Power Spectrum of WDM signal

frequency

On each polarization they launched two independent channels !!!!

Is it realistic ?

In the opinion of many, polarization multiplexing is unrealistic

to detect the two polarization-multiplexed channels traveling at the same frequency complex polarization-separating optics is

needed dynamic tracking of the output

polarization states is necessary

New approaches: EDC

An electronic equalizer is placed right after the RX

C-N + 1

Is EDC effective? very hot topic research is ongoing “as we speak” current results show that IM/DD can almost double

its resilience (65 ps/nm 120 ps/nm) breakthrough new techniques are being studied

(non-linear post-distortion, extinction ratio control) EDC looks much less effective with DPSK and

almost not at all useful with duobinary MLSE is also emerging as a forerunner, to be

applied to all of the previously mentioned formats the race is on to break increasingly larger

dispersion limits

This talk

How did we get this far? The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and

Conclusion Out of the many formats that we have

investigated: IM/DD is inadequate for high and dispersion

tolerance other exotic formats were discarded duobinary looks very good as a potential candidate

for ultra-dense WDM metro and extended metro systems, with efficiency up to 1 bit/s/Hz and very good dispersion tolerance

DPSK looks very promising as well, especially for long haul and submarine thanks to non-linearity resilience

next step: DQPSK potential to be fully assessed but astounding 1.6 bit/s/Hz already obtained in the lab

EDC and MLSE are the next big thing…

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