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Advanced Modulation Formats for High-Capacity Optical Transmission
Pierluigi PoggioliniPolitecnico di Torino
Winter School Winter School 20052005
www.optcom.polito.it
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special thanks to:
Roberto Gaudino, Andrea Carena, Vittorio Curri, Gabriella
Bosco
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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
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Global Components & Networks Market (Actual and Forecast)
$8.0
$4.4
$1.6 $1.5 $1.9 $2.2 $2.5
$35.6
$28.2
$10.5$8.3 $9.0
$10.2$11.2
$0
$5
$10
$15
$20
$25
$30
$35
$40
2000 2001 2002 2003 2004 2005 2006
$ in
bill
ion
s
Optical Components Optical Networks
US
D B
illio
ns
www.nren.nasa.gov/workshop7/pps/03.birkner.keynote.ppt
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...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
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Internet Edge Traffic
0.23 0.470.89
1.57
2.57
3.78
0.17
0.26
0.30
0.34
0.41
0.46
0.460.73
1.19
1.91
2.98
4.24
0.110.050.20
0.230.22 0.31
0.00
1.00
2.00
3.00
4.00
5.00
1999 2000 2001 2002 2003 2004 2005 2006
IP Traffic Non-IP Traffic
Edg
e T
raff
ic,
Tbp
s
www.nren.nasa.gov/workshop7/pps/03.birkner.keynote.ppt
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Hitting the capacity lid
41
65 72
5 7 13 21
76
54
35
2000 2001 2002E 2000 2001 2002E 2003E 2004E 2005E 2006EEnd of Year
74
69
15
26
42
60
93
50
81
Range of Estimate
Lit capacity does not intersect with required
capacity until 2005installed lit
capacity
required capacity
www.nren.nasa.gov/workshop7/pps/03.birkner.keynote.ppt
U.S. Long-haul capacity (Terabps)U.S. Long-haul capacity (Terabps)
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Capacity utilization in the US, early 2003
41.8% of total capacity
already lit
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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?
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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
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Fiber attenuation vs
S C L0.2 dB/km
wavelength [nm]
att
enu
ati
on [
dB
/km
]
4 THz
it is difficult to use thisband because RAMANamplification is needed
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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…
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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
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This talk
Preface The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and
DPSK Future prospects: multilevel systems Conclusion
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This talk
Preface The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and
DPSK Future prospects: multilevel systems Conclusion
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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
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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
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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
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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
ASE
POSNR
P
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Signal and noise
dB [mW/THz]
f
dB [mW/THz]
f
dB [mW/THz]
f
noise must beassociated to a
bandwidth
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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
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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
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5.8 dB
3.3 dB uncoded
RS(255,239)
FEC Performance
Optical Signal to Noise Ratio OSNR
BitErrorRate
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This talk
Preface The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and
DPSK Future prospects: multilevel systems Conclusion
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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
BB
R
f
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Frequency spacing f
Power Spectrum of a WDM signal
frequency
ff
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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
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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)
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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!
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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
=0.8
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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
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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
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f=75 GHz
An example of 3-channel IM/DD NRZ spectrum
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f=75 GHzwith ASE
noise
An example of 3-channel IM/DD NRZ spectrum
Pierluigi Poggiolini:
perché non 50 GHz????
Pierluigi Poggiolini:
perché non 50 GHz????
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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”?
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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)
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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
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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
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This talk
Preface The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and
DPSK Future prospects: multilevel systems Conclusion
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IM/DD
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
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The IM/DD spectrum
op
tica
l pow
er
time
1 01 00 11 1 0100
optical power at the TX output
-30
-20
-10
0
10
20
frequency
op
tica
l pow
er
densi
ty [
dB
]
optical spectrum at the TX output
IM/DD - NRZ FORMAT
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The IM/DD spectrum
-30
-20
-10
0
10
20
frequency [THz]
optical power spectral density [dB (mW/THz)]
optical spectrum at the TX output
2 BR
BR
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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
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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
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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??
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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
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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
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5-Pole Bessel TX filter
-3dB bandwidth equal to 1.0*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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5-Pole Bessel TX filter
-3dB bandwidth equal to 0.9*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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5-Pole Bessel TX filter
-3dB bandwidth equal to 0.8*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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5-Pole Bessel TX filter
-3dB bandwidth equal to 0.7*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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5-Pole Bessel TX filter
-3dB bandwidth equal to 0.6*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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5-Pole Bessel TX filter
-3dB bandwidth equal to 0.5*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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5-Pole Bessel TX filter
-3dB bandwidth equal to 0.4*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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5-Pole Bessel TX filter
-3dB bandwidth equal to 0.3*RB
timefrequency
spect
rum
at
TX
puls
e p
ow
er
at
TX
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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
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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!!!
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Need tight filter
not OKbetterbest
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Filters in an actual system
optical RX filter
electrical TX filter electrical RX filter
scope
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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
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scope
EDFA noise
RX for center channel
center channel
right channel
left channel
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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
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Optimized IM/DD NRZ
0
1
2
3
4
5
30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pe
na
lty
[d
B]
TX electrical BW: 50 GHxRX optical BW: 50 GHz
RX electrical BW: 40 GHz
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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
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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
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scope
center channel
right channel
left channel
EDFA noise
RX for center channelTX filter
TX filter
TX filter
RX filter
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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
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Optimized NRZ with optical TX filter
0
1
2
3
4
5
30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pe
na
lty
[d
B]
NRZ
NRZ with TX filter
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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
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Return-to-Zero
RZ
1 1 1 0 1 1 0 0 1 0 1 0 0 1 0 0 1 1 0 1 0 1 0 0
NRZ
1 1 1 0 1 1 0 0 1 0 1 0 0 1 0 0 1 1 0 1 0 1 0 0
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Is RZ presumably better?RZ
NRZ
transmitted pulse transmitted power spectrum
frequency
pow
er
densi
ty
pow
er
bit time slot
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This talk
Preface The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and
DPSK Future prospects: multilevel systems Conclusion
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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…
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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
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DPSK
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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)
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DPSK transmitter
DPSK transmission can be done either at constant power or going
through the origin of the complex phasor plane:
Pierluigi Poggiolini:
animare
Pierluigi Poggiolini:
animare
0
1
requires a phase modulator
TXE
TX opticalfield phasor 0
1
can be done with a conventionalMZ modulator, biased at zero
power transmission
TXE
TX opticalfield phasor
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DPSK: pulse-shape and spectrum
Optical Spectrum
Optical Spectrum
Optical Phase vs. t
Optical Phase vs. t
Using a Phase Modulator
Optical Power vs. t
Optical Power vs. t
Using a MZ Amplitude Modulator
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MZ modulator bias and swing
op
tica
l FI
ELD
driving voltage V
bias for DPSK
swing for DPSK
bias for NRZ swing for IM/DD
0TXE
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DPSK Receiver
1 bit delay line 0.5 cm at 40 Gbit
3 dB coupler
+
Balanced receiver
optical amplifie
r
RX filter
delay is equalto 1 bit
Asymmetric MZ interferometer
-
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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.
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How does it work?
The RX therefore performs differential demodulation
A differential encoder at the TX is then needed
1
2
2
0out
out
E E E E
E E E
1
2
( ) 0
( ) 2out
out
E E E
E E E E
When two subsequent bits have the same phase, we get:
When they have opposite phases, we get:
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Differential Encoder at the TX
The coder is relatively simple to implementeven at 42.65 Gbit/s
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DPSK sensitivityDPSK sensitivity
8
9
10
11
12
13
14
15
16
17
18
QBER
6 7 8 9 10 11 12 13
OSNR (over RB) [dB]
14 15
IMDD
6.e-3
2.e-3
8.e-4
2.e-4
3.e-5
4.e-6
3.e-7
9.e-9
1.e-10
7.e-13
1e-16
BER
2.7 dB
DPSK
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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
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Eye asimmetry
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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 !
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Optimization results: optical TX filter
-3
-2
-1
0
1
2
3
4
5
30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pen
alty
[d
B]
NRZ with TX filter DPSK with TX filter
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Optimization results: optical TX filter
0
1
2
3
4
5
30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pen
alty
[d
B]
NRZ with TX filter
DPSK with TX filter + 2.5 dB
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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
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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
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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
Out2
0( ) cos / BH f f f R
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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
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Balanced Photo-Detector
The BPD must ideally be perfectly balanced
The responsivity of the two photodiodes and subsequent electronics should be identical
+
-
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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!
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System Robustness
What about fiber chromatic dispersion...?
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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?
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NRZ with TX filter optimized for 50 GHz
Pen
alt
y [d
B]
Dispersion [ps/nm]
3dBtolerance:
+- 65 ps/nm
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DPSK with optical TX filter optimized for 50 GHz
Pen
alt
y [d
B]
Dispersion [ps/nm]
3dBtolerance:
+- 140 ps/nm
It is this goodonly withbalanceddetector.Otherwisesame as NRZ/RZ!!!
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Intermission II
What about non-linearities...?
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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.
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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
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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
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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
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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)
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Duobinary
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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
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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
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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
0 T
1( )r t
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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
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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
0 T
1( )r t
2T
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bit “interference”
So when two bits with opposite sign are emitted in sequence, they “destructively interfere” at least in part:
0 T
( )r t
2T
3T 4T
0
1
( )r t T
( )DUOE t
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A three-level signal As a result of the “interference”, the resulting
signal has in fact not two, but three possible levels:
0 T
2T 3T 4T
0
1
3 levels
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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
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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
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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
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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
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Field and power eyes
( )RXE t2
( )RXE t
three levels two levels
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Experimental eye
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Differential Encoder at the TX
This confirms the very close similarity with DPSK
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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
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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
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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)
Pierluigi Poggiolini:
se c’e’ il filtro, allora il delay-and-add logico non serve più
Pierluigi Poggiolini:
se c’e’ il filtro, allora il delay-and-add logico non serve più
TX filter
part of the bit “correlation” is introduced here
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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!
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Comparison, with TX optical filters
-1
0
1
2
3
4
5
30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pe
na
lty
[d
B]
NRZ with TX optical filter
Duobinary with TX filter
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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?
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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
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duobinary sensitivity
8
9
10
11
12
13
14
15
16
17
18
QBER
6 7 8 9 10 11 12 13
OSNR (over RB) [dB]
14 15
IM/D
D
6.e-3
2.e-3
8.e-4
2.e-4
3.e-5
4.e-6
3.e-7
9.e-9
1.e-10
7.e-13
1e-16
BER
DPSK
0.91 dB
duobinary
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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
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The minimum BW pulse
)(tr
T
t
“ripples” decayas 1/t2
T
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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
T21
T21
the minimum total BW is1/T, equal to the bit rate
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Comparing more realistic pulses
2T
3T
NRZ ISI-freepulse
t
t f
f-1/2T
-1/2T 1/2T
1/2T
Duobinary ISI-freepulse
raisedcosineroll-off 0.5
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The bottom line...
0
1
2
3
4
5
30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pe
na
lty
[d
B]
NRZ with TX filter
Duobinary with TX filter
DPSK with TX filter
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Duobinary vs. NRZ spectra
TX spectra withfilters optimized
for 50 GHz spacing
IM/DD NRZ
duobinary
The spectrum isnoticeablynarrower!
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Eye diagrams, filters optimized for f=50 GHz
Duobinary
IM/DD NRZ
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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
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Duobinary Robustness
What about fiber chromatic dispersion...?
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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
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NRZ with TX filter optimized for 50 GHz
Pen
alt
y [d
B]
Dispersion [ps/nm]
3dBtolerance:
+- 65 ps/nm
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DPSK with optical TX filter optimized for 50 GHz
Pen
alt
y [d
B]
Dispersion [ps/nm]
3dBtolerance:
+- 140 ps/nm
It is so goodonly withbalanceddetector.Otherwisesame as NRZ/RZ!!!
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Duobinary with TX filter optimized for 50 GHz
Pen
alt
y [d
B]
Dispersion [ps/nm]
3dBtolerance:
+- 140 ps/nm!!!!
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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
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Results for optimized duobinary
6
7
8
9
10
11
12
13
14
15
OS
NR
, [d
B]
DPSK
DB
NRZ
65
Chromatic Dispersion, [ps/nm]
230140
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Why?
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
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Zero forcing on zero level
0 T
2T 3T 4T
0
1
zero level crossed on every transition
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Zero forcing on zero level
0 T
2T 3T 4T
0
1
zero level crossed on every transitioneven when pulses spread out
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NRZ Duobinary
no dispersion
65 ps/nm
no dispersion
65 ps/nm
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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
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Intermission IV
What about non-linearities...?
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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
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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
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This talk
How did we get this far? The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and
DPSK Future prospects: multilevel systems Conclusion
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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
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4-level Phase Modulation
Four “signal points” are used
This way, each “phase”, or “signal point”, carries two bits
2
0
23 bits “00”
bit “10”
bits “01”bits “11”
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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
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RX General Structure
2-DPSK
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RX General Structure
4-DPSK
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How does it work?
when a new symbol arrivesit has a “component”with respect to both
which is detected by theBPD
2
0
23
IQ
+
-
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How does it work?
when a new symbol arrivesit has a “component”with respect to both
which is detected by theBPD
2
0
23
IQ
- -
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DQPSK sensitivity
8
9
10
11
12
13
14
15
16
17
18
QBER
6 7 8 9 10 11 12 13
OSNR over RB [dB]14 15
IM/D
D
6.e-3
2.e-3
8.e-4
2.e-4
3.e-5
4.e-6
3.e-7
9.e-9
1.e-10
7.e-13
1e-16
BER
2-DPSK
4-DPSK
1 dB
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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.
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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
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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
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no narrow optical TX filter
Filters wereoptimized foreach value ofdispersion !
Still DQPSKfalls shortof duobinary(230 ps/nm)
Chromatic Dispersion, [ps/nm]
55110 175
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Results for optimized duobinary
6
7
8
9
10
11
12
13
14
15
OS
NR
, [d
B]
DPSK
DB
NRZ
65
Chromatic Dispersion, [ps/nm]
230140
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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.
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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
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DQPSK OSNR penalty
0
1
2
3
4
5
20 30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pe
na
lty
[d
B]
NRZ with TX filter
Duobinary with TX filter
DPSK with TX filter
DQPSK with TX filter
= 1.23
1.33
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Orthogonal launch
vertical polarization
horizontal polarization
Power Spectrum of WDM signal
frequency
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TX center channel
TX right channel
TX left channel
2
+
2
+ X
MOST IMPORTANTCROSSTALK TERMUSEFUL CROSSTALK
RX filter
All channels on the same polarization
same polarization
2
+
2
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TX center channel
TX right channel
TX left channel
2
+
2
MOST IMPORTANTCROSSTALK TERM
USEFUL CROSSTALK
RX filter
Left and Right channels on same, center on orthogonal polarization
orthogonal polarizations
+ 2 X
+
2
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The final showdown…
0
1
2
3
4
5
20 30 40 50 60 70 80 90 100 110 120
Channel spacing [GHz]
Pe
na
lty
[d
B]
NRZ with TX filter
Duobinary with TX filter
DPSK with TX filter
DQPSK with TX filter
DQPSK with TX filter orth. launch
1.45
1.6
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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…
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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:
= 2
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Polarization multiplexing
vertical polarization
horizontal polarization
Power Spectrum of WDM signal
frequency
On each polarization they launched two independent channels !!!!
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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
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New approaches: EDC
An electronic equalizer is placed right after the RX
C-N + 1
CN
C
-NC
0C
N -1
x(t)
y(t)
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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
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This talk
How did we get this far? The system context What is “bandwidth efficiency”? Conventional IM/DD Alternative formats: Duobinary and
DPSK Future prospects: multilevel systems Conclusion
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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…