Transcript slides

Next Generation
Coherent Technologies
Geoff Bennett: Director, Solutions and Technology
A Review of High-Speed Coherent Transmission Technologies for Long-Haul
DWDM Transmission at 100G and Beyond
Geoff Bennett, Kuang-Tsan Wu, Anuj Malik, Soumya Roy, and Ahmed Awadalla
IEEE Communications Magazine, October 2014
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A high level summary of optical transmission evolution
2010
2015
Pre-Coherent Era
First Coherent Era
1
2
3
• Phase modulation
• Coherent detection
• Receiver-based DSP
• 1st gen SD-FEC
• Super-channels emerge
• Flexible grid emerges
• Advanced phase modulation
• Coherent detection
• Rx and Tx-based DSP
• 2nd gen SD-FEC
• Super-channels dominate
• Flexible grid dominates
9.5Tb/s
12, 18, 24Tb/s
• Simple intensity modulation
• Direct detection
• No real signal processing
• HD-FEC
• Single wave transponders
• Fixed grid
800Gb/s
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Second Coherent Era
The Coherent Receiver
(First Generation)
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How does a coherent detector work?
In commercial DWDM systems
Digital Signal Encoded
as Phase Changes
Coherent Detector
•
•
•
•
Local
Oscillator
Demodulator
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Photodetect
Amplify
Digitize
Compensate
Solving for Linear Impairments
 To a first approximation linear impairments can be
characterized by a transfer function describing:
• Chromatic Dispersion
• PMD
• Tx and Rx filtering
Linear equalizer
transfer function
 As the signal propagates, noise is accumulated
• Amplified Spontaneous Emissions, Shot Noise, DRB, etc.
Forzati et al: Non-linear compensation techniques for coherent fibre transmission.
Proc. of SPIE-OSA-IEEENol. 8309 830911-1
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Linear transfer
function
describing effects
of CD and PMD
The Challenge of Compensating for Noise
 In other media the solution to noise is to
increase signal strength
 But optical fiber is a non-linear medium
• The non-linear component of refractive index changes
with the intensity of the optical power
• Refractive index at a given power level (I) given by:
n(I) = n0 + n2 .n ILinear refractive index
0
n2 2nd order non-linear refractive index
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Kerr Effect
Non-Linear Effects
Pulse “compression”
 Self Phase Modulation
 Cross Phase Modulation

If symbols in adjacent channels
“line up” their power level can
exceed the NL threshold
 Four Wave Mixing
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
3 1
2
4

Non-linear interaction of 1 and 2 causes
the appearance of 3 and 4
The Coherent Transmitter
(Second Generation)
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Tx DSP and DAC Are Key Additions
4 Important functions
1: High order modulation
Laser
2: Pulse Shaping
S1
3: Pre-dispersion
90
DAC
S2
DSP
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4: Non-linear compensation
Tx DSP and DAC Functions:
Higher Order Modulation
 Higher order modulation has “fewer photons per bit”,
and requires higher symbol resolution
QPSK
2 bits
16QAM
4 bits
• A given symbol has the same absolute
optical power limit, but 16QAM is
carrying twice the number of bits
• The resolution between symbol states is
much tighter for 16QAM
Result: 16QAM has 5X shorter reach vs QPSK,
for only 2X increase in spectral efficiency
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Tx DSP and DAC Functions:
Intelligent Pulse Shaping
With DSP and DAC: signal from the side
lobes has been intelligently incorporated
into the main pulse
Without DSP and DAC
Too close a channel spacing
will result in a reach penalty
ICI
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We can decrease channel
spacing, and increase total
fiber capacity, without a
reach penalty
Terminology: “Nyquist DWDM”
Transmitter
Pulse-Shaped
QPSK Spectrum
Laser
Mod
DSP
DAC
Alpha
 Shaped pulses can be spaced “at the Baud rate”
50GHz
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•
Eg: 32GBaud signal could be spaced at just over 32GHz
•
The additional spacing is known as the alpha, and the
practical limit for alpha is 3-4% before OSNR penalties
are incurred
Fixed Grid Coherent Transmission
PM-QPSK
100Gb/s
50GHz
In 1st generation coherent, 100G PM-QPSK
fits efficiently into 50GHz grid spacing
These fixed grids are
spaced so that individual
carriers can be optically
added or dropped in
ROADM
We could choose to
drop these carriers
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Pulse Shaped Carriers
PM-QPSK
100Gb/s
50GHz
We now become limited by the fixed grid plan
What can we gain if:
We have narrower carriers?
and…
We don’t need to optically
add/drop?
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Moving to Flexible Grid
Example shows a 12 carrier
super-channel
The width of this super-channel can be flexible, but not totally
flexible because of the need to manage optical spectrum
Super-Channel
Let’s forget about fixed grids
We can move the carriers to
“Nyquist” spacing
If we need to optically
add/drop we create a guard
band at the ends of this superchannel
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We can create sliceable super-channels
These carriers are generated on the same line card…so does that
mean they have to start and terminate at the same points?
Super-Channel 1
No…they can
be sliced
Super-Channel 2
1.2T PM-QPSK
462.5GHz
400G PM-QPSK
400G PM-QPSK
400G PM-QPSK
162.5GHz
162.5GHz
162.5GHz
3 x 400 Gb/s
Super-Channels
1.2 Tb/s
Super-Channel
100G PM-QPSK
(3 x 162.5
= 487.5GHz)
100G PM-QPSK
12 X
50GHz
50GHz
12 x 100 Gb/s
Individual Channels
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ITU-T G.694.1 Flexible Grid
 Flexible slots have a total width granularity of 12.5GHz, but a center frequency
granularity of 6.25GHz
 193.1THz reference frequency, and all flexible slots are defined with respect to
this reference frequency (outlined in red)
•
•
The flexible slot is defined by two numbers, n and m
The center of a given flexible slot is n x 6.25GHz from the reference frequency,
and is m x 12.5GHz total width
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For Flexible Grid: Super-Channels are Mandatory
Key point:
Super-Channel
To achieve the claimed fiber capacity in
next generation coherent systems, superchannels are mandatory
This is a super-channel
• Whether it’s based on one card or many
• Whether it’s based on PICs or discretes
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Tx DSP and DAC Functions:
Pre-dispersion
We know a signal will be dispersed as
it travels along the fiber
Rx
Tx
Using DSP and DAC we can apply
negative direction of dispersion
before transmission
In reality we would divide up the
job of dispersion compensation
between Tx and Rx
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1 or 0?
Dispersion behavior
can be tightly
controlled
Tx DSP and DAC Functions:
Non-Linear Compensation
 Promising demonstrations of NL Compensation gains of >1dB
in “real-world” OSNR improvements
 An array of complex algorithms:
•
•
•
•
•
NPCC: Nonlinear Polarization Crosstalk Correction (for XPM)
AFCPR: Adaptive Fee-Forward Carrier Phase Recovery
RF Pilot waves
DBP: Digital Back Propagation, Perturbed DBP (for SPM)
Tx Pulse Pre-dispersion (aka pre-compensation, pre-emphasis)
Commercial implementation over the next few years
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Solving for Non-Linear Effects:
This is not a trivial excercise!
Each equation represents the complex
envelope, A, of two orthogonal polarization
components of the electric field
Forzati et al: Non-linear compensation techniques for coherent fibre transmission.
Proc. of SPIE-OSA-IEEENol. 8309 830911-1
21 | © 2015 Infinera Confidential & Proprietary
2: attenuation coefficient
2: dispertion parameter
: nonlinear coefficient
z: propagation direction
t: time
Next Generation Coherent: The End Result
Tx Pulse Shaping
Extended C-Band capacity increases from 9.5Tb/s to 12Tb/s using PMQPSK. PM-8QAM delivers 18Tb/s, and PM-16QAM delivers 24Tb/s.
Dispersion
management
Most applicable on submarine links, but allows for a 5X increase in
Chromatic Dispersion compensation.
Next Gen SD-FEC
Seems to be saturating out at about 12dB NGC
(about 1dB improvement)
Non-linear
compensation
Algorithms are still evolving. Longer term expectation is to deliver
around a 1dB OSNR improvement.
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Summary: Next Generation Coherent Technologies
Outlined three distinct eras in
optical transmission
We’re in transition to the
Second Coherent Era
Flexible Grid and Super-Channels become mandatory
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Thank You!
Geoff Bennett
[email protected]
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