© Agilent Technologies, Inc. 2010
Dr. Bernd Nebendahl
Research and Development
Digital & Photonic Test
Agilent Technologies
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 1 of 74
© Agilent Technologies, Inc. 2010
Part I
• Trends in 40/100 GbE
• Introduction to Advanced Optical
Modulation
– IQ Modulation
– Polarization Division Multiplexing
– Orthogonal Frequency Domain Multiplexing
• Receiver Technologies
– Measurement Principles
– Frequency or Time Domain
– Delay Line Interferometer or Coherent Receiver
– Real Time or Equivalent Time Sampling
Part II
• Signal Processing for Coherent
Receivers
– Carrier Phase Recovery
– Polarization Demultiplexing
• Quality Rating
– Error Vector Magnitude, Phase Error, Magnitude
Error, Quadrature Error, Gain Imbalance
– Bit Error Rate and Error Vector Magnitude
• Conclusion
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
February 2010Page 2 of 74
© Agilent Technologies, Inc. 2010
February 2010
4 Fibers, 1 wavelength
with 10, 20 or 25Gbit/s,
on-off modulation
50 GHz 50 GHz
1 Fiber, 192 wavelength with
40/100Gbit/s in each ITU-T channel,
advanced modulation
1 Fiber 4 wavelength
with 10, 20 or 25Gbit/s,
on-off modulation
Focus of this
Tutorial
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 3 of 74
© Agilent Technologies, Inc. 2010
• Keep existing 50GHz spacing to improve return on investment
• Maximize ROI by increasing the capacity of today„s 10G systems by 4x,
i.e. from 80x10G to 80x40G and 80x100G in the C-band
• Necessitates positioning 40G/100G channels on 50GHz frequency grid
while also allowing mixing with 10G
• Transparent reach of at least 1,000km
• System design
• No change in line system design with the introduction of 40G and 100G
on a 10G system
• High tolerance to in-line optical filtering (ROADMs)
• High tolerance to chromatic dispersion
• High tolerance to polarization-mode dispersion (PMD)
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 4 of 74
© Agilent Technologies, Inc. 2010
Transmission of 100-Gb/s binary (duo binary) optical signals is
extremely challenging• Modulator bandwidth not wide enough / drive voltage too high
• Electrical amplifier bandwidth and output voltage not sufficient
• High-performance integrated photo-receivers not available
• Bandwidth not compatible with 100-GHz (50-GHz) channel spacing
• Large transmission penalties expected from CD and PMD
New modulation formats for 100G long haul transmission• Multi-level coding: QPSK, QAM, M-ary ASK, …
• Relaxed bandwidth/voltage requirements for modulator
• Compatible with 100-GHz (50-GHz) channel spacing
• Polarization multiplexing instead of single polarization
• Compatible with 50-GHz channel spacing
• Coherent instead of direct detection
• Electronic dispersion mitigation (PMD and CD)
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 5 of 74
© Agilent Technologies, Inc. 2010
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 6 of 74
© Agilent Technologies, Inc. 2010
1. Standardize 100G Short-Range/ Client Interfaces
2. Develop T&M Equipment for Short-Range Transmission
3. Identify (Standardize?) Modulation Format(s) for Long-Haul
4. T&M Equipment for Long-Haul Transmission
Constellation analyzers for advanced M-ary formats, etc.
5. T&M Equipment for Long-Haul Transport
In-band OSNR, CD and PMD monitoring
6.Enhanced PMD and CD Compensation Techniques
February 2010
Focus
of this
Tutorial
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 7 of 74
© Agilent Technologies, Inc. 2010
Part I
• Trends in 40/100 GbE
• Introduction to Advanced Optical
Modulation
– IQ Modulation
– Polarization Division Multiplexing
– Orthogonal Frequency Domain Multiplexing
• Receiver Technologies
– Measurement Principles
– Frequency or Time Domain
– Delay Line Interferometer or Coherent Receiver
– Real Time or Equivalent Time Sampling
Part II
• Signal Processing for Coherent
Receivers
– Carrier Phase Recovery
– Polarization Demultiplexing
• Quality Rating
– Error Vector Magnitude, Phase Error, Magnitude
Error, Quadrature Error, Gain Imbalance
– Bit Error Rate and Error Vector Magnitude
• Conclusion
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
February 2010Page 8 of 74
© Agilent Technologies, Inc. 2010
Every carrier signal can be described with two parameters
• Amplitude and Phase or
• In-phase and Quadrature
Both parameters can be modulated to carry information
I (in-phase or real part)
Q (quadrature, imaginary part)
Phase φ
(I,Q)
Q value
I value
The position of the vector end point is called a constellation point and
therefore the diagram is called a constellation diagram
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 9 of 74
© Agilent Technologies, Inc. 2010
Vectors defined by amplitude and phase are separated
by the transition of the transmission clock
I (in-phase or real part)
Q (quadrature, imaginary part)
I
Q
Transmitted signal
Data clock
Sample points
Transition between
sampling points
Switch to
next vector
10
11
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 10 of 74
© Agilent Technologies, Inc. 2010
Modulating amplitude and/or phase does not necessarily
increase spectral efficiency compared to OOK
I
0 1
Q
I
0 1
Q
Traditional On-Off (OOK) format displayed with
two vectors. Information is coded in 2 symbols 0
and 1 only in the amplitude
In this BPSK format, information is coded only in
the phase instead of amplitude. Number of
symbols is still 2 !
I
0 1
Q
In this artificial format amplitude and phase is
modulated but still only 2 symbols are available.
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 11 of 74
© Agilent Technologies, Inc. 2010
The data stream can be coded into a 4 symbol alphabet
like { A, B, C, D, ….}
We can code also in the following way:
00 a sin(ωt+45)
01 a sin(ωt+135)
10 a sin(ωt+225)
11 a sin(ωt+315)
0 0 1 0 1 1 1 0 0 1 0 1 0 0 1 0 1 1 1 1 0 0 1 0
A DB B C C A B D D A B
Original
data stream
Possible
Symbol alphabet
Increasing the number of symbols that are coded in an alphabet
increases the number of bits that are transmitted in one clock
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 12 of 74
© Agilent Technologies, Inc. 2010
a sin(ωt+45) 00
a sin(ωt+135) 10
a sin(ωt+225) 01
a sin(ωt+315) 11
I (in phase)
Q (quadrature)
0010
01 11
4 symbol alphabet codes 2 bits
in one transmission clock cycle
As a result one vector position in the polar plane codes 2 bits,
which increases the spectral efficiency by a factor of 2
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 13 of 74
© Agilent Technologies, Inc. 2010
February 2010
I
Q
Signal
1
1 1
1
0
00
0
11 00 01 10 QPSK constellation map
11
00
01
10
I
Q
Laser
/2
1 0 0 1
DemuxBinary Bit stream QPSK Signal
1 0 1 0
1 1 0 0 0 1 1 0
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 14 of 74
© Agilent Technologies, Inc. 2010
0 0 1 0 1 1 1 0
Data rate: [bit/s]
A B D B
Symbol rate: [Symbols transmitted/s ]
Symbols coded and transmitted
as vectors 4 symbols
Original data 8 bits
Symbol rate or baud rate describes the symbol transmission
clock rate not the transmission data clock rate
Symbol rate (baud rate) is equal to or smaller than data rate
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 15 of 74
© Agilent Technologies, Inc. 2010
I
Q
QAM 16
0000 0100 0011 1000
0001 0101 1011 1001
0011 0111 1111 1011
0010 0110 1111 1010
16 symbol alphabet coding 4 bits
Symbol (Baud) rate is 4 times lower than clock rate
Savings in speed translates to savings in power !
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 16 of 74
© Agilent Technologies, Inc. 2010
February 2010
frequency
y-polarization
x-polarization
• the optical wave can be split into two orthogonal polarizations
• each polarization can carry an independent signal and can be viewed
as a virtual transmission channel within the fiber
• these signals can be seperated using polarization diversity receivers
PDM increases the spectral efficiency by a factor of 2
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 17 of 74
© Agilent Technologies, Inc. 2010
February 2010
112 Gb/s Optical spectra50 GHz ITU channel
PDM NRZ-QPSK
NRZ-QPSK
NRZ-OOK
-200G -100G 0G 100G 200G
Offset from Carrier
Factor of 2 from OOK to QPSK
Factor of 2 from SinglePol to PDM
Using advanced modulation and PDM allows to transmit
more than 100 Gb/s in a 50 GHz wide ITU channel!
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 18 of 74
© Agilent Technologies, Inc. 2010
• widely used in wireless and electrical wireline
• uses more than one coherent subcarrier for the transmission at lower symbol rates
• can be generated optical (typically low number of subcarriers) or electrically (high
number of subcarriers are possible, but requires fast DAC)
• Pilot carriers can be used to estimate the channel parameters
• Receiver synchronization is simplified using guard intervalls
• typically very sensitive to nonlinearities (fiber effects?)
• signals typically have a high peak to valley ratio
• does not necessarilly increase spectral efficiency but can improve robustness
against certain impairments
February 2010
OFDM might be an interesting opportunity for optical
transmission but is currently still in research
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 19 of 74
© Agilent Technologies, Inc. 2010
Part I
• Trends in 40/100 GbE
• Introduction to Advanced Optical
Modulation
– IQ Modulation
– Polarization Division Multiplexing
– Orthogonal Frequency Domain Multiplexing
• Receiver Technologies
– Measurement Principles
– Frequency or Time Domain
– Delay Line Interferometer or Coherent Receiver
– Real Time or Equivalent Time Sampling
Part II
• Signal Processing for Coherent
Receivers
– Carrier Phase Recovery
– Polarization Demultiplexing
• Quality Rating
– Error Vector Magnitude, Phase Error, Magnitude
Error, Quadrature Error, Gain Imbalance
– Bit Error Rate and Error Vector Magnitude
• Conclusion
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
February 2010Page 20 of 74
© Agilent Technologies, Inc. 2010
Until now
OOK (RZ or NRZ), maybe multilevel
information encoded in power
direct detection (electrical or
optical) and real-time or equivalent
time sampling are possible
In future
QPSK, QAM, ....
information encoded in power and
phase
since phase is not an absolute
quantity, a phase reference has to
be introduced
February 2010
Photodiode IphotoS
Photodiode Iphoto
S
R
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 21 of 74
© Agilent Technologies, Inc. 2010
Local oscillator
• Coherent detection
• Time domain or frequency domain
• Intradyne or heterodyne
• Real- or equivalent time sampling
Self referencing
• Delay line interferometer
• Time domain
• Real- or equivalent time sampling
February 2010
Delay Photodiode
Photodiode
I1
I2
I1-I2
S
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 22 of 74
© Agilent Technologies, Inc. 2010
February 2010
Time domain or frequency domain
Time domain:
Coherent or delay line interferometer
Coherent:
Real time or equivalent time
sampling
Equivalent time sampling:
Optical or Electrical sampling
Delay line interferometer:
Real time or Equivalent time
sampling
Equivalent time sampling:
Optical or Electrical sampling
Frequency Domain:
Static or swept LO
Swept LO:
narrowband detection
Static LO:
wideband detection (ESA)
Covered in this Tutorial
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 23 of 74
© Agilent Technologies, Inc. 2010
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 24 of 74
© Agilent Technologies, Inc. 2010
Con
• Only works for repetitive signal
• Symbol clock required (either from CDR or from TX)
• Pattern length limited by linewidth/jitter of TX and LO
• polarization multiplexed signals require an optical polarization controller and a
stable input SOP
• Only phase differences are measured. Reconstruction of phase requires
integration. Might lead to large errors
Pro
• Virtually „infinite“ bandwidth (only limited by tuning range of LO, can be many THz)
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 25 of 74
© Agilent Technologies, Inc. 2010
Con
• Delay needs to be adjusted to actual symbol rate
• polarization multiplexed signals require an optical polarization controller and a
stable input SOP
• Reconstruction of transistions difficult
• Lower sensitivity compared to heterodyne/intradyne detection
• Linear distortions cannot be removed by signal processing
Pro
• Only minor impact from phase noise of TX
• Using equivalent time sampling is possible higher bandwith than real time
sampling
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 26 of 74
© Agilent Technologies, Inc. 2010
February 2010
IQ Demodulator
Phase shifter 90°
Photodiode
Photodiode
Photodiode
Photodiode
I1
I2
Q1
Q2
Signal
Local Oscillator
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 27 of 74
© Agilent Technologies, Inc. 2010
February 2010
IQ Demodulator, x-Polarization
Phase shifter 90°
Photodiode
Photodiode
Photodiode
Photodiode
Photodiode
Photodiode
Photodiode
Photodiode
IQ Demodulator, y-PolarizationPolarization
Splitter
Local Oscillator Phase shifter 90°
I1
I2
Q1
Q2
I3
I4
Q3
Q4
Signal
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 28 of 74
© Agilent Technologies, Inc. 2010
Signal
PBS 50/50
90° OpticalHybrid
90° OpticalHybrid
LO atti?
BalancedReceiver
BalancedReceiver
BalancedReceiver
BalancedReceiver
ADC ADC ADC ADC
Clock recovery & re-timing
Equalizer (remove CD and PMD)
Carrier recovery
Slicer & decoder
I/Q plot
Spectrum
Time Series
...
February 2010
• Polarization diversity coherent
receiver generates 4 electrical outputs
• AD convertes sample the 4 channels
• Postprocessing recovers the carrier
• Postprocessing demultiplexes the
polarization multiplexed signals
• Recovered signals are demodulated
and displayed in various formats
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 29 of 74
© Agilent Technologies, Inc. 2010
Optical sampling uses nonlinear effects to mix the signal and the probe pulse
Coherent reception and optical sampling typically involves a local oscillator probe
pulse and to generate the pulsed I and Q signals
These signals are electrically detected at lower speed. To be able to recover the
original signal, it needs to
• be repetitive (i.e. real datastreams cannot be measured, BER cannot be
measured)
• contain the symbol clock and pattern clock in the raw signals
• have a carrier phase noise small enough for tracking with equivalent time
sampling
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 30 of 74
© Agilent Technologies, Inc. 2010
Con
• Only works for repetitive signal
• Pattern synchronization requires measuring many cycles of the pattern
• Higher level QAM formats are more difficult to synchronize to
• Carrier phase recovery requires tighter limits for transmitter laser phase noise than
in real systems
• Postprocessing can only be applied after the signal has been recoverd
Pro
• Higher detection bandwidth compared to real time sampling but not as high as for
frequency domain based methods with swept LO
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 31 of 74
© Agilent Technologies, Inc. 2010
February 2010
Con
• Bandwidth limited by analog bandwith of ADC
Pro
• Works for any signal (repetitive and true random data signals)
• Does not require better phase noise than RX in the network
• Less complex processing to yield results
• Can detect bit errors due to improper TX realisation
• Signal processing can be identical to final RX
• Polarization demultiplexing in SW
• Real time sampling with 30 GHz electrical bandwidth allow to cover 60 GHz
(more than one channel in the 50 GHz ITU grid)
• Detection of OFDM signals can be done using proper signal processing
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 32 of 74
© Agilent Technologies, Inc. 2010
Currently the modulation formats are targeted to fit into the 50GHz ITU grid
• effective signal bandwidth is less (depending on the number of add-drop
multiplexers ~40 GHz)
Using modulation formats that need more bandwidth means:
• Current network infrastructrure must be replaced or upgraded
• Chromatic dispersion and polarization mode dispersion become more important
• TX and RX hardware require ultra-high speed electronics, which is not available
today
• increase of power dissipation in linecards
• As soon as higher speed ADCs are available, real-time scopes will be faster as
well
For the 50 GHz ITU channel spacing there is no real need for bandwidth in
excess of 20 GHz
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 33 of 74
© Agilent Technologies, Inc. 2010
This is the end of Part I of our Tutorial!
Please make sure to continue with Part II and complete the
evaluation form at the end of the presentation.
You‟ll be entered into a drawing with a chance to win a $75
Amazon.com gift card!
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 34 of 74
© Agilent Technologies, Inc. 2010
Part I
• Trends in 40/100 GbE
• Introduction to Advanced Optical
Modulation
– IQ Modulation
– Polarization Division Multiplexing
– Orthogonal Frequency Domain Multiplexing
• Receiver Technologies
– Measurement Principles
– Frequency or Time Domain
– Delay Line Interferometer or Coherent Receiver
– Real Time or Equivalent Time Sampling
Part II
• Signal Processing for Coherent
Receivers
– Carrier Phase Recovery
– Polarization Demultiplexing
• Quality Rating
– Error Vector Magnitude, Phase Error, Magnitude
Error, Quadrature Error, Gain Imbalance
– Bit Error Rate and Error Vector Magnitude
• Conclusion
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
February 2010Page 35 of 74
© Agilent Technologies, Inc. 2010
• The frequency difference between the transmitter laser and the local oscillator
leads to a „rotating“ constellation
February 2010
I
Q
0010
01 11
I
Q
0010
01 11
t t+Tsymbol
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
must not change faster than
/4 per symbol time
• Frequency offset must be
smaller than 1/8 of the
symbol clock for QPSK
t
linearphase drift
t
Page 36 of 74
© Agilent Technologies, Inc. 2010
• To be able to track the phase, the signal must be sampled at times with predictable phase values (i.e.
at the symbol times)
• For a bandwidth limited signal the sampling rate of the phase with predictable values is smaller than the
actual sampling rate
• To be able to recover the phase, the carrier phase noise and offset must be within very tight limits
(tighter than required in a real transmission system)
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
t
linearphase drift
t
Transmitter lasers are not likely
to meet these tighter
specifications since they are not
required in real line cards that
use real time acquisition
Page 37 of 74
© Agilent Technologies, Inc. 2010
sample #
Ca
rrie
r ph
ase [
rad
]
Symbols are artificially
narrow. The phase tracking
reduces angular width of
symbols.
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 38 of 74
© Agilent Technologies, Inc. 2010
Nice constellation with
“round” symbols.
High
Medium
sample #
Carr
ier
phase [ra
d]
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 39 of 74
© Agilent Technologies, Inc. 2010
Medium
Low
Possible loss
of track.
sample #
Carr
ier
phase [ra
d]
February 2010
Constellation is
effected by the phase
noise.
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 40 of 74
© Agilent Technologies, Inc. 2010
February 2010
y
x
y
xy
x
y
x
y
x
• standard single mode fiber (SMF) does not preserve the state of
polarization (SOP)
• polarization maintaining fiber (PMF) preserves the SOP but is not
deployed for data transmission
the effect of propagation through fiber is described using
the Jones matrix
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 41 of 74
© Agilent Technologies, Inc. 2010
February 2010
x-polarization
(Transmitter)
y-polarization
(Transmitter)
x-polarization
(Receiver)
y-polarization
(Receiver)
Ideal transmission channel
sx and sy are time dependent complex numbers describing the time dependet field of
the optical wave (not only the optical power!)
If transmitter and receiver are aligned the received signals are identical to the sent
signals, there is no crosstalk between the channels
rx 1 0 sx
ry 0 1 sy= .
Jones matrix of ideal channel
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 42 of 74
© Agilent Technologies, Inc. 2010
February 2010
x-polarization
(Transmitter)
y-polarization
(Transmitter)
x-polarization
(Receiver)
y-polarization
(Receiver)
Real transmission channel (SMF)
There is crosstalk between the polarization channels. The receiver „sees“ a linear
combination of the „horizontal“ and „vertical“ signals sx and sy.
Without loss, PDL and PMD the channel matrix has only one independent parameter:
angle of rotation of reference frames (single mode fiber cannot preserve the SOP)
rx cos( ) sin( ) sx
ry -sin( ) cos( ) sy= .
Jones matrix
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 43 of 74
© Agilent Technologies, Inc. 2010
February 2010
x-polarization
(Transmitter)
y-polarization
(Transmitter)
x-polarization
(Receiver)
y-polarization
(Receiver)
In the most general form the Jones matrix is a complex valued 2x2 matrix
(with 8 independent real parameters)
rx Jxx Jxy sx
ry Jyx Jyy sy= .
Jones matrix
Transmission with Loss, PDL and PMD
J. C. Geyer, et al., “Channel Parameter Estimation for Polarization Diverse
Coherent Receivers,” PTL, Vol. 20, No. 10, May 15, 2008, p. 776
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 44 of 74
© Agilent Technologies, Inc. 2010
February 2010
To calculate the original signal sx and sy from measurement we need:
1. An estimation of the Jones matrix of the transmission channel.
2. A measurement of the rx and ry (the field of the optical wave including phase)
sx J-1xx J-1
xy rx
sy J-1yx J-1
yy ry= .
Inverse Jones matrix J-1
J-1xx J-1
xy 1 Jyy -Jxy
J-1yx J-1
yy JxxJyy-JxyJyx -Jyx Jxx=
J. C. Geyer, et al., “Channel Parameter Estimation for Polarization Diverse
Coherent Receivers,” PTL, Vol. 20, No. 10, May 15, 2008, p. 776
Coherent receivers detect the field and not only the power
of an optical wave
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 45 of 74
© Agilent Technologies, Inc. 2010
February 2010
• Training symbols allow
efficient estimation of the
channel
• Detection needs to know the
training sequence and detect it
Not feasable for T&M gear
• “blind” estimation does not
require prior knowledge about
signal (except the modulation
format)
• Might not be able to recover
the full information
Better choice for T&M
There is no ultimate solution for the problem!
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 46 of 74
© Agilent Technologies, Inc. 2010
February 2010
Phase differences between x- and y-signals:
• 00/00, 10/10, 11/11, 01/01 0 (linear 45°)
• 00/10, 10/11, 11/01, 01/00 /2 (right circular)
• 00/11, 10/01, 11/00, 01/10 (linear -45°)
• 00/01, 10/00, 11/10, 01/11 - /2 (left circular)
These 4 SOPs define a plane in the Stokes space!
Normal can be used to estimate Jones matrix
PDM vs. Single Polarization
Other formats?
Time dependent Jones matrix ?
x y
Real measurements!
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 47 of 74
© Agilent Technologies, Inc. 2010
February 2010
In the Stokes space, all points of PDM 64QAM are within the
boundaries of a lens-like object.
Still defines a plane (method does not depend on format)
1.0 0.5 0.5 1.0
1.0
0.5
0.5
1.0
Example: 64QAM
Stokes SpaceConstellation
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 48 of 74
© Agilent Technologies, Inc. 2010
February 2010
Constellation
map
rx
ry
J-1xx J-1
xy
J-1yx J-1
yy
sx
sy
Calculate vector to
nearest symbol
Update
estimation
• Needs knowledge of constellation
• Update step can be taylored
• Additional restrictions for J-1 are
possible
• Might not converge for bad initial
guess
• Locking speed?
Decision!
Iterate per
symbol
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 49 of 74
© Agilent Technologies, Inc. 2010
February 2010
J-1xx and J-1
yy J-1yx and J-1
xy
Constellation
SOP
Inverse
Jones
Matrix
Before and after correction
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 50 of 74
© Agilent Technologies, Inc. 2010
February 2010
y-Polarization
x-Polarization
IQ-Plot Symbols/Errors Spectrum
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 51 of 74
© Agilent Technologies, Inc. 2010
February 2010
y-Polarization
x-Polarization
IQ-Plot Symbols/Errors Spectrum
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 52 of 74
© Agilent Technologies, Inc. 2010
February 2010
y-Polarization
x-Polarization
IQ-Plot Symbols/Errors Spectrum
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 53 of 74
© Agilent Technologies, Inc. 2010
Part I
• Trends in 40/100 GbE
• Introduction to Advanced Optical
Modulation
– IQ Modulation
– Polarization Division Multiplexing
– Orthogonal Frequency Domain Multiplexing
• Receiver Technologies
– Measurement Principles
– Frequency or Time Domain
– Delay Line Interferometer or Coherent Receiver
– Real Time or Equivalent Time Sampling
Part II
• Signal Processing for Coherent
Receivers
– Carrier Phase Recovery
– Polarization Demultiplexing
• Quality Rating
– Error Vector Magnitude, Phase Error, Magnitude
Error, Quadrature Error, Gain Imbalance
– Bit Error Rate and Error Vector Magnitude
• Conclusion
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
February 2010Page 54 of 74
© Agilent Technologies, Inc. 2010
How can these impairments be measured?
How do they distort the constellation diagram?
π/2
Bias Voltages
Gain imbalance
π/2 shift error
Line width
Phase noise
Path delay between I and Q
Offset in symbol rate between receiver and transmitter
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 55 of 74
© Agilent Technologies, Inc. 2010
Constellation diagram Eye diagram of I and Q
path with equal scaling
Quantitative Analysis
Gain imbalance is
quantified with: 2.938 dB
Gain imbalance in the IQ paths of modulator lead to a “rectangular” constellation
Gain imbalance:
= 20 * log10
I
Q
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 56 of 74
© Agilent Technologies, Inc. 2010
• Eye diagram gives no
indication about a
possible root cause
• Spectrum show‟s also
the typical QPSK
pattern
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 57 of 74
© Agilent Technologies, Inc. 2010
70.23 deg
instead of 90 deg !
Quadrature error describes the deviation from orthogonality of the I and Q
modulation and leads to a rhombic constellation shape
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 58 of 74
© Agilent Technologies, Inc. 2010
Ideal
(square)
Measured
(square)
IQ offset: -31.043 dB
given as ratio between
signal and offset
I-Q offset is caused by a DC offset in the I and/or Q path.
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 59 of 74
© Agilent Technologies, Inc. 2010
DFB source External Cavity Laser Typical line width
DFB 2-3 MHz
ECL 0.1 MHz
How does this difference influence the constellation diagram ?
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 60 of 74
© Agilent Technologies, Inc. 2010
I Eye
Q Eye
Instruments based on optical
power detection for I and Q path
High resolution spectrum
gives also no indication
of the error source
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 61 of 74
© Agilent Technologies, Inc. 2010
Constellation left
indicates phase noise
Analyzing the phase error
show low frequency phase
noise
Constellation diagram with DFB
transmitter laser and low phase
tracking bandwidth
Constellation and phase error
with external cavity laser or with DFB
laser and higher tracking bandwidth
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 62 of 74
© Agilent Technologies, Inc. 2010
I (in-phase or real part)
Q (quadrature, imaginary part)
Phase φ
Ideal
constellation
point
Measured
constellation
point
Error vector
The error vector magnitude (EVM) represents the Euclidian distance between the
ideal symbol coordinate and the actual recorded symbol
Note:
EVM is part of the 802.11
WLAN standard
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 63 of 74
© Agilent Technologies, Inc. 2010
February 2010
Demodulate
test
signal
Calculate the
reference
Reconstruct
I-Q signal
calculate
complex
Error Vector
bits
Reference
trace
In-phase ref
Quadrature ref
Additional error analysis tools
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 64 of 74
© Agilent Technologies, Inc. 2010
V shape
Structured constellation points
Eye diagram gives
no clear indication,
but indicates a
problem
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 65 of 74
© Agilent Technologies, Inc. 2010
π/2Symbol clock
Error vector magnitude
Small errors in symbol rate cause a typical V shape
in EVM versus detected symbols
de
co
de
Recovered symbol clock phase using the
user input of the clock frequency
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 66 of 74
© Agilent Technologies, Inc. 2010
t
t +Δt where Δt < symbol length
Small delay errors between I and Q path might look like a bandwidth
limited modulator driver, also look at the IQ eye diagrams in this case
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 67 of 74
© Agilent Technologies, Inc. 2010
Why is the transition missing?
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 68 of 74
© Agilent Technologies, Inc. 2010
February 2010
t
t + 4 bits delay
0010
01 11
Choosing the wrong PRBS length and delay between I and Q
might lead to missing transitions
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 69 of 74
© Agilent Technologies, Inc. 2010
• Assume a gaussion noise distribution
• Calculate the probability to detect the wrong
symbol using an ideal decision
BER estimation
Note:
Any distortion (Gain Imbalance, Quadrature Error,
...) of the constellation will increase the EVM
BER estimation is an upper limit, with
additional distortion the same EVM will lead
to a lower BER!
Specifying the EVM of the transmitter
ensures that a certain BER can be achieved
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 70 of 74
© Agilent Technologies, Inc. 2010
February 2010
5 10 15 20 25 30
1E-12
1E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
QPSK
8PSK
8PSK GRAY
16QAM
16QAM GRAY
BE
R
EVM / %
FEC limit
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 71 of 74
© Agilent Technologies, Inc. 2010
Part I
• Trends in 40/100 GbE
• Introduction to Advanced Optical
Modulation
– IQ Modulation
– Polarization Division Multiplexing
– Orthogonal Frequency Domain Multiplexing
• Receiver Technologies
– Measurement Principles
– Frequency or Time Domain
– Delay Line Interferometer or Coherent Receiver
– Real Time or Equivalent Time Sampling
Part II
• Signal Processing for Coherent
Receivers
– Carrier Phase Recovery
– Polarization Demultiplexing
• Quality Rating
– Error Vector Magnitude, Phase Error, Magnitude
Error, Quadrature Error, Gain Imbalance
– Bit Error Rate and Error Vector Magnitude
• Conclusion
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
February 2010Page 72 of 74
© Agilent Technologies, Inc. 2010
• Coherent reception with real time sampling offers the best fit for
measuring advanced optical modulation formats targeted for 50 GHz
channels
• Carrier phase recovery and polarization demultiplexing are the
challenges for coherent reception
• Quality rating of transmitters for advanced modulation formats is more
challenging than for on-off keying transmitters. This is even more true
for higher order modulation formats
• Constellation diagram analysis along with additional tools gives
significantly more information about the signal quality that is emitted
than observing just an I and Q eye and often leads directly to the
error source
• Specifying the signal quality in terms of EVM, Quadrature Error, Gain
Imbalance along with other numbers will allow to predict the limit for
system BER independently of the receiver
February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
Page 73 of 74
© Agilent Technologies, Inc. 2010
Thank you for attending our Tutorial!
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February 2010
Metrology of Advanced Optical Modulation
Formats for 40/100G and beyond
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