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customer-presentation----cd-pmd-fundamentals-sept-2014

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CD / PMD Fundamentals
Jean-Sébastien Tassé
Product Line Manager
Contributions from Gwennael Amice
and Francis Audet
August 2014
What is dispersion?
• Dispersion = pulse broadening or pulse spreading as
light propagates in a fiber.
• In other words, dispersion increases pulse duration.
Impact of dispersion
• Dispersion causes pulses to broaden which might lead
to pulses that overlap.
• In extreme cases, we get inter-symbol interference (ISI)
and higher Bit Error Rate.
Dispersion consequences
Dispersion CD/PMD
Pulse Broadening
Bit Errors and BERT issues
Significant delays and expense in
installation and commissioning
SLA Penalties
Inability to operate at high speed
(extreme)
Two main types of dispersion
• Chromatic Dispersion (CD):
• Different wavelengths travel at different velocities
Pulse Spreading
• Polarization Mode Dispersion (PMD):
• Two polarization modes travel at different velocities
Pulse Spreading
CD Fundamentals
Chromatic Dispersion (CD)
• Different wavelengths travel at
different speeds in a fiber, causing
pulse broadening
• Limits how fast and how far a signal
will travel
• Higher bit rates are less robust
t
z,t
Effects of Chromatic Dispersion
Low chromatic dispersion
High chromatic dispersion
Facts about chromatic dispersion
•
•
•
•
Chromatic dispersion is linear.
It increases proportionally with distance.
Does not change over time.
Since CD properties are set during manufacturing,
there is no FAIL in the field. Values measured are
used to adjust the dispersion compensation modules
and to select the optimum set of transmitter/receiver.
Chromatic dispersion
• Total chromatic dispersion is the sum of
• Material dispersion +
• Waveguide dispersion
Be-an-Expert Training Program
Cause 1: Material Dispersion
• White light is composed of all colors.
• Different colors travel at different speeds in fiber because
the index of refraction depends on the wavelength.
• Just like a prism decomposes white light because the
angle of refraction depends on the index of refraction,
which depends on the wavelength.
Be-an-Expert Training Program
Cause 2: Waveguide Dispersion
• Different index profiles of the fiber (i.e. the waveguide)
affect the fiber CD properties.
Be-an-Expert Training Program
Cause 2: Waveguide Dispersion
Be-an-Expert Training Program
Chromatic Dispersion
(ps/nm-km)
CD for different fiber types
+
+7
dispersion unshifted
G.652
+


non-zero
dispersion





dispersion shifted
G.653
non-zero dispersion shifted
G.655
(nm)


CD key measurements:
1. CD at 1550 nm
Example: G.652 fiber is ~ +17 ps/nm/km at 1550 nm.
2. Zero dispersion wavelength
Example: G.652 fiber has zero dispersion at ~ 1310 nm.
CD example
• A pulse is sent in a SMF28 fiber (CD=17ps/[email protected])
• Source is DFB laser:
• Linewidth = 7pm, central wavelength = 1550 nm.
• WDM network is 600 km long.
• Pulse duration t1 and t2:
• t1: depends on the opto-electronics
• t2 = t1 + (17ps/nm.km * 7pm * 600 km)
= t1 + 71.4ps!!
t1
t2
• This means that the pulse duration
increased by 71.4 ps due to CD.
How to compensate?
CD
?
20
18
16
14
What compensation
should you apply?
12
1525
Be-an-Expert Training Program
1550
1565
CD accumulation with distance
CD
Distance
Be-an-Expert Training Program
CD: Good Compensation
CD
Distance
CD: Bad Compensation
CD
Distance
Be-an-Expert Training Program
CD compensation
• If measured accurately, CD can be compensated for with
dispersion compensation modules (DCM).
No Compensation
With Compensation
OC192 Delay Threshold
Delay
Length
TX
DCM
DCM
RX
Fiber characterization and CD
• Regardless if the services are to be deployed
on a new build or existing fiber network, a
series of measurements must be performed to
certify that the link is appropriate for use.
• The series of measurements performed is
known as “fiber characterization”.
• The specific requirements are identified by
the ITU-T SG15, ref: G.650.3.
• CD is one of the measurements specified in
this standard.
Tolerance to CD versus Bit Rate
• NRZ pulses (on-off keying)
• 1 dB power penalty
Bit rate
CD Tolerance
CD Tolerance
(km of G.652)
2.5Gbits (OC48/STM16)
18468 ps/nm
970 km
10Gbits (OC192/STM64)
1193 ps/nm
63 km
40 Gbits
75 ps/nm
4 km
100 Gbits
12 ps/nm
0.6 km
Maximum reachable distance
• Maximum reachable distance for NRZ pulses (on-off keying)
without CD compensation as a function of bit rate and fiber type
PMD Fundamentals
Definition of polarization
• Light is electromagnetic wave composed of
• Electric Field Vector E
• Magnetic Field Vector H
• Propagating in time and in space in the z direction
• Oscillating in the x-z and y-z planes
• Polarization is a property of an electromagnetic wave that
describes the orientation of its oscillation.
Polarization Mode Dispersion (PMD)
• Pulses travel at different speeds depending on the polarization.
This is called PMD.
 If PMD = 0: polarization vectors travel at same speed.
 If PMD ≠ 0: polarization vectors do not travel at same speed.
Differential Group Delay (DGD)
•
•
•
•
Fast axis: polarization axis such that the pulse travels the fastest.
Slow axis: polarization axis such that the pulse travels the slowest.
Fast and slow axis are called principal states of polarization.
Delay between fast and slow axis is called Differential Group
Delay (DGD).

T
t
fast axis
slow axis
z,t

PMD and DGD
 DGD is specific to one
wavelength and it
varies over time.
 PMD: average of DGD,
for all wavelengths.
PMD and DGD
•
•
•
•
Probability density function: distribution of all DGD values.
DGD varies over time, but PMD is more stable.
At a specific time, 0 ps < DGD < DGD max.
Typically, DGD max = 3 times PMD.
Visualizing PMD
• Let’s visualize a light pulse traveling into a fiber and
segment it into 9 quadrants.
Visualizing PMD
• Fiber section:
• Light pulse:
Visualizing PMD
If we transmit 1-0-1:
1 0
1
With PMD, this becomes:
1 0 1
The « 1 » is dimmer, the « 0 » can have light: BER
PMD causes
Geometric
Internal Stress
External Stress
Environmental constraints
Wind (aerial fibers)
Heat
Bend
PMD causes
• Asymmetries in fiber core geometry and/or stress distribution
create local fiber birefringence.
• A "real" fiber is a randomly distributed addition of these local
birefringent portions.
Older fibers exhibit worse PMD
Source: John Peters, Ariel Dori, and Felix Kapron, Bellcore
Fiber characterization and PMD
• Regardless if the services are to be deployed
on a new build or existing fiber network, a
series of measurements must be performed to
certify that the link is appropriate for use.
• The series of measurements performed is
known as “fiber characterization”.
• The specific requirements are identified by
the ITU-T SG15, ref: G.650.3.
• PMD is one of the measurements specified in
this standard.
PMD thresholds: how much PMD causes failures?
•
•
•
•
•
Instantaneous DGD (DGDinst): DGD at specific time.
DGDmax: max DGD a system can suffer from without outage.
Therefore, outage occurs when DGDinst > DGDmax.
However, PMD analyzers measure PMD, not DGDinst.
Relationship between PMD and outage probability is given below for 1 dB
OSNR penalty.
DGDmax to PMD
ratio
Probability that
DGDinst > DGDmax
Time per year that
DGDinst > DGDmax
2.5
1.5 x 10-3
13.1 h
3.0
4.2 x 10-5
22 min
3.2
9.2 x 10-6
5 min
3.5
7.7 x 10-7
24 s
3.8
5.1 x 10-8
1.6 s
4.0
7.4 x 10-9
0.23 s
DGDmax thresholds
• Systems fail when DGDinst > DGDmax
• Status: refers to whether this value has been adopted by
standards, or is at the proposal stage.
Data rate
Modulation format
DGDmax
Status
2.5G
SDH/SONET NRZ
120 ps
Adopted
10G
SDH/SONET NRZ or OTN
30 ps
Adopted
40G
NRZ-DPSK (noncoherent)
8 ps
Proposal
40G
RZ-QPSK (noncoherent)
9 ps
Proposal
40G
DP-QPSK (coherent)
75 ps
Proposal
100G
NRZ (noncoherent)
2.9 ps
Proposal
100G
DP-QPSK (coherent)
27 ps
Proposal
PMD threshold example
• Let’s do an example of PMD threshold calculation.
• First, you need to know the data rate and modulation format of the
channel. Let’s assume 40G RZ-QPSK.
• Second table says DGDmax = 9 ps.
• According to the first table, that means if the measured PMD is
3.6 ps (DGDmax/PMD ratio = 2.5), then it should be expected that
outages due to PMD will occur during 13.1 hours in a given year.
• Similarly, if the measured PMD is 3 ps (DGDmax/PMD = 3), then it
should be expected that outages due to PMD will occur during 22
minutes in a given year.
• The PMD threshold therefore depends on the acceptable outage
probability, as determined by each service provider.
• A DGDmax/PMD ratio of 3 is commonly chosen for determining the
PMD threshold.
Questions?
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