Lecture: 8 Physical Layer Impairments in Optical
Networks
Ajmal Muhammad, Robert Forchheimer
Information Coding Group
ISY Department
Outline
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Introduction to Physical Layer Impairments (PLIs)
PLIs Classification
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Linear and non-linear
Signal Quality Estimation
PLIs Aware Routing and Wavelength Assignment
Physical Layer Impairments
Optical signals traverse the optical fibre links, passive and/or active
optical components
Signals encounter many impairments that affect their intensity level,
as well as their temporal, spectral and polarization properties
If the received signal quality is not within the receiver sensitivity
threshold, the receiver may not be able to correctly detect the
optical signal
Physical Layer Impairment Awareness
Important for network designers and operators to know:
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Various important Physical Layer Impairments (PLIs)
Their effects on lightpath (connection) feasibility
PLI analytical modeling, monitoring and mitigation techniques
Techniques to communicate PLI information to network layer and
control plane protocols
How to use all these techniques to dynamically set-up and
manage optically feasible lightpaths
PLIs Dependence
PLIs and their significance depend on: network type, reach, type of
network applications
Network type: opaque (signal undergoes OEO conversion at all
intermediate nodes along its path), translucent (undergoes OEO at
some intermediate nodes), transparent (lightpaths are switched
completely in the optical domain)
Reach: Access, metro, or core/long-haul network
Type of applications: Real-time, non-real time, mission-critical, etc
Maximum Transparency Length
The maximum distance an optical signal can travel and be detected by
a receiver without requiring OEO conversion
The maximum transparency length of an optical path depends on:
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The optical signal power
The fibre distance
Type of fibre and design of links (e.g., dispersion compensation)
The number of wavelengths on a single fibre
The bit-rate per wavelength
The amplification mechanism and the number of amplifiers
The number and type of switching elements through which the
signals pass before reaching the egress node or before
regeneration
PLIs Classification
PLIs are broadly classified into two categories: linear and non-linear
Optical systems that operate below a certain input power threshold
exhibit a linear relationship between the input and output signal power
The loss and refractive index (n) of the fibre are independent of the
signal power, i.e., static in nature
Important linear impairment are: fibre attenuation, component insertion
loss, Amplifier Spontaneous Emission (ASE) noise, Chromatic
Dispersion (CD) (or Group Velocity Dispersion (GVD)), Polarization
Mode Dispersion (PMD), crosstalk and Filter Concatenation (FC)
PLIs Classification: Non-linear
Non-linear impairments refer to phenomena that only occur when the
signal energy propagating in a medium attains sufficiently high
intensities
This can be due to high launch power and/or the confinement of energy
in extremely small areas, i.e., fibre core
Non-linear impairments induce phase variation and introduce noise into
the optical signal
Important non-linear impairments are: Self Phase Modulation (SPM),
Cross Phase Modulation (XPM), Four Wave Mixing (FWM)
Outline

Introduction to Physical Layer Impairments (PLIs)

PLIs Classification
 Linear and non-linear

Signal Quality Estimation

PLIs Aware Routing and Wavelength Assignment
Signal Attenuation & Insertion Loss
Signal attenuation: refers to the loss of power of a signal propagating
through optical fibre as distance increases
Causes: material absorption, Rayleigh scattering
Material absorption: impurities within fibre absorb propagating signal
power, often convert the energy into heat
Rayleigh scattering: photons can interact with the atoms in the fibre
causing energy to be scattering in all directions
If a scattered photon does not propagate in the same direction as the
original signal, then signal attenuation or loss occur
Insertion loss: loss of signal power resulting from the insertion of a
device in an optical fibre and is usually expressed in decibels (dB)
Amplified Spontaneous Emission (ASE)
Amplifiers are used to overcome fibre losses
Optical noise is added by each amplifier
-spontaneously emitted photons have random characteristics and manifest in
the amplified signal as noise
ASE noise within the signal bandwidth cannot be removed and is subject to
gain from any other amplifier downstream in the optical link
Optical Signal to Noise Ratio (OSNR)
Power in optical signal divided by the power in 0.1 nm of the noise
spectrum
Expressed in dB
For amplifiers and a line system, delivering a high ONSR is good
For a receiver, tolerating a low OSNR is good
dB
Km
Chromatic Dispersion
Material Dispersion: Since refractive index (n) is a
function of wavelength, different wavelengths travel at
slightly different velocities.
Waveguide Dispersion: Signal in the cladding travels with
a different velocity than the signal in the core. This
phenomenon is significant in single mode conditions.
Group Velocity (Chromatic) Dispersion
= Material Disp. + Waveguide Disp.
Group Velocity Dispersion
Effects of Dispersion and Attenuation
Polarizations of Fundamental Mode
Two polarization states exist in the fundamental mode in
a single mode fiber
Polarization Mode Dispersion (PMD)
Each polarization state has a different velocity  PMD
Crosstalk
Optical switches are prone to signal leakage, giving rise to crosstalk
Inter-channel crosstalk: occurs between signals on adjacent
channels. Can be eliminated by using narrow pass-band receivers.
Intra-channel crosstalk: occurs among signals on the same
wavelengths, or signals whose wavelengths fall within each other’s
receiver pass-band.
Outline

Introduction to Physical Layer Impairments (PLIs)

PLIs Classification
 Linear and non-linear

Signal Quality Estimation

PLIs Aware Routing and Wavelength Assignment
Kerr Effect
The refractive index (n=c/v) of optical fibre dependent on the optical
signal intensity, I: n=n0 + n2I = n0 + n2 P/Aeff
Where P is optical signal power, Aeff is the effective area of the fibre
core cross section, n0 is the linear refractive index, n2 is the “nonlinear
index coefficient”
When I is large, the nonlinear component of the refractive index becomes
significant, resulting in the kerr effect (change in the refractive index of a
material in response to an applied electric field)
Kerr effect
Self and Cross Phase Modulation (SPM & XPM )
The refractive index changes induced by the kerr effect cause phase
changes in different parts of the optical pulse to travel at different
speeds, resulting in new frequencies being introduced into the pulse
The kerr effect inducing phase changes of a signal due to its own
intensity variation is known as self phase modulation
The kerr effect induces phase modulation in a signal due to intensity
variations in other channels, this effect is known as cross phase
modulation
Four Wave Mixing (FWM)
Multiple channels at different wavelengths (frequencies) propagate
down a single fibre. The signals of these channels interact to produce
new signals
FWM generated by
two signals f1 & f2
FWM generated by
three signals
In general, for N signal channels, the number of generated mixing
product M will be:
M= N2.(N-1)/2
And the generated FWM frequencies are given by: fijk=fi+fj-fk , i!=k, j!=k
Digital Processing for Impairments Compensation
Encoding for
Error correction
Customer
Traffic coming
Into the chip
Compensation for
Imperfection in the
Compensation DSP for compensating modulator
for nonlinearity dispersion & shaping
the spectrum
Tx Processing
22 M Gates
DSP= 20 Mgates
Receive Processing
Undoing the
polarization
effects
Inverting the difference
b/w the transmitter
laser &the receiver laser
70 T ops/s
32 nm CMOS
150 M gates
3.7 km wire (copper)
Outline

Introduction to Physical Layer Impairments (PLIs)

PLIs Classification
 Linear and non-linear

Signal Quality Estimation

PLIs Aware Routing and Wavelength Assignment
Eye Diagram
Overlay the received bit stream in the time domain over a
three-bit sliding window
Eight 3-bit sequence
Superimposition of multiple
instances of the eight 3-bit
binary sequences
Eye Diagram in the Presence of
Signal Degradation
When a received signal
is degraded by optical
impairments, the eye diagram
becomes partially closed and
distorted
For ASE, this
corresponds to an
increase in the
standard deviation
of the levels
For PMD and CD, this
corresponds to distortions in
the slope of the bit transitions
and an increase in the timing
jitter
Bit Error Rate (BER) and Q-factor
BER: number of bits received in error as a ratio of the total
number of transmitted bits
idec is the signal level at the decision instant
For Gaussian distributions with mean and standard
deviations given by
and
Perror minimized when
Q-factor
Optimal decision
threshold value
Q-factor and BER
Typical BER levels range from 10-9 to 10-12, correspond to Qfactor of 6 to 8, respectively
Using forward error correction (FEC), a system may tolerate up
to levels of 10-3 corresponding to a Q-factor of 3
1
Q exp(-Q2 / 2)
BER = erfc( ) »
2
2
Q 2p
I -I
Q= 1 0
s1 +s 0
Outline

Introduction to Physical Layer Impairments (PLIs)

PLIs Classification
 Linear and non-linear

Signal Quality Estimation

PLIs Aware Routing and Wavelength Assignment
PLI-RWA Proposals
When selecting a lightpath (route and wavelength), a PLI-RWA
algorithm for a transparent or translucent network has to take
into account the physical layer impairments as well as
wavelength availability
The PLIs are either considered as constraints for the RWA
decisions (i.e., physical layer impairment constrained or PLICRWA) or the RWA decisions are made considering these
impairments (i.e., physical layer impairment aware or PLIARWA)
In PLIA-RWA, it is possible to find alternate routes considering
the impairments, while in PLIC-RWA the routing decisions are
constrained by PLIs
Approach 1
Compute the route and the wavelength in the traditional way
and finally verify the selected lightpath considering the physical
layer impairments
Approach 2
Considering the physical layer impairments values in the
routing and/or wavelength assignment decisions
Approach 3
Considering the physical layer impairments values in the
routing and/or wavelength assignment decisions and finally
also verify the quality of the candidate lightpath
PLI
verification