Lecture: 8 Physical Layer Impairments in Optical Networks

Ajmal Muhammad, Robert ForchheimerInformation Coding Group

ISY Department

Outline

Introduction to Physical Layer Impairments (PLIs) PLIs Classification

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:

Various important Physical Layer Impairments (PLIs)Their effects on lightpath (connection) feasibilityPLI analytical modeling, monitoring and mitigation techniquesTechniques to communicate PLI information to network layer and control plane protocolsHow 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:

The optical signal powerThe fibre distanceType of fibre and design of links (e.g., dispersion compensation)The number of wavelengths on a single fibreThe bit-rate per wavelengthThe amplification mechanism and the number of amplifiersThe 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 dBFor amplifiers and a line system, delivering a high ONSR is goodFor a receiver, tolerating a low OSNR is good

Km

dB

Chromatic DispersionMaterial 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 ina single mode fiber

Polarization Mode Dispersion (PMD)

Each polarization state has a different velocity PMD

CrosstalkOptical 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

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

FWM generated by two signals f1 & f2

FWM generated by three signals

Digital Processing for Impairments Compensation

Tx Processing

CustomerTraffic comingInto the chip

Encoding forError correction

Compensationfor nonlinearity

DSP for compensatingdispersion & shaping the spectrum

Compensation forImperfection in the modulator

22 M GatesDSP= 20 Mgates

Receive Processing

Undoing the polarization effects

Inverting the differenceb/w the transmitter laser &the receiver laser

70 T ops/s32 nm CMOS150 M gates3.7 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 multipleinstances of the eight 3-bitbinary 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 anincrease in thestandard deviationof the levels

For PMD and CD, thiscorresponds to distortions inthe slope of the bit transitionsand an increase in the timingjitter

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 standarddeviations given by

Q-factor

and

Optimal decision threshold value

Perror minimized when

Q-factor and BER

Typical BER levels range from 10-9 to 10-12, correspond to Q-factor 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

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 PLIC-RWA) or the RWA decisions are made considering these impairments (i.e., physical layer impairment aware or PLIA-RWA)

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

PLIverification