IEC TR 61282-13:2014

Fibre optic communication system design guides - Part 13: Guidance on in-service PMD and CD characterization of fibre optic links

IEC/TR 61282-13:2014 which is a technical report, presents general information about in-service measurements of polarization mode dispersion (PMD) and chromatic dispersion (CD) in fibre optic links. It describes the background and need for these measurements, the various methods and techniques developed thus far, and their possible implementations for practical applications. Key words: polarization mode dispersion (PMD), chromatic dispersion (CD), fibre optic links

General Information

• Status: Published
• Publication Date: 05-May-2014
• ICS: 33.180.01 - Fibre optic systems in general
• Technical Committee: SC 86C - Fibre optic systems, sensing and active devices
• Drafting Committee: WG 1 - TC 86/SC 86C/WG 1
• Current Stage: PPUB - Publication issued
• Start Date: 06-May-2014
• Completion Date: 15-Aug-2014

Overview

IEC TR 61282-13:2014 - "Fibre optic communication system design guides - Part 13: Guidance on in-service PMD and CD characterization of fibre optic links" - is a technical report from the IEC that provides state‑of‑the‑art guidance on performing in-service measurements of polarization mode dispersion (PMD) and chromatic dispersion (CD) in live fibre optic links. Rather than prescribing mandatory test procedures, the report surveys background, measurement needs, available methods and practical implementation options for non‑intrusive and semi‑intrusive fibre characterization.

Key topics covered

• Purpose and background - why in‑service PMD and CD characterization is essential for high‑speed fibre networks and link qualification.
• Measurement approaches:
  • Non‑intrusive techniques such as polarization‑sensitive spectral analysis, high‑speed intensity detection, RF spectral analysis and coherent/heterodyne detection.
  • Semi‑intrusive probe‑signal methods including multi‑tone and multi‑wavelength probe signals for CD and PMD assessment.
• Practical measurement considerations:
  • Instrumentation examples and set‑ups (e.g., coherent receivers, high‑speed sampling), measurement accuracy and calibration methods (including use of calibrated CD/DGD compensators).
  • Signal types and analysis methods (asynchronous waveform sampling, phase‑portrait and RF spectral analysis) - the report includes worked examples for typical modulation formats (examples include 10 Gbit/s NRZ/RZ cases).
• Implementation issues - transmission of probe signals in live networks, spectral filtering, and limitations of each technique.
• Patents and IP considerations - the report notes that some methods described may be subject to patent rights.

Practical applications

• Network performance monitoring and degradation diagnosis (identifying PMD/CD as root causes of bit‑errors or margin loss).
• Commissioning and acceptance testing of fibre links where service continuity must be maintained (in‑service characterization without taking channels out of service).
• Capacity planning and migration to higher data rates (evaluating if legacy fibres meet new dispersion budgets).
• Vendor and test equipment selection - understanding tradeoffs between non‑intrusive vs semi‑intrusive methods and when to use coherent detection or probe signals.

Who should use this report

• Optical network designers and transmission engineers
• Field test and maintenance teams for metropolitan and long‑haul networks
• Test equipment manufacturers and R&D teams developing PMD/CD measurement solutions
• Service providers planning upgrades to 10 Gbit/s and beyond

Related standards

• IEC TR 61282 series and the wider IEC 61280 family of fibre‑optic communication test procedures.
  Note: IEC TR 61282‑13 is a technical report (guidance/state‑of‑the‑art) rather than a prescriptive standard; users should consult normative IEC standards and patent databases where applicable.

Keywords: IEC TR 61282-13:2014, polarization mode dispersion (PMD), chromatic dispersion (CD), in‑service measurements, fibre optic links, non‑intrusive, probe signal, asynchronous sampling, coherent detection.

Standards Content (Sample)

IEC TR 61282-13 ®
Edition 1.0 2014-05
TECHNICAL
REPORT
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Fibre optic communication system design guides –
Part 13: Guidance on in-service PMD and CD characterization of fibre optic links
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IEC TR 61282-13 ®
Edition 1.0 2014-05
TECHNICAL
REPORT
colour
inside
Fibre optic communication system design guides –

Part 13: Guidance on in-service PMD and CD characterization of fibre optic links

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
X
ICS 33.180.01 ISBN 978-2-8322-1572-2

– 2 – IEC TR 61282-13:2014  IEC 2014
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Symbols, acronyms and abbreviated terms . 7
4 Background . 9
5 Non-intrusive fibre characterization . 12
5.1 PMD measurement via polarization-sensitive spectral analysis . 12
5.1.1 Introductory remark . 12
5.1.2 Measurement principle. 13
5.1.3 Methods for measuring ∆τ via polarization analysis . 15
eff
5.1.4 Measurement accuracy . 19
5.1.5 Measurement set-up example . 21
5.2 CD and PMD measurements based on high-speed intensity detection . 22
5.2.1 Introductory remark . 22
5.2.2 Asynchronous waveform sampling . 24
5.2.3 RF spectral analysis . 29
5.3 CD and PMD measurements based on high-speed coherent detection . 32
5.3.1 Introductory remark . 32
5.3.2 Heterodyne detection . 33
5.3.3 Direct detection with optical CD or PMD compensation . 33
5.3.4 Electronic CD and PMD compensation in intradyne coherent receiver. 35
6 Semi-intrusive fibre characterization with special probe signals . 37
6.1 CD measurement using multi-tone probe signal . 37
6.1.1 Introductory remark . 37
6.1.2 Differential phase shift method with narrowband probe signals . 37
6.1.3 Issues of transmitting alien probe signals . 41
6.1.4 Exemplary procedure for in-service CD measurements . 42
6.2 PMD measurement with special probe signals. 43
6.2.1 Introductory remark . 43
6.2.2 Probe signal generator for PMD measurements . 43
Bibliography . 45

Figure 1 – Out-of-service fibre characterization with broadband optical probe signal . 9
Figure 2 – In-service fibre characterization with non-intrusive method . 10
Figure 3 – Semi-intrusive in-service fibre characterization using narrowband probe
signal . 11
Figure 4 – Rayleigh PDF for ∆τ compared with Maxwellian PDF for ∆τ . 14
eff
Figure 5 – PMD-induced polarization rotation within the spectrum of a modulated signal . 15
Figure 6 – Set-up for measuring PMD-induced polarization rotations in optical signals. 16
Figure 7 – Modified set-up for measuring PMD-induced polarization rotations . 16
Figure 8 – Sequence of polarization transformations leading to a scan with P ≈ P at
p s
ν=0 (left) and corresponding power ratios (right) . 17

Figure 9 – Sequence of polarization transformations with P ≈ P at ν=0 (left) and
p s
corresponding rotation angles (right) . 18
Figure 10 – Apparatus using coherent detection to measure ∆τ . 21
eff
Figure 11 – Apparatus for GVD measurements in a transmitted signal using a high-
speed receiver with time-domain waveform analysis or, alternatively, RF spectrum
analysis . 23
Figure 12 – Set-up for determining the sign of the GVD in the fibre link with an
additional optical CD element of known GVD magnitude and sign . 24
Figure 13 – Asynchronous sampling of the waveform of a 10 Gbit/s NRZ-OOK signal . 25
Figure 14 – Asynchronously sampled waveform histograms of a 10 Gbit/s NRZ-OOK
signal without dispersion, with 1 000 ps/nm GVD, and with 48 ps DGD . 26
Figure 15 – Asynchronous waveform analysis with two successive samples per symbol
period . 26
Figure 16 – Apparatus for asynchronous waveform analysis with time-delayed dual
sampling . 27
Figure 17 – Phase portraits of a 10 Gbit/s NRZ-OOK signal with various amounts of
GVD and DGD . 28
Figure 18 – Phase portraits of a 10 Gbit/s NRZ-OOK signal wherein the time delay
between each sample pair is set to half the symbol period . 29
Figure 19 – RF spectra of directly detected 10 Gbit/s NRZ- and RZ-OOK signals
distorted by various amounts of GVD . 30
Figure 20 – Magnitude of the clock frequency component in the RF spectra of 10 Gbit/s
NRZ- and RZ-OOK signals as a function of GVD . 30
Figure 21 – Impact of PMD on the RF spectra of directly detected 10 Gbit/s NRZ- and
RZ-OOK signals . 31
Figure 22 – Apparatus for simultaneous GVD and DGD measurements on NRZ- or RZ-
OOK signals using separate detectors for upper and lower modulation sidebands . 31
Figure 23 – Optical filtering of a 10 Gbit/s NRZ-OOK signal for separate detection of
upper and lower modulation sidebands . 32
Figure 24 – RF power spectrum of a 10 Gbit/s NRZ-OOK signal detected with an
optical heterodyne receiver . 33
Figure 25 – Apparatus for measuring GVD with calibrated optical CD compensator . 34
Figure 26 – Apparatus for measuring PMD with calibrated optical DGD compensator. 35
Figure 27 – Coherent optical receiver with high-speed digital signal processing and
electronic CD and PMD compensation . 36
Figure 28 – Spectrum of an amplitude modulated dual-wavelength probe signal . 38
Figure 29 – Signal generator and analyser for dual-wavelength probe signal . 39
Figure 30 – Four-wavelength probe signal generator using high-speed modulator . 39
Figure 31 – Example of end-to-end CD measurements in 6 unused WDM channels . 40
Figure 32 – In-service CD measurement with broadband probe signal . 41
Figure 33 – Modified dual-wavelength probe signal with un-modulated carrier . 42
Figure 34 – Probe signal generator for PMD measurements . 44

– 4 – IEC TR 61282-13:2014  IEC 2014
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FIBRE OPTIC COMMUNICATION SYSTEM DESIGN GUIDES –

Part 13: Guidance on in-service PMD and
CD characterization of fibre optic links

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 61282-13, which is a technical report, has been prepared by subcommittee 86C: Fibre
optic systems and active devices, of IEC technical committee 86: Fibre optics.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
86C/1201/DTR 86C/1236/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 61280 series, published under the general title Fibre-optic
communication subsystem test procedures, can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

– 6 – IEC TR 61282-13:2014  IEC 2014
INTRODUCTION
The International Electrotechnical Commission (IEC) draws attention to the fact that it is
claimed that compliance with this document may involve the use of a patent concerning
optical frequency-sensitive analyser given in 5.1.3.4 and concerning CD measurement using
multi-tone probe signal given in 6.1.
IEC takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured the IEC that he/she is willing to negotiate licences
either free of charge or under reasonable and non-discriminatory terms and conditions with
applicants throughout the world. In this respect, the statement of the holder of this patent
right is registered with IEC. Information may be obtained from:
Exfo Electro-Optical Engineering Inc.
400 Avenue Grodin
QC G1M 2K2
CANADA
JDS Uniphase Corporation
430 N. McCarthy Blvd.
Milpitas, CA 95035
USA
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights other than those identified above. IEC shall not be held responsible for
identifying any or all such patent rights.
ISO (www.iso.org/patents) and IEC (http://patents.iec.ch) maintain on-line data bases of
patents relevant to their standards. Users are encouraged to consult the data bases for the
most up to date information concerning patents.

FIBRE OPTIC COMMUNICATION SYSTEM DESIGN GUIDES –

Part 13: Guidance on in-service PMD and
CD characterization of fibre optic links

1 Scope
This part of IEC 61282, which is a technical report, presents general information about in-
service measurements of polarization mode dispersion (PMD) and chromatic dispersion (CD)
in fibre optic links. It describes the background and need for these measurements, the various
methods and techniques developed thus far, and their possible implementations for practical
applications.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60793-1-42, Optical fibres – Part 1-42: Measurement methods and test procedures –
Chromatic dispersion
IEC 61280-4-4, Fibre optic communication subsystem test procedures – Part 4-4: Cable
plants and links– Polarization mode dispersion measurement for installed links
3 Symbols, acronyms and abbreviated terms
D(λ) group velocity dispersion coefficient at optical wavelength λ
F frequency of amplitude modulation in CD measurement
L length of arc of the SOP rotation on the Poincaré sphere
L length of fibre or fibre link
f
P , P optical signal powers in two orthogonal SOPs
p s

P normalized optical power

∆P normalized optical power difference
S , S , S Stokes parameter
1 2 3
Ŝ normalized Stokes vector
N number of statistically independent effective DGD measurements
N number of statistically independent effective DGD measurements in time
t
N number of statistically independent signal wavelengths
ν
c speed of light in vacuum
∆f optical frequency interval or spacing
f electrical signal frequency in dual-wavelength frequency generator
clock frequency of digital data modulation
f
clock
∆t time interval between effective DGD measurements or differential time delay in
CD measurement
– 8 – IEC TR 61282-13:2014  IEC 2014
∆t correlation time of effective DGD variations
corr
∆φ differential phase shift in CD measurement
δλ wavelength increment (interval, spacing or step size)
δν optical frequency increment (interval, spacing or step size)
∆λ optical source spectral width or linewidth (FWHM unless noted otherwise)
∆ν optical frequency interval or spacing
∆τ differential group delay value
∆τ effective or partial DGD value, ∆τ = ∆τ sinϕ , where ϕ is the angle between
eff eff
PSP vector and signal SOP vector on the Poincaré sphere
<∆τ> average DGD over a wavelength range or time interval
<∆τ > average effective DGD over a wavelength range or time interval
eff
2 1/2
<∆τ > average RMS DGD over a wavelength range or time interval
λ optical wavelength
v optical light frequency
ϕ angle between PSP and signal SOP vector on the Poincaré sphere
Φ(ν) optical phase shift introduced by GVD in the spectral components of a
modulated signal
ψ angle between two Stokes vectors
σ standard deviation of DGD measurements
θ polarization rotation angle on the Poincaré sphere
ACF autocorrelation function
ADC analogue-to-digital converter
AM amplitude modulation
ASE amplified stimulated emission (from optical amplifiers)
BPF optical or electrical band-pass filter
CD chromatic dispersion
CW continuous wave
DGD differential group delay
DMUX wavelength division de-multiplexer
DOP degree of polarization
DPSK differential phase shift keying
DSP digital signal processing or processor
GVD group velocity dispersion
JME Jones matrix eigenanalysis (PMD test method)
LO local oscillator or local oscillator laser
MT monitoring port or tap
MUX wavelength division multiplexer
NRZ non-return-to-zero modulation
OA optical amplifier
OOK on-off keying
OTDR optical time-domain reflectometry
PDF probability density function

PC variable polarization controller
PBS polarization beam splitter
PD photo detector
PM phase modulation
PMD polarization mode dispersion
PSK phase shift keying
PSP principal SOP
QPSK quadrature phase shift keying
ROADM reconfigurable optical add-drop multiplexer
RF radio frequency
RZ return-to-zero modulation
SOP state of polarization
WDM wavelength division multiplexing or multiplexer
4 Background
Excessive chromatic or polarization mode dispersion in fibre optic links may severely impair
the transmission of high-speed optical signals. It is therefore important to accurately
characterize the end-to-end optical properties of a fibre link before it is put into service. CD or
PMD in a fibre link may be characterized using any of the measurement methods described in
international standards, such as IEC 60793-1-42 for CD measurements and IEC 61280-4-4 for
PMD measurements. A common feature of these methods is that they require either
broadband or broadly tuneable optical probe signals to be injected into one end of the link
while the optical properties of the fibre are analysed at the other end (see Figure 1).
Consequently, the fibre link cannot carry any traffic during the duration of the measurement
and has to be taken out of service.

Broad-
band Signal
OA OA
probe
analyser
Fibre Fibre Fibre
signal
span 1 span 2 span n
IEC  1505/14
Key
OA optical amplifier
Figure 1 – Out-of-service fibre characterization with broadband optical probe signal
Such out-of-service measurements are usually acceptable when a new fibre link is installed.
However, they are highly undesirable when the fibre dispersion needs to be re-measured in a
link that already carries commercial traffic [1] . This situation may occur, for example, when a
link is considered to be upgraded to a higher bit rate, e.g. from transmitting 10-Gb/s NRZ-
OOK to 40-Gb/s DPSK signals, or during occasional troubleshooting. When conventional fibre
characterization methods are used, all signals carried by the link have to be re-routed to other
links before the measurement can be performed.
______________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC TR 61282-13:2014  IEC 2014
To avoid such time-consuming re-configuration of network traffic, various methods have been
developed for measuring fibre properties in transmission links that carry live commercial
traffic [2-11]. An important requirement for in-service fibre characterization is that the
measurement procedure must not at any time interrupt or otherwise impair the transmission of
traffic signals through the fibre.
This technical report describes the measurement principle and application of seven different
in-service fibre characterization methods as well as their impact on network operation.
Signal
OA
analyser
WDM
signals
Rx 1
Tx 1
D
Tap Rx 2
Tx 2
M
M
OA
U OA
U
Rx 3
Tx 3
X
X
ROADM
Rx n
Tx n
OA
IEC  1506/14
Key
Tx optical transmitter
Rx optical receiver
OA optical amplifier
MUX WDM multiplexer
DMUX WDM demultiplexer
ROADM reconfigurable optical add-drop multiplexer
Figure 2 – In-service fibre characterization with non-intrusive method

OA Signal
Probe
analyser
signal
Rx 1
Tx 1
D
M
M
OA
U OA
U
Rx 3
Tx 3
X
X
Tap
ROADM
Rx n
Tx n
WDM
signals
OA
Signal
analyser
IEC  1507/14
Key
Tx optical transmitter
Rx optical receiver
OA optical amplifier
MUX WDM multiplexer
DMUX WDM demultiplexer
ROADM reconfigurable optical add-drop multiplexer
Figure 3 – Semi-intrusive in-service fibre characterization
using narrowband probe signal
In-service fibre characterization methods may be divided into three categories:
a) Completely non-intrusive methods which measure the desired fibre property by analysing
the transmitted traffic signals at pre-installed monitoring ports (or taps) along the link, as
shown in Figure 2. These methods do not interfere with the normal operation of the fibre
link (just like in-service OSNR measurement techniques). Some of these methods employ
high-speed optical receivers which recover or analyse the transmitted data [2-6], whereas
others only analyse the spectral or polarization characteristics of the transmitted signal [7].
Non-intrusive methods have been employed to measure end-to-end CD and PMD.
b) Semi-intrusive methods which employ special optically narrow-band probe signals to
measure the desired fibre property [8-12]. These probe signals are usually injected into
unused (i.e. empty) WDM channels at the input of the link, via a pre-installed WDM
multiplexer, and co-transmitted with the normal traffic signals, as shown in Figure 3.
Methods using probe signals are generally considered to be intrusive, even though the
measurement may not interrupt transmission of the traffic signals, because modern
networks often require provisioning of the transport system to allow alien signals to pass
through optical amplifiers and ROADM nodes. In addition, the co-transmission of probe
signals may adversely affect the quality of the traffic signals through nonlinear interactions
in the fibre (such as four-wave mixing or cross-phase modulation). Probe signals are often
employed to measure end-to-end CD and PMD in fibre links and can be designed to be
particularly sensitive to the fibre property to be measured [8, 10, 12]. However, the signals
must meet the required optical power levels and/or spectral shape expected by optical
channel monitors in ROADM nodes, as they otherwise may be blocked [1].
c) Out-of-band measurement methods using probe signals at optical frequencies that are
outside of the band used for transmission of traffic signals. Pre-installed WDM couplers
are required to inject and extract the probe signals without traffic interruption. Usually, out-
of-band signals do not pass through optical amplifiers and/or ROADM nodes along the
link. This method is predominantly used for in-service OTDR measurements to monitor
fibre and connector losses during operation.

– 12 – IEC TR 61282-13:2014  IEC 2014
The non-intrusive fibre characterization methods of category a) avoid any interference with
the network operation and, hence, may be performed at any time and over any desired length
of time. This aspect is important if the fibre property to be measured fluctuates with time, like
in the case of PMD, and needs to be monitored over a longer period of time [13]. Furthermore,
all non-intrusive methods are single-ended measurements and, hence, require test equipment
only at the receiving end of the fibre link, whereas the semi-intrusive methods of category b)
need an additional probe signal generator at the input of the fibre link.
5 Non-intrusive fibre characterization
5.1 PMD measurement via polarization-sensitive spectral analysis
5.1.1 Introductory remark
This clause describes a truly non-intrusive method and apparatus for in-service PMD
measurements on fibre links carrying conventional single-polarized WDM signals, i.e. signals
that are transmitted in a single state of polarization (SOP). Just like many other out-of-service
or intrusive PMD measurement methods, this method assumes that the PMD in the fibre link
is composed of a large number of birefringent sections, which are randomly oriented and
randomly distributed along the fibre link, so that the instantaneous DGD, measured at
different optical frequencies and/or different times, is randomly distributed with a Maxwellian
probability density function (PDF) [14].
It should be noted that the assumption of a Maxwellian PDF for the statistical distribution of
the DGD is widely used to assess the PMD-induced transmission impairments of a fibre link.
In fact, the main reason for measuring the mean DGD in fibre links is to estimate the
probability of PMD-induced transmission outages, which can occur when the randomly varying
DGD exceeds a certain maximal value, beyond which the transmitted signals may become
severely distorted, so that they cannot be received without errors [14].
The likelihood of transmission outages in a fibre link can be determined from the measured
value of the mean DGD only if the statistical distribution of the DGD is known. This
distribution is normally assumed to have a Maxwellian PDF, and this assumption has been
preponderantly confirmed in numerous investigations of medium- and long-distance fibre
links. Therefore, the assumption of a Maxwellian-distributed DGD in the method described
below does not restrict its applicability for measuring the mean DGD in fibre links to assess
the likelihood of PMD-induced transmission outages.
The method employs a combined optical spectrum and polarization analyser, i.e. a spectrally
narrowband polarimeter whose centre frequency can be tuned continuously over a sufficiently
large range. This analyser is connected to a broadband monitoring port at the end of a fibre
link and measures the optical frequency dependence of the polarization state in each
transmitted optical signal. The optical resolution bandwidth of this analyser has to be
substantially smaller than the spectral bandwidth of each data-carrying signal transmitted over
the fibre link. This polarization-sensitive spectral analysis may be performed on any single-
polarized signal, having arbitrary launch SOP, and does not require knowledge of the
particular modulation format or symbol rate of the transmitted signals. Thus, it may be readily
applied in mixed transmission systems carrying signals of different symbol rates and/or
modulation formats.
The polarization analysis shall be performed − either simultaneously or consecutively − on all
WDM signals that traverse the fibre link under test, but shall not include polarization-
multiplexed signals or signals that have traversed other fibre links prior to entering the
selected link. From this set of measurements one can then estimate the mean DGD in the
fibre link, as explained in more detail in 5.1.2. Just like for conventional PMD measurements,
the uncertainty of this estimate depends on the frequency range covered by the analysed
WDM signals, as well as on the number of WDM signals included in the set of measurements.
In general, the uncertainty is smallest when the WDM signals are equidistantly distributed
over the largest possible frequency range (see 5.1.4 for more details).

The uncertainty of the estimated mean DGD may be reduced further by repeating the
polarization analysis on the transmitted WDM signals periodically over a sufficiently long time
interval. The mean DGD in the fibre link is then determined from the time- and frequency-
average of the measured frequency dependence of the polarization state variations in the
individual signal spectra. In the extreme case, the mean DGD in a fibre link may be assessed
from a set of periodically repeated polarization analyses on just one selected WDM signal.
The uncertainty of the mean DGD derived from these single-signal measurements depends on
the total measurement time and may be estimated from the speed and magnitude of the DGD
fluctuations observed in the measurements (see 5.1.4).
Therefore, this method may be applied to directly measure the end-to-end PMD of individual
signal paths in ROADM networks, wherein the various WDM signals may traverse different
fibre spans, because they are added (and dropped) at different locations. As a result, one
may find fibre links where only a small number of the received signals have traversed the
exact same signal path. Only these signals should be included in the polarization analysis and
used to calculate the mean DGD. Because the uncertainty of a PMD measurement generally
increases inversely with the number of analysed signals, it is important to include all signals
in the analysis that have traversed the same signal path. The benefits of using more signals
is limited by their correlations, as explained in more detail in 5.1.4.
End-to-end PMD measurements of signal paths generally avoid errors associated with the
concatenation of span-by-span PMD characterization. Furthermore, because the PMD
analysis may be performed at other points along the fibre link where a monitoring tap is
installed, it may thus be possible to identify fibre sections with particularly high PMD values.
In either case, performing these in-service PMD measurements has absolutely no impact on
the operation of the network. The accuracy of the method has been asserted in lab
experiments as well as in field trials and found to be within a few per cent of that of standard
methods over a wide range of DGD values [6-7].
5.1.2 Measurement principle
End-to-end PMD in a fibre link may be characterized by the mean DGD, <∆τ >, or alternatively
2 1/ 2
by the RMS DGD, < ∆τ > , which is closely related to <∆τ >. For example, <∆τ > can be
readily determined by measuring the DGD, ∆τ, at various optical frequencies across the
transmission band and averaging the results (see also IEC 61280-4-4). However, <∆τ > may
also be obtained by averaging a set of ∆τ measurements which are taken at the same optical
frequency and repeated several times over a sufficiently long time interval, or from the
average of a set of ∆τ measurements taken at different times and frequencies [6, 14]. In either
case, such DGD measurements typically require a special probe signal as well as knowledge
or even control of the launch polarization state of the probe signal, whereas commercial WDM
signals are usually launched with arbitrary polarization states which may not be controlled,
varied or aligned.
The PMD-induced waveform distortion or pulse spreading in a WDM signal with arbitrary
launch SOP depends on the orientation of the launch SOP relative to the usually unknown
and randomly oriented input principal state of polarization (PSP) of the fibre at the signal
wavelength. It may be characterized by a parameter commonly referred to as “effective” or
“partial” DGD, ∆τ This quantity is defined as the magnitude of the component of the PMD
eff
vector in Stokes space that is orthogonal to the launch polarization state of the optical signal
[6, 15]. Its relation to the instantaneous DGD ∆τ is
∆τ = ∆τ sinϕ (1)
eff
wherein ϕ denotes the aforementioned angle between the Stokes vectors representing the
launch SOP of the signal and the input PSP of the fibre.
It is easily seen that ∆τ = ∆τ if the launch SOP is an equal mix of the two input PSPs, and
eff
∆τ  = 0 if the launch SOP is identical with one of the two PSPs. For fibre links having
eff
– 14 – IEC TR 61282-13:2014  IEC 2014
preponderantly randomly distributed and oriented birefringence, the statistical distribution of
∆τ, measured at different optical frequencies and/or at different times, is described by a
Maxwellian PDF, whereas the PSPs are randomly oriented in Stokes space [14].
Consequently, the statistical distribution of the angle ϕ in Equation (1) is uniform (e.g. in the
interval between 0 and π), even when the various WDM signals are launched in random,
τ is
mutually different polarization states, and the corresponding statistical distribution of ∆
eff
given by a Rayleigh PDF [6, 14, 15],
 
∆τ ∆τ
eff eff
 
exp −
(2)
 
2 2
 
< ∆τ > 2 < ∆τ >
 
which depends only on the parameter < ∆τ >, just like the Maxwellian PDF for ∆τ, even
though the two distributions are substantially different, as shown in Figure 4.
Rayleigh PDF
∆τ
eff
100 Maxwellian PDF
∆τ
0 10 20 30 40 50
Delay  (ps)
IEC  1508/14
Figure 4 – Rayleigh PDF for ∆τ compared with Maxwellian PDF for ∆τ
eff
Since both statistical distributions depend only on the parameter < ∆τ >, it is therefore
possible to deduce the mean DGD <∆τ > from the mean effective DGD <∆τ >, which may be
eff
determined from a sufficiently large statistical ensemble of ∆τ , measurements, as explained
eff
in more detail in 5.1.3 and 5.1.4. In fact, the mean DGD <∆τ > is directly proportional to the
mean value <∆τ > [6, 14, 15], i.e.
eff
< ∆τ > = (4 π ) < ∆τ > (3)
eff
Thus, the mean DGD in a fibre link may be determined from a set of in-service measurements
of ∆τ on the transmitted optical signals. These measurements do not require knowledge or
eff
control of the launch SOPs of the analysed signals and, hence, are truly non-intrusive. In fact,
the launch SOPs can be all identical, and therefore highly correlated, or completely random
and mutually uncorrelated. The statistical distribution of ϕ and hence ∆τ in Equation (1) is
eff
the same in either case, because of the random orientation of the PSPs at the various signal
frequencies.
To minimize the measurement uncertainty of the mean value <∆τ >, the polarization
eff
analysis should be performed, either simultaneously or consecutively, on all WDM signals that
Frequency of occurrence  (a.u.)

traverse the link under test, but should not include signals that have traversed other fibre links
prior to entering the link under test. If the number of analysed signals is small and/or if their
frequencies are not spaced sufficiently far apart (see 5.1.4.2), the ∆τ measurements shall
eff
be repeated several times at predetermined time intervals ∆t over a sufficiently long time
period, which in some cases, may be several hours or even several days, depending on the
speed and magnitude of the PMD fluctuations in the fibre link. A more detailed discussion of
the measurement accuracy is provided in 5.1.4.
5.1.3 Methods for measuring ∆τ via polarization analysis
eff
5.1.3.1 Introductory remark
In general, PMD introduces frequency-dependent variations in the SOP of a polarized optical
signal, so that the various spectral components of a modulated optical signal passing through
the fibre link are transformed into different SOPs. Within a sufficiently narrow optical
bandwidth ∆ν (typically less than 0,16/<∆τ >), the frequency-dependent polarization
transformations may be approximated by a uniform rotation about a fixed axis on the Poincaré
sphere, as shown schematically in Figure 5.
S
PSP
ϕ
S
L
S
IEC  1509/14
Key
S , S , S Stokes parameter
1 2 3
L length of polarization rotation trace
ϕ  angle between PSP and signal SOP
Figure 5 – PMD-induced polarization rotation within the spectrum of a modulated signal
The rotation axis is defined by the orientation of the PSPs, while the rotation rate is
proportional to ∆τ. For a signal with optical bandwidth ∆ν, the full rotation angle is given by
Φ=2π ∆τ ∆ν, whereas the length, L, of the arc traced by the SOP rotation is L=2π ∆τ ∆ν.
eff
5.1.3.2 Frequency-selective polarimeter
A general method for measuring ∆τ in a modulated optical signal is to analyse the
eff
frequency-dependent SOP variations across the spectrum of the signal. This can be
accomplished, for example, with the help of a narrowband, frequency-tuneable optical
polarimeter, like the apparatus shown schematically in Figure 6 [15]. With this instrument, it
may even be possible to directly determine ∆τ from the Poincaré sphere analysis. However,
such measurements of the actual DGD become very unreliable when the launch SOP is nearly
identical with one of the two PSPs of the fibre at the centre wavelength of the signal (see
5.3.3.2 below).
– 16 – IEC TR 61282-13:2014  IEC 2014
MT S (ν)
BPF PA
S (ν)
S (ν)
ν
IEC  1510/14
Key
MT monitoring tap
BPF tuneable band-pass filter
PA complete polarization analyser (polarimeter)
Figure 6 – Set-up for measuring PMD-induced polarization rotations in optical signals
5.1.3.3 Frequency- and polarization-selective analyser
An alternative method and apparatus for measuring ∆τ is shown in Figure 7. It employs a
eff
simple polarization splitter in combination with a variable polarization transformer and a
tuneable optical band-pass filter to analyse the SOP variations in the signal spectrum. The
advantage of this apparatus is that it is significantly easier to calibrate than the full
polarimeter of Figure 6. The function of the polarization transformer in Figure 7 is to adjust the
relative orientation of the PMD-induced polarization rotation so that
a) the SOP at the centre of the signal spectrum (ν =0) is a 50/50 mix of the two eigenstates
of the polarization splitter, and
b) the axis of the PMD-induced rotation (on the Poincaré sphere) is orthogonal to the
eigenstates of the PBS.
PBS
MT
P (ν)
PC BPF P
ν
P (ν)
S
IEC  1511/14
Key
MT monitoring tap
BPF tuneable band-pass filter
PC variable polarization controller
PBS polarization beam splitter
Figure 7 – Modified set-up for measuring PMD-induced polarization rotations
The transformation described above yields the highest sensitivity of the detector signals, P
p
and P , to the PMD-induced polarization rotation and, hence, may be found by scanning the
s
tuneable filter repeatedly across the signal spectru
...