IEC 61290-4-2:2011

Optical amplifiers - Test methods - Part 4-2: Gain transient parameters - Broadband source method

IEC 61290-4-2:2011 applies to optical amplifiers (OAs) and optically amplified elementary sub-systems. More specifically, it applies to OAs using active fibres (optical fibre amplifiers, OFAs) containing rare-earth dopants, such as erbium doped fibre amplifiers (EDFAs), presently commercially available, as indicated in IEC 61291-1. The object of this part of IEC 61290-4 is to establish uniform requirements for accurate and reliable measurements, by means of the broadband source method, of the transient response of OFAs to dynamic changes in their input power, as defined in IEC 61290-4-1:2011.
This publication is to be read in conjunction with IEC 61291-1:2006.

Amplificateurs optiques - Méthodes d'essai - Partie 4-2: Paramètres de gain transitoire - Méthode par source large bande

La CEI 61290-4-2:2011 s'applique aux amplificateurs optiques (AO) et aux sous-systèmes élémentaires à amplification optique. Plus précisément, Elle s'applique aux AO utilisant des fibres actives (amplificateurs à fibres optiques, AFO), contenant des dopants aux terres rares, tels que les amplificateurs à fibre dopée à l'erbium (EDFA), actuellement disponibles sur le marché, comme l'indique la CEI 61291-1. L'objet de la présente partie de la CEI 61290-4 est d'établir des exigences uniformes en vue de mesures précises et fiables, au moyen de la méthode par source large bande, de la réponse transitoire des AFO aux variations dynamiques de leur puissance d'entrée, comme défini dans la CEI 61290-4-1:2011.
Cette publication doit être lue conjointement avec la CEI 61291-1:2006.

General Information

• Status: Published
• Publication Date: 12-Jul-2011
• ICS: 33.180.30 - Optic amplifiers
• Technical Committee: SC 86C - Fibre optic systems, sensing and active devices
• Drafting Committee: WG 3 - TC 86/SC 86C/WG 3
• Current Stage: PPUB - Publication issued
• Start Date: 31-Oct-2011
• Completion Date: 13-Jul-2011

Overview

IEC 61290-4-2:2011 is a critical international standard published by the International Electrotechnical Commission (IEC) that specifies test methods for optical amplifiers, focusing on their gain transient parameters using the broadband source method. This standard specifically addresses optical fiber amplifiers (OFAs) using rare-earth dopants such as erbium-doped fiber amplifiers (EDFAs), which are commonly deployed in dense wavelength division multiplexing (DWDM) systems.

The key objective of IEC 61290-4-2:2011 is to establish uniform requirements for accurate and reliable measurement of transient gain behavior during dynamic input power changes, using a broadband light source spread across the transmission band (e.g., the C-Band). This method complements the two-wavelength method described in IEC 61290-4-1:2011 by providing an alternative approach when the saturating signal spans the full DWDM spectrum.

Key Topics

• Gain transient parameters
  The standard defines parameters critical to evaluating transient gain, including:

  • Transient gain response time constant (settling time)
  • Gain offset
  • Transient net gain overshoot
  • Transient net gain undershoot

• Broadband source method
  Uses a saturating signal spread uniformly over the transmission band instead of a single wavelength, simulating real-world dynamic changes in multi-channel DWDM networks.

• Transient events
  Focuses on accurately measuring transient gain responses to:

  • Channel addition (input power rises)
  • Channel removal (input power falls)

• Definitions and test procedures
  Standardizes terminology such as surviving signals, saturating signals, add/drop levels, and rise/fall times to ensure consistent measurement and interpretation of transient responses.

• Test apparatus and setup
  Specifies instrumentation requirements and test configurations essential for implementing the broadband source measurement technique.

Applications

IEC 61290-4-2:2011 provides invaluable guidelines for:

• Manufacturers of optical amplifiers
  Design and validate optical amplifiers with optimized transient gain performance to ensure signal integrity in DWDM communication systems.

• Network equipment manufacturers (NEMs)
  Assess transient gain overshoot and undershoot to minimize cascading distortion and ensure robust operation of optical networks.

• Telecommunication service providers
  Verify that deployed optical amplifiers meet transient response requirements, guaranteeing high-quality, reliable optical signal amplification under dynamic channel load conditions.

• Research and development
  Facilitate advancements in amplifier technology by providing standardized measurement techniques for transient gain dynamics.

Related Standards

• IEC 61290-4-1:2011 - Two wavelength method
  Focuses on transient gain measurement using a saturating signal at a single wavelength, complementing the broadband source approach.

• IEC 61291-1:2006 - Optical fibre amplifiers - Generic specification
  Provides general performance specifications and test procedures for optical fiber amplifiers, serving as a foundational reference for IEC 61290-4-2.

• Other parts of the IEC 61290 series
  Cover various test methods and performance parameters related to optical amplifiers, offering a comprehensive framework for amplifier characterization.

---

This standard is essential for ensuring accurate, consistent, and reproducible measurement of gain transient parameters in optical amplifiers, which directly impact the performance and reliability of modern optical communication networks. By adhering to IEC 61290-4-2:2011, stakeholders can better manage the challenges of dynamic DWDM channel configurations and maintain high-quality optical signal amplification.

Standards Content (Sample)

IEC 61290-4-2 ®
Edition 1.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical amplifiers – Test methods –
Part 4-2: Gain transient parameters – Broadband source method

Amplificateurs optiques – Méthodes d’essai –
Partie 4-2: Paramètres de gain transitoire – Méthode par source large bande

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IEC 61290-4-2 ®
Edition 1.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical amplifiers – Test methods –
Part 4-2: Gain transient parameters – Broadband source method

Amplificateurs optiques – Méthodes d’essai –
Partie 4-2: Paramètres de gain transitoire – Méthode par source large bande

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX R
ICS 33.180.30 ISBN 978-2-88912-573-9

– 2 – 61290-4-2 © IEC:2011
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope and object . 6
2 Normative references. 6
3 Terms, definitions and abbreviations . 6
3.1 General . 6
3.2 Terms and definitions . 9
3.3 Abbreviated terms . 10
4 Apparatus . 10
5 Test sample . 12
6 Procedure . 12
7 Calculations . 13
8 Test results . 14
Annex A (informative) Comparison between two-wavelength method and broadband
method . 15
Bibliography . 17

Figure 1 – Definitions of rise and fall times for (a) a channel addition event, and (b) a
channel removal event . 7
Figure 2 – OFA transient gain response for (a) a channel removal event, and (b) a

channel addition event . 8
Figure 3 – Transient measurement test set-up for broadband source method . 11
Figure A.1 – Effect of non-flat gain spectrum on gain offset . 15
Figure A.2 – Different transient suppression response for different types of saturating
signals . 16

Table 1 – Examples of “add” and “drop” scenarios for transient control measurement . 13
Table 2 – Typical results of transient control measurement for a C-Band EDFA . 14

61290-4-2 © IEC:2011 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL AMPLIFIERS –
TEST METHODS –
Part 4-2: Gain transient parameters –
Broadband source method
FOREWORD
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61290-4-2 has been prepared by subcommittee 86C: Fibre optic
systems and active devices, of IEC technical committee 86: Fibre optics.
This standard shall be used in conjunction with IEC 61291-1. It was established on the basis of
the second (2006) edition of that standard.
Future standards in this series will carry the new general title as cited above. Titles of existing
standards in this series will be updated at the time of the next edition.

– 4 – 61290-4-2 © IEC:2011
The text of this standard is based on the following documents:
CDV Report on voting
86C/957/CDV 86C/991/RVC
Full information on the voting for the approval of this standard 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 of the IEC 61290 series, published under the general title Optical amplifiers –
Test methods 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

61290-4-2 © IEC:2011 – 5 –
INTRODUCTION
This part of IEC 61290-4 is devoted to the subject of optical amplifiers. The technology of
optical amplifiers is quite new and still emerging; hence amendments and new editions to this
standard can be expected.
Each abbreviation introduced in this standard is explained in the text at least the first time it
appears. However, for an easier understanding of the whole text, a list of all abbreviations used
in this standard is given in 3.3.

– 6 – 61290-4-2 © IEC:2011
OPTICAL AMPLIFIERS –
TEST METHODS –
Part 4-2: Gain transient parameters –
Broadband source method
1 Scope and object
This part of IEC 61290-4 applies to optical amplifiers (OAs) and optically amplified elementary
sub-systems. More specifically, it applies to OAs using active fibres (optical fibre amplifiers,
OFAs) containing rare-earth dopants, such as erbium doped fibre amplifiers (EDFAs), presently
commercially available, as indicated in IEC 61291-1.
The object of this part of IEC 61290-4 is to establish uniform requirements for accurate and
reliable measurements, by means of the broadband source method, of the transient response of
OFAs to dynamic changes in their input power, as defined in IEC 61290-4-1:2011.
The broadband source method is different from the two-wavelength method described in
IEC 61290-4-1:– in that the saturating signal is not located at a single wavelength, but is rather
spread out across the entire specified DWDM transmission band of the OFA-under-test (e.g. the
C-Band, 1 525 nm to 1 565 nm). Thus, this method may be relevant to the characterization of
transient events where the DWDM signals that are added or dropped are more or less uniformly
spread across the transmission band. The difference between the two measurement methods is
discussed in more detail in Annex A.
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 61290-4-1:2011, Optical amplifiers – Test methods – Part 4-1: Gain transient parameters –
Two wavelength method
IEC 61291-1, Optical fibre amplifiers – Part 1: Generic specification
3 Terms, definitions and abbreviations
3.1 General
When the input power to an OFA operating in saturation changes sharply, the gain of the
amplifier will typically exhibit a transient response before settling back into the required gain.
This response is dictated both by the optical characteristics of the active fibre within the OFA,
as well as the performance of the automatic gain control (AGC) mechanism.
Since a change in input power typically occurs when part of the DWDM channels within the
specified transmission band are dropped or added, definitions are provided that describe a
dynamic event leading to a transient response. Rise and fall time definitions are shown in
Figure 1.
61290-4-2 © IEC:2011 – 7 –
100 %
of change
90 %
of change
10 %
of change
Rise
time
Time
Channel
Channel
addition start
addition end
(a) IEC  1582/11
10 %
of change
90 %
of change
100 %
of change
Fall
time
Time
Channel
Channel
removal start
removal end
(b)
IEC  1583/11
Figure 1 – Definitions of rise and fall times for (a) a channel addition event,
and (b) a channel removal event
The parameters generally used to characterize the transient gain behaviour of a gain controlled
OFA for the case of channel addition/removal are defined in Figure 2. Figure 2(a) specifically
represents the time dependence of the gain of one of the surviving channels when channels
are removed. Likewise the transient gain behaviour of a pre-existing channel for the case when
channels are added is shown in Figure 2(b). The main transient parameters are: transient gain

Input power to EDFA
Input power to EDFA
(linear a.u.)
(linear a.u.)
Transient gain response time
constant (settling time)
– 8 – 61290-4-2 © IEC:2011
response time constant (setting time), gain offset, transient net gain overshoot, and transient
net gain undershoot. The transient gain overshoot and undershoot are particularly critical to
carriers and network equipment manufacturers (NEMs) given that the speed and amplitude of
gain fluctuations compound through the network as the optical signal passes through an
increasing number of cascaded amplifiers. Properly designed optical amplifiers have very small
values for these transient parameters.
Net gain
overshoot
Gain
Gain
overshoot
stability
Final
gain
Gain offset
Initial
Gain
Net gain
gain
undershoot
undershoot
Time
(a)
IEC  1584/11
Net gain
Gain
Overshoot
overshoot
Initial
Gain offset
gain
Final
gain
Gain
stability
Gain
Net gain
undershoot
undershoot
Transient gain response time
constant (settling time)
Time
(b)
IEC  1585/11
Figure 2 – OFA transient gain response for (a) a channel removal event,
and (b) a channel addition event

Gain (dB)
Gain (dB)
61290-4-2 © IEC:2011 – 9 –
3.2 Terms and definitions
For the purposes of this document, the following terms, definitions and abbreviations apply.
3.2.1
surviving (pre-existing) signal
optical signal that remains (exists) after (before) a drop (add) event
3.2.2
saturating signal
optical signal that is switched off (on), thus triggering the drop (add) event
3.2.3
drop (add) level (dB)
amount in dB by which the input power decreases (increases) due to dropping (adding) of
channels
3.2.4
add rise time
time it takes for the input power to rise from 10 % to 90 % of the total difference between the
initial and final input power levels during an add event (see Figure 1a)
3.2.5
drop fall time
time it takes for the input power to fall from 10 % to 90 % of the total difference between the
initial and final input power levels during a drop event (see Figure 1b)
3.2.6
initial gain
gain of the surviving (pre-existing) channel before a drop (add) event
3.2.7
final gain
steady state gain of the surviving (pre-existing) channel a very long time (i.e. once the gain has
stabilized) after a drop (add) event
3.2.8
gain offset
change in dB of the gain between initial and final state, defined as final gain – initial gain
NOTE Gain offset may be positive or negative for both channel addition and removal events.
3.2.9
gain stability
specified peak-to-peak gain fluctuations of the OFA under steady state conditions (i.e. not in
response to a transient event)
3.2.10
transient gain response time constant (settling time)
amount of time required to bring the gain of the surviving (pre-existing) channel to the final gain
NOTE 1 This parameter is the measured time from the beginning of the drop (add) event that created the transient
gain response, to the time at which the surviving (pre-existing) channel gain first enters within the gain stability
band centred on the final gain.
NOTE 2 Hereon this will also be referred to as settling time.

– 10 – 61290-4-2 © IEC:2011
3.2.11
transient gain overshoot
difference in dB between the maximum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the lowest of either the initial gain and final
gain
NOTE Hereon this will also be referred to as gain overshoot.
3.2.12
transient net gain overshoot
difference in dB between the maximum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the highest of either the initial gain and final
gain
NOTE 1 The transient net gain overshoot is just the transient gain overshoot minus the gain offset, and represents
the actual transient response not related to the shift of the amplifier from the initial steady state condition to the
final steady state condition.
NOTE 2 Hereon this will also be referred to as net gain overshoot.
3.2.13
transient gain undershoot
difference in dB between the minimum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the highest of either the initial gain and final
gain
NOTE Hereon this will also be referred to as gain undershoot.
3.2.14
transient net gain undershoot
difference in dB between the minimum surviving (pre-existing) channel gain reached during the
OFA transient response to a drop (add) event, and the lowest of either the initial gain and final
gain
NOTE 1 The transient net gain undershoot is just the transient gain undershoot minus the gain offset, and
represents the actual transient response not related to the shift of the amplifier from the initial steady state
condition to the final steady state condition.
NOTE 2 Hereon this will also be referred to as net gain undershoot.
3.3 Abbreviated terms
AGC automatic gain control
DFB distributed feedback
DWDM dense wavelength division multiplexing
EDFA erbium-doped fibre amplifier
NEM network equipment manufacturer
OA optical amplifier
OFA optical fibre amplifier
SHB spectral hole burning
VOA variable optical attenuator
WDM wavelength division multiplexing
4 Apparatus
Figure 3 shows a generic setup to characterize the transient response properties of OFAs
using the broadband source method.

61290-4-2 © IEC:2011 – 11 –
Laser
source VOA1
OFA
Optical Pass
under
coupler filter
test
Block Optical
Broadband
VOA2
filter modulator
source
Detector
Trigger
Pulse generator Oscilloscope
IEC  1586/11
Figure 3 – Transient measurement test set-up for broadband source method
The test equipment listed below, with the required characteristics, is needed.
a) A laser source for supplying the surviving signal, with the following characteristics
1) Ability to support the range of surviving signal wavelengths for which the OFA under
test is to be tested. This could be provided for example by a tunable laser, or a bank of
distributed feedback (DFB) lasers
2) An achievable average output power such that at the input to the OFA under test the
power will be above the maximum specified input power of the OFA
b) A broadband source for supplying the saturating signal, with the following characteristics
1) At least 95 % of the output power should be contained within the specified transmission
band of the OFA under test
2) A variation of not more than 1dB peak-to-peak of the power level across the specified
transmission band of the OFA under test
3) An achievable output power such that at the input to the OFA under test the power will
be above the maximum specified input power of the OFA
c) VOA1 – A variable optical attenuator (VOA) with a dynamic range sufficient to support the
required range of surviving signal levels at which the OFA under test is to be tested
NOTE 1 If the output power of the laser source can be varied over the required dynamic range, then VOA1
may not be needed.
d) VOA2 – A VOA with a dynamic range sufficient to support the required range of saturating
signal powers (dictated by the sum of the surviving signal levels and drop level) at which
the OFA under test is to be tested.
NOTE 2 If the output power of the broadband source can be varied over the required dynamic range, then
VOA2 may not be needed.
e) Block filter – A filter designed to block the broadband signal in the vicinity of the surviving
signal wavelength, with the following characteristics
1) Ability to support the range of surviving signal wavelengths for which the OFA under
test is to be tested. This could be provided for example by a tunable filter, or a series of
discrete filters.
2) Uniform insertion loss to within 0,5 dB over the entire specified transmission band of
the OFA under test except in a range of ±125 GHz of the surviving signal wavelength.
3) Attenuation of at least 15 dB over the uniform Insertion Loss in a range of ±75 GHz of
the surviving signal wavelength
f) Optical modulator to switch the saturating signal “on” and “off”, with the following
characteristics
– 12 – 61290-4-2 © IEC:2011
1) Extinction ratio at least 5 dB higher than the maximum drop level for which the OFA
under test is to be tested
2) Switching time fast enough to support the fastest drop time for which the OFA under
test is to be tested
g) Optical coupler – Any optical coupler selected to support requirements a)2) and b)3) above
h) Pass filter – A filter designed to pass only the surviving signal wavelength, with the
following characteristics
1) Ability to support the range of surviving signal wavelengths for which the OFA-under-
test is to be tested. This could be provided for example by a tunable filter, or a series of
discrete filters
2) 1-dB passband of at least ±20 GHz centered around the surviving signal wavelength
3) At least 20 dB attenuation level below the minimum insertion loss across the entire
specified transmission band of the OFA under test except within a range of ±100 GHz
centered around the surviving signal wavelength
i) Detector – to detect the filtered output of the OFA under test, with the following
characteristics
1) A sufficiently wide bandwidth to support the fastest drop time for which the OFA is to be
tested
2) A linear response within a ±5 dB range of all surviving signal levels for which the OFA
under test is to be tested
j) Oscilloscope – to measure the transient response of the filtered output of the OFA under
test, with a sufficiently wide bandwidth to support the fastest drop time for which the OFA is
to be tested
k) Pulse generator – To generate the “on”-“off” signal to the optical modulator, with a pulse
width short enough to support the fastest drop time for which the OFA under test is to be
tested
5 Test sample
The OFA shall operate at nominal operating conditions. If the OFA is likely to cause laser
oscillations due to unwanted reflections, optical isolators should be used to bracket the OFA
under test. This will minimize signal instability and measurement inaccuracy.
6 Procedure
In the setup shown, the input signal power to the OFA under test is the combination of a
discrete wavelength representing the surviving signal, and a broadband source representing a
saturating signal. The power of each of these two sources can be adjusted via the appropriate
VOA to achieve the desired power ratio at the input to the OFA necessary to simulate the add
and drop events to be tested. The broadband source is filtered by a block filter such that the
radiation in the vicinity of the surviving signal wavelength is always well below the surviving
signal level. Furthermore, the broadband source is turned “on” and “off” via the pulse generator
driving the optical modulator, thus simulating add and drop event.
The output of the OFA under test is filtered such that only the surviving signal power is
detected by the detector and the oscilloscope (taking into account that radiation from the
broadband source in the vicinity of the surviving signal wavelength was already filtered by the
block filter). Utilizing the pulse generator as a trigger, the oscilloscope can be configured to
show the transient response of the power of the surviving signal after both drop and add events.
Measurement of the various transient response parameters from the oscilloscope display is
described in detail in IEC 61290-4-1:–.
To perform a single transient measurement, the following steps should be followed

61290-4-2 © IEC:2011 – 13 –
a) Set the wavelength of the laser source, block filter and pass filter according to the surviving
signal wavelength to be tested
b) Set the gain of the OFA under test to the required operating gain for the measurement. The
gain can be measured either using an internal calibrated gain measurement function of the
OFA, or directly according to one of the following standards: IEC 61290-1-1, IEC 61290-1-2,
IEC 61290-1-3
c) Set the power levels of the laser source and broadband source (using the VOAs and with
the modulator at the “on” position) such that the following conditions are satisfied at the
input to the OFA under test
1) The total input power is equal to the required operating input power for the
measurement
2) The difference between the total input power and the laser source power is drop level
NOTE The input power to the OFA under test can typically be measured using an internal detector within the
OFA module especially calibrated for this purpose. Should such a detector not be available, then a calibrated
optical power meter can be connected in place for the OFA under test for the purpose of measuring the input
power.
d) Activate the pulse generator and set the fall and rise times of the pulses to the required
drop and add times to be tested. Using the oscilloscope, verify that the measured fall and
rise times (10 % to 90 %) are between 0,5 and 1 times the required drop and add times to
be tested
e) Set the trigger function of the oscilloscope to display a drop event, and record the display.
Then set the trigger function to display an add event and record the display (see
IEC 61290-4-1:–)
Several transient control measurements can be performed, according to the operating
conditions and specifications that are provided. Measurements may also be taken for various
“add” and “drop” scenarios as shown in Table 1. These measurements are typically performed
over a broad range of input power levels.
Table 1 – Examples of “add” and “drop” scenarios for transient control measurement
Channels
Scenario Total channels Surviving channels
added/dropped
20 dB add/drop 100 1 99
16 dB add/drop 40 1 39
13 dB add/drop 40 2 38
10 dB add/drop 40 4 36
6 dB add/drop 40 10 30
3 dB add/drop 40 20 20
7 Calculations
The results of the transient measurement are the following parameters
• Channel addition/removal transient gain overshoot and transient net gain overshoot
• Channel addition/removal transient gain undershoot and transient net gain undershoot
• Channel addition/removal gain offset
• Channel addition/removal transient gain response time constant (setting time)
These parameters can be extracted from the oscilloscope display, as described in Figure 2.

– 14 – 61290-4-2 © IEC:2011
8 Test results
Table 2 shows typical measurement conditions and transient control measurement results of
C-band EDFAs. The measurement conditions include gain, surviving channel wavelength, input
power, transient type (e.g. 3 dB drop, 1 dB add), and different transient parameters. In order to
characterize the OFA transient, the user should choose the measurement conditions to
adequately characterize the dynamic range of the OFA under test.
Typical values of transient parameters are listed in the last row of the table.
Table 2 – Typical results of transient control measurement for a C-Band EDFA
Amplifier gain _____ (dB)                                 Surviving channel wavelength ____ (nm)
Transient gain
Input Transient net Transient net gain
Transient event
response time Gain offset
power gain overshoot undershoot
description constant dB
dBm db dB
µsec
3 dB add or drop -4 0,5 0,2 10 -0,2
x dB add or drop
y dB
Typical values <1 <0,5 <100 <0,5

61290-4-2 © IEC:2011 – 15 –
Annex A
(informative)
Comparison between two-wavelength method and broadband method

The transient suppression response of an EDFA depends on both the initial state before the
event and the final state after the event. Thus even if the final state (e.g. given surviving
channel in a drop event) is identical, the transient suppression of the EDFA may differ for two
different initial states. This difference is reflected both in the gain offset, and the dynamic
transient gain response.
The gain offset may be different for two different initial states due to spectral hole burning
(SHB) effects (see IEC 61290-4-1:–, Annex A), as well as a non-flat gain spectrum. In the
former case, the SHB effect means that the Initial gain of the surviving channel (e.g. in a drop
event), is changed due to the spectral composition of the saturating signal. Thus, the gain
offset will be different for different types of saturating signals.
In the case of a non-flat gain spectrum, the AGC mechanism of the EDFA closes the gain loop
on the average gain for the initial input signal, comprising both the saturating signal and the
surviving channel. If the gain spectrum of the amplifier is non-flat, and the saturating signal is
broadband, then the initial gain of the surviving channel may be different from the average gain.
On the other hand, once the saturating signal disappears and only the surviving channel
remains, then the final gain of the surviving channel is always equal to the average gain. Since
the AGC mechanism of the EDFA always maintains the same average gain, then a gain offset
will result for the surviving channel, as shown in Figure A.1. If instead of a broadband
saturating signal, a single wavelength saturating signal located near the surviving channel is
used, then the Initial gain and final gain of the surviving channel would both be equal to the
average gain, and no gain offset would result.
18,5
18,4
Initial gain spectrum
18,3
18,2
Average gain
18,1
18,0
17,9
Gain offset
17,8
17,7
17,6
Initial surviving channel gain
17,5
1 528 1 533 1 538 1 543 1 548 1 553 1 558 1 563
Wavelength  (nm)
IEC  1587/11
Figure A.1 – Effect of non-flat gain spectrum on gain offset

Gain  (dB)
– 16 – 61290-4-2 © IEC:2011
With respect to the dynamic transient gain response, this depends critically on the transient
suppression mechanism of the EDFA. Typically, this mechanism is implemented by a
combination of electronic feed-forward and feed-back control loops. In particular, the feed-
forward control changes the pump current by a given predicted amount when the input
conditions change. For a very fast event (e.g. 1-μs fall/rise time), the amount by which the
current is changed is determined by the total power before the event and the total power
following the event. Since different initial conditions (i.e. different types of saturating signal) will
result in different initial pump currents even for the same given total input power, this means
that the required current change will be different. Thus, when the feed-forward mechanism is
calibrated, it is important to perform the calibration for initial conditions which best simulate the
actual operating conditions of the EDFA. Furthermore, the test method by which the gain
transient parameters are measured should also best reflect the actual operating conditions of
the EDFA.
If the EDFA is designated to operate in a WDM network with typically full and uniform channel
loading across the operating wavelength band, the saturating signal used to measure gain
transient parameters should best reflect this condition. Thus, we should expect that a test
method based on a broadband saturating signal would better reflect the transient performance
with initial full channel loading, compared to a single wavelength saturating signal. This is
illustrated in Figure A.2, where the transient suppression performance is compared for different
saturating signals. As can be seen, the transient gain response in the case of the broadband
ASE source saturating signal is almost identical to the case of a uniformly distributed
16-channel WDM saturating signal. On the other hand, the transient gain response for the case
of a single wavelength saturating signal placed 1 nm apart from the surviving channel is
markedly different. In this figure, the EDFA gain is 26 dB, the input power of the saturating
signal is -7 dBm, while the drop level is 10 dB
Conversely if the EDFA is designated to operate within a narrow wavelength band (e.g. 4 nm),
then a single wavelength saturating signal will best reflect this condition, and the two
wavelength method should preferably be used.
3,0
2,8
2,6
2,4
2,2
2,0
1,8
1,6
1,4
1,2
1,0
0 20 40 60 80 100
Time  (µs)
IEC  1588/11
Single wavelength 1nm from surviving channel
Broadband ASE source
16 channel WDM signal
Figure A.2 – Different transient suppression response for different types
of saturating signals
Relative power  (dB)
61290-4-2 © IEC:2011 – 17 –
Bibliography
IEC 61290-1-1, Optical amplifiers – Test methods – Part 1-1: Power and gain parameters –
Optical spectrum analyzer method
IEC 61290-1-2, Optical amplifiers – Test methods – Part 1-2: Power and gain parameters –
Electrical spectrum analyzer method
IEC 61290-1-3, Optical amplifiers – Test methods – Part 1-3: Power and gain parameters –
Optical power meter method
____________
– 18 – 61290-4-2 © CEI:2011
SOMMAIRE
AVANT-PROPOS . 19
INTRODUCTION . 21
1 Domaine d’application et objet . 22
2 Références normatives . 22
3 Termes, définitions et abréviations . 22
3.1 Généralités . 22
3.2 Termes et définitions . 25
3.3 Abréviations . 27
4 Matériel . 27
5 Echantillon d’essai . 29
6 Mode opératoire. 29
7 Calculs . 31
8 Résultats d’essai . 31
Annexe A (informative) Comparaison entre la méthode à deux longueurs d'onde et la
méthode large bande . 33
Bibliographie . 36

Figure 1 – Définitions des temps de montée et de descente pour (a) un événement
d’ajout de canal, et (b) un événement de suppression de canal . 23
Figure 2 – Réponse du gain transitoire d’un AFO pour un événement (a) de suppression

de canal, et un événement (b) d’ajout de canal . 25
Figure 3 – Montage d’essai de mesure transitoire pour la méthode par source large
bande . 28
Figure A.1 – Effet du spectre d’un gain non régulier sur le décalage de ce gain . 34
Figure A.2 – Différentes réponses de suppression des transitoires pour différents types
de signaux de saturation . 35

Tableau 1 – Exemples de scénarios « d’ajout » et de « suppression » pour la mesure de
commande transitoire . 31
Tableau 2 – Résultats typiques de mesure du gain transitoire pour un EDFA Bande C . 32

61290-4-2 © CEI:2011 – 19 –
COMMISSION ÉLECTROTECHNIQUE INTERNATIONALE
____________
AMPLIFICATEURS OPTIQUES –
MÉTHODES D’ESSAI –
Partie 4-2: Paramètres de gain transitoire –
Méthode par source large bande

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