IEC 61290-4-1:2016
(Main)Optical amplifiers - Test methods - Part 4-1: Gain transient parameters - Two-wavelength method
Optical amplifiers - Test methods - Part 4-1: Gain transient parameters - Two-wavelength method
IEC 61290-4-1:2016 applies to optical amplifiers (OAs) using active fibres (optical fibre amplifiers (OFAs)) containing rare-earth dopants including erbium-doped fibre amplifiers (EDFAs) and optically amplified elementary sub-systems. These amplifiers are commercially available and widely deployed in service provider networks. The object of document is to provide the general background for OFA transients and related parameters, and to describe a standard test method for accurate and reliable measurement of the following transient parameters:
- channel addition or removal transient gain overshoot and transient net gain overshoot;
- channel addition or removal transient gain undershoot and transient net gain undershoot;
- channel addition or removal gain offset;
- channel addition or removal transient gain response time constant (settling time). This second edition cancels and replaces the first edition published in 2011. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
- extended the applicability from only EDFAs to all OFAs;
- updated definitions for consistency with other documents in the IEC 61290-4 series. Keywords: optical amplifiers (OAs), optical fibre amplifiers (OFAs), rare-earth dopants, erbium-doped fibre amplifiers
Amplificateurs optiques - Méthodes d'essai - Partie 4-1: Paramètres de gain transitoire - Méthode à deux longueurs d'onde
L'IEC 61290-4-1:2016 s'applique aux amplificateurs optiques (OA) utilisant des fibres actives (amplificateurs à fibres optiques (OFA)), dopées aux terres rares. Elle concerne également les amplificateurs à fibre dopée à l'erbium (EDFA) et les sous-systèmes élémentaires à amplification optique. Ces amplificateurs sont disponibles sur le marché et sont largement déployés dans les réseaux de prestataires de service. La présente partie de l'IEC 61290-4 a pour objet de fournir le contexte général pour les transitoires d'OFA et les paramètres associés, et de décrire une méthode d'essai normalisée de mesure exacte et fiable des paramètres transitoires suivants:
- dépassement positif du gain transitoire lors de l'ajout ou de la suppression de canaux et dépassement positif du gain net transitoire;
- dépassement négatif du gain transitoire lors de l'ajout ou de la suppression de canaux et dépassement négatif du gain net transitoire;
- décalage de gain lors de l'ajout ou de la suppression de canaux;
- constante de temps de réponse du gain transitoire lors de l'ajout ou de la suppression de canaux (temps de stabilisation). Cette deuxième édition annule et remplace la première édition parue en 2011. Cette édition constitue une révision technique. Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
- extension de l'application des seuls EDFA à l'ensemble des OFA;
- mise à jour des définitions dans un objectif de cohérence avec les autres documents de la série IEC 61290-4. Mots clés: amplificateurs optiques (OA), amplificateurs à fibres optiques (OFA), dopées aux terres rares, des amplificateurs à fibre dopée à l'erbium
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IEC 61290-4-1 ®
Edition 2.0 2016-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical amplifiers – Test methods –
Part 4-1: Gain transient parameters – Two-wavelength method
Amplificateurs optiques – Méthodes d’essai –
Partie 4-1: Paramètres de gain transitoire – Méthode à deux longueurs d'onde
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IEC 61290-4-1 ®
Edition 2.0 2016-09
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Optical amplifiers – Test methods –
Part 4-1: Gain transient parameters – Two-wavelength method
Amplificateurs optiques – Méthodes d’essai –
Partie 4-1: Paramètres de gain transitoire – Méthode à deux longueurs d'onde
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 33.180.30 ISBN 978-2-8322-3659-8
– 2 – IEC 61290-4-1:2016 © IEC 2016
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references. 6
3 Terms, definitions and abbreviated terms . 6
3.1 Terms and definitions . 6
3.2 Abbreviated terms . 8
4 Measurement apparatus . 8
5 Test specimen . 11
6 Procedure . 11
7 Calculations . 12
8 Test results . 12
Annex A (informative) Background on transient phenomenon in optical amplifiers . 13
Annex B (informative) Slew rate effect on transient gain response . 16
B.1 The importance of rise time and fall time of input power . 16
B.2 Measured data and explanation . 16
Bibliography . 19
Figure 1 – Definitions of rise and fall times . 9
Figure 2 – OFA transient gain response . 10
Figure 3 – Generic transient control measurement setup. 11
Figure A.1 – OFA pump control for a chain of 5 OFAs and 4-fibre spans . 14
Figure A.2 – EDFA spectral hole depth for different gain compression . 15
Figure A.3 – EDFA spectral hole depth for different wavelengths . 15
Figure B.1 – Transient gain response at various slew rates . 17
Figure B.2 – 16 dB add and drop (rise and fall time = 10 µs) . 18
Figure B.3 – 16 dB add and drop (rise and fall time = 1 000 µs) . 18
Table 1 – Examples of add and drop scenarios for transient control measurement . 12
Table 2 – Typical results of transient control measurement . 12
Table B.1 – Transient gain response for various rise times and fall times (16 dB add or
drop) . 17
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
OPTICAL AMPLIFIERS –
TEST METHODS –
Part 4-1: Gain transient parameters –
Two-wavelength method
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61290-4-1 has been prepared by subcommittee 86C: Fibre optic
systems and active devices, of IEC technical committee 86: Fibre optics.
This second edition cancels and replaces the first edition published in 2011. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) extended the applicability from only EDFAs to all OFAs;
b) updated definitions for consistency with other documents in the IEC 61290-4 series.
– 4 – IEC 61290-4-1:2016 © IEC 2016
The text of this standard is based on the following documents:
CDV Report on voting
86C/1347/CDV 86C/1397/RVC
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document 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 website 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.
INTRODUCTION
This part of IEC 61290-4 is devoted to optical amplifiers (OAs). The technology of OAs is
quite new and still emerging; hence amendments and new editions to this document can be
expected.
Background information on the transient phenomenon in erbium-doped fibre amplifiers and
the consequences on fibre optic systems is provided in Annex A and on slew rate effects in
Annex B.
– 6 – IEC 61290-4-1:2016 © IEC 2016
OPTICAL AMPLIFIERS –
TEST METHODS –
Part 4-1: Gain transient parameters –
Two-wavelength method
1 Scope
This part of IEC 61290-4 applies to optical amplifiers (OAs) using active fibres (optical fibre
amplifiers (OFAs)) containing rare-earth dopants including erbium-doped fibre amplifiers
(EDFAs) and optically amplified elementary sub-systems. These amplifiers are commercially
available and widely deployed in service provider networks.
The object of document is to provide the general background for OFA transients and related
parameters, and to describe a standard test method for accurate and reliable measurement of
the following transient parameters:
a) channel addition or removal transient gain overshoot and transient net gain overshoot;
b) channel addition or removal transient gain undershoot and transient net gain undershoot;
c) channel addition or removal gain offset;
d) channel addition or removal transient gain response time constant (settling time).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements 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 61291-1, Optical amplifiers – Part 1: Generic specification
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61291-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1.1
surviving channel
optical signal that remains after a drop event
3.1.2
rise time
time it takes for the input optical signal to rise from 10 % to 90 % of the total difference
between the initial and final signal levels during an add event
Note 1 to entry: See Figure 1(a).
3.1.3
initial gain
gain of the surviving or pre-existing channel before a drop or add event
3.1.4
final gain
steady-state gain of the surviving or pre-existing channel after a long period of time (i.e. once
the gain has stabilized) after a drop or add event
3.1.5
gain offset
change of the gain between initial and final state
Note 1 to entry: Gain offset is expressed in dB.
Note 2 to entry: Gain offset = final gain (in dB) ‒ initial gain (in dB).
Note 3 to entry: Gain offset may be positive or negative for both channel addition and removal events.
3.1.6
gain stability
specified peak-to-peak gain fluctuations of the OFA under steady state conditions (i.e. not in
response to a transient event)
3.1.7
transient gain response time constant
settling time
amount of time required to bring the gain of the surviving or pre-existing channel to the final
gain
Note 1 to entry: This parameter is the measured time from the beginning of the drop or add event that created the
transient gain response to the time at which the surviving or pre-existing channel gain first enters within the gain
stability band centred on the final gain.
Note 2 to entry: Hereon, this will also be referred to as "settling time".
3.1.8
transient gain overshoot
difference between the maximum surviving or pre-existing channel gain reached during the
OFA transient response to a drop or add event, and the lowest of either the initial gain and
final gain
Note 1 to entry: Transient gain overshoot is expressed in dB.
Note 2 to entry: Hereon, this will also be referred to as "gain overshoot".
3.1.9
transient net gain overshoot
difference between the maximum surviving or pre-existing channel gain reached during the
OFA transient response to a drop or add event, and the highest of either the initial gain and
final gain
Note 1 to entry: The transient net gain overshoot is expressed in dB.
Note 2 to entry: The transient net gain overshoot is 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 3 to entry: Hereon, this will also be referred to as "net gain overshoot".
– 8 – IEC 61290-4-1:2016 © IEC 2016
3.1.10
transient gain undershoot
difference between the minimum surviving or pre-existing channel gain reached during the
OFA transient response to a drop or add event, and the highest of either the initial gain and
final gain
Note 1 to entry: The transient gain undershoot is expressed in dB.
Note 2 to entry: Hereon, this will also be referred to as "gain undershoot".
3.1.11
transient net gain undershoot
difference between the minimum surviving or pre-existing channel gain reached during the
OFA transient response to a drop or add event and the lowest of either the initial gain and
final gain
Note 1 to entry: The transient net gain undershoot is expressed in dB.
Note 2 to entry: The transient net gain undershoot is 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 3 to entry: Hereon this will also be referred to as "net gain undershoot".
3.2 Abbreviated terms
AGC automatic gain control
AOM acousto-optic modulator
BER bit error rate
DFB distributed feedback
DWDM dense wavelength division multiplexing
EDFA erbium-doped fibre amplifier
FWHM full-width half-maximum
NEM network equipment manufacturer
NSP network service provider
O/E optical-to-electronic
OA optical amplifier
OFA optical fibre amplifier
OSNR optical signal-to-noise ratio
SHB spectral-hole-burning
VOA variable optical attenuator
WDM wavelength division multiplexing
4 Measurement apparatus
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 dense wavelength division
multiplexing (DWDM) channels within the specified transmission band are dropped or added,
definitions are provided that describe a dynamic event leading to transient response. Rise and
fall time definitions are shown in Figure 1.
Rise time
Time
Channel
Channel
addition start
addition end
IEC
(a) Definitions of rise and fall times in the case of a channel addition event
Fall time
Time
Channel
Channel
removal start
removal end
IEC
(b) Definitions of rise and fall times in the case of a channel removal event
Figure 1 – Definitions of rise and fall times
The parameters generally used to characterize the transient gain behaviour of a gain
controlled OFA for the case of channel removal are defined in Figure 2(a). The figure
specifically represents the time dependence of the gain of one of the surviving channels when
channels are removed. Likewise, the transient gain behaviour for the case when channels are
added is shown in Figure 2(b). The main transient parameters are: transient gain response
time constant (settling time), gain offset, transient net gain overshoot, and transient gain net
undershoot. The transient gain overshoot and undershoot are particularly critical to carriers
Input power to EDFA
Input power to EDFA
(linear a.u.)
(linear a.u.)
10 % of change
100 % of change
10 % of change
90 % of change
90 % of change
100 % of change
Transient gain response time
constant (settling time)
– 10 – IEC 61290-4-1:2016 © IEC 2016
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.
Final gain
Initial
Gain offset
gain
Net gain
undershoot
Time
IEC
(a) OFA transient gain response for a channel removal event
Net gain
Gain overshoot
Overshoot
Gain offset
Initial
gain
Final gain
Transient gain response time
constant (settling time)
Time
IEC
(b) OFA transient gain response for a channel addition event
Figure 2 – OFA transient gain response
Figure 3 shows a typical setup to characterize the transient response properties of OFAs.
Gain
stability
Gain (dB)
Gain (dB)
Gain overshoot
Gain undershoot
Net gain
overshoot
Gain stability
Gain
undershoot
Net gain undershoot
DDFFBB Las Laserer
VOVOA1A1
λ1
Optical OOOFFFAAA Pass
Optical Pass
under
coupler underunder filter
coupler filter
test
tteestst
Pol. Optical
DDFFBB Las Laserer
Pol. Optical
VOVOA2A2
modulator
scrambler modulator
λ2
DetDecettecor tor
TTrriiggergger
Pulse generator OOsciscillllooscoscoppee
Pulse generator
IEC
Figure 3 – Generic transient control measurement setup
5 Test specimen
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 in Figure 3, the input signal power into the amplifier being tested is the
combination of two distributed feedback (DFB) lasers with wavelengths approximately 1 nm
apart. One of the wavelengths represents add or drop channels while the other represents
pre-existing or surviving channels. Each wavelength channel is subsequently adjusted with a
variable optical attenuator (VOA) to the desired optical input power levels. One optical
modulator driven by a function generator acts as an on/off switch, to simulate add and drop
events. The two optical channels are subsequently combined onto the same fibre before the
signal is directed to the amplifier being tested. A tuneable filter, an optical-to-electronic (O/E)
converter and an oscilloscope are placed in tandem at the output of the amplifier. The pre-
existing or surviving channel is selected with the tuneable filter and its transient response is
monitored with the O/E converter and oscilloscope. A waveform similar to the one shown in
Figure 2 is displayed on the oscilloscope’s screen.
To simulate a drop event at the input of the amplifier being tested, the two lasers are set so
that their total input power is equal to the amplifier’s typical input power (e.g. 1 dBm).
Therefore, the two lasers at –2 dBm each represent 20 optical channels having –15 dBm
power per channel. When the function generator turns the modulator into the “off” position,
the second laser is completely suppressed, changing the system’s channel loading. For
instance, when one laser is switched off, it simulates a 3 dB "drop" or a change in the
system’s channel loading from 40 channels to 20 channels. Similarly, when the modulator is
changed into an "on" state, the addition of a second laser simulates a 3 dB add in optical
power, or a change in the system’s channel loading from 20 channels to 40 channels. For
other transient control measurements, the VOAs can be adjusted accordingly so that the input
power levels will differ by an appropriate value.
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.
– 12 – IEC 61290-4-1:2016 © IEC 2016
Table 1 – Examples of add and drop scenarios
for transient control measurement
Pre-existing or Channels added or
Scenario Total channels
surviving channels dropped
20 dB add or drop 100 1 99
16 dB add or drop 40 1 39
13 dB add or drop 40 2 38
10 dB add or drop 40 4 36
6 dB add or drop 40 10 30
3 dB add or drop 40 20 20
7 Calculations
The results of the transient measurement are the following parameters:
• Channel addition or removal transient gain overshoot and transient net gain overshoot
• Channel addition or removal transient gain undershoot and transient net gain undershoot
• Channel addition or removal gain offset
• Channel addition or removal transient gain response time constant (settling time)
These parameters can be extracted from the oscilloscope display, as described in Figure 2.
8 Test results
Table 2 shows typical measurement conditions and transient control measurement results of
C-band OFAs. 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. The values of amplifier gain, pre-
existing or surviving channel wavelength and added or dropped channel wavelength shall be
provided by the specifier.
Typical values of transient parameters are listed in the last row of Table 2.
Table 2 – Typical results of transient control measurement
a
Pre-existing or surviving channel wavelength
a (___nm)
Amplifier gain
(___dB) a
Added or dropped channel wavelength
(___nm)
Transient event Input power Transient net Transient net Transient gain Gain offset
description gain overshoot gain undershoot response time
constant
dBm dB dB µs dB
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
a
Values of gain and wavelength to be provided by the specifier.
Annex A
(informative)
Background on transient phenomenon in optical amplifiers
Optical power transients are sub-millisecond fluctuations in network power levels that are
caused by events such as channel loading changes, passive loss variations, and network
protection switching. In a dynamic networking environment, optical amplifiers need to be able
to compensate for such power variations in order to avoid potential degradation of quality of
service. For instance, in a network reconfiguration scenario, the number of DWDM channels
at the input of an OFA may suddenly decrease, increasing the amplifier’s inversion and
therefore its gain, in a matter of microseconds. This gain change is detrimental to network
service providers (NSPs) given that their networks will no longer operate in the gain level for
which they were optimized, potentially impacting service quality. An increase in bit error rate
(BER) is a typical manifestation of quality of service degradation. A reduction in channel
power can decrease the optical signal-to-noise ratio (OSNR), while an increase in the power
can enhance degradation due to non-linear effects in transmission fibre and increased signal
shot noise factor (F ) from shot noise from amplified input signal.
shot,sig
Three factors determine the gain in OFAs: input optical power, optical pump power, and the
inversion level of the optical amplifier. The inversion level of an OFA characterises the
fraction of erbium atoms that are available to provide energy to the input optical signal,
resulting in optical gain. Typically, the inversion level increases with the increase in optical
pump power and decreases with the increase in input optical power. For that reason, if
wavelengths are added to an OFA input, increasing its optical input power, the optical power
of the pumps will also need to be increased in order to maintain the inversion level and,
therefore, a constant gain per channel. Constant gain per channel is important to optimise the
performance of optical networks. Similarly, if wavelengths are dropped from an OFA input, the
pumps will need to be rapidly decreased in order to maintain a constant gain per channel.
The gain of an OFA can be controlled by adjusting its pump current. The basic scheme for the
pump control is shown in Figure A.1 and involves making measurement of input and output
power of the OFA through signal taps and monitor photodiodes. Early reported work
addressed pump control on time scales of the spontaneous lifetime in OFAs. One of the
studies demonstrated low frequency feed forward compensation with a low frequency control
loop. The results of pump power control on time scales much shorter than the erbium
spontaneous lifetime that were demonstrated to arrest the power excursion in the surviving
channels are shown in Figure A.1. The necessary response time was characterised by
monitoring the power of the surviving channel as a function of the delay after the cutoff of the
dropped channels. The second stage pump power of the amplifier was then decreased by an
amount suitable to restore the gain of the surviving channels. This experiment demonstrates
that the dynamic timescales for changes in signal power and pump power are comparable and
the power excursion of the surviving channels can be arbitrarily limited if the pump power is
decreased with sufficiently short delay. For example, in the last trace, negligible power
excursion occurs when a correction is applied after a delay of a few microseconds. This
shows that with standard pumps, if the decision to take the corrective action can be reached
in time, the pump power can be turned down quickly enough to control the excursions of
surviving channels. These measurements demonstrate that, for the pump control to minimize
the variations in the power of the surviving channels in case of channel loss, the response of
the control scheme must be at the most a few tens of microseconds.
– 14 – IEC 61290-4-1:2016 © IEC 2016
Gain control off
1 2 3 4
1 span, control off
2 spans, control off
P
3 spans, control off
in
4 spans, control off
EDF
0 0,5 1,0 1,5 2,0
P
P
p Time (ms)
out
Surviving channel power after 4 spans, 320 km
G
Gain control on
Gain control
circuit
0 0,5 1,0 1,5 2,0
Time (ms)
IEC
NOTE Half of the channels are added and dropped periodically. Surviving channel relative power is shown on the
right hand side for both the cases with and without pump control on all the OFAs.
Figure A.1 – OFA pump control for a chain of 5 OFAs and 4-fibre spans
In lightwave transmission applications, OFAs are operated in saturation mode. The gain
saturation in OFAs is predominantly homogeneous, which means that in a multi-channel WDM
system, once the gain of one of the channels is known, the gain of other channels can be
calculated directly. This result comes from the homogeneous property of the OFA model.
While the gain spectrum of OFAs is predominantly homogeneous, however, a small amount of
inhomogeneity has been observed. The inhomogeneous broadening gives rise to spectral-
hole-burning (SHB) in the gain spectra of optical amplifiers. Using an accurate difference
measurement technique, the SHB in OFAs has been measured at room temperature. The
result of SHB measurement for different saturation levels is shown in Figure A.2, which shows
the existence of a spectral hole having a full-width half-maximum (FWHM) of 8 nm. The depth
of the hole increases linearly at a rate of 0,027 dB per 1 dB increase in gain compression
relative to small signal gain. For 10 dB gain compression, a dip of 0,28 dB in the gain spectra
due to SHB is observed. The SHB is strongly dependent upon the wavelength and has been
shown to be four times larger at 1 532 nm than at 1 551 nm. The dependence of the spectral
hole width on the saturating wavelength is shown in Figure A.3. The FWHM of the hole
increases as the saturating wavelength is increased.
Surviving channel relative power (dB) Surviving channel relative power (dB)
0,5
Compression
–0,5
1,00 dB
2,00 dB
3,54 dB
5,25 dB
–1,0 7,32 dB
10,91 dB
13,41 dB
–1,5
1 520 1 530 1 540 1 550 1 560
Wavelength (nm)
IEC
Figure A.2 – EDFA spectral hole depth for different gain compression
λ = 1 545 nm
sat
4 nm
λ = 1 551 nm
sat
8 nm
λ = 1 562 nm
sat
10 nm
1 535 1 540 1 545 1 550 1 555 1 560 1 565 1 570 1 575
Wavelength (nm)
IEC
Figure A.3 – EDFA spectral hole depth for different wavelengths
The SHB effect impacts the gain shape of the long-haul optical transmission systems. The
effect manifests itself such that each WDM channel in the system reduces the gain of the
neighbouring channels within the spectral hole-width but does not significantly affect channels
far removed in wavelength. While characterizing the gain spectra of the amplifiers it is
therefore important that multi-wavelength input signal with channel separation less than the
SHB width be employed. The SHB effect observed in an individual amplifier is small (0,2 dB to
0,3 dB) but in long chain of amplifiers such as in a long haul or submarine system it can add
up to produce a significant and observable change in the overall spectrum. The importance of
SHB was noted in long-haul transmission over 9 300 km.
Gain difference (0,1 dB/div.)
Hole depth (dB)
– 16 – IEC 61290-4-1:2016 © IEC 2016
Annex B
(informative)
Slew rate effect on transient gain response
B.1 The importance of rise time and fall time of input power
When channels are either added at add event or removed at drop event, it must be
considered how fast the input power will be changed while measuring transient gain. Gain
control of the OFA is generally realized by power monitors of input and output levels and by
way of power adjustment method through driving pump laser current. Optical design and
control algorithms affect the transient response of gain at add or drop events, as explained in
Annex A. Additionally, the input power slew rate of changing conditions also affects transient
gain response.
If the input power to the OFA changes slowly, then the gain control mechanism may be able to
compensate transient gain phenomena with the fast gain control mechanism of OFA. In
addition, the pump power adjustment process can minimize transient gain under the gradual
sloped input power variation. Thus, the transient response of the OFA will be suppressed with
small transient gain.
If input power to the OFA changes rapidly, such as in the case of a step input, then the gain
control mechanism may not be able to suppress transient gain, since the gain control
mechanism of the OFA is not fast enough to compensate transient gain under the steep input
power variation. In addition, the transient response of the OFA will be increased as a result.
B.2 Measured data and explanation
Measured data for various rise time and fall time conditions are provided with typical
experimental data. Transient gain responses at 16 dB add or drop conditions are evaluated
for the case of a single stage OFA. Rise times and fall times are varied from 10 µsec to
1 000 µsec to observe effect of various rise time and fall time conditions on transient gain
response.
A schematic diagram of the experimental setup is described in Figure 3. In this experiment,
the pre-existing or surviving channel wavelength and the add or drop channel wavelength are
1 561 nm and 1 545 nm, respectively. An acousto-optical modulator (AOM) is used as a
modulator. Rise time and fall time is adjusted using an arbitrary function generator so that the
slew rate will provide rise time and fall time from 10 µs to 1 000 µs. The output of the function
generator is connected to the electrical input to the AOM. Transient gain responses are
recorded by an oscilloscope to quantify the transient gain response and absolute value of the
gain offset. Table B.1 summarizes transient gain response for various rise time and fall time
conditions. A positive value means an overshoot of the transient gain response at the drop
event, and a negative value means an undershoot of the transient gain response at the add
event. The overshoot level and undershoot level of transient gain responses are plotted in
Figure B.1.
Transient gain response is mitigated at larger rise and fall times as presumed in Figure B.1.
Figure B.2 and Figure B.3 display transient gain responses at 10 µs rise or fall time and
1 000 µs rise or fall time.
Table B.1 – Transient gain response for various
rise times and fall times (16 dB add or drop)
Pre-existing or Transient gain response
Add or drop
surviving
Rise time
channel Steady gain
channel
16 dB add 16 dB drop
wavelength response
Fall time
wavelength
event event
λ2
λ1
nm nm µs dB dB dB
10 –0,76 0,74
50 –0,76 0,58
1 561 1 545 0,29
100 –0,63 0,52
1000 –0,56 0,29
1,0
0,8
0,6
0,4
0,2
0,0
-0,2
-0,4
-0,6
-0,8
-1,0
0 100 200 300 400 500 600 700 800 900 1 000 1 100
Rise time at add event, fall time at drop event (µs)
Under shoot at add condition Overshoot at drop condition
IEC
Figure B.1 – Transient gain response at various slew rates
Transient gain response
(overshoot, undershoot) (dB)
– 18 – IEC 61290-4-1:2016 © IEC 2016
Pre-existing channel output SPreurv-eivxinisg ticng channelhanne outpul outt put
Transient gain response
Transient gain response
Transient gain response
Total input power
Total input power Total input power
IEC
IEC
16 dB add case (rise time = 10 µs) 16 dB drop case (fall time = 10 µs)
Figure B.2 – 16 dB add and drop (rise and fall time = 10 µs)
Pre-existing channel output
Surviving channel output
Transient gain response
Transient gain response
Total input power
Total input power
IEC
IEC
16 dB add case (rise time = 1 000 µs) 16 dB drop case (fall time = 1 000 µs)
Figure B.3 – 16 dB add and drop (rise and fall time = 1 000 µs)
Bibliography
IEC 61290-1 (all parts), Optical amplifiers – Test methods – Part 1-1: Power and gain
parameters
IEC 61290-3-1, Optical amplifiers – Test methods – Part 3-1: Noise figure parameters –
Optical spectrum analyzer method
IEC 61290-3-2, Optical amplifiers – Test methods – Part 3-2: Noise figure parameters –
Electrical spectrum analyzer method
IEC 61290-4-2, Optical amplifiers – Test methods – Part 4-2: Gain transient parameters –
Broadband source method
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LEOS Summer Topic
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