IEC TR 62343-6-1:2011

IEC/TR 62343-6-1 ®
Edition 1.0 2011-02
TECHNICAL
REPORT
colour
inside
Dynamic modules –
Part 6-1: Dynamic channel equalizers

IEC/TR 62343-6-1:2011(E)
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IEC/TR 62343-6-1 ®
Edition 1.0 2011-02
TECHNICAL
REPORT
colour
inside
Dynamic modules –
Part 6-1: Dynamic channel equalizers

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
M
ICS 33.180 ISBN 978-2-88912-365-0

– 2 – TR 62343-6-1  IEC:2011(E)
CONTENTS
FOREWORD . 3
1 Scope . 5
2 Terms and definitions . 5
3 Background . 6
4 Gain equalized EDFAs . 7
5 OSNR in WDM systems . 8
6 System impact of amplifier gain flatness . 9
7 Benefits of dynamic channel equalization . 10
8 DCE technologies . 10
Bibliography . 13

Figure 1 – ROADM architecture . 7
Figure 2 – Gain spectrum of an EDFA with GEF . 8
Figure 3 – OSNR penalty caused by optical gain non-uniformity . 10

Table 1 – An example of DCE specifications . 12

TR 62343-6-1  IEC:2011(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DYNAMIC MODULES –
Part 6-1: Dynamic channel equalizers

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
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example "state of the art".
IEC 62343-6-1, 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/969/DTR 86C/994/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.

– 4 – TR 62343-6-1  IEC:2011(E)
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TR 62343-6-1  IEC:2011(E) – 5 –
DYNAMIC MODULES –
Part 6-1: Dynamic channel equalizers

1 Scope
This part of IEC 62343 is a technical report and deals with dynamic channel equalizers (DCE).
The report includes a description of the dynamic channel equalization and its benefits in a
wavelength division multiplexed (WDM) transmission system and also covers different DCE
component technologies that are being used.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
channel non-uniformity
difference (in dB) between the powers of the channel with the most power (in dBm) and the
channel with the least power (in dBm). This applies to a multichannel signal across the
operating wavelength range
2.2
in-band extinction ratio
within the operating wavelength range, the difference (in dB) between the minimum power of
the non-extinguished channels (in dBm) and the maximum power of the extinguished
channels (in dBm)
2.3
out-of-band attenuation
attenuation (in dB) of channels that fall outside of the operating wavelength range
2.4
operating wavelength range
specified range of wavelengths from λ to λ about a nominal operating wavelength λ ,
imin imax I
within which a dynamic optical module is designed to operate with a specified performance
2.5
channel frequency range
frequency range within which a device is expected to operate with a specified performance
NOTE For a particular nominal channel central frequency, f , this frequency range is from f = (f - ∆f )
nomi imin nomi max
to fi = (f + ∆f ), where ∆f is the maximum channel central frequency deviation.
max nomi max max
2.6
ripple
peak to peak difference in insertion loss within a channel frequency (or wavelength) range
2.7
channel spacing
centre-to-centre difference in frequency (or wavelength) between adjacent channels in a
device
– 6 – TR 62343-6-1  IEC:2011(E)
2.8
channel response time
elapsed time it takes a device to transform a channel from a specified initial power level to a
specified final power level desired state, when the resulting output channel non-uniformity
tolerance is met, measured from the time the actuation energy is applied or removed
3 Background
The capacity of dense wavelength division multiplexed (DWDM) networks has grown
exponentially since 2000 to meet the bandwidth demand created by the Internet. The highest
demonstrated transmission capacity over a single fibre now exceeds 10 Tb/s. There is also a
push to reduce the overall capital expenditure of building networks and lower the cost of
transmitting data.
In order to reduce capital expenditure, the networks are evolving such that high-capacity
transmission can be carried out over ultra-long distances of several thousand kilometres
without optical-electronic-optical (OEO) regeneration. One of the challenges in ultra-long-haul
transmission systems is to equalize the power of WDM channels in order to provide an
acceptable optical signal-to-noise ratio (OSNR) and deliver a high quality of service for all
optical channels. It is currently difficult to equalize the power of the various wavelengths
present in a system because of wavelength dependence in the gain/loss of different elements
forming the WDM transmission system.
The key elements that contribute to the wavelength dependent gain/loss include erbium-
doped fibre amplifiers (EDFAs), transmission fibre, dispersion compensators and passive
optical elements in a fibre optic transmission system. The problem of wavelength-dependent
gain/loss becomes more critical in ultra-long-haul networks where signals will have to pass
through up to 50 EDFAs and fibre spans without OEO regeneration. Next-generation networks
will require some method of dynamic channel equalization to provide uniform OSNR for all the
channels in the WDM system and thereby improve the system margin which can be used to
lower the cost of ultra-long haul-systems.
Recently, point-to-point systems have evolved towards ring and mesh networks.
Reconfigurable optical add-drop multiplexer (ROADM)-based architectures have emerged to
provide flexible and reconfigurable networks.
An example of the ROADM node architecture is shown in Figure 1a. A multichannel DWDM
fibre enters the node and the optical power is immediately split to provide paths for
wavelengths that transit through the node and dropped wavelengths that get routed to a
demultiplexer. The through traffic enters a 1 × 1 WSS (i.e. it has just one input and one output
port so there is no switching) that under remote control either passes through, equalizes, or
blocks (extinguishes) any or all wavelengths. New wavelengths are added by passive
combination after the WSS. The WSS blocks any wavelengths identical to the added
wavelengths so that there are no duplicate wavelengths carrying traffic in the same channel.
Discrete variable optical attenuators (VOAs) are used to equalize the optical power of the
added wavelengths and an optical power monitor (OPM) provides feedback for the optical
power equalization controls of the WSS and VOAs. Figure 1b shows a variation on this
architecture where the locally added wavelengths are still combined at a multiplexer but are
now directed to the Add port of a 2 × 1 WSS. The WSS selects specific wavelengths from
either the In or Add port and routes these to the Out port for transmission to the next network
node. The WSS in this architecture also equalizes the optical power of the added wavelengths,
eliminating the need for discrete VOAs.
Both architectures of Figures 1a and 1b are termed fixed add/drop because the dropped and
added wavelengths are associated with specific or fixed ports on the multiplexers. While these
wavelengths are still connected manually to specific service line cards (e.g. 10 Gb Ethernet or
SAN protocol), one school of thought holds that this is of no major concern because it is
usually done in conjunction with the manual provisioning of the service line cards themselves.
The main advantage of these ROADM architectures is that the multiple wavelengths passing

TR 62343-6-1  IEC:2011(E) – 7 –
through the node are routed and equalized in an automated fashion. Figures 1c and 1d show
two-degree ROADM configurations that eliminate the fixed physical associations for the
dropped and added wavelengths with the demux and mux ports. The industry calls this feature
colourless because any colour (frequency) or wavelength can be directed to any Drop port
and from any Add port.
In Out In Out
80 % 60 %
1x1
2x1
40 %
20 %
WSS
WSS
OPM
OPM
Demux Add
Mux Demux
Mux
Local Drop Local Drop
Local Add
Local Add
Figure 1a – Fixed Add/Drop Figure 1b – Fixed Add/Drop
IEC  297/11 IEC  298/11
In Ou
In
1x1 Out 1xN
WSS
WSS
OPM
OPM
TL TL TL TL
TR TR
Local Drop
Local Drop Local Add Local Add
Figur 1c – Colourless Add/Drop Figure 1d – Colourless Add/Drop
IEC  299/11 IEC  300/11
Figure 1 – ROADM architecture
This technical report explains how the wavelength dependent gain in EDFAs can impair the
system performance of a long haul system and how the use of dynamic channel equalization
devices such as dynamic gain equalization filters (GEFs) can improve the end of system
OSNR to extend their reach to ultra long distances.
4 Gain equalized EDFAs
Manufacturers of wideband EDFAs insert static gain equalization filters (GEFs) between the
stages of an EDFA to flatten the gain spectrum. The most commonly used GEFs, based on
thin film technology, consist of translucent multi-layer structures of materials with different
indices of refraction that create interference effects.

– 8 – TR 62343-6-1  IEC:2011(E)
35nm
Gain
(dB)
1 530 1 560
1 540
1 550
IEC  301/11
Figure 2 – Gain spectrum of an EDFA with GEF
The ideal GEF would have a transmission spectrum that resembles the inverse of the EDFAs
gain spectrum. Despite the sophisticated thin film technology of the GEFs, they do not
compensate for the spectral gain variation perfectly and therefore leave the power of the
various channels somewhat unequal. In other words, the gain spectrum of the integrated
EDFA and GEF subsyste
...