IEC TR 62343-6-4:2017

Dynamic modules - Part 6-4: Design guides - Reconfigurable optical add/drop multiplexer

IEC TR 62343-6-4:2017(E) which is a Technical Report on reconfigurable optical add/drop multiplexers (ROADMs), provides a description of the ROADMs in dynamic optical networks and related optical component and module technologies, including wavelength selective switches (WSSs).

General Information

• Status: Published
• Publication Date: 26-Jan-2017
• ICS: 33.180.01 - Fibre optic systems in general
• ICS: 33.180.99 - Other fibre optic equipment
• Technical Committee: SC 86C - Fibre optic systems, sensing and active devices
• Drafting Committee: WG 5 - TC 86/SC 86C/WG 5
• Current Stage: PPUB - Publication issued
• Start Date: 27-Jan-2017
• Completion Date: 27-Feb-2017

Overview

IEC TR 62343-6-4:2017 is a Technical Report from the International Electrotechnical Commission (IEC) that provides design guidance for reconfigurable optical add/drop multiplexers (ROADMs) used in dynamic optical networks. As part of the IEC 62343 “Dynamic modules” series, this report describes ROADM node functions, architectures and the related optical component and module technologies - in particular wavelength selective switches (WSSs) - to support reconfigurable Dense Wavelength Division Multiplexing (DWDM) systems.

Key topics covered

• ROADM background and network evolution - rationale for reconfigurable nodes in metro, regional and long-haul optical networks and how ROADMs enable capacity scaling without optical–electrical–optical conversion.
• ROADM architectures - descriptions of common architectures such as broadcast-and-select and route-and-select, and architectures that provide colourless, directionless and contentionless (CDC) functionality.
• Wavelength Selective Switch (WSS) technologies - role of WSS in ROADMs and comparisons of switching engines.
• WSS performance characteristics - key parameters that affect ROADM design and operation (for example: insertion loss, isolation/crosstalk, channel count and granularity, switching speed, wavelength range, polarization-dependent loss (PDL), polarization mode dispersion (PMD), power handling and port count).
• Component technologies (Annex A) - overview of switching engines and module technologies including PLC (planar lightwave circuit), MEMS, LCD/LC, LCOS (liquid crystal on silicon) and DLP implementations, with a feature comparison to guide technology selection.
• Optical monitoring and amplification - node-level considerations such as optical channel monitoring (OCM), variable optical attenuation (VOA) and integration with amplifiers (EDFA).

Practical applications and users

IEC TR 62343-6-4 is useful for:

• Network architects and system designers planning DWDM networks that require flexible wavelength routing and provisioning.
• Equipment manufacturers and module vendors designing ROADM subsystems and selecting WSS or switching engine technologies.
• Carrier operations, planning and test teams evaluating performance trade-offs (loss, isolation, channel granularity) when deploying ROADMs in metro, regional and long-haul networks.
• Standards bodies and research teams studying ROADM evolution, CDC implementations and component-level technology choices.

Related standards

• Part of the IEC 62343 “Dynamic modules” series.
• Prepared by IEC TC 86C (Fibre optic systems and active devices); consult the IEC webstore for related IEC publications and updates.

Keywords: ROADM, wavelength selective switch, WSS, reconfigurable optical add/drop multiplexer, DWDM, dynamic optical networks, CDC, MEMS, PLC, LCOS.

Standards Content (Sample)

IEC TR 62343-6-4 ®
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Dynamic modules –
Part 6-4: Design guides – Reconfigurable optical add/drop multiplexer
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IEC TR 62343-6-4 ®
Edition 1.0 2017-01
TECHNICAL
REPORT
colour
inside
Dynamic modules –
Part 6-4: Design guides – Reconfigurable optical add/drop multiplexer

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 33.180.01, 33.180.99 ISBN 978-2-8322-3879-0

– 2 – IEC TR 62343-6-4:2017 © IEC 2017
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and abbreviated terms . 6
3.1 Terms and definitions . 6
3.2 Abbreviated terms . 6
4 Reconfigurable optical add/drop multiplexer . 7
4.1 Background. 7
4.2 Optical network evolution . 8
4.3 ROADM subsystem technologies evolution . 11
4.3.1 General . 11
4.3.2 Wavelength blocker based ROADMs . 11
4.3.3 Integrated planar lightwave circuits (IPLC) based ROADMs . 12
4.3.4 Wavelength selective switches . 13
4.4 ROADM architecture . 14
4.4.1 Broadcast and select architecture . 14
4.4.2 Route and select architecture . 16
4.4.3 Colourless, directionless and contentionless (CDC) functionality . 17
5 WSS performance characteristics . 22
Annex A (informative) ROADM (WSS) component technologies . 23
A.1 General description . 23
A.2 PLC technology . 23
A.3 MEMS (micro-electromechanical system) technology . 24
A.4 LCD (liquid crystal device) technology . 25
A.5 LCOS (liquid crystal on silicon) . 25
A.6 DLP (digital light processor) mirror arrays . 26
A.7 Feature comparison of each switching engine technology . 26
Bibliography . 28

Figure 1 – Reconfigurable optical network .7
Figure 2 – Evolution of optical networks from point-to-point to reconfigurable WDM .9
Figure 3 – Schematic of a ROADM node showing functions of wavelength pass-
through add or drop, channel power equalization, and optical channel monitoring
(OCM) . 10
Figure 4 – Evolution of ROADM technologies . 11
Figure 5 – Wavelength blocker based ROADM architecture . 12
Figure 6 – 2-degree ROADM node architecture with wavelength blocker, dynamic OAs
and shared OCMs . 12
Figure 7 – PLC based ROADM architecture . 13
Figure 8 – ROADM architecture incorporating a WSS . 14
Figure 9 – Broadcast and select architecture of ROADMs . 15
Figure 10 – ROADM architectures . 16
Figure 11 – ROADM route and select architectures . 17
Figure 12 – 8x8 WSS and large port count 1x24 WSS based colourless, directionless
and contentionless ROADM architecture . 18

Figure 13 – Coloured and directional ROADM architecture including WSS, splitters,
AWGs and transceivers (TRx) . 19
Figure 14 – Colourless, directionless and contentionless ROADM architecture . 20
Figure 15 – Technologies for contentionless architecture . 21
Figure A.1 – Example of PLC devices for ROADM systems . 23
Figure A.2 – Generic internal configuration of WSS (example, MEMS based) . 24
Figure A.3 – Switching engine of MEMS . 24
Figure A.4 – LC switching engine . 25
Figure A.5 – LCOS switching engine . 26
Figure A.6 – DLP switching engine . 26

Table 1 – List of key WSS parameters . 22
Table A.1 – WSS switch engine feature comparison . 27

– 4 – IEC TR 62343-6-4:2017 © IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
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DYNAMIC MODULES –
Part 6-4: Design guides –
Reconfigurable optical add/drop multiplexer

FOREWORD
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The main task of IEC technical committees is to prepare International Standards. However, a
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data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC TR 62343-6-4, 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/1400/DTR 86C/1420/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 document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 62343 series, published under the general title Dynamic modules,
can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
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• reconfirmed,
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– 6 – IEC TR 62343-6-4:2017 © IEC 2017
DYNAMIC MODULES –
Part 6-4: Design guides –
Reconfigurable optical add/drop multiplexer

1 Scope
This part of IEC 62343, which is a Technical Report on reconfigurable optical add/drop
multiplexers (ROADMs), provides a description of the ROADMs in dynamic optical networks
and related optical component and module technologies, including wavelength selective
switches (WSSs).
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
No terms and definitions are listed in this document.
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
3.2 Abbreviated terms
AWG arrayed waveguide grating
CDC colourless, directionless and contentionless
demux demultiplexer
DWDM dense wavelength division multiplexing
DLP digital light processor
EDFA erbium doped fibre amplifier
IPLC integrated planar lightwave circuit
LC liquid crystal
LCD liquid crystal device
LCOS liquid crystal on silicon
LH long haul
MEMS micro-electromechanical systems
MPD monitor photo-diode
mux multiplexer
OA optical amplifier
OEO optical-electrical-optical
OCM optical channel monitor
OADM optical add drop multiplexer
PDL polarization dependent loss
PLC planar lightwave circuit
PMD polarization mode dispersion
ROADM reconfigurable optical add/drop multiplexer
TF tuneable filter
TRx transceiver
Rx receiver
3R regeneration, retiming and reshaping
Tx transmitter
ULH ultra long haul
VOA variable optical attenuator
WSS wavelength selective switch
WB wavelength blocker
WDM wavelength division multiplexing
4 Reconfigurable optical add/drop multiplexer
4.1 Background
Optical networks are evolving to address both the rapid growth in capacity demand and highly
efficient and seamless connectivity requirements. While high data rate DWDM channels at
40 Gb/s and 100 Gb/s are being introduced in the network to grow the capacity to multiple
Tb/s per fibre, the uncertainty in traffic demand and the emergence of bandwidth-hungry
applications like video-on-demand have turned the industry’s focus to dynamic, reconfigurable
optical networks. Telecommunication carriers and content providers require switching nodes
at their central offices in order to route, switch and monitor the optical wavelength channels
as they traverse the optical network. These switching nodes, as shown in Figure 1, are called
reconfigurable optical add/drop multiplexers (ROADMs), and they are the key nodal sub-
systems used in implementing modern optical communication infrastructure.
Long haul
Regional
Optical signal being
transported across a Metro
cascade of multiple
reconfigurable nodes
Access
IEC
Figure 1 – Reconfigurable optical network

– 8 – IEC TR 62343-6-4:2017 © IEC 2017
Different segments of the optical network, long haul (LH)/ultra long haul (ULH), regional,
metro and access, are schematically shown in Figure 1. Generally, the long haul network is
optimized for point-to-point traffic with a predictable traffic pattern. The regional and metro
networks are characterized by having the bandwidth scalability of long haul with the service
flexibility of the access network. In this segment of the network, the traffic pattern tends to be
more dynamic and less predictable, requiring the network to have greater flexibility. While
ROADMs were first introduced in the LH/ULH part of the network, it is the metro and regional
segment where they offer the highest value proposition.
In addition to ease of service provisioning and network reconfigurability, optically routed
networks reduce the need for unnecessary processing of through-traffic by eliminating the
signal conversion from the optical to the electronic domain and back to the optical domain for
retransmission, thereby significantly reducing cost. Elimination of signal conversion to the
electronic domain makes ROADM nodes transparent to traffic data rate and modulation format,
enabling easy network capacity upgrade without impacting the live traffic, a key requirement
of service providers. They also include the important function of signal monitoring and power
balancing. For dynamic optical networks, it is increasingly important to co-optimize different
networking aspects, such as optical layer flexibility and signal impairments.
4.2 Optical network evolution
Evolution of wavelength division multiplexing (WDM) transmission networks is illustrated in
Figure 2. Networks have evolved from transmission systems consisting of point-to-point WDM
links to modern dynamic and reconfigurable networks. As illustrated in part (a) of Figure 2, the
earliest WDM systems included point-to-point high capacity links interconnecting terminal
equipment. Transmission links consisted of periodic fibre spans and optical amplifiers for
compensating link loss. All wavelengths entering the node are terminated via optical-
electrical-optical (O-E-O) conversion at the network nodal points, where the optical channels
are demultiplexed via an arrayed waveguide grating (AWG) element, for example, and each
wavelength is directed to a receiver of a separate transponder that converts the DWDM
signals to the electrical domain, and then to a client optical signal at 1 310 nm for short reach
interconnect. Similarly, the egress traffic from the node is sent on a fibre link and is originated
from multiplexed DWDM wavelengths from transponders connected to 1 310 nm client short
reach interfaces.
The node is equipped to handle two types of traffic:
a) express traffic, which after passing through the node is directed to its final destination via
another WDM link intersecting the node;
b) add/drop traffic, which is either terminated at the node or originates from the node.
As mentioned earlier, all traffic through the node is mediated via 1 310 nm links, and the
express and add/drop wavelength channels are predetermined by hard-wired connections.
The benefits of this architecture include full 3R (regeneration, retiming and reshaping)
regeneration and wavelength conversion of all the optical signals leading to pristine signals
from the node, multi-vendor equipment interoperability via a commonly used 1 310 nm client
interface, and signal quality monitoring in the electrical domain, for example via overhead
bytes.
The main drawback of this architecture is that any reconfiguration of the node will require
manual intervention by changing the patch cords. In addition, the architecture is not scalable,
since the transponders are data rate specific. The network cannot therefore be upgraded to
handle higher data rate traffic without replacing all the transponders. The rapid and
unpredictable growth of network traffic from today’s internet requires the network to be
dynamic and flexible in supporting new growth areas.

a)
b)
c)
Fiber amplifier
Fixed wavelength add/drop filter
Wavelength mux/demux
ROADM
IEC
Key
a) point-to-point transport network
b) fixed wavelength add/drop filter based network
c) ROADM networks
Figure 2 – Evolution of optical networks from point-to-point
to reconfigurable WDM
In order to increase network flexibility for growing traffic demand in the networks, optical add
drop multiplexers (OADMs) were introduced. The initial OADMs that were commercially
available in the mid-1990s were not configurable and were achieved via fixed filters. As
shown in part (b) of Figure 2, these provided fixed WDM connectivity between multiple nodes.
The introduction of fixed OADMs helped reduce the cost of the network by eliminating the
need of OEO conversion for the express optical channels. However, the network designer
needed to predetermine which wavelengths would be dropped at a node, since the OADM
would remain fixed in this configuration. This posed a severe limitation because the service
providers could not adapt to unpredictable deviations to network capacity demand. Moreover,
provisioning of new channels created the additional complication of adequately balancing the
power of all the WDM channels without affecting service on the live channels, through optical
amplifier transients, for example. This led to cumbersome controlled and manual turning up of
channels, negating the benefits derived from the elimination of transponders at the nodes.
Subsequent availability of fully configurable OADMs (part (c) of Figure 2) enabled network
operators to configure any wavelength as transiting or add/drop, without affecting any of the
existing traffic on the OADM. OADMs with this flexibility and configurability were termed
reconfigurable-OADMs (ROADMs).
The functionality of a ROADM node is illustrated in Figure 3, which shows a two-degree node
consisting of a pair of input and output fibres entering a network node, for example east to
west. Usually, there is another pair of fibres (not shown in the figure) carrying traffic the other
direction (west to east). The wavelength channels entering the node from the east side are
shown to have different power levels. The channels are dispersed by the first module and
selectively routed either to the express path to continue further or to the local drop ports. The
module also includes variable optical attenuators that adjust the power of the channels in
conjunction with an optical power monitor shown in the second module. The express channels
out of the first module and the locally added channels are combined by the second module.
Optical attenuators in the add path are used to adjust the power of these channel so that all
the channels egressing from the node have equal power. The ROADM node is thus able to
accomplish selective routing of the wavelength channels to the express path, carry out the

– 10 – IEC TR 62343-6-4:2017 © IEC 2017
wavelength drop and add function, and finally monitor and balance the power of optical
channels sent on the output fibre.
The above example as shown in Figure 3 refers to a two-degree node. ROADMs enable
wavelength routing in higher degree nodes consisting of fibre connections in other directions.
A four-degree node, for example, will have fibres connecting east, west, north and south
directions. ROADMs can interconnect wavelengths coming from different directions in the
optical domain. This enables mesh networking at the optical node, which can be managed
cost effectively and is agnostic to data rate and modulation format. ROADMs are a key
enabler of the modern 100 Gbit/s and 400 Gbit/s coherent transport networks, which use
different modulation formats such as QPSK and 16QAM.
ROADM ROADM
module module
DROPs ADDs
Variable optical
attenuators (VOAs)
IEC
Figure 3 – Schematic of a ROADM node showing functions of wavelength
pass-through add or drop, channel power equalization,
and optical channel monitoring (OCM)
ROADM networks are architected with the goal of having a dynamic photonic layer capable of
rapid wavelength routing. ROADMs have been widely deployed in intercity and metro core
networks in the last decade. Optical bypass and remote configurability in ROADM networks
led to fewer router interfaces and reduction in manual fibre patching, thereby lowering the
overall cost per bit compared to static networks. When a new wavelength connection is
needed in a static network, the transponders and regenerators are individually wired into the
network manually via a laborious process. Some of the barriers and limitations to ROADM
introduction in the network include the inflexible introduction of interconnections on the client
side between the transponders and subtending electronic equipment such as the routers and
switches. Another barrier has been the network control software, which is designed without
the concept of a dynamic wavelength and can be very difficult to change and update. The
third major barrier is related to the business model for ROADM based networks: at present,
monetizing these dynamic networks to pay for the additional cost remains a challenge.
Deployment of ROADMs has increased rapidly in recent years. Essentially all new metro,
regional and long haul WDM systems developed by equipment manufacturers and new
deployments offer ROADM-based wavelength agility as a key feature. The deployments
planned by Tier-1 carriers globally require reconfigurable wavelength agility to reduce
operational expenses and increase service flexibility. In order to avoid failures due to signal
degradation, the optical amplifiers (OAs) for these networks need to be “agile” by
incorporating fast gain control and ability to adjust the OA operating conditions quickly in
response to changes in the network and number of wavelength channels. It was noted in an
industry report that deployment of ROADMs and agile EDFAs is correlated and has enabled
the transition from fixed to dynamic optical networks. Since 2010, ROADM and EDFA module
deployments were predominantly (over 85 %) dynamic and agile, and only a small number
(~15 %) had fixed characteristics.

4.3 ROADM subsystem technologies evolution
4.3.1 General
The evolution of ROADM component technologies is depicted in Figure 4. As shown,
ROADMs have progressed several technology generations: starting from simple filters in the
1990s to wavelength blockers (WBs) and planar lightwave circuits (PLC) based devices in the
early 2000s to the current wavelength selective switches (WSS). The number of ports in
wavelength selective switch based ROADMs has increased from 1 x 2 to 1 x 9 to 1 x 20. The
ROADM subsystem technologies are described below.
1 × 20+
1 × 9
1 × 4
1 × 2
Wavelength
selective switch
(WSS)
Fixed Planar lightwave
wavelength circuit (PLC)
filter
Wavelength
blocker(WB)
1998 2000 2002 2004 2006 2008 2011
IEC
Source: Brandon Collings, OFC 2011
Figure 4 – Evolution of ROADM technologies
4.3.2 Wavelength blocker based ROADMs
The era of ROADM began with wavelength blocker based ROADMs as shown in Figure 5.
These are capable of blocking individual wavelengths and passing the rest. This is very
convenient for 2-degree ROADMs. The add/drop function is passive, and there are no
cascaded AWGs in the express path. This improved cascadability because of broader
passbands. Moreover, built-in attenuation provided additional functionality, which is very
useful in equalizing the channel powers. A 2-degree ROADM is shown in Figure 6.
However, these devices still have coloured and directional add/drop fibres. Wavelength
blockers are two-port devices with one input port and one output port. The multi-wavelength
signal entering the input port is demultiplexed by a diffraction grating, and each wavelength
can be independently attenuated by using a MEMS or LC elements. The wavelength channels
can be suitably blocked by attenuation to greater than 30 dB.
These were the first devices to enable ROADMs in 2000. Control of wavelength channel
power and support of broadcast and select architecture made them very successful.
Increasing functionality
– 12 – IEC TR 62343-6-4:2017 © IEC 2017

In Out
WB
Add
OCM
AWG AWG
demux mux
Local drop Local add
IEC
Figure 5 – Wavelength blocker based ROADM architecture
OA
OA
WB
OPM
OPM 2 × 1 switch
2 × 1 switch
WB
OA OA
IEC
Figure 6 – 2-degree ROADM node architecture with wavelength blocker,
dynamic OAs and shared OCMs
4.3.3 Integrated planar lightwave circuits (IPLC) based ROADMs
A very popular early design for ROADMs is based on planar lightwave circuits (PLC)
technology. In this approach, an array of PLC based 2 x 2 switches is nested between a
demultiplexer and multiplexer (Figure 7) to provide optical bypass functionality based on
arrayed waveguide gratings. In AWGs, the multiplexing and demultiplexing of WDM channels
(shown as 1 to n) is done in waveguides integrated onto a single substrate, so fewer fibre
connections are needed. As technology advanced, so did the level of integration, with some
component vendors eventually integrating all the key functions required into a single substrate
using PLC technology.
West
East
2 × 2 switch
In
2 MPD
Out
n
VOA
Demux Mux
Local drop/ add
IEC
Figure 7 – PLC based ROADM architecture
The main limitation of this design is that it supports only two fibre route directions or pairs.
Moreover, because of the large number of parallel optical paths inside the device, multi-path
interference occurs. Because of the passband characteristics of the AWGs, cascading can
cause significant bandwidth narrowing. Finally, the add/drop fibres are predefined to be a
specific wavelength channel and a specific direction (coloured and directional). Despite
...