As an unprecedented opportunity to dramatically increase the bandwidth capacity, WDM (Wavelength Division Multiplexing) technology is an ideal solution to get more bandwidth and lower cost in nowaday telecommunications networks. By virtue of fame, WDM becomes a household word now. Yet, most of the time, we only know what is "WDM" but do not really know WDM technology. Actually, there are various of terminologies used in WDM that are always a headache for us.

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Table of Contents
Overview··········································································································· 3
Basic Terminologies of WDM··················································································· 3
WDM (Wavelength Division Multiplexing)································································· 3
CWDM (Coarse Wavelength Division Multiplexing)·······················································3
DWDM (Dense Wavelength Division Multiplexing)······················································· 4
WDM Transmission System····················································································· 4
Single Fiber Transmission····················································································· 4
Dual Fiber Transmission······················································································· 4
Upstream (Return) & Downstream (Forward)····························································· 5
WDM System Topologies························································································ 5
Network Topologies····························································································5
Ring Topology··································································································· 5
Node··············································································································6
Filtration Technologies Applied to WDM····································································· 6
Arrayed Waveguide Grating (AWG)··········································································6
Fiber Bragg Grating (FBG)·····················································································6
Thin Film Filter (TFF)··························································································· 7
WDM Equipment··································································································7
Mux (Multiplexer)······························································································ 7
DeMux (De-Multiplexer)······················································································ 7
OADM (Optical Add-Drop Multiplexer)····································································· 8
FWDM (Filter-Based Wavelength Division Multiplexer)··················································8
Compact WDM································································································· 8
Banded Skip Filters·····························································································8
Ports on WDM Equipment······················································································ 8
Common Port··································································································· 9
Express or Upgrade Port······················································································ 9
1310nm Port···································································································· 9
1550nm Port···································································································· 9
Monitor Port···································································································· 9
Do You Know All These
Terminologies of WDM
Technology?

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Parameters of WDM Systems···················································································10
Wavelengths···································································································· 10
Channel·········································································································· 10
Pass Band········································································································10
Insertion Loss··································································································· 11
Polarization Dependent Loss (PDL)·········································································· 11
Polarization Mode Dispersion (PMD)········································································11
Return Loss······································································································11
Passband Ripple································································································ 11
Isolation··········································································································12
Operation Temperature······················································································· 12
Storage Temperature·························································································· 12
Water Peak······································································································ 12
New Network Technologies Based on WDM·································································13
Wavelength Division Multiplexing-Passive Optical Network (WDM-PON)···························· 13
Optical Transport Network (OTN)············································································13

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Overview
As an unprecedented opportunity to dramatically increase the bandwidth capacity, WDM
(Wavelength Division Multiplexing) technology is an ideal solution to get more bandwidth and lower
cost in nowaday telecommunications networks. By virtue of fame, WDM becomes a household
word now. Yet, most of the time, we only know what is "WDM" but do not really know WDM
technology. Actually, there are various of terminologies used in WDM that are always a headache
for us. Now, let's see what are they.
Basic Terminologies of WDM
First of all, there are three basic terminologies you should clearly know.
 WDM (Wavelength Division Multiplexing)
A technology that multiplexes a number of optical carrier signals onto a single optical fiber by using
different optical wavelengths (i.e., colors) of laser light. It breaks white light passing through fiber
optic cable into all the colors of the spectrum, much like light passed through a prism creates a
rainbow. Every wavelength carries an individual signal that does not interfere with the other
wavelengths.
 CWDM (Coarse Wavelength Division Multiplexing)
CWDM is a specific WDM technology defined by the ITU (International Telecommunication Union)
in ITU-T G.694.2 spectral grids, using the wavelengths from 1270 nm to 1610 nm within a 20nm
channel spacing. It is a technology of choice for cost efficiently transporting large amounts of data
traffic in telecoms or enterprise networks.

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 DWDM (Dense Wavelength Division Multiplexing)
DWDM is a specific WDM technology also defined by the ITU but in ITU-T G.694.1 spectral grids.
The grid is specified as frequency in THz, anchored at 193.1 THz, with a variety of specified channel
spacing from 12.5 GHz to 200 GHz, among which 100 GHz is common. In practice, DWDM frequency
is usually converted to wavelength. DWDM typically has the capability to transport up to 80
channels (wavelengths) in what is known as the Conventional band (C-band) spectrum, with all 80
channels in the 1550 nm region.
WDM Transmission System
When referring to fiber optic transmission in WDM system, you should know these:
 Single Fiber Transmission
Single fiber, namely bi-directional communication on one single fiber. This system utilizes two
identical sets of wavelengths for both directions over a single fiber. Individual channels residing on
the single fiber system may propagate in either direction.
 Dual Fiber Transmission
Dual fiber, namely comprised of two single fibers, one fiber is used for the transmit direction and
the other is used for the receive direction. In dual fiber transmission system, the same wavelength is
normally used in both the transmit and receive directions. The second fiber may serve as a backup
fiber as in a redundant system, or it may provide an optical path in the opposite direction.

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 Upstream (Return) & Downstream (Forward)
The direction of a communication signal can be refered using these two terminologies. The
downstream direction is defined as communication originating at a service provider and sent to the
service user. Upstream is in the opposite direction.
WDM System Topologies
The topology in which WDM systems are used plays a key role in determining the extent to which
the WDM network is utilized. The related terminologies are:
 Network Topologies
WDM products bring higher efficiency to fiber networks through multiple channel usage of fiber.
Networks are identified by their fiber layout or topology. Network topologies such as Mesh, Ring,
P2P (Point-to-Point), and P2MP (Point-to-Multipoint) will sometimes use WDM products particularly
designed for the network. So, it is important to understand the intended network use when
selecting WDM products. Entire networks are often comprised of several kinds of sub-network
topologies.
 Ring Topology
In metropolitan area networks, infrastructures are generally organized over a ring topology. Ring
topology is a type of network topology consisting of a closed loop. Fiber ring networks are
comprised of a series of fiber spans that terminate at network nodes spread throughout the loop.
Each node in the ring will connect to two, and only two, adjacent nodes. Ring networks are often
dual fiber systems. Contrast ring topology with an unclosed, end-to-end or point-to-point fiber
span.

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 Node
In network topology, a node is a termination of a single branch or multiple branches of the network.
A WDM network consists of a set of nodes, physically interconnected by optical fiber (the physical
topology), upon which a logical topology is overlaid by establishing lightpath interconnections
between the nodes. The use of WDM on the fiber side allows the node to be segmented or divided
into additional serving areas thus expanding the customer base and available bandwidth.
Filtration Technologies Applied to WDM
There are three competing filtration technologies applied to WDM. They are:
 Arrayed Waveguide Grating (AWG)
AWG, including Athermal AWG (AAWG) and Thermal AWG (TAWG), is commonly used as optical
MUX/DeMUX in WDM systems. AAWG have equivalent performance to standard TAWG but require
no electrical power, software or temperature.
 Fiber Bragg Grating (FBG)
FBGs are versatile wavelength filters for multiplexing and demultiplexing WDM signals. They also
can compensate for chromatic dispersion that can degrade the quality of the WDM signal in an
optical fiber.

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 Thin Film Filter (TFF)
Thin film filters were adopted very early on and have been widely deployed since because they have
the unique attributes that meet the stringent requirements of optical communication systems. The
main advantage of thin film filters is its ability to achieve high accuracy in processing in small device
sizes when compared it to competing technologies.
WDM Equipment
To build a WDM system, these WDM equipment are required:
 Mux (Multiplexer)
WDM multiplexer is a device that multiplexes or combines optical signals of different wavelengths
(colors) together on one single fiber.
 DeMux (De-Multiplexer)
In contrast to multiplexer, DeMux is a device that
de-multiplexes or splits optical transmission
comprised of multiplexed wavelengths onto
individual fibers assigned to each wavelength.
Note: In today's market, there are CWDM
Mux/DeMux products and DWDM Mux/DeMux
products. These products have the Mux and DeMux
inside and comes in a package like 1RU 19"
rackmont, LGX box and ABS module etc.

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 OADM (Optical Add-Drop Multiplexer)
OADM is a device used in WDM systems for multiplexing and routing different channels of light into
or out of a single fiber.
 FWDM (Filter-Based Wavelength Division Multiplexer)
Filter-based Wavelength Division Multiplexer (FWDM) is a kind of WDM multiplexer based on the
Thin Film Filter (TFF) technology. FWDM combines or separates light at different wavelengths in a
wide wavelength range and is extensively used in EDFA, Raman amplifiers, and WDM optical
networks.
 Compact WDM
As the name suggests, these are multi-channel WDM products that have relatively small footprints
so that they can provide more channels with a device footprint small enough to fit within a FOSC
(Fiber Optic Splice Closure), splice tray or splice holder. These products utilize a free-space multiple
bounce technology in which light reflects from each filter element directly onto the next filter
element instead of being collimated and launched into a fiber as in individual, discrete TFF
components. In addition, bend insensitive fiber permits the use of smaller housings for joining
(concatenating) individual TFFs into a multi-channel product.
 Banded Skip Filters
Banded skip filters are used to build BWDM (Band WDM) products. These filters are TFFs that have
wide pass bands, which contain multiple channels. For example, DWDM Red/Blue C-band Filter is
used to separate or combine Red and Blue band wavelength signals in C-band DWDM systems and
high-power amplification systems. It is just like a regular FWDM, with the only difference that the
wavelengths are split in Red/Blue filter while bonded in WDM.
Ports on WDM Equipment
Ports on WDM equipment, do you really know their functions?

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 Common Port
The connection point of a WDM product where combined channels appear. For a MUX product,
combined channels are transmitted from the common port. For a DEMUX, the combined channels
are received at the common port.
 Express or Upgrade Port
For CWDM products, there will normally be either an upgrade or an express port, but not both. The
upgrade or express port on a CWDM Mux or DeMux is used to add, drop, or pass through additional
channels which enables the cascading of two CWDM Mux/DeMux modules, doubling the channel
capacity on the common fiber link.
For DWDM products, the purpose of an upgrade port is to be able to add, drop, or pass through
C-band DWDM channels not already in use, namely only channels that reside in the band 1530 –
1565 nm. If the DWDM product also has an express port, then that port is normally used for
additional channels residing outside the C-band, such as most of the CWDM channels.
 1310nm Port
The 1310nm port is a wide band optic port added to other specific CWDM wavelengths in a module.
For example if an 8 channel CWDM is called out it may use wavelengths 1470 nm to 1610 nm and
request the 1310nm port. The 1310nm port is used in some legacy networks and sometimes as a
return path. If an existing legacy network is using 1310nm port and they have exhausted all fibers
and are looking for ways to increase their network capacity they can add in other CWDM
wavelengths on to the same fiber while still allowing the use of the 1310nm port. Meanwhile, it can
carry LR optics, LX optics etc.
 1550nm Port
Similar to 1310nm port, allows a legacy 1550nm signal to pass and can carry ER optics, ZR optics, LX
optics, ZX optics etc.
 Monitor Port
This port is used to monitor or test the power signal coming out of a Muxed CWDM or before it gets
demuxed from the signal coming through the fiber network usually at a 5% or less power level.
Generally, it can be connected with measurement or monitoring equipment, such as power meters
or network analyzers. Network administrators will use this to test of monitor if a signal has failed or
changed without having to interrupt the existing network.

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Parameters of WDM Systems
You should also know these parameters when operating a WDM system:
 Wavelengths
Wavelength is the distance, measured in the direction of propagation, between two points of the
same phase in consecutive cycles of a wave. The wavelength λm of monochromatic light travelling
in a optical fiber is expressed:
 λm = λ / n = v / f
 λ = optical wavelength in a vacuum
 n = the refractive index of the dielectric medium
 v = phase velocity, given by c / n
 c = the speed of light in a vacuum: 2.99792458 X 108 m/s
 f = the optical frequency.
Note: In WDM practice, wavelengths such as the wavelength of a communications laser, the
wavelength specifications for optical filters, and the wavelengths of optical transmission channels
over fiber are all given as λ, the wavelength in nanometers as would occur in a vacuum.
 Channel
In WDM systems, each input channel is assigned a unique wavelength (i.e. color of light), thus the
channels can traverse the fiber "in parallel".
 Pass Band
A pass band is the range of frequencies or wavelengths that can pass through a filter. It is one of the
parameters of WDM filters. In practice, it is the tolerance of the filter for laser drift away from the
center wavelength. For example, a typical pass band for CWDM filters is ± 6.5 nm about the center
wavelength. So a 1551nm laser could operate within a range of 1544.5 nm to 1557.5 nm without
encountering extra channel loss.

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 Insertion Loss
Insertion loss is the attenuation caused by the insertion of WDM filter in an optical transmission
system. It is normally specified as the maximum insertion loss occurring across the filter pass band.
The insertion loss of a WDM product is given as the maximum insertion loss occurring at the
channel port with the highest loss. In WDM networks, insertion loss is one of several contributors to
the total loss of the communication link. Thin film filters exhibit fairly wide manufacturing variance
in their insertion loss values and are screened prior to use in WDM products.
 Polarization Dependent Loss (PDL)
The loss exhibited by a WDM filter is dependent on the optical polarization of the light. PDL is the
largest difference in maximum insertion loss occurring at all states of optical polarization. PDL for a
WDM product is specified as the largest allowed PDL for any channel.
 Polarization Mode Dispersion (PMD)
PMD is an important linear phenomenon occurring inside optical fibers, which can cause the optical
receiver to be unable to interpret the signal correctly, and results in high bit error rates. It is another
polarization effects that lead to impairments in the long-haul optical fiber transmission systems.
 Return Loss
Return loss is the loss of power in the signal returned/reflected by a discontinuity in a transmission
line or optical fiber of WDM systems. A large value of return loss is desirable for preventing
problems with source lasers and reducing transmitted loss. Return loss for a WDM product is the
smallest, measured return loss at all ports.
 Passband Ripple
Passband ripple is defined as the maximum peak-to-peak loss variation within the passband of one
channel.

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 Isolation
Isolation is a measure of light at an undesired wavelength at any given point. Expressed in dB, it is
the difference of the maximum insertion loss within the filter pass band and the minimum loss
occurring within other filtering pass bands. Isolation is measured by applying a swept optical power
source to the filter's common port and measuring the loss within the filter's pass band and the pass
bands of other filters. When other filters are those with pass bands nearest to the filter's pass band,
it is called the Adjacent Channel Isolation. For the remaining ports, it is called the Non-Adjacent
Channel Isolation.
 Operation Temperature
Operating Temperature (°C) is the ambient temperature range over which the device's performance
spec can be met.
 Storage Temperature
Storage Temperature (°C) is the ambient temperature range over which the device can be stored
without affecting its intended application afterwards.
 Water Peak
At 1385 nm, overtones of the OH- contamination-related molecular absorption manifest themselves
in the well known "water peak". Attenuation at and near the water peak can exceed 2 dB/km
compared with attenuation values of <0.2 dB/km as can occur in the C-band (1530 – 1565 nm).

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New Network Technologies Based on WDM
On the basic of WDM, some new network technologies have emerged, such as:
 Wavelength Division Multiplexing-Passive Optical Network (WDM-PON)
WDM-PON is an innovative concept for access and backhaul networks. It uses WDM over a physical
P2MP fiber infrastructure that contains no active components (i.e., PON). WDM-PON allows
operators to deliver high bandwidth to multiple endpoints over long distances.
 Optical Transport Network (OTN)
OTN was designed to provide support for optical networking using wavelength-division multiplexing
(WDM) unlike its predecessor SONET/SDH. It is able to provide functionality of transport,
multiplexing, switching, management, supervision and survivability of optical channels carrying
client signals.

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The knowledge is endless, thus, the word you see on this page as only the tip of an iceberg. But
once you know all of them, you may better know about WDM systems.
Warm Tips: Fiberstore offers a variety of WDM Mux/DeMux equipment that are in stock and can be
shipped in 12 hrs. Visit www.fs.com or contact us over sales@fs.com for more details.
Contact Us
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China
Tel: +86 (755) 8300 3611
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Email: sales@fs.com
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Email: sales@fs.com
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W1B 3HH, United Kingdom
Tel: +44 2081441980
Notice: This document is for informational purposes only and does not set forth any warranty, expressed or
implied, concerning any equipment, equipment features, or service offered or to be offered by Fiberstore.
Fiberstore reserves the right to make changes to this document at any time, without notice, and assumes no
responsibility for its use. This information document describes features that may not be currently available.
Contact a Fiberstore sales team for information on feature and product availability.
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