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Table of Contents

Overview..............................................................................................................................................3Basic Terminologies of WDM...............................................................................................................3

WDM (Wavelength Division Multiplexing)....................................................................................3CWDM (Coarse Wavelength Division Multiplexing)......................................................................3DWDM (Dense Wavelength Division Multiplexing).......................................................................4

WDM Transmission System..................................................................................................................4Single Fiber Transmission...............................................................................................................4Dual Fiber Transmission.................................................................................................................4Upstream (Return) & Downstream (Forward)................................................................................5

WDM System Topologies......................................................................................................................5Network Topologies.......................................................................................................................5Ring Topology.................................................................................................................................5Node...............................................................................................................................................6

Filtration Technologies Applied to WDM.............................................................................................6Arrayed Waveguide Grating (AWG)................................................................................................6Fiber Bragg Grating (FBG)...............................................................................................................6Thin Film Filter (TFF).......................................................................................................................7

WDM Equipment..................................................................................................................................7Mux (Multiplexer)..........................................................................................................................7DeMux (De-Multiplexer)................................................................................................................7OADM (Optical Add-Drop Multiplexer)..........................................................................................8FWDM (Filter-Based Wavelength Division Multiplexer).................................................................8Compact WDM..............................................................................................................................8Banded Skip Filters.......................................................................................................................8

Ports on WDM Equipment...................................................................................................................8Common Port.................................................................................................................................9Express or Upgrade Port.................................................................................................................91310nm Port..................................................................................................................................91550nm Port..................................................................................................................................9Monitor Port..................................................................................................................................9

Do You Know All TheseTerminologies of WDMTechnology?

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Parameters of WDM Systems............................................................................................................10Wavelengths...............................................................................................................................10Channel.......................................................................................................................................10Pass Band....................................................................................................................................10Insertion Loss..............................................................................................................................11Polarization Dependent Loss (PDL).............................................................................................11Polarization Mode Dispersion (PMD).........................................................................................11Return Loss..................................................................................................................................11Passband Ripple..........................................................................................................................11Isolation......................................................................................................................................12Operation Temperature..............................................................................................................12Storage Temperature...................................................................................................................12Water Peak...................................................................................................................................12

New Network Technologies Based on WDM.....................................................................................13Wavelength Division Multiplexing-Passive Optical Network (WDM-PON)..................................13Optical 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 lowercost in nowaday telecommunications networks. By virtue of fame, WDM becomes a householdword now. Yet, most of the time, we only know what is "WDM" but do not really know WDMtechnology. Actually, there are various of terminologies used in WDM that are always a headachefor 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 usingdifferent optical wavelengths (i.e., colors) of laser light. It breaks white light passing through fiberoptic cable into all the colors of the spectrum, much like light passed through a prism creates arainbow. Every wavelength carries an individual signal that does not interfere with the otherwavelengths.

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 20nmchannel spacing. It is a technology of choice for cost efficiently transporting large amounts of datatraffic 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 channelspacing from 12.5 GHz to 200 GHz, among which 100 GHz is common. In practice, DWDM frequencyis usually converted to wavelength. DWDM typically has the capability to transport up to 80channels (wavelengths) in what is known as the Conventional band (C-band) spectrum, with all 80channels 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 twoidentical sets of wavelengths for both directions over a single fiber. Individual channels residing onthe 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 andthe other is used for the receive direction. In dual fiber transmission system, the same wavelength isnormally used in both the transmit and receive directions. The second fiber may serve as a backupfiber 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. Thedownstream direction is defined as communication originating at a service provider and sent to theservice 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 whichthe 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 particularlydesigned for the network. So, it is important to understand the intended network use whenselecting WDM products. Entire networks are often comprised of several kinds of sub-networktopologies.

Ring Topology

In metropolitan area networks, infrastructures are generally organized over a ring topology. Ringtopology is a type of network topology consisting of a closed loop. Fiber ring networks arecomprised 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 oftendual fiber systems. Contrast ring topology with an unclosed, end-to-end or point-to-point fiberspan.

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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 physicaltopology), upon which a logical topology is overlaid by establishing lightpath interconnectionsbetween the nodes. The use of WDM on the fiber side allows the node to be segmented or dividedinto 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 opticalMUX/DeMUX in WDM systems. AAWG have equivalent performance to standard TAWG but requireno electrical power, software or temperature.

Fiber Bragg Grating (FBG)

FBGs are versatile wavelength filters for multiplexing and demultiplexing WDM signals. They alsocan compensate for chromatic dispersion that can degrade the quality of the WDM signal in anoptical 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 havethe unique attributes that meet the stringent requirements of optical communication systems. Themain advantage of thin film filters is its ability to achieve high accuracy in processing in small devicesizes 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 thatde-multiplexes or splits optical transmissioncomprised of multiplexed wavelengths ontoindividual fibers assigned to each wavelength.

Note: In today's market, there are CWDMMux/DeMux products and DWDM Mux/DeMuxproducts. These products have the Mux and DeMuxinside 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 intoor 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 theThin Film Filter (TFF) technology. FWDM combines or separates light at different wavelengths in awide wavelength range and is extensively used in EDFA, Raman amplifiers, and WDM opticalnetworks.

Compact WDM

As the name suggests, these are multi-channel WDM products that have relatively small footprintsso 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 multiplebounce technology in which light reflects from each filter element directly onto the next filterelement instead of being collimated and launched into a fiber as in individual, discrete TFFcomponents. 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 havewide pass bands, which contain multiple channels. For example, DWDM Red/Blue C-band Filter isused to separate or combine Red and Blue band wavelength signals in C-band DWDM systems andhigh-power amplification systems. It is just like a regular FWDM, with the only difference that thewavelengths 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 channelsare 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. Theupgrade or express port on a CWDM Mux or DeMux is used to add, drop, or pass through additionalchannels which enables the cascading of two CWDM Mux/DeMux modules, doubling the channelcapacity on the common fiber link.

For DWDM products, the purpose of an upgrade port is to be able to add, drop, or pass throughC-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 foradditional 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 andrequest the 1310nm port. The 1310nm port is used in some legacy networks and sometimes as areturn path. If an existing legacy network is using 1310nm port and they have exhausted all fibersand are looking for ways to increase their network capacity they can add in other CWDMwavelengths on to the same fiber while still allowing the use of the 1310nm port. Meanwhile, it cancarry 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, LXoptics, 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 getsdemuxed 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 metersor network analyzers. Network administrators will use this to test of monitor if a signal has failed orchanged without having to interrupt the existing network.

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Parameters of WDM SystemsYou 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 thesame phase in consecutive cycles of a wave. The wavelength λm of monochromatic light travellingin 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, thewavelength specifications for optical filters, and the wavelengths of optical transmission channelsover 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 thechannels 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 theparameters of WDM filters. In practice, it is the tolerance of the filter for laser drift away from thecenter wavelength. For example, a typical pass band for CWDM filters is ± 6.5 nm about the centerwavelength. So a 1551nm laser could operate within a range of 1544.5 nm to 1557.5 nm withoutencountering extra channel loss.

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Insertion Loss

Insertion loss is the attenuation caused by the insertion of WDM filter in an optical transmissionsystem. 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 thechannel port with the highest loss. In WDM networks, insertion loss is one of several contributors tothe total loss of the communication link. Thin film filters exhibit fairly wide manufacturing variancein 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 thelargest difference in maximum insertion loss occurring at all states of optical polarization. PDL for aWDM 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 opticalreceiver to be unable to interpret the signal correctly, and results in high bit error rates. It is anotherpolarization 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 transmissionline or optical fiber of WDM systems. A large value of return loss is desirable for preventingproblems with source lasers and reducing transmitted loss. Return loss for a WDM product is thesmallest, measured return loss at all ports.

Passband Ripple

Passband ripple is defined as the maximum peak-to-peak loss variation within the passband of onechannel.

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Isolation

Isolation is a measure of light at an undesired wavelength at any given point. Expressed in dB, it isthe difference of the maximum insertion loss within the filter pass band and the minimum lossoccurring within other filtering pass bands. Isolation is measured by applying a swept optical powersource to the filter's common port and measuring the loss within the filter's pass band and the passbands 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-AdjacentChannel Isolation.

Operation Temperature

Operating Temperature (°C) is the ambient temperature range over which the device's performancespec can be met.

Storage Temperature

Storage Temperature (°C) is the ambient temperature range over which the device can be storedwithout affecting its intended application afterwards.

Water Peak

At 1385 nm, overtones of the OH- contamination-related molecular absorption manifest themselvesin the well known "water peak". Attenuation at and near the water peak can exceed 2 dB/kmcompared 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 WDMOn 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 physicalP2MP fiber infrastructure that contains no active components (i.e., PON). WDM-PON allowsoperators 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 carryingclient signals.

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The knowledge is endless, thus, the word you see on this page as only the tip of an iceberg. Butonce 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 beshipped in 12 hrs. Visit www.fs.com or contact us over sales@fs.com for more details.

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Notice: This document is for informational purposes only and does not set forth any warranty, expressed orimplied, 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 noresponsibility 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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