Wavelength -Division Multiplexing (WDM) and Dense -WDM (D -WDM) technologies: an overview Author s: Topliceanu Mihai -Adrian Fotache Dan Mihai… [602831]

Optoelectric Devices
Wavelength -Division Multiplexing (WDM) and Dense -WDM (D -WDM)
technologies: an overview

Author s: Topliceanu Mihai -Adrian
Fotache Dan Mihai
Professor coordinator: Valentin Feies

Contents

1. Introduction ………………………….. ………………………….. ……….. 1
1.1 Fiber -optic communication ………………………….. ……………. 1
1.2 Optical Multiplexing ………………………….. ……………………… 2
2. Wave division multiplexing ………………………….. ……………….. 4
2.1 WDM in telecommunications ………………………….. ………… 5
3. Dense -wavelength division multiplexing …………………………. 7
4. Conclusions ………………………….. ………………………….. ……… 10
5. References ………………………….. ………………………….. ………. 11

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1. Introduction

1.1 Fiber -optic communication
Fiber -optic communication is a method of transmitting information from one place
to another by sending pulses of light through an optical fiber. The light forms an
electromagnetic carrier wave that is modulated to carry information.Fiber is p referred
over electrical cabling when high bandwidth, long distance, or immunity to
electromagnetic interference are required.
Optical fiber was first developed in the 1970s as a transmission medium and it has
replaced other transmission media such as copp er wire.
Optical fiber is used by many telecommunications companies to transmit
telephone signals, Internet communication, and cable television signals. Due to
much lower attenuation and interference, optical fiber has large advantages over
existing copper wire in long -distance and high -demand applications. However,
infrastructure development within cities was relatively difficult and time -consuming,
and fiber -optic systems were complex and expensive to install and operate. Due to
these difficulties, fiber -optic communication systems have primarily been installed in
long-distance applications, where they can be used to their full transmission
capacity, offsetting the increased cost.
Today, optical fiber has been used to develop new high speed communication
systems that transmit information as light pulses. The development of optical fiber
has revolutionized the telecommunications industry.

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1.2 Optical Multiplexing
Multiplexing is the process of combining two or more input signals into a single
transmission. At receiver’s end, the combined signals are separated into distinct
separate signal s.Multiplexing enhances efficiency use of bandwidth.
Multiplexers are hardware components that combine multiple analog or digital
input signals into a single l ine of transmission. And at the receiver’s end, t he
multiplexers are known as de multiplexers – performing reverse function of
multiplexers .
Optical multiplexer and de multiplexer a re required to multiplex and de multiplex
various wavelengths onto a single fiber link.Each specific I/O will be used for a single
wavelength.One optical filter system can act as both multiplexer and de multiplexer .

Multiplexing system scheme

Optical multiplexer and de multiplexer are basically passive optical filter systems,
which are arranged to process specific wavelengths in and out of the transport
system.
Laser 1
Laser 2
Laser 3
Multiplexer
Optical Fiber
Demultiplexer
Regenerator +
Receiver

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There are different techniques in multiplexing light signals o nto a single optical
fiber link:
• Optical Time Division Multiplexing (OTDM) , s eparating
wavelengths in time
• Wavelength division multiplexing (WDM) , each channel is
assigned a unique carrier frequency
• Coarse Wavelength Division Multiplexing (CWDM)
• Dense Wavelength Division Multiplexing (D-WDM), us es a much
narrower channel spacing, therefore, many more wavelengths are
supported.
• Code Division Multiplexing , a lso used in microwave
transmission.Spectrum of each wavelength is assigned a unique
spreading code.Channels overlap both in time and frequency
domains but the code guide each wavelength.

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2. Wave division multiplexing

One of the most promising concepts for high capacity communication systems is
waveleng th division multiplexing (WDM). Each communication channel is allocat ed to
a different frequency and multiplexed onto a single fiber. At the destination
wavelengths are spatially separated to different receiver locations.
WDM is mostly used for optical fiber communications to transmi t data in several
channels with slightly different wavelengths. It is achieved through refraction and
diffraction technique for combining and separating optical signals of different wave
lengths . In this way, the transmission capacities of fiber -optic links can be increased
strongly, so that most efficient use is made not only of the fibers themselves but also
of the active components such as fiber amplifiers. Apart from telecom, wavelength
division multi plexing is also used for interrogating multiple fiber -optic s ensors within a
single fiber.

Basic WDM scheme

First wavelength division multiplexing systems combined only two signals but
modern WDM systems can handle up to 160 signals .A multiplexer is used at the
transmi tter end system to combine the s ignals together and a demultiplexer is used
at the receiver end s ystem to split the signals .

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The WDM innovation represents a revolution inside the optical communications
revolution, allowing the latter to continue its exponential growth. From last few years
wavelength divi sion multiplexing plays a vital role for large capacity transmission
systems .

2.1 WDM in telecommunications
Theoretically, the full data transmission capacity of a fiber could be exploited with
a single data channel of very high data rate, corresponding to a very large channel
bandwidth. However, given the enormous availabl e bandwidth of the low -loss
transmission window of single -mode fibers, this would lead to a data rate which is
far higher than what can be handled by optoelectronic send ers and receivers. Also,
various types of dispersion in the transmission fiber would have very detrimental
effects on such wide -bandwidth channels, so that the transmission distance would
be strongly restricted. Wavelength division multiplexing solves thes e problems by
keeping the transmission rates of each c hannel at reasonably low levels and
achieving a high total data rate by com bining several or many channels.
Two different versions of WDM, defined by standards of the International
Telecommunication Union (ITU), are distinguished:
• Coarse wavelength division multiplexing (CWDM) , uses a relativel y small
number of channels and a large channel spacing .The nominal wavelengths
range from 1310 nm to 1610 nm. The wavelength tolerance for the
transmitters is fair ly large so that unstabilized DFB lasers can be used. The
single -channel bit rate is usually between 1 and 3.125 Gbit/s. The resulting
total data rates are useful within metropolitan areas, as long as broadband
technologies are not widespread in households ( fiber to the home).

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• Dense wavelength division multiplexing (DWDM) is the extended method
for very large da ta capacities, as required e in the Internet backbone. It uses
a large n umber of channels ( 40, 80, or 160), and a correspondingly small
channel spacing of 12.5, 25, 50 or 100 GHz. All optical channel frequencies
refer to a reference frequency which has been fixed at 193.10 THz (1552.5
nm). The transmitters have to meet tight wavelength tolerances. Typically,
they are temperature -stabilized DFB lasers. The single -channel bit rate can
be between 1 and 100 Gbit/s, and in the future even higher.
Due to the wide amplification bandwidth of erbium -doped fiber amplifiers, all
channels can often b e amplified in a single device, except in cases where the full
range of CWDM wavelengths is used . However, problems can arise from the
variation of gain with wavelength or from interaction of the data channels (crosstalk,
channel inte rference) via fiber nonlinearities. Enormous progres s has been achieved
with a combination of various techniques, such as the development of very
broadband (double -band) fiber amplifiers, gain flattening filters, nonlinear data
regeneration and the like. The system parameters such as channel bandwidth,
chan nel spacing, transmitted power levels, fiber and amplifier types, modulation
formats, dispersion compensation schemes, etc., need to be well balanced to
achie ve optimum overall performance.
Even for existing fiber links with only one or a few channels per fiber, it can make
sense to replace senders and receivers for operation with more channels, as this can
be cheaper than replacing the whole system with a system with a higher
transmission capacity. In fact, this approach often eliminates the need to instal l
additional fibers, even though the demand on transmission capac ities is increasing
enormously.
Apart from increasing the transmission capacity, wavelength division multiplexing
also adds flexibility to complex communication systems. In particular, differ ent data
channels can be injected at different locations in a system, and other channels can
be extracted. For such operations, add –drop multiplexers can be used, which allow
one to add or drop data channels based on their wavelengths. Reconfigurable add –
drop multiplexers make it possible to reconfigure the system flexibly so as to provide
data connections between a larg e number of different stations.

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In many cases, time division multiplexing (TDM) can be an alternative to
wavelength division multiplexing. Here, different channels are distinguished by arrival
time rather than by wavelength.
3. Dense -wavelength division multiplexing

Dense wavelength division multiplexing (DWDM) is wavelength division
multiplexing (WDM) with typical channel spacing of 100 GHz for 40 channels and 50
GHz for 80 channels. Each channel contains a TDM (time division multiplex) signal.
And each of up to 80 channels can carry 2.5 Gbps for a total of 200 billion bits per
second by the optical fiber. These signals use the 3rd transmiss ion window, called
the C -Band, meaning the light beam wavelengths are between 1530nm to 1565nm.
DWDM is sometimes called wave division multiplexing (WDM) and WDM is
growing denser as the technology evolves. Thus, the two terms are often used
synonymously.
The difference between WDM and dense wavelength division multi plexing
(DWDM) is fundamentally one of only degree. DWDM spaces the wavelengths more
closely than does WDM, and therefore has a greater overall capacity .
DWDM has a number of other notable f eatures. These include the ability to
amplify all the wavelengths at onc e without first converting them to electrical signals,
and the ability to carry signals of different spee ds and types simultaneously and
transparently over the fiber (protocol and bit rate independence).
A basic Dense Wavelength Division Multiplexing contains five main components:
• DWDM Terminal Multiplexer: This device contains a one wavelength
converting transponder for each wavelength carried. It receives an input
optical signal, converts it to an electrical signal and then retransmits it as an
optical signal (a process abbreviated as O/E/O) using a 1550 nm laser beam.
The MUX (multiplexer) takes a number of 1550 nm optical signals and places
them on a single optical fiber. This terminal m ultiplexer may also contain an
EDFA (Erbium Doped Fiber Amplifier) to amplify the optical signal.

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• Intermediate Line Repeater: These are amplifiers placed every 80 to 100
kilometers to compensate for loss of optical power; amplification is done by an
EDFA, usually consisting of several amplifier stages.
• Intermediate Optical Terminal, or Optical Add/Drop Multiplexer: This is a
remote site amplifier placed where the signal may have traveled up to 140
kilometers; diagnostics and telemetry signals are extracted or inserted.
• DWDM Terminal Demultiplexer: This device breaks the multi -wave signal
back into individual signals; these may be sent to O/E/O output transponders
before being relayed to their intended destinations, i.e. client -layer systems.
• Optical Supervis ory Channel (OSC): This channel carries information about
the multi -wave optical signal and may provide data about conditions at the site
of the intermediate line repeater (component 2 above).

Basic D -WDM scheme

DWDM systems have to maintain more stable wavelength or frequency than those
needed for CWDM because of the closer spacing of the wavelengths. Precision
temperature control of laser transmitter is required in DWDM systems to prevent
"drift" off a very narr ow frequency window of the order of a few GHz. In addition,
since DWDM provides greater maximum capacity it tends to be used at a higher
level in the communications hierarchy than CWDM, for example on the Internet
backbone and is therefore associated with higher modulation rates, thus creating a

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smaller market for DWDM devices with very high performance. These factors of
smaller volume and higher performance result in DWDM systems typically being
more expensive than CWDM.
Use of DWDM allows providers to off er services such as e -mail, video, and
multimedia carried as Internet Protocol(IP) data over asynchronous transfer
mode(ATM) and voice carried over SONET/SDH.Despite the fact that this formats
provide unique capabilities, all three can be transported over optical layer using
DWDM.This unifying capability allow the providers to respond to user demands using
only one network.
Recent innovations in DWDM transport systems include pluggable and software –
tunable transceiver modules capable of operating on 40 or 8 0 channels. This
dramatically reduces the need for discrete spare pluggable modules, when a handful
of pluggable devices can handle the full range of wavelengths.

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4. Conclusions

We live in a time of technological growth, in which users need high speed access
to information.There is no other physical medium that can carry more data over
greater distances than optic fiber.But fiber is precious.That is why we use WDM and
DWDM as means of transmission.
Wavelength Division Multiplexing (WDM) uses multiple wavelengths (colors of
light) to transport signals over a single fiber. WDM 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 ( color) carries an individual signal that
does not interfere with the other wavelengths (colors). In simple terms: WDM creates
virtual fibers – the best and simplest way to multiply fiber capacity.
WDM is now recognized as the Layer 1 transport technology i n all tiers of the
network. It offers low -cost transport for all applications and services, scales easily in
terms of capacity and reach and provides rapid protection against any fiber plant
failure. Fully transparent to any bit rate and protocol, WDM is t he natural integration
layer for modern networks. This allows networks to become more manageable,
operate more efficiently and transport considerably higher bandwidth for high -volume
data transmission.
WDM has revolutionized the cost per bit of transport. But thanks to DWDM, fiber
networks can carry multiple Terabits of data per second over thousands of
kilometers – at cost points unimaginable less than a decade ago. State -of-the-art
DWDM systems support up to 192 wavelengths on a single pair of fiber, with each
wavelength transporting up to 100Gbit/s capacity – 400Gbit/s and one Terabit/s on
the horizon.
DWDM provides ultimate scalability and reach for fiber networks. Without the
capacity and reach of DWDM systems, most Web 2.0 and cloud -computing solutions
today would not be feasible. Establishing transport connections as short as tens of
kilometers to enabling nationwide and transoceanic transport networks, DWDM is
the workhorse of all the bit -pipes keeping the data highway alive and expanding.

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5. Referen ces

1. Agrawal, G.P., Lightwave Technology Telecommunication Systems , Wiley (2005)
2. Agrawal, G.P., Fiber -Optic Communications Systems , 3rd ed., Wiley (2002)
3. WDM technologies: Optical Networks , vol. III, Elsevier Academic Press (2004)
4. Introduction to DWDM Technology, CISCO (2001)
5. Optical WDM Networks, Concepts and Design Principles, Jun Zheng, Hussein Mouftah
6. https://www.rp -photonics.com/wavelength_division_mul tiplexing.html
7. https://en.wikipedia.org/wiki/Wavelength -division_multiplexing