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Wavelength Division Multiplexing (WDM) –
Concepts and Components
Stavros Iezekiel
Department of Electrical and
Computer Engineering
University of Cyprus
• HMY 455
• Lecture 14
• Fall Semester 2014
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THE CONCEPT OF WAVELENGTH
DIVISION MULTIPLEXING
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In the history of optical fibre communications, multiplexing has played an important role in
exploiting the bandwidth potential of this medium.
TDM
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Multiplexing is an old technique, having been widely used in radio communications and
telephony, for example. The aim is for multiple channels to share the same medium.
Frequency division
multiplexing (FDM)
Time division
multiplexing (TDM)
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Early generations of optical fibre links used single-wavelengths, and multiplexing was
performed in the time-domain (TDM). However, the development of EDFAs has made
available a large enough spectral bandwidth so as to enable wavelength division
multiplexing (WDM).
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WHY WDM?
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In the 1550 nm window, there is several THz of potential bandwidth, some of which
also coincides with the spectral gain profile of an EDFA
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•
In early generations of optical fibre communication systems, only a single
wavelength was used, i.e. a very small fraction of the available optical bandwidth in
fibre
• N.B. Optical sources can only be modulated to a few tens of GHz (or Gb/s)
at the very most. For example, 40 Gb/s is considered a high end spec.
•
These limitations can be overcome with wavelength division multiplexing (WDM),
in which many different wavelengths share the same fibre.
•
In early WDM, wavelengths were widely separated. The ITU G.694.2 standard for
coarse WDM (CWDM) uses the wavelengths from 1270 nm through 1610 nm with a
channel spacing of 20 nm. Most CWDM systems cannot be supported by EDFAs and
so cover short ranges.
•
In dense WDM (DWDM), the channel spacing is much closer (e.g. the G.694.1
frequency grid, in which wavelengths are positioned in a grid having exactly
100 GHz (about 0.8 nm) spacing in optical frequency, with a reference frequency
fixed at 193.10 THz (1,552.52 nm).
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•
Optical amplifiers allow amplification over a wide enough wavelength range to support
many wavelength channels, each one of which can carry the same signal or be
modulated separately:
Attenuation (dB/km)
2.0
100 GHz
spacing
(0.8 nm)
1.0
0
1300
1500
1700
λ (nm)
λ (nm)
•
A spacing of 25 GHz has also
been demonstrated, leading
to a 160 channel system.
•
Wavelength spacing will be
limited by laser spectral width
and optical filter bandwidth.
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• Advantageous features of WDM include:
– Capacity upgrades. If each wavelength can support a bit rate of BT (e.g.
40 Gb/s), then system capacity is increased by a factor of N by using N
wavelengths. It is possible to upgrade a link by upgrading the terminal
equipment, not by replacing the fibre.
– Transparency. Each optical channel (i.e. wavelength) can support any
signal format (e.g. digital or analogue, time-division multiplexed etc.)
– Wavelength rerouting and switching. Can switch wavelengths and
route signals by wavelength, adding an extra dimension to network
design.
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WDM point-to-point link
In effect, each wavelength channel is like a separate link
λ1
λ1
RX1
TX1
TX2
TX3
λ2
λ2
EDFA
λ3
λ3
DMUX
MUX
RX2
RX3
λ1, λ2, λ3 ..... λN
λN
λN
TXN
Multiplexer
RXN
Demultiplexer
Tuneable laser diodes, bit rate of BT.
(or laser diodes with different fixed wavelengths)
Optical receivers
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WDM COMPONENTS
WDM Components
λ1
λ1
RX1
TX1
λ2
λ2
λ3
λ3
TX2
TX3
DMUX
MUX
RX2
RX3
λ1, λ2, λ3 ..... λN
λN
λN
RXN
TXN
WDM Multiplexer
• used to combine several different wavelengths
onto one fibre;
• should have low insertion loss.
WDM Components
λ1
λ1
RX1
TX1
λ2
λ2
λ3
λ3
TX2
TX3
DMUX
MUX
RX2
RX3
λ1, λ2, λ3 ..... λN
λN
λN
RXN
TXN
WDM demultiplexer
• used to remove several
different wavelengths from one fibre;
• need low loss and high selectivity.
WDM Components
λ1
λ1
RX1
TX1
λ2
TX2
TX3
λ2
EDFA
λ3
λ3
DMUX
MUX
RX2
RX3
λ1, λ2, λ3 ..... λN
λN
λN
RXN
TXN
Optical amplifier (EDFA)
• provides amplification over wavelength window at 1550 nm
• need broad spectral bandwidth, low crosstalk & flat gain
WDM Components
λ1
λ1
RX1
TX1
λ2
λ2
EDFA
TX2
λ3
λ3
TX3
DMUX
MUX
RX2
RX3
λ1, λ2, λ3 ..... λN
λN
λN
TXN
Tuneable laser diodes
• need large wavelength tuning range, high-speed tunability, high data rate
transmission, rigid wavelength stability and repeatability.
RXN
WDM Components: Filters
• Tuneable optical filter: used to filter out a single
wavelength for a photodetector to produce a
tuneable receiver:
λ
Input
λ
Passband tuned to
third wavelength
Output
Tuneable filter: Fabry-Perot resonator
Mirror
Input light
E(0)
RE(d)
Mirror: reflectivity R
R2E(2d)
Output light
d
Round-trip phase condition: 2kd = 2mπ
Piezoelectric
transducer
m = integer.
λ2
0
2d
Transmission
1
0
1 2 3 4 5
λ
channel 2 selected
WDM Broadcast & Select Link
TX1
λ1
λ1
λ1
RX1
λ2
TX2
TX3
λ2
EDFA
λ1
λ3
MUX
λN
TXN
1×N
RX2
RX3
λ5
RXN
1 × N coupler:
• all input wavelengths on each
o/p fibre, but at 1/N power
• has 10 log N dB loss
Optical receivers can tune
in to any broadcast wavelength
WDM Components: 2 × 2 fused-fibre coupler
P1
P0
P3
P2
 P2 
 × 100%
Splitting ratio = 
 P1 + P2 
e.g. 50% split is equivalent
to 3 dB coupler
 P0 

Excess loss =10 log10 
 P1 + P2 
Ideally, require 0 dB
excess loss
 Pi
Insertion loss =10 log10 
P
 j
 P3 
Crosstalk = 10 log10  
 P0 




Ideally, require
- ∞ dB crosstalk
Passive N × N star coupler
Power split out
equally amongst
all output fibres;
power in any one
output is:
P1
P2
N×N
PN
N/B. All input wavelengths are multiplexed onto each output
(P1 + P2 + ..... + PN)
N
1
Splitting loss = − 10 log10   = 10 log10 N
N




PIN 

Fibre star excess loss = 10 log10 N


 ∑ POUT ,i 
 i =1

Expressions for insertion loss and crosstalk same
as for 2 × 2 coupler
Fused-fiber star coupler
• Made by twisting, heating and
pulling fibres to fuse them
together
Star coupler made from 2 × 2 (3-dB) couplers:
PIN
0.25 PIN
0.25 PIN
0.25 PIN
0.25 PIN
WDM Components: Add/drop MUX
Add/drop multiplexers for selective wavelength
routing/extraction:
λ add
Add
λ
Input
λ
λadd
Drop
λ
Input
λ
λ drop
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WDM LOCAL AREA NETWORKS
WDM LANs: Basic Architectures
Dual rail (bus) configuration
Fibre
λ1
λ2
λN
TX
TX
RX
RX
Fibre
coupler
......
TX
RX
WDM LANs: Basic Architectures
Passive star configuration
λ1
λ2
TX
RX
RX
Star
Coupler
λN
TX
RX
TX
λ3
TX
RX
Examples of WDM Local Area Networks
(i) LAMBDANET
λ16
λ1
Node
1
Node
16
16 × 16
STAR
COUPLER
λ2
Node
2
λ3
Node
3
λ4
λ5
Node
4
Node
5
Individual LAMBDANET node:
Laser λ1
Electronics
interface
Photoreceivers
WDM
DMUX
In LAMBDANET, each node is equipped with one fixed transmitter (DFB
laser) emitting a unique wavelength and N fixed receivers. (N = no. of
nodes in the network). Incoming wavelengths are separated using a
wavelength demultiplexer, and each individual wavelength is sent to a
photoreceiver.
Each node’s transmitter is fixed on that node’s home wavelength.
• No tuneable components needed: relatively simple
system to build. Other advantage is contention-free
broadcast capability and support for one-to-one links as
well as multicasting.
• Disadvantage: not easily scaleable, needs N data
wavelengths for N nodes ⇒ cost per node is high.
(ii) Rainbow (IBM)
NODE 1
λ2
λ1
LASER TX.
STAR
COUPLER
NODE 2
TUNABLE FILTER RX.
λ1,λ2 …. λN
λ1,λ2 …. λN
λ1,λ2 …. λN
λN
NODE N
• Broadcast and select architecture.
• Each node broadcasts a unique wavelength and is able to select any
one of the wavelengths present in the network via a tuneable filter.
• Protocol used to set up connections is as follows:
(a) Idle receivers continually scan across all wavelengths.
(b) If node wishes to transmit to node , then it continually
sends a request (using λ1) to for a connection.
(c) When detects the request from , it locks its filter onto λ1.
(d) Node then sends a connection accept (using λ2) to node .
(e) When detects ’s acceptance, it locks its filter to λ2, and a
full duplex connection is established.
• LAMBDANET and Rainbow are star topologies with N
wavelengths assigned to N nodes (i.e. no wavelength re-use).
• Alternative topologies are possible, e.g. chain and ring.
• The following diagram shows a four-node ring network where
add-drop multiplexers (ADM) are employed to allow
wavelength re-use.
λ1 λ2 λ3
λ1 λ2 λ3
ADM
λ4,λ5,λ6
λ1
λ4
λ5
λ1
λ4
λ5
NODE 1
λ2,λ3,λ6
NODE 2
RING
NETWORK
NODE 3
λ1,λ3,λ5
λ2 λ4 λ6
λ2 λ4 λ6
λ1,λ2,λ4
NODE 4
λ3
λ5
λ6
λ3
λ5
λ6
Wavelength assignment table
• Alternative assignments are possible, as long as wavelengths
are not in contention with one another; e.g. next diagram
λ2 λ4 λ6
λ2 λ4 λ6
λ1,λ3,λ5
λ3
λ5
λ6
λ3
λ5
λ6
NODE 1
λ2,λ3,λ6
λ1,λ2,λ4
NODE 4
NODE 2
NODE 3
λ4,λ5,λ6
λ1 λ2 λ3
λ1 λ2 λ3
λ1
λ4
λ5
λ1
λ4
λ5
• Number of wavelengths added at each node equals the
number that are received: all add/drop multiplexers are
the same.
• Advantages: full interconnection between nodes is
possible, i.e. any node can talk to any other. One might
expect N 2 wavelengths would be needed to achieve this,
but by re-using wavelengths as shown above, need far
fewer. (e.g. for N = 4, only need 6, not 16).
Broadcast & Select Multihop Networks
• Disadvantage of single-hop networks such as Rainbow is the
need for rapidly tuneable lasers or receiver filters.
• Multi-hop networks overcome this problem by not having a
direct connection between all node pairs. This allows each
node to have a small number of fixed wavelength transmitters
and receivers, i.e. node complexity is reduced.
Example: Shufflenet Multihop Network
Passive star configuration
λ1 λ2
λ6 λ8
Node 1
Node 2
λ5 λ7
λ7 λ8
Star
Coupler
λ3 λ4
λ3 λ1
Node 3
Node 4
λ2 λ4
λ5 λ6
Example: Shufflenet Multihop Network
Dual rail configuration
λ1 λ2
TX
Node
RX
λ5 λ7
λ3 λ4
λ7 λ8
λ5 λ6
TX
TX
TX
RX
RX
RX
λ6 λ8
λ1 λ3
λ2 λ4
Logical interconnection pattern and wavelength assignment:
λ1
1
2
λ2
λ3
λ4
λ5
3
λ6
1
λ7
4
2
λ8
k columns each have pk nodes, where p is the number of fixed transceiver
pairs per node.
The total number of nodes is then N = kpk
The total number of wavelengths is Nλ = pN = kpk + 1
The maximum number of hops needed to reach a given node is Hmax = 2k - 1
For example, for p = 2, k =2, we have:
λ9
λ1
1
2
3
λ2
λ3
5
6
λ11
λ4
λ12
λ5
λ13
7
λ6
4
λ10
1
Total number of wavelengths is
Nλ = pN = kpk + 1 = 16
2
The maximum number of hops
needed to reach a given node is
Hmax = 2k - 1 = 2.2 - 1 = 3
3
λ14
λ7
λ8
8
λ15
λ16
Total number of nodes is
N = kpk = 2.22 = 8
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