Design and Simulation WDM

 With the development of telecommunication, the requirements of the
transmission capacity and service categories are becoming bigger and bigger,
under this background, WDM technology emerged. What is WDM?

 Understand and master the basic concepts and transmission modes, structure of WDM;
 Understand WDM transmission media;
 Understand technical principle and key technologies of WDM;
 Understand technology specification for WDM system.
 Simulation WDM system.

What's WDM？
.
Free Way
Gas Station
Patrol Car

What's WDM？
 A short definition of a WDM system, which both applies to
Dense Wavelength-Division Multiplexing (DWDM), and
Coarse Wavelength-Division Multiplexing (CWDM) is this:
 In fiber-optic communications, wavelength-division multiplexing (WDM)
is a technology which multiplexes multiple optical carrier signals on a
single optical fiber by using different wavelengths (Lambda’s) of laser light
to carry different signals.

Increasing transmission capacity in optical
fiber communication systems:
 laying new fibers
 Costly
 Increasing the effective bandwidth of existing fibers
 Increasing the bit rate
 Increasing the number of wavelengths on a fiber

Time Division Multiplexing(TDM)
SDH is the standardized TDM based
hierarchical model where the following
transmission rates are defined:
STM-1: 155 Mbps
STM-4: 622 Mbps
STM-16: 2.5 Gbps
STM-64: 10 Gbps
STM-256: 40 Gbps
Gigabit Ethernet: 1 Gbps, 10 Gbps

Problems for high bit rates
 complex and costly
 chromatic dispersion
 nonlinear effects
 Polarization Mode Dispersion

So
 Combining many wavelength onto a single fiber using
Wavelength Division Multiplexing(WDM)

WDM Concept
 Different signals with specific wavelength are
multiplexed into a fiber for transmission.
1
2
┋
1 2 n
┉
n
SDH signal
IP package
ATM cells

Transmission modes – Unidirectional
transmission
 Single fiber unidirectional transmission
MUX DMUX
O
T
U
O
T
U
STATION A STATION B

Transmission modes - Bi-directional
transmission
 Single fiber bi-directional transmission
STATION A STATION B
MUX/DMUX DMUX/MUX
O
T
U
O
T
U

Wavelength Division Multiplexing
 Two developments have allowed us to start to access the massive
bandwidth potential of optical fiber:
 Optical amplifiers:
 Provide amplification in the low loss window of optical fiber, and over a
broad range of wavelengths (typically 40 nm centred on 1550 nm).
 Provide “transparent” operation, i.e. “photons in, photons out”.
 Wavelength division multiplexing:
 WDM components allow us to multiplex together different wavelength
channels.

How much bandwidth is there?
 Assume that our system will operate around 1550 nm and use optical
amplifiers. The BW is given by the spectral range over which we get
adequate amplification, say 10 dB for 50 km spacing (50 km  0.2
dB/km).

From earlier diagram, the following spectral bandwidth falls well within
the 10 dB gain requirement:
1550 nm 1570 nm1530 nm
1 = c/f1
0 2 = c/f2
 = 2 - 1
f = f1 - f2
1 = 1530 nm, 2 = 1570 nm, i.e. 1  2  0

 
21
12
21
21





ccc
fff
2
0


c
f
In this case,  = 40 nm, 0 = 1550 nm which gives:
f = 5 THz (= 5000 GHz)
No laser exists that is capable of being modulated over this range.

WDM
 These limitations can be overcome with wavelength division
multiplexing (WDM).
 Many different wavelengths (from different lasers) share the same
optical fiber.
 In early WDM, wavelengths were separated by 10 nm; current
separation is down to 0.8 nm, and is called dense WDM (i.e.
DWDM).

WDM
 Spectral windows in single-mode fiber offer phenomenal
bandwidths:

WDM
 Present-day optical devices have bandwidths well below this (< 100
GHz), so to take advantage of the available fiber bandwidth, we can
use many wavelengths in the 1.55 m window (where we have the
advantage of optical amplification):
Loss,
dB/km
1.3m 1.55m 0.1
0.2
1 2 n

WDM
 ITU (International Telecommunication Union) have set a standard
spacing of 100 GHz for WDM systems (originates from FDM
technology). For the 1550 nm window, this is 0.8 nm spacing.
 Therefore in our example of 5 THz bandwidth, we have 50 wavelength
channels available. If each channel supports 10 Gb/s bit rate for example,
then the aggregate bit rate would be 500 Gb/s or 0.5 Tb/s.
 It’s estimated that with 1 Tb/s we could transmit all the world’s TV
station output simultaneously.

Advantages of WDM
 Capacity upgrades. If each wavelength can support a bit rate of a few Gb/s (40
Gb/s in state-of-the-art), then system capacity is increased massively by using
many wavelengths.
 Transparency. Each optical channel (i.e. wavelength) can support any signal
format (e.g. digital or analogue, TDM etc.)
 Wavelength rerouting and switching. Can switch wavelengths and route signals
by wavelength, adding an extra dimension to network design.

Advantages of WDM
 Transparent media
 Long haul transmission
 High capacity
 Use existing optical fibers
 High performance-to-cost ratio
 Reliability
 Easy upgrading

Brief Introduction to CWDM
 CWDM (Coarse Wavelength Division Multiplex)
 The CWDM greatly reduces the system cost while
providing certain amount of wavelengths and
transmission distance within 100 km.
 Difference between CWDM and DWDM:

WDM Components: MUX
 WDM multiplexer: used to combine several different wavelengths onto one
fiber:
 Should have low insertion loss.
MUX
1
4
3
2
1 2, 3 ,4

WDM Components: DMUX
 WDM demultiplexer: used to remove several different wavelengths from one
fiber:
 Should have low insertion loss, high selectivity
DEMUX
1
4
3
2
1 2, 3 ,4

WDM Components: EDFAs and Filters
 Optical amplifier (EDFA): used to provide gain to wavelengths in the
1.55 m band.
 Tuneable optical filter: used to filter out a single wavelength for a
photodetector to produce a tuneable receiver:
Input
Passband tuned to
third wavelength Output

WDM Components: Tunable lasers,
add/drop MUX
 Tuneable laser diodes (figures of merit are large wavelength tuning
range, high-speed tunability, high data rate transmission, rigid
wavelength stability and repeatability).
 Add/drop multiplexers for selective wavelength routing/extraction:
Input add
Add

Application: Wavelength-selective WDM
 Provides fixed wavelength links for N transmitter/receiver pairs over a
single fiber:
MUX
1
4
3
2
1 2, 3 ,4
DEMUX
1
4
3
2

Application: Broadband WDM
 More flexible: offers broadcast and select:
 Receivers can “tune in” to any of the broadcast wavelengths
 Power splitter/combiner has 10 log N dB loss: need EDFA
N x 1 1 x N
Filter
1
2
n
1
1
3
Power
combiner
Power
splitterEDFA

Experimental WDM Local Area Networks
 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.

Ring Network
4,5,6
2,3,6
1,2,4
1,3,5
1 2 3 1 2 3
2 4 62 4 6
1
4
51
4
5
3
5
6
3
5
6
NODE 1
NODE 2 NODE 4
NODE 3
RING
NETWORK
ADM

Wavelength assignment table
• Alternative assignments are possible, as long as wavelengths
are not in contention with one another; e.g. next diagram

Wavelength assignment
1,3,5
1,2,4
2,3,6
4,5,6
2 4 6 2 4 6
1 2 31 2 3
3
5
63
5
6
1
4
5
1
4
5
NODE 1
NODE 2 NODE 4
NODE 3

Ring Networks
 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).

Choosing the right system
 When having to move traffic on spans strecthing more than 800 km, it is
obvious for many reasons to select the DWDM system over the CWDM or
an SDH system for that matter.

OptiSystem
 OptiSystem’s wide range of applications includes:
 Optical communication system design from component to system level at the physical layer
 CATV or TDM/WDM network design
 Passive optical networks (PON) based FTTx
 Free space optic (FSO) systems
 Radio over fiber (ROF) systems
 SONET/SDH ring design
 Transmitter, channel, amplifier, and receiver design
 Dispersion map design
 Estimation of BER and system penalties with different receiver models
 Amplified system BER and link budget calculations

OptiSystem graphical user interface (GUI)

20 Gbps, WDM transmission over 600 km

Simulation Parameter
Bit rate Parameters System limitation
20 Gbps (Optical Link with optical
amplifier + WDM)
Loss, chromatic dispersion, optical
signal to noise ratio (OSNR), four
wave mixing (FWM), Self-phase
modulation (SPM), Polarization
mode dispersion (PMD)
• OSNR limited
• Eye distortion limited

Transmitter
 Transmitter consists
 Pseudo-Random Bit Sequence
Generator at 2.5 Gbps
 NRZ Pulse Generator (convert
binary to electrical pulse)
 Continuous Wave Laser
 Mach-Zender Modulator
 8 transmitter with 100 GHz
channel spacing.

Optical Receiver
 Receiver Consists
 Photodetector
 Filter
 3R (Re-amplify + Regenerator + Reshaping)

Transmitted and Received Pulse
Tx
Rx

Optical Spectrum Analyzer
At
MUX
After travel
600 Km

Result
 Min BER is 2.85e-60
We have successfully transmitted
20 Gbps bit with very low bit error
8x2.5 Gbps = 20 Gbps

Design and Simulation WDM

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