WDM Technologies &
Optical Concepts
CWDM/DWDM & Components
Global Services Division
ECI Training Department
1.110
DWDM Technology
Agenda
 Optics Introduction
 Why optics?
 Trends in optics
 WDM – overview
 WDM standards & transmission windows
 DWDM components
2.110
DWDM Technology
Introduction: Why Optics?
 High bit rate
 High immunity from disturbances
 No aging problems
 High security
 For any distance
 Number of channels theoretically unlimited
3.110
DWDM Technology
Additional Capacity – How?
 Three ways to increase transmission capacity:
 Deployment of new, additional fibers (very expensive)
 Increasing TDM bit rate (more bits per second)
 Transmitting several channels via the same fiber
(on single mode fibers only)
4.110
DWDM Technology
Optics Today
 100Gb/s per channel

Today: 88 channels of 100Gb/s = 8.8Tb/s
 For any distance
 (Using Amplifiers, DCFs, Regenerators…)
5.110
DWDM Technology
WDM – Overview
 Wavelength Division Multiplexing multiplexes a number
of optical carrier signals into a single optical fiber by using
different wavelengths (Lambdas) of laser light
 The WDM principle is similar to a prism of light in the
visible spectrum
l1, l2, l3…. ln
6.110
l1
l2
l3
ln
DWDM Technology
WDM – Overview
 WDM splits and combines different wavelengths into a
single fiber using a multiplexer at the transmitting end (to
join the signals together), and a demultiplexer at the
receiver end (to split them apart again)
MUX
DEMUX
 Capacity of a given link can be expanded simply by
upgrading the multiplexers and demultiplexers at each end
7.110
DWDM Technology
WDM Transmission Bands
 Two different wavelength patterns:
 Dense (DWDM)
 Coarse/Conventional (CWDM)
 DWDM uses the C or L band for transmission with
denser channel spacing
 Typical system uses 44ch at 100GHz (0.8nm) interchannel
spacing or 88ch with 50GHz(0.4nm) interchannel spacing
 CWDM systems provide up to 8 or 16 channels of
transmission in the O, E, S, C and L bands
8.110
DWDM Technology
CWDM – Coarse WDM
 Uses 200-250GHz interchannel spacing
 10Gb networks supporting 8 channels
 2.5Gb network supporting 16 channels
 Runs over both SMF and MMF
 Distances are generally shorter than DWDM
 Costs of deploying CWDM are significantly lower than
DWDM
 Mostly used for the several applications:
 10 Gigabit Ethernet LANs and some 10xGbE WAN applications
9.110
DWDM Technology
CWDM Spectrum
 Standard G.694.2 defines 18 channels in 5 bands:
 O, E, S, C and L
Fiber attenuation (DB/km)
CWDM wavelength grid as specified by ITU-T G.694.2
Wavelength (nm)
10.110
DWDM Technology
CWDM 8 Channels
Mux
Demux
 Upgrade to 16 channels is done by adding an extra
1271-1451nm range
11.110
DWDM Technology
DWDM
 Developed in the early 1990s to add capacity to
undersea and transcontinental routes
 Uses the 1500nm to 1600nm band which has minimum
attenuation for long distance routes (3rd window)
 Today typically transmits from 2.4Gbps - 200Gbps
12.110
DWDM Technology
Third Window Spectrum – DWDM
L
C
Blue
1529.94nm
Red
1568.77nm
1570.01nm
1610.92nm
 Interchannel spacing of 50 & 100GHz
 Examples of conversion from λ in nm to frequencies in THz



13.110
1529.94(nm) = 195.95(THz)
1568.77(nm) = 191.10(THz)
1570.01(nm) = 190.95(THz)
1610.92(nm) = 186.10(THz)
DWDM Technology
DWDM 44 Channel Grid [ THz ]
 44ch – 100GHz spacing
 First ch: 191.70THz
 Last ch: 196.00THz
191.70 191.75 191.80 191.85 191.90 191.95 192.00
192.05 192.10 192.15 192.20 192.25 192.30 192.35 192.40 192.45 192.50
192.55 192.60 192.65 192.70 192.75 192.80 192.85 192.90 192.95 193.00
193.05 193.10 193.15 193.20 193.25 193.30 193.35 193.40 193.45 193.50
193.55 193.60 193.65 193.70 193.75 193.80 193.85 193.90 193.95 194.00
194.05 194.10 194.15 194.20 194.25 194.30 194.35 194.40 194.45 194.50
194.55 194.60 194.65 194.70 194.75 194.80 194.85 194.90 194.95 195.00
195.05 195.10 195.15 195.20 195.25 195.30 195.35 195.40 195.45 195.00
195.55 195.60 195.65 195.70 195.75 195.80 195.85 195.90 195.95 196.00
196.05
14.110
DWDM Technology
DWDM 88 Channel Grid [ THz ]
1563.86nm
 88ch – 50GHz spacing
 First ch: 191.70THz
 Last ch: 196.05THz
15.110
1529.16nm
DWDM Technology
CWDM Vs. DWDM
 CWDM Advantages:
 Simple technology
 Cost effective
 The best solution for shorter
distances
 Fiber Channel – for Data
Center Storage
 CWDM Disadvantages:
 Less capacity than DWDM
 Less transmission range
 Less bit rate per channel
16.110
 DWDM Advantages:
 Maximum system channel
capacity available
 Maximum distance capability with
EDFA and Raman amplifiers
 Pay as you grow expansion
 DWDM Disadvantages:
 Needs more power
 High accuracy lasers & wave
filters
 Expensive EDFA & Raman
amplifiers
 Start up costs are higher than
equivalent CWDM
DWDM Technology
DWDM Network
Mux
Demux
Amplifier
DCF
Interfaces
Interfaces
Fiber
C/T filter
ROADM






17.110
Transmitters, Receivers
Mux, Demux, OADM, GOADM & ROADM
EDF-Amplifiers:
 (Booster, PreAmp)
Fiber
DCF & DCM
C/T Filter
DWDM Technology
Single Mode Vs. Multi Mode Fiber
D = 125±2µm
D = 125±2µm
d = 8.6-9.5µm
d = 50-62.5 µm
Core
Cladding
 SMF has a yellow jacket
18.110
 MMF has an orange jacket
DWDM Technology
Attenuation
 Attenuation:
 The reduction of signal strength during transmission - sometimes
called loss
 The relation between power in to power out
 If a signal attenuates too much, the destination device cannot
identify it, or, it may not even reach the destination.
 The power is measured in mw (milli watts), the calculation is in
dB
19.110
DWDM Technology
20.110
DWDM Technology
DCFs & DCMs
 Dispersion Compensating Fiber (DCF)
 For all composite signals in the C Band (external equipment)
 Large amount of special fiber with the dispersion opposite to
that of the fiber
 Dispersion Compensating Module (DCM)
 For single channel or for the whole C band (Internal module)
 Needed for compensation of chromatic dispersion
 Depends on the type of fiber
21.110
DWDM Technology
DCFs & DCMs
DCF:
DCM:
 Large insertion loss
 Low attenuation
 Big
 Small
 Expensive
 Cheap
 Problems with slope
compensation
 Attenuation is independent of the
compensation length
 Tunable compensation is possible
 Dispersion ripple can result in power
penalty
22.110
DWDM Technology
Insertion Loss of DCFs Vs. DCMs
 Insertion loss 40km
 DCF – 5 dB
 DCM – 3 dB
 Insertion loss 80km
 DCF – 8 dB
 DCM – 3 dB
 Insertion loss 100km
 DCF – 10 dB
 DCM – 3 dB
23.110
DWDM Technology
24.110
DWDM Technology
DWDM: Optical Amplifiers
 A device that amplifies an optical signal directly,
without converting it into an electrical signal
 Three major optical amplifier types:
 Booster: At the transmitter site
 Inline Amplifier: Between 2 NEs
 Preamplifier: At the receiver site
25.110
DWDM Technology
EDFA Amplifiers
 Used to amplify the signal and to compensate for the
attenuation of the passive documents
 What are the important parameters?
 Gain: G = Pout – Pin
G
 Amplifiers are the source of optical noise
 What is Noise Figure (NF) ?
 The amount of noise emitted by an amplifier
26.110
DWDM Technology
EDFA Amplifiers
 Psat – the maximum optical power an amplifier can
provide at the Output
G
Psat
 Gain Flatness:
 Difference between the maximum and the minimum
channel power in the working region
27.110
DWDM Technology
Gain Tilt and Flatness
 Gain Tilt is a phenomenon that occurs in amplifiers
 In reality there is no ideal amplifier or ideal amplification
 Example:
Amp
20.75dB
+0.75
20dB
19.25dB
-0.75
λ
 Two ways to improve the flatness:
 V-mux
 Dynamic Gain Equalizer
28.110
DWDM Technology
Gain Tilt Filter
 What for?
 To compensate for the fiber gain tilt phenomenon
 The cause of gain tilt:
 Optical passive components and optical fibers
 The result:
 In the Optical fibers different channels are attenuated differently
during propagation
 There is a negative slope in insertion loss (channels close to the
1529 nm attenuated less than channels close to the 1560 nm)
29.110
DWDM Technology
Gain Tilt Filter
 GTF - exists in the output of the amplifiers
 Straightens the power of each channel at the output of
the amplifiers
 In ECI we edit Gain Tilt Correction of 0.1dB per 10km
 Gain Tilt Correction can be done before or after the
range of the distance. (Over the entire link, must
maintain consistency! )
30.110
DWDM Technology
Cause of BR in the Optical Network
 Dirt in the fiber core
 Inaccurate fiber splice
 Inaccurate physical contact between male and female
optical connectors
31.110
DWDM Technology
Back Reflection
 BR Threshold = 20 dB
 BR < 20dB  High Back Reflection (Alarm)
 BR > 20dB  No Alarm
 How to Reduce BR
 Check physical fiber contacts
 Clean fibers
 Angled Polished Connector (APC) is used to minimize
attenuation and back reflection
32.110
DWDM Technology
33.110
DWDM Technology
Transponder & Regenerator
Transponder:
 Device that receives a single signal from the client side
and retransmits the signal on a different frequency
toward the network (Line)
Regenerator:
 Device that regenerates a weak optical signal from line
to line (within the network)
 Improves OSNR
 Can keep or change frequency of regenerating channel
34.110
DWDM Technology
Muxponders
 Multiplexing several lower client signals into a higher
DWDM signal toward the network
 Today multi bit rate Muxponders are supported
35.110
DWDM Technology
Photo-detector
Processing
Receiver
36.110
DWDM Technology
Receivers: Photodiode
Bit Error (BER) - number of correct bits per one error
 BER = 10-12 – 1 error per 1012 bits
 2.5 Gb/s – one error in 400 sec
 10 Gb/s – one error in 100 sec
 Sensitivity – minimum power required for the receiver to
receive the signal with an acceptable amount of errors
(OSNR)
 Reason for this – electrical noises in the photodiode
37.110
DWDM Technology
OSNR Defined
 OSNR (dB) = Optical Signal to Noise Ratio
S
– =
N
signal power (Photo-detector current)
–––––––––––––––––––––––––––
noise power (Photo-detector) + noise power (Amplifier)
 The measure of the ratio of signal power to noise power
in an optical channel (the amount of noise in the signal)
 The higher the OSNR –the better
38.110
DWDM Technology
OSNR Threshold
 OSNR threshold value is set and influenced by:
 Rate (2.5Gb, 10Gb, 40Gb and 100Gb)
 Type of the transmitter
 EFEC, FEC or no FEC
39.110
DWDM Technology
Forward Error Correction – FEC
 The principle: transmit with an extra band to fix errors
 Implemented in the transmission interfaces
 Allows us to have the same BER in lower OSNR
 Can extend the transmission distance of the system by
200 - 500km
40.110
DWDM Technology
Forward Error Correction – FEC
FEC Types:
 Standard FECs
 G.709 – 7% FEC – 56 dB gain
 E-FEC – 7% FEC – 8 dB gain
 S-FEC – 25% FEC – 9 dB gain
 E-FEC is the most popular solution
41.110
DWDM Technology
42.110
DWDM Technology
Coupler
 Coupler – device that combines 2 inputs into a single
output, no wavelength filtering in the mixing process
 In most cases the output signal contains half of the
power from each one of the input signals
43.110
DWDM Technology
Splitters
 Splitters - device that separates a signal into two
independent streams, no wavelength filtering in the
process. Each output signal contains same structure but
less power.
 50%-50% Splitter
For protection
 95%-5% Splitter
For monitoring
44.110
DWDM Technology
Multiplexer – C/T Filters
 Supervisory channel is used to manage the network
 Two standards: 1510 nm and 1310 nm
 1510 nm – less attenuation, more expensive
 C/T filters are used to separate the C-band from the
supervisory channel
C/T in
45.110
C/T out
DWDM Technology
Management Channel
 Option 1: GCC - in band management channel used per
 (OTN)
 Option 2: OSC - Optical Supervisory Channel Out of
third window:
 1510 nm – used for long distances (most common)
 1480nm – water peak
 1310nm – used for short distances (second window)
46.110
DWDM Technology
47.110
DWDM Technology
Components: Multiplexers
Multiplexer
Composite
 Multiplexes separate optical channels into one fiber
48.110
DWDM Technology
Mux Principals
Filter
1
Filter
2
Filter
Mirror
3
Filter
4
49.110
DWDM Technology
Demux
 Demultiplexes the DWDM signal into
separate optical channels
 The same technologies as in Mux
are utilized except Star coupler since there are no filters
 Every Mux, besides the star can be a
Demux
50.110
DWDM Technology
Demux Principals
Filter
1
Filter
2
Filter
Mirror
3
Filter
4
51.110
DWDM Technology
TFA
 TFA - Tunable Filter Array
 Replaces the classical Mux/Demux for ROADM nodes
 Each port has a single channel reconfigurable port
 Based on same DLP technology as 2-degree ROADMs
 Can add/drop colorlessly up to 88 50GHz wavelengths
per card
 Expandable to xx ports with additional splitter/coupler
 40Gb/s and 100Gb/s compatible
52.110
DWDM Technology
53.110
DWDM Technology
OADM / GOADM
 Optical Add/Drop Multiplexer
 The OADM adds and drops single or multiple wavelengths
without interfering with the other wavelengths
OADM
Drop
channels
Add
channels
 Group Optical Add/Drop Multiplexer
 The GOADM follows the same purpose for neighbouring
wavelengths.
 Lower insertion loss for the through wavelengths
1
54.110
2
3
4
After
t
DWDM Technology
OADM Types
 Fixed – the drop channel numbers are fixed
 Grouped – only a group of successive channels can be
dropped
 Random – any combination can be dropped
 Equalization – possibility to control the power levels at
the output to equalize the channels
 Reconfigurable OADM
55.110
DWDM Technology
ROADM – Reconfigurable OADM
 Flexible
 Dynamically and remotely reconfigurable in 10ms
 Managed (not passive)
 No problems with filters concatenation
 Expensive
 Limited number of ports
 Includes separate Add & Drop sides
56.110
DWDM Technology
Typical Optical Link
Transmitter
Mux
Signal
input
X
BA
Transmitter
Signal
input
drop
IL
DCF
OADM
IL
add
Y
PA
Demux
Photo-detector
Signal
output
Photo-detector
Signal
output
57.110
DWDM Technology
Summary
 WDM standards & transmission windows
 Optical concepts
 Light propagation
 Dispersion, Attenuation, OSNR
 WDM standards & transmission windows
 OSC channels
 DWDM Components:





58.110
Couplers & Splitters
Muxes & Demuxes
Transceivers
Amplifiers
Fiber types
DWDM Technology