WDM principles

WDM Principles
February 2014

1

Anuradha Udunuwara | udunuwara@ieee.org | www.linkedin.com/in/anuradhau | @AnuradhU

How to increase network
capacity?
Space
Division
Multiplexing
(SDM)
• Add fiber &
equipment
• Time & Cost

Time
Division
Multiplexing
(TDM)
• PDH/SDH (STM16->STM-64(10G)>STM-256(40G)
• Cost & Complexity

Wavelength
Division
Multiplexing
(WDM)
• Economical, mature
& quick

2


What’s WDM?
• A technology that utilizes the properties of
refracted light to both combine and
separate optical signals based on their
wavelengths within the optical spectrum
• Different signals with specific wavelength
are multiplexed into a fiber for
transmission

3


What’s WDM? , Contd.,
Gas Station

Free Way

Petrol Car

Freeway
Petrol Car
Gas Station
Gray Car
Colored Car
Driveway

: Fiber
: Supervisory Signal
: Optical relay
: Client Service
: Service in different channels (wavelength)
: Optical wavelength
4


Advantages of WDM
• Ultra high capacity
• Data transparency transmission
– Doesn’t change the structure or any byte in the frame
for the client signal

• Long haul transmission
• Compatible with existing optical fibers
• High performance-to-cost-ratio (unit traffic cost)
– However, projects get long time to break-even

• High networking flexibility, economy and reliability
• Smooth expansion
5


WDM system key technologies
• Optical multiplexer (MUX) and demultiplexer
• Optical Amplifier (Amp)
• Supervisory channel
• Optical Source

6


Optical Multiplexer Unit:
Multiplex several services
with different wavelength
into one main path signal

Optical Amplifier:
Amplifies the optical
signal

1

A

P

A
OLA

nm
OSC

Optical Transponder
Unit: Access the client
services & convert the
wavelength compiled with
ITU standard

OTU1

P
P

OA

n

A

P

P
OA

OTUn

Optical Line
Amplifier

O
M
U

2

1

A

1, 2..n

P
OTU2

System structure

1, 2..n

A
OTU1

Optical De-multiplexer
Unit: De-multiplex one
main path signal into
several individual signals

nm
OSC

O
D
U

2

n

OTU2

OTUn

OSC

Optical Supervisory
Channel: Terminate &
Re-generation. Not
amplification.

A
P

Active
Passive
7


OTU- Optical Transponder Unit
Optical to
Electrical
conversion

O
Non-color
E
(Not defined by ITU-T)
E
O
Ex:1310 nm short reach SMF
1550 nm long reach SMF Wavelength conversion
850 nm MMF

Electrical to
Optical
conversion

Color
(Defined by ITU-T)
Ex:1: 1550.51 nm
2 :1551.23 nm

Can’t use these in WDM
without OTU

SMF-Single Mode Fiber
MMF-Multi Mode Fiber
8


Loss
• Passive => Loss (power reduction)
– Ex:- Input power to the MUX 0 dB. Output power from
the MUX -6 dB. Therefore the loss is 6 dB

• Loss can be due to splicing, distance, bending,
aging, connectors

Source: http://www.thefoa.org/tech/lossbudg.htm


9

Transmission Modes
• Single fiber unidirectional
– 2 optical fibers

• Single fiber bidirectional
– Only 1 optical fiber
– Ex:- CWDM, to reduce cost

Coarse WDM

10


Application modes
• Open system
– NO special requirements for multiplex terminal
optical interface
– Only requirement is that these interfaces meet
the optical interface standards defined in ITU-T

• Integrated system
– Doesn’t adopt wavelength conversion technology
– Requires that the wavelength of the optical signal
at the multiplex terminal confirms to the
specifications for the WDM system
11


c=f
• c = velocity of light in a vacuum = 3 x 108 m/s (constant)
• f = frequency (Hz)
•  = wavelength (m)
• f1/
• Refractive index n = c / v
• v = speed of light in a material
• In an optical fiber, since the n of core is higher than n of
cladding the light refracts

12


Fiber cable types
• G.652
• G.653
– Main application: submarine

• G.655
– Best fiber for WDM
– Expensive

13


Single vs. multi mode

Source: http://osd.com.au/multimode-versus-singlemode/

High Attenuation (3 dB/km)
High dispersion
Expensive today (because of less demand)

Attenuation = 0.22 dB.km (G.652 @ 1550nm)
No mode dispersion

Mode=Path of light
14


WDM network topologies
• Point to Point
• Ring
• Mesh

Cost 
Complexity 
Reliability 

15


CWDM vs. DWDM
DWDM
CWDM

Source: http://www.cable360.net/tech/strategy/businesscases/30007.html

CWDM- Coarse WDM, DWDM-Dense WDM


16

Since f  1 / ,
channel
spacing can
be denotes as
both distance
and frequency

CWDM vs. DWDM, cont.,
Types

CWDM

DWDM

Channel spacing (Grid)

20 nm (fixed)

100 GHz/ 50 GHz/ 25 GHz

1311~1611 nm
(All bands)

C-band:
1529nm~1561nm
L-band:
1570nm~1603nm

18 x 10 Gbps

192 x 10 Gbps

Laser

Un-cooled Laser

Cooled Laser

Cost

70%

100%

100 km (max)

5000 km

Band

Capacity (max)

Application
As CWDM
works in all 5
bands,
amplification is
NOT possible

17


Linear and non-linear effects
• Linear ( distance)
– Attenuation
– Dispersion

• Non-Linear
– Four Way Mixing
(FWM)

• Chromatic
• Polarization

18


Attenuation

Lowest loss
band

Water peak

Source: http://osd.com.au/multimode-versus-singlemode/

19


Dispersion
•

Physical phenomenon of signal distortion caused when various modes
carrying signal energy or different frequencies of the signal have different
group velocity and disperse from each other during propagation

•

Digital modulation -> carrier frequency+ multiple other frequencies ->

different speeds -> Inter Symbol Interference (ISI) -> Bit errors
•
Color

2 types
–

Mode dispersion
•

–

Dominant in MMF

Chromatic dispersion (CD)
•
•

Dominant in SMF
Ex:- Rainbow
–

•

Light through water traverse at different speeds

Dispersion affects the own channel
Source: http://www.bubblews.com/news/2058509-somewhere-over-the-rainbow
20


Chromatic dispersion
A phenomenon that the phase velocity and group velocity of light propagating
in a transparent medium depend on the optical frequency. A related
quantitative measure is the Group Velocity Dispersion (GVD)

Source: https://www.upc.edu/patents/TO/ict-and-electronic-technologies/chromatic-dispersion-1.jpg
21


Chromatic dispersion, cont.,
G.655: little
dispersion to
avoid FWM

G.652: widely
used, need
DCF for high
rate
transmission

Dispersion
coefficient

G.653

17 ps/nm/km

4.5 ps/nm/km

1310

1550

Wavelength/nm

22


Dispersion Compensation Fiber
(DCF)

DCM (Dispersion Compensation
Module) . Usually placed at
bottom of rack

Source: http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=5719

Dispersion-> DCF ->Dispersion
longer fiber distance -> attenuation  -> Optical Amplifiers -> noise  -> S/N
If the total accumulated dispersion (ps/nm) is less than 800 for 10 Gbps/STM-64, then DCM is not required
23


Polarization Mode Dispersion
(PMD)
• Resulting from different propagation
velocities of 2 states of cross polarization of
optical signal in fiber
• Can’t avoid
• Due to
– Manufacturing process
– Installation/usage (temperature, vibration,
bending (DCM)

Source: http://www.fiberoptics4sale.com/wordpress/optical-fiber-dispersion/

• Both PMD and CD are sensitive at higher bit
rates
24


Optical source
• Key requirements
– Large dispersion tolerance value
– Standard and stable wavelength
• ITU-T recommends the maximum deviation of the
channel frequency to be <=10% of channel
spacing

25


Modulators
• Direct
• Electro-Absorption (EA) External
– Ex:- for 40 G

• Mach-Zehnder (M-Z) External
– Ex:- for 40 G

• Coherent
– Ex:- for 100 G
– No DCM required
26


Comparison of modulators
Types

Direct

EA

M-Z

Coherent

Max.
dispersion
tolerance
(ps/nm)

1200-4000

7200-12800

>12800

40000

Cost

moderate

expensive

Very
expensive

Very
expensive

good

better

best

best

Wavelength
stability

27


S (signal)
OA

Amplifier

• Compensates the loss
• Any analog signal system has noise. Optical signal is
also analog
• More Amps-> more accumulated noise (N)->S/N->BIR
– Amp keeps Signal (S) constant.

• Solution: re-generation
• Amplification and regeneration gives unlimited distance,
theoretically

28


Amplifier types
• EDFA - Erbium Doped Fiber Amplifier
– Widely used

• RFA - Raman Fiber Amplifier
– Uses non-linearity effect
– Uses high power class 4 laser
• Use APC (Angular Physical Contact) connectors instead of
PC
– Ex:-LC/APC (Lucent Connector), SC/APC, FC/APC

– 20 km distance
• Need to maintain splice loss <0.1dB within 1st 10 km and
<0.2dB within next 10 km

– Low noise
– Low gain efficiency (10~12 dB)
29


EDFA

Source: http://www.tlc.unipr.it/bononi/ricerca/edfa.html

Source: http://spie.org/x33612.xml

30


Raman

Source: http://www.globalspec.com/RefArticleImages/157D4FA155A471EB0023715782A949C2_04_04_DWDM-10.jpg

31


Optical Multiplexer and demultiplexer
• TFF - Thin Film Filter
– when no. of channels<16

• AWG - Arrayed Waveguide Grating
– when no. of channels>=16
– expensive

32


TFF
0.1 dB loss. Therefore max. of 16
channels

Has the
lowest power

Source: http://www.fiberoptics4sale.com/wordpress/what-is-multilayer-dielectric-thin-film-filter/

33


AWG

All have the same
power

Source: http://docstore.mik.ua/univercd/cc/td/doc/product/mels/cm1500/dwdm/dwdm_ovr.htm

34


Supervisory technologies
• OSC - Optical Supervisory Channel
– Often used in backbone systems
– Uses OTN (G.709) framing (similar to SDH) on
OUT board
– Costly

• ESC - Electrical Supervisory Channel
– Often used in metropolitan systems
– OTU is mandatory at every site
• OLA sites don’t have OUT. Therefore can’t mange
OLAs with ESC

35


Related ITU-T recommendations
•
•
•
•
•
•

G.652
- SMF
G.655
- Dispersion-shifted SMF
G.661/G.662/G.663
- OAs
G.671
- Passive optical components
G.957
- SDH optical interfaces
G.691
- Optical interfaces for single channel STM64, STM-256 systems & other SDH systems
with OA
G.692
- Optical interfaces for multi-channel
systems with OA
G.709
- OTN interfaces
• G.975
- FEC for submarine systems
36


About the Author
37

Eng. Anuradha Udunuwara is a Chartered Engineer by profession based in Sri Lanka. He has
over a decade industry experience in strategy, architecture, engineering, design, plan,
implementation and maintenance of CSP Networks using both packet-switched (PS) and
Circuit-Switched (CS) technologies, along with legacy to NGN migration. Eng. Anuradha is a
well-known in the field of CSP industry, both locally and internationally.
Graduated from University of Peradeniya, Sri Lanka in 2001 with an honors in Electrical &
Electronic Engineering, Eng. Anuradha is a corporate member of the Institution of Engineers
Sri Lanka, a professional member of British Computer Society, a member of Institution of
Electrical & Electronic Engineers, a member of Institution of Engineering & Technology
(formerly Institution of Electrical Engineers), a member of the Computer Society of Sri Lanka,
a life member of Sri Lanka Association for the Advancement of Science, a senior member of
the Carrier Ethernet Forum, a member of the Internet Society, a member of the Internet
Strategy Forum, a member of the Internet Strategy Forum Network, a member & a senior
contributor of the Ethernet Academy, a member of the NGN/IMS forum and a member of the
Peradeniya Engineering Faculty Alumni Association. He is also an ITIL foundation certified
and the only MEF-CECP in the country.
In his spare time Anuradha enjoys spending time with his family, playing badminton,
photography, reading and travelling.


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