Multiplexing Techniques in Optical Networks: WDM

1
Multiplexing Techniques in Optical Networks WDM

• Dr Manoj Kumar
• Professor Head(ECE)
• DAVIET, Jalandhar

2
Multiple Access Methods

• TDMA Time Division Multiple Access
• Done in the electrical domain
• SCMA Sub Carrier Multiple Access
• FDM done in the electrical domain
• CDMA Code Division Multiple Access
• Not very popular
• WDMA Wavelength Division Multiple Access (The
  most promising)

3
Sub Carrier Multiplexing

• Widely used in CATV distribution

4
A Closer Look.
Transmitting End
Baseband Data
Baseband-RF Modulation
RF-Optical Modulation
Two different Modulations for each RF Carrier !
Single Mode Fiber
Gain BPF
RF-Baseband Demodulation
Optical - RF Demodulation
Baseband Data
Receiving End
200 THz
1.8 GHz
5
Sub Carrier Multiplexing
Unmodulated (main) carrier
f2
f2
f1
f1
f0
Frequency
Sub-carriers

• Each modulating RF carrier will look like a
  sub-carrier
• Unmodulated optical signal is the main carrier
• Frequency division multiplexed (FDM) multi
  channel systems also called as SCM

6
Sub Carrier Multiplexing

• Ability to both analog and digitally modulated
  sub-carriers
• Each RF carrier may carry voice, data, HD video
  or digital audio
• They may be modulated on RF carriers using
  different techniques
• Performance analysis is not straightforward

7
CATV Distribution

• 50-88 MHz and 120-550 MHz spectrum is allocated
  for CATV
• Either AM or FM technique for RF ? Optical
  conversion
• AM Simple implementation, but SNR gt 40 dB for
  each channel, high linearity required
• FM The information is frequency modulated on RF
  before intensity modulating the laser, better SNR
  and less linearity requirement

8
TDMA

• Signals are multiplexed in time
• This could be done in electrical domain (TDMA) or
  optical domain (OTDMA)
• Highly time synchronized transmitter/receiver
• Stable and precise clocks
• Most widely used (SONET, GPON etc.)

9
Wavelength Division multiplexing
Each wavelength is like a separate channel (fiber)
10
TDM Vs WDM
SONET
11
Wavelength Division Multiplexing

• Passive/active devices are needed to combine,
  distribute, isolate and amplify optical power at
  different wavelengths

12
Why WDM?

• Capacity upgrade of existing fiber networks
  (without adding fibers)
• Transparency Each optical channel can carry any
  transmission format (different asynchronous bit
  rates, analog or digital)
• Scalability Buy and install equipment for
  additional demand as needed
• Wavelength routing and switching Wavelength is
  used as another dimension to time and space

13
Evolution of the Technology
14
Review of Modes

• Multimode Fiber There are several
  electro-magnetic modes that are stable within the
  fiber, Ex TE01, TM01
• The injected power from the source is distributed
  across all these modes
• WDM is not possible with multimode fibers
• Single Mode Fiber Only the fundamental mode will
  exist.
• All the coupled energy will be in this mode. This
  mode occupies a very narrow spectrum making
  Wavelength Division Multiplexing possible

15
Multimode Laser Spectrum
Multimode Lasers are not suitable for DWDM
systems (two wide spectrum)
16
Photo detector Responsivity

• Photo detectors are sensitive over wide spectrum
  (600 nm).
• Hence, narrow optical filters needed to separate
  channels before the detection in DWDM systems

17
Optical Amplifiers are key in DWDM systems
18
WDM, CWDM and DWDM

• WDM technology uses multiple wavelengths to
  transmit information over a single fiber
• Coarse WDM (CWDM) has wider channel spacing (20
  nm) low cost
• Dense WDM (DWDM) has dense channel spacing (0.8
  nm) which allows simultaneous transmission of 16
  wavelengths high capacity

19
WDM and DWDM

• First WDM networks used just two wavelengths,
  1310 nm and 1550 nm
• Today's DWDM systems utilize 16, 32,64,128 or
  more wavelengths in the 1550 nm window
• Each of these wavelength provide an independent
  channel (Ex each may transmit 10 Gb/s digital or
  SCMA analog)
• The range of standardized channel grids includes
  50, 100, 200 and 1000 GHz spacing
• Wavelength spacing practically depends on
• laser linewidth
• optical filter bandwidth

20
ITU-T Standard Transmission DWDM windows
21
Principles of DWDM

• BW of a modulated laser 10-50 MHz ? 0.001 nm
• Typical Guard band 0.4 1.6 nm
• 80 nm or 14 THz _at_1300 nm band
• 120 nm or 15 THz _at_ 1550 nm
• Discrete wavelengths form individual channels
  that can be modulated, routed and switched
  individually
• These operations require variety of passive and
  active devices

Ex. 10.1
22
Nortel OPTERA 640 System
64 wavelengths each carrying 10 Gb/s
23
Key components for WDM

• Passive Optical Components
• Wavelength Selective Splitters
• Wavelength Selective Couplers
• Active Optical Components
• Tunable Optical Filter
• Tunable Source
• Optical amplifier
• Add-drop Multiplexer and De-multiplexer

24
DWDM Limitations

• Theoretically large number of channels can be
  packed in a fiber
• For physical realization of DWDM networks we need
  precise wavelength selective devices
• Optical amplifiers are imperative to provide long
  transmission distances without repeaters

25
(No Transcript)
26
(No Transcript)
27
Types of Fiber

• Dispersion Optimized Fiber
• Non-zero dispersion shifted fiber (NZ-DSF)? 4
  ps/nm/km near 1530-1570nm band
• Avoids four-way mixing
• Dispersion Compensating Fiber
• Standard fiber has 17 ps/nm/km DCF has -100
  ps/nm/km
• 100 km of standard fiber followed by 17 km of DCF
  ? zero dispersion

28
Summary

• DWDM plays an important role in high capacity
  optical networks
• Theoretically enormous capacity is possible
• Practically wavelength selective (optical signal
  processing) components decide it
• Passive signal processing elements like FBG are
  attractive
• Optical amplifications is imperative to realize
  DWDM networks