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Multiplexing Techniques in Optical Networks: WDM. Dr Manoj Kumar Professor & Head(ECE) DAVIET, Jalandhar. Multiple Access Methods . TDMA – Time Division Multiple Access Done in the electrical domain SCMA – Sub Carrier Multiple Access FDM done in the electrical domain

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Multiplexing Techniques in Optical Networks: WDM

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multiplexing techniques in optical networks wdm

Multiplexing Techniques in Optical Networks: WDM

Dr Manoj Kumar

Professor & Head(ECE)

DAVIET, Jalandhar

multiple access methods
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)
sub carrier multiplexing
Sub Carrier Multiplexing

Widely used in CATV distribution


A Closer Look….

Transmitting End

Baseband Data



RF-Optical Modulation

Two different Modulations

for each RF Carrier !

Single Mode


Gain BPF

RF-Baseband Demodulation

Optical - RF Demodulation

Receiving End

Baseband Data

200 THz

1.8 GHz

sub carrier multiplexing5
Sub Carrier Multiplexing

Unmodulated (main) carrier



  • 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






sub carrier multiplexing6
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
catv distribution
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 > 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

  • 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.)
wavelength division multiplexing
Wavelength Division multiplexing

Each wavelength is like a separate channel (fiber)

wavelength division multiplexing11
Wavelength Division Multiplexing
  • Passive/active devices are needed to combine, distribute, isolate and amplify optical power at different wavelengths
why wdm
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
review of modes
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

multimode laser spectrum
Multimode Laser Spectrum

Multimode Lasers

are not suitable

for DWDM systems

(two wide spectrum)

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

Photo detector Responsivity
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
wdm and dwdm
  • First WDM networks used just two wavelengths, 1310 nm and1550 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
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 @1300 nm band

120 nm or 15 THz @ 1550 nm

Discrete wavelengths form individual channels that can be modulated, routed and switched individually

These operations require variety of passive and active devices

Principles of DWDM

Ex. 10.1

nortel optera 640 system
Nortel OPTERA 640 System

64 wavelengths each carrying 10 Gb/s

key components for wdm
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
dwdm limitations
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

types of fiber
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
  • 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