Multiplexing Techniques in Optical Networks: WDM by rM4xIRHL

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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
• CDMA – Code Division Multiple Access
  – Not very popular
• WDMA – Wavelength Division Multiple
  Access (The most promising)
Sub Carrier Multiplexing




Widely used in CATV distribution
               A Closer Look….
                                                          Transm
                                                          itting
       Baseband    Baseband-RF         RF-Optical         End
         Data       Modulation         Modulation




           Two different Modulations
           for each RF Carrier !                         Single
                                                         Mode
                                                         Fiber




Baseband    RF-Baseband         Gain      Optical - RF   Receiving
  Data      Demodulation        BPF       Demodulation   End

                           1.8 GHz            200 THz
      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
   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
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
                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.)
 Wavelength Division multiplexing




Each wavelength is like a separate channel (fiber)
             SONET



TDM Vs WDM
 Wavelength Division Multiplexing




• Passive/active devices are needed to
  combine, distribute, isolate and amplify
  optical power at different wavelengths
             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
Evolution of the Technology
           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 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
Optical
Amplifiers
are key in
DWDM
systems
   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 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
ITU-T Standard Transmission DWDM
             windows
        c 
     2  
        
         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
                           c 
                        2           Ex. 10.1
                           
Nortel OPTERA 640 System




     64 wavelengths each carrying 10 Gb/s
   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
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
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
                 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

								
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