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 Receiving End Baseband Data 200 THz 1.8 GHz

5. Sub Carrier Multiplexing Unmodulated (main) carrier f2 f2 • 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 f1 f1 f0 Frequency Sub-carriers

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 > 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 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

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 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

20. ITU-T Standard Transmission DWDM windows

21. 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

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

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