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Chapter 4 Medium Access Technologies

Chapter 4 Medium Access Technologies. Time domain medium access Frequency domain medium access Code domain medium access. Time Domain Medium Access. Basic concept of optical TDMA. time slot. MOD. S P L I T T E R. M U L T I P L E X E R. LD. MOD. MOD. AMP. Pulsed

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Chapter 4 Medium Access Technologies

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  1. Chapter 4 Medium Access Technologies • Time domain medium access • Frequency domain medium access • Code domain medium access

  2. Time Domain Medium Access • Basic concept of optical TDMA time slot MOD S P L I T T E R M U L T I P L E X E R LD MOD MOD AMP Pulsed Laser MOD Delay Line Data input

  3. Demultiplexing for OTDM NOLM Receiver Optical TDM Signal Input To end user Pulse Generator Clock Recovery NOLM: Non-linear optical loop mirror

  4. Optical Sources in OTDM Network --- Fiber ring laser

  5. --- Semiconductor mode-locked laser

  6. --- DFB/Electroabsorption modulator

  7. --- Gain-switched laser diode

  8. Optical switch Optical switch Vπ Out t t 2Vπ In Optical signal Clock • Optical Demultiplexing in OTDM Network --- Lithium niobate modulator demultiplexing

  9. kms Same group velocity around loop Data PC Clock Clock out Clock in λ ck 3db coupler out Data IN Onward transmission Dropped channel Time --- Fiber nonlinear optical loop mirror (NOLM)

  10. Semiconductor laser amplifier nonlinearity 10 GHz clock Variable delay t WDM 3dB 40 Gb/s data Demultiplexed data channel Through traffic --- Semiconductor amplifier demultiplexer TOAD/SLALOM

  11. Synchronization and Clock Recovery --- Optical phase-locked-loop clock recovery Bit Stream Input Variable Attenuator MZM To Demultiplexer VCO

  12. --- SOA fiber mode-locked ring lasers Isolator SOA Filter OSC EA Coupler Polarizer Output --- Self-pulsating laser diodes

  13. Crosstalk in OTDM --- Crosstalk-signal mixing at a directional coupler Signal, power = Ps Isolation = x Crosstalk, power = Px Output power = where A depends critically on the relative coherence, optical Frequency and polarization state of the inputs.

  14. --- Coherent Crosstalk : Coherent signal and crosstalk wavelengths interact according to coupled mode theory, is the phase difference between the input. where --- Incoherent noise-free crosstalk:incoherent signal and crosstalk waveforms from different source couple independently, where s and x are the respective optical frequencies, whilst s and x represent the phase noise of each source. If the beat

  15. Frequency s - x exceeds the receiver bandwidth the intensity noise is filtered out leaving a simple sum of the individual signal and crosstalk intensities. --- Incoherent beat noise crosstalk : intensity noise from incoherent wavelengths will be present if either, they originate from very closely wavelength matched sources such that their beat frequency falls within the the receiver bandwidth, or if they originate from a single source but suffer a differential delay, , greater than the coherence time. In the latter case And the phase noise in the source is converted to intensity noise At the receiver.

  16. Optical Packet Switched Networks --- Basic principle of packet switching User A User E User B User F A H User C User G User D User H

  17. --- Functions to implement a packet switching structure a. Logic b. Storage c. Flow and Congestion control At the present time each one of the above functions is extremely difficult to perform completely optically.

  18. Frequency Domain Medium Access • Wavelength-Division Multiplexing Access (WDMA) • Subcarrier Multiplexing Access (SCMA)

  19. Wavelength Division Multiplexing Access (WDMA) • Basic concept of WDM Message Signal SN(x) N O M U X Optical Fiber SMF-28 LD Message Signal S1(x) 1 123 …N OMUX : Optical Multiplexer

  20. Features of WDMA --- Independence of channels --- Transparency and more efficient modulation format --- Small receiver bandwidth (sum of channel bandwidth) --- High sensitivity per channel --- Low multiplexing loss

  21. WDM Devices • Tunable Lasers --- External cavity lasers --- Integrated lasers • Tunable Filters --- Passive TF --- Active TF --- Semiconductor laser amplifiers • Multiplexers/Demultiplexers --- Dispersive elements --- Mach-Zehnder integrated waveguide

  22. Tunable Lasers --- External cavity lasers a. Acousto-optical tuning method b. Electro-optical tuning method --- Integrated tunable lasers a. DFB/DBR lasers b. 3-section vertical coupler filter (VCF)

  23. Summary of The Tuned Laser Characteristics

  24. Tunable Filters (TF) • --- Passive TF a. mirror position b. etalon angle • --- Active TF • a. Fabry-Perot filter • b. tunable Mach-Zehnder integrated interferometer • filters • c. wavelength-selective polarization by electro-optic • or acoustic-optic • --- Semiconductor laser amplifiers • a. DFB laser amplifier

  25. Summary of The Tuned Filter Characteristics

  26. Multiplexers/Demultiplexers • --- Dispersive elements a. Prism b. Diffraction grating  Thin film interference filters, 200/100 GHz •  Array waveguide gratings, 100/50 GHz •  Fiber Bragg gratings, 100/50 GHz •  With Interleaver, 25 GHz --- Mach-Zehnder integrated waveguide

  27. WDM Types --- Coarse wavelength-division multiplexing (CWDM) a.1310 nm/ 1550 nm b.Channel width up to 20 nm --- Dense wavelength-division multiplexing (DWDM) a. Channel width 1.6 nm ( 200 GHz) , 0.8 nm (100 GHz) 0.4 nm (50 GHz)

  28. Four Channel DWDM Using Micro-Optics • and Interference Filter Thin-film filter Collimators 1 1234 GRIN Lens 3 Collimators 2 4 GRIN: Grade-Index Fiber

  29. Four Channel DWDM Using Waveguide Gratings Waveguide Array 1 2 3 4 1234 Silicon Substrate

  30. Four Channel DWDM Using Fiber Bragg Gratings Input Channels Bragg Grating 12 34 Bragg Grating Dropped Channel 1 Dropped Channel 2 Dropped Channel Dropped Channel 3 4

  31. Unidirectional WDMA 1 1 A LD1 DWDM Device Single Fiber DWDM Device PIN A 2 2 B LD2 PIN B 12 LD : Distributed Feedback Laser (DFB Laser) PIN : PIN photodiode

  32. Bidirectional WDMA 1 1 A LD CWDM Device Single Fiber CWDM Device PIN A 2 2 B PIN 1 LD B 2 LD : Distributed Feedback Laser (DFB Laser) PIN : PIN photodiode

  33. WDM-based Passive Optical Network Optical Network Unit (ONU) Laser 1 Receiver WDM Star Coupler NN : : Optical Network Unit (ONU) Laser N Receiver WDM

  34. Direct Detection WDM Systems --- Fabry-Perot filter (FPF) receiver Direct detector isolator  Amp. Tunable FPF Adv. The direct detection receiving system offer simplicity, low cost and minimum complexity.

  35. --- Free spectral range (FSR) of FPF receiver 1 FSR=(N-0.5)fc T B B fc: minimum channel spacing B: bandwidth of the filter

  36. Coherent Detection WDM Systems --- Basic Concept Received Optical Signal Combiner Photo- detector Receiver Electronics Data Local Oscillator

  37. Critical Parameters in WDMA System --- Channel band a. Channel central wavelength b. Channel spacing c. Bandwidth at –3 dB d. Isolation and crosstalk e. Flatness (i.e. ripple on the peak of the channel power) f. Channel uniformity --- Polarization Dependent Effects a. polarization dependent loss b. polarization mode dispersion --- Insertion Loss (IL) --- Directivity --- Optical Return Loss

  38. --- Channel band a. Channel central wavelength Central wavelength -3 dB A comparison between two traces shows how an apparently minor change can affect the central wavelength  Channel Central wavelength -3 dB  Channel

  39. b. Channel spacing Spaced channels are most commonly in accordance with the ITU grid, spaced at 100 GHz (1.6 nm) or 50 GHz (0.8 nm) intervals. c. Bandwidth at –3 dB Bandwidth defines the spectral range over which The device can be used effectively or called full Width half maximum (FWHM).

  40. d. Isolation and crosstalk Channel isolation describes the rejection of signal power from or to another channel. Adjacent-channel isolation of 25 dB or greater were sufficient. Crosstalk describes the power leaking through a channel band from other channel. e. Flatness The flatness is the possible variations in transmitted power as transmission wavelength varies within the normal bandpass.

  41. f. Channel uniformity Channel uniformity refers to the amount of variation in transmittance or the insertion loss from channel to channel in a mux/demux. --- Polarization Dependent Effects Channel characteristics such as insertion loss, central wavelength, and bandwidth will vary with polarization. Therefore, to guarantee reliable performance, a system designer must accommodate the worst-case polarization dependence of all the passive components used in the system.

  42. a. Polarization dependent loss (PDL) PDL is the variation in loss over the range of possible polarization states. It obtained from the ratio between the transmittances in the Best and worst polarization states. In general, PDL < 0.1 dB. -7 SOP = 90 SOP = 45 SOP = 0 Transmittance -18 1548.7 1547.8 Wavelength

  43. b. Polarization mode dispersion (PMD) PMD is the two polarization components of a signal travel at different velocities, and fall out of phase along a fiber link. This Effect distorts signal pulses, broadens and affects error rates in Digital systems, and can introduce seriours harmonic distortion in analog systems.

  44. --- Insertion Loss (IL) The IL of a component is the difference between the powr entering and leaving it, i.e., --- Directivity Directivity is a measure of the isolation between the input ports Of a multi-input device. 1 Non-reflective termination 2 3 4 WDM

  45. --- Optical Return Loss a. It contributes to overall power loss. b. High-performance laser transmitters are very sensitive to reflected light which can significantly degrade the stability of the laser and S/N of the system. c. Reflected light can be reflected in the forward direction, and cause multi-path interference (MPI).

  46. System Engineering in the WDMA • Stabilising the wavelength of sensitive components • Control of non-linear effects • Control of dispersion • Control of cross-talk • Control of system noise (especially ASE)

  47. Stabilising Wavelengths --- Temperature control --- Laser chirp and relaxation oscillation --- Laser drift --- Frequency referencing a. Fiber Bragg gratings b. Fabry-Perot filters (or Etalons)

  48. Control of non-linear effects --- Four-Wave Mixing (FWM) Signal Wavelengths Spurious Side Modes 21- 2 1 2 22- 1 --- Stimulated Brillouin Scattering (SBS)

  49. --- Stimulated Raman Scattering (SRS) --- Carrier-Induced Phase Modulation (CIP) a. Self Phase Modulation (SPM) b. Cross Phase Modulation (XPM)

  50. Control of Dispersion --- The Using of dispersion-shifted fiber --- Nonlinear Prechirp compensation --- Using long chirp fiber Bragg gratings

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