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EE 230: Optical Fiber Communication Lecture 13

EE 230: Optical Fiber Communication Lecture 13. Dispersion Compensation. From the movie Warriors of the Net. Pulse Dispersion. Definition of chirp. The chirp C is defined by the change in frequency d  due to the rate of change of the phase:  is the initial 1/e duration of the pulse.

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EE 230: Optical Fiber Communication Lecture 13

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  1. EE 230: Optical Fiber Communication Lecture 13 Dispersion Compensation From the movie Warriors of the Net

  2. Pulse Dispersion

  3. Definition of chirp The chirp C is defined by the change in frequency d due to the rate of change of the phase:  is the initial 1/e duration of the pulse

  4. Spread of Gaussian Pulse

  5. Dispersion Power Penalty at different Bit Rates

  6. Degradation of a 40 Gb/s Signal

  7. Ideal Dispersion Compensation Device • Large negative dispersion coefficient • Low attenuation • Minimal nonlinear contributions • Wide bandwidth • Corrects dispersion slope as well • Minimal ripple • Polarization independent • Manufacturable

  8. Various Dispersion Compensation Techniques

  9. Propagation of Gaussian Pulses Input Pulse Output Pulse chirped and broadened b2<0 for standard single mode silica fiber and Ld ~ 1800 km at 2.5 Gb/s and ~115 km at 10 Gb/s Upon further propagation the pulse will continue to broaden and acquire chirp. Input Pulse Already Positively Chirped After some distance the chirp is removed and the pulse assumes its minimum possible width Optical Networks a Practical Perspective-Ramaswami and Sivarajan

  10. Spectral Shaping at the Transmitter Optical Fiber Telecommunications IIIA

  11. Compensation at Receiver • Adjust decision point on the fly based on previous few bits • Mathematically extrapolate signal back to what it presumably was at origin • These techniques can be used only if calculations can be done much faster than bit rate

  12. Dispersion Properties of Various Fibers

  13. Chromatic Dispersion Properties of Various Fibers

  14. Conventional Dispersion Compensating Fiber Fiber Optic Communications Technology- Mynbaev & Scheiner

  15. Dispersion Compensating Fiber

  16. Use of Dispersion Compensating Fiber Understanding Fiber Optics-Hecht

  17. Problem with Conventional Dispersion Shifted Fiber

  18. Importance of Slope Matching

  19. Link Distance Dependence on Slope Matching

  20. Higher order Mode DispersionProperties LaserComm

  21. High-Order-Mode Dispersion Compensation Device

  22. Compensation with Optical Filters

  23. Chirped fiber Bragg grating dispersion where  is the difference between Bragg wavelengths at ends of grating. For n=1.45 and =0.2 nm, D=4.8x107 ps/(km-nm) as compared to 18 for fiber

  24. Chirped Fiber Bragg Gratings Optical Networks A Practical Perspective-Ramaswami & Sivarajan

  25. Pulse Spreading due to Self Phase Modulation

  26. Four-wave Mixing

  27. Taylor Series expansion of β(ω) Through the cubic term: where

  28. Importance of Taylor Series terms Group velocity Vg, dispersion D, and dispersion slope S

  29. Four-Wave Mixing Phase-Matching Requirement Phase mismatch M needs to be small for FWM to occur significantly

  30. Spectral Inversion • Add pump signal whose wavelength is ideally at zero-dispersion point • Four-wave mixing generates phase conjugate signal at 2p-s • Phase conjugate undoes both GVD and SPM over second half of link • Filter out pump beam at end

  31. Mid-Span Spectral Inversion Optical Fiber Telecommunications IIIA

  32. Dispersion Managed Network

  33. Summary of Techniques • At transmitter: prechirping, coding • At receiver: signal analysis, decision point adjustment • Fiber: DCF, DSF, dual-mode fiber • Filters: Bragg gratings, Mach-Zehnders • Spectral inversion

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