EE 230: Optical Fiber Communication Lecture 13

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