EE 230: Optical Fiber Communication Lecture 13From the movieWarriors of the NetDispersion Compensation
Pulse Dispersion
Definition of chirpThe 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
Spread of Gaussian Pulse
Dispersion Power Penalty at different Bit Rates
Degradation of a 40 Gb/s Signal
Ideal Dispersion Compensation DeviceLarge negative dispersion coefficientLow attenuationMinimal nonlinear contributionsWide bandwidthCorrects dispersion slope as wellMinimal ripplePolarization independentManufacturable
Various Dispersion Compensation Techniques
Spectral Shaping at the TransmitterOptical Fiber Telecommunications IIIA
Compensation at ReceiverAdjust decision point on the fly based on previous few bitsMathematically extrapolate signal back to what it presumably was at originThese techniques can be used only if calculations can be done much faster than bit rate
Dispersion Properties of Various Fibers
Chromatic Dispersion Properties of Various Fibers
Conventional Dispersion Compensating FiberFiber Optic Communications Technology- Mynbaev & Scheiner
Dispersion Compensating Fiber
Use of Dispersion Compensating FiberUnderstanding Fiber Optics-Hecht
Problem with Conventional Dispersion Shifted Fiber
Importance of Slope Matching
Link Distance Dependence on Slope Matching
Higher order Mode DispersionPropertiesLaserComm
High-Order-Mode Dispersion Compensation Device
Compensation with Optical Filters
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
Chirped Fiber Bragg GratingsOptical Networks A Practical Perspective-Ramaswami & Sivarajan
Pulse Spreading due to Self Phase Modulation
Four-wave Mixing
Taylor Series expansion of ()Through the cubic term:
where
Importance of Taylor Series termsGroup velocity Vg, dispersion D, and dispersion slope S
Four-Wave Mixing Phase-Matching RequirementPhase mismatch M needs to be small for FWM to occur significantly
Spectral InversionAdd pump signal whose wavelength is ideally at zero-dispersion pointFour-wave mixing generates phase conjugate signal at 2p-sPhase conjugate undoes both GVD and SPM over second half of linkFilter out pump beam at end
Mid-Span Spectral InversionOptical Fiber Telecommunications IIIA
Dispersion Managed Network
Summary of TechniquesAt transmitter: prechirping, codingAt receiver: signal analysis, decision point adjustmentFiber: DCF, DSF, dual-mode fiberFilters: Bragg gratings, Mach-ZehndersSpectral inversion