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

Propagation of Gaussian PulsesInput PulseOutput Pulse chirped and broadenedInput PulseAlready PositivelyChirpedAfter some distancethe chirp is removedand the pulse assumes its minimum possible widthUpon further propagation the pulse will continue to broaden and acquire chirp.b2

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