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EE 230: Optical Fiber Communication Lecture 13
Dispersion Compensation From the movie Warriors of the Net
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Pulse Dispersion
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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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Spread of Gaussian Pulse
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Dispersion Power Penalty at different Bit Rates
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Degradation of a 40 Gb/s Signal
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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
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Various Dispersion Compensation Techniques
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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 Input Pulse Already Positively Chirped After some distance the chirp is removed and the pulse assumes its minimum possible width Upon further propagation the pulse will continue to broaden and acquire chirp. Optical Networks a Practical Perspective-Ramaswami and Sivarajan
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Spectral Shaping at the Transmitter
Optical Fiber Telecommunications IIIA
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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
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Dispersion Properties of Various Fibers
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Chromatic Dispersion Properties of Various Fibers
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Conventional Dispersion Compensating Fiber
Fiber Optic Communications Technology- Mynbaev & Scheiner
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Dispersion Compensating Fiber
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Use of Dispersion Compensating Fiber
Understanding Fiber Optics-Hecht
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Problem with Conventional Dispersion Shifted Fiber
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Importance of Slope Matching
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Link Distance Dependence on Slope Matching
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Higher order Mode DispersionProperties
LaserComm
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High-Order-Mode Dispersion Compensation Device
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Compensation with Optical Filters
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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
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Chirped Fiber Bragg Gratings
Optical Networks A Practical Perspective-Ramaswami & Sivarajan
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Pulse Spreading due to Self Phase Modulation
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Four-wave Mixing
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Taylor Series expansion of β(ω)
Through the cubic term: where
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Importance of Taylor Series terms
Group velocity Vg, dispersion D, and dispersion slope S
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Four-Wave Mixing Phase-Matching Requirement
Phase mismatch M needs to be small for FWM to occur significantly
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Spectral Inversion Add pump signal whose wavelength is ideally at zero-dispersion point Four-wave mixing generates phase conjugate signal at 2p-s Phase conjugate undoes both GVD and SPM over second half of link Filter out pump beam at end
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Mid-Span Spectral Inversion
Optical Fiber Telecommunications IIIA
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Dispersion Managed Network
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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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