EE 230: Optical Fiber Communication Lecture 13

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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
b2lt0 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
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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 n1.45 and ??0.2 nm, D4.8x107 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 ß(?)

• 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 2?p-?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