EE 230: Optical Fiber Communication Lecture 4

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EE 230 Optical Fiber Communication Lecture 4
Fiber Dispersion
From the movie Warriors of the Net
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Types of Dispersion

• Modal dispersion different modes propagate at
  different group velocities
• Material dispersion the index of refraction of
  the medium changes with wavelength
• Waveguide dispersion index change across
  waveguide means that different wavelengths have
  different delays
• Polarization mode dispersion if waveguide is
  birefringent

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Definitions

• Group velocity
• Group delay
• Dispersion

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Wave Packet Construction
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Group and Phase Velocity
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Transform Limited Frequency Spread
Pulse modulated Sine wave
Rectangular Pulse
Optics-Hecht Zajac
Gaussian modulated Sine wave
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Spectral properties of digitally modulated
signals1
No spectral component at the clock frequency For
Non-Return to Zero Encoded Data
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Pulse Propagation through Fibers
Response of a multi-mode fiber to a single short
pulse
Broadening of a short pulse after transmission
through different types of fibers
Fundamentals of Photonics - Saleh and Teich
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Material Dispersion
Refractive Index of Pure Silica and its
derivatives
Material Dispersion Parameter for pure silica and
germanium doped silica
Optical Fiber Communications-Keiser
Fiber Optic Communications-Palais
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Zero Dispersion Slope
Fiber Optics Communication Technology-Mynbaev
Scheiner
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Fiber Dispersion
DispersionThe change in speed of propagation
with wavelength
Pulse Degradation from Dispersion
Typical fiber Dispersion Curve
Ray optics View of the origin of waveguide
dispersion
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Polarization Mode Dispersion
Polarization Maintaining Fiber
Non-Polarization Maintaining Fiber
Random power transfer occurs between different
polarization modes as the pulse propagates, if
the two modes have different propagation
velocities then the pulse will broaden
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Representative Fiber Characteristics
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Dispersion for various Fiber Types
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Dispersion Shifted and Dispersion Flattened Fibers
Waveguide Dispersion mechanism As l Ý, Mode
field diameter Ý More of the mode travels in the
cladding. Cladding index is lower so pulse
velocity Ý resulting in dispersion ß
Chromatic Dispersion Parameters of a singlemode
fiber
Fundamentals of Photonics - Saleh and Teich
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Comparison of Dispersion Characteristics
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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