Download EE 230: Optical Fiber Communication Lecture 4

EE 230: Optical Fiber Communication Lecture 4
Fiber Dispersion
From the movie
Warriors of the Net
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
Definitions
Group velocity
Group delay
Dispersion
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vg   
  
1
 1 


 c k

D
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Wave Packet Construction
Group and Phase Velocity
Transform Limited Frequency Spread
Rectangular Pulse
Pulse modulated Sine wave
Gaussian modulated Sine wave
Optics-Hecht & Zajac
Spectral properties of digitally modulated signals1
No spectral component at the clock frequency
For Non-Return to Zero Encoded Data
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
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
Zero Dispersion Slope
Fiber Optics Communication Technology-Mynbaev & Scheiner
Fiber Dispersion
Dispersion:The change in speed of propagation with wavelength
Pulse Degradation from Dispersion
Typical fiber Dispersion Curve
Ray optics View of the origin of waveguide dispersion
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
Representative Fiber Characteristics
Dispersion for various Fiber Types
Dispersion Shifted and Dispersion
Flattened Fibers
Chromatic Dispersion Parameters of
a singlemode fiber
Fundamentals of Photonics - Saleh and Teich
Waveguide Dispersion mechanism
As   Mode field diameter 
More of the mode travels in the cladding.
Cladding index is lower so pulse velocity 
resulting in dispersion 
Comparison of Dispersion Characteristics
Propagation of Gaussian Pulses
Input Pulse
Output Pulse
chirped and broadened
2<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