This document discusses different types of dispersion in optical fibers, including modal dispersion, material dispersion, waveguide dispersion, and polarization mode dispersion. It defines important terms related to dispersion like group velocity and group delay. It also examines how dispersion causes pulse broadening over distance as different wavelengths within a pulse propagate at different speeds through the fiber. Finally, it compares the dispersion characteristics of different fiber types like dispersion shifted and flattened fibers which are designed to reduce dispersion effects.

1. EE 230: Optical Fiber Communication Lecture 4
From the movie
Warriors of the Net
Fiber Dispersion

2. 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

3. Definitions
Group velocity
Group delay
Dispersion
1−






∂
∂
=
ω
β
gv
kc ∂
∂
=
∂
∂
=
β
ω
β
τ
1
λ
τ
∂
∂
=D

4. Wave Packet Construction

5. Group and Phase Velocity

6. Transform Limited Frequency Spread
Optics-Hecht & Zajac
Rectangular Pulse Pulse modulated Sine wave
Gaussian modulated Sine wave

7. Spectral properties of digitally modulated
signals1
Bit
TimeT
Time
RZ
Encoded
Data
NRZ
Encoded
Data
1 0 1 1 0 0 1
Frequency
1/T 2/T 3/T
P
o
w
e
r
No spectral component at the clock frequency
For Non-Return to Zero Encoded Data

8. Pulse Propagation through Fibers
Fundamentals of Photonics - Saleh and Teich
Response of a multi-mode fiber to a single
short pulse
Broadening of a short pulse after transmission
through different types of fibers

9. Material Dispersion
Refractive Index of Pure Silica and its derivatives
Fiber Optic Communications-Palais
Material Dispersion Parameter for pure silica
and germanium doped silica
Optical Fiber Communications-Keiser

10. Zero Dispersion Slope
Fiber Optics Communication Technology-Mynbaev & Scheiner

11. Fiber Dispersion
Pulse Degradation from Dispersion
Ray optics View of the origin of waveguide dispersion
Typical fiber Dispersion Curve
Dispersion:The change in speed of propagation with wavelength

12. 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

13. Representative Fiber Characteristics

14. Dispersion for various Fiber Types

15. Dispersion Shifted and Dispersion
Flattened Fibers
Fundamentals of Photonics - Saleh and Teich
Waveguide Dispersion mechanism
As λ ⇑, Mode field diameter ⇑
More of the mode travels in the cladding.
Cladding index is lower so pulse velocity ⇑
resulting in dispersion ⇓
Chromatic Dispersion Parameters of
a singlemode fiber

16. Comparison of Dispersion Characteristics

17. Propagation of Gaussian Pulses
Input Pulse Output Pulse
chirped and broadened
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.
β2<0 for standard single mode silica fiber
and Ld ~ 1800 km at 2.5 Gb/s and ~115 km at 10 Gb/s
Optical Networks a Practical Perspective-Ramaswami and Sivarajan