EE 230: Optical Fiber Communication Lecture 4

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Fiber Dispersion

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

1−

∂∂=ωβ

gv

kc ∂∂=

∂∂= βωβτ 1

λτ

∂∂=D

Wave Packet Construction

Group and Phase Velocity

Transform Limited Frequency Spread

Optics-Hecht & Zajac

Rectangular Pulse Pulse modulated Sine wave

Gaussian modulated Sine wave

Spectral properties of digitally modulated signals1

.

BitTime T

Time

RZEncodedData

NRZEncodedData

1 0 1 1 0 0 1

Frequency1/T 2/T 3/T

P o w e r

No spectral component at the clock frequencyFor Non-Return to Zero Encoded Data

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

Material Dispersion

Refractive Index of Pure Silica and its derivatives

Fiber Optic Communications-Palais

Material Dispersion Parameter for pure silicaand germanium doped silica

Optical Fiber Communications-Keiser

Zero Dispersion Slope

Fiber Optics Communication Technology-Mynbaev & Scheiner

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

Polarization Mode Dispersion

Polarization Maintaining Fiber

Non-Polarization Maintaining Fiber

Random power transfer occurs between different polarization modesas the pulse propagates, if the two modes have different propagation velocitiesthen the pulse will broaden

Representative Fiber Characteristics

Dispersion for various Fiber Types

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 ofa singlemode fiber

Comparison of Dispersion Characteristics

Propagation of Gaussian Pulses

Input Pulse Output Pulse chirped and broadened

Input PulseAlready PositivelyChirped

After some distancethe chirp is removedand 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