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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

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EE 230: Optical Fiber Communication Lecture 4

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Ee 230 optical fiber communication lecture 4 l.jpg

EE 230: Optical Fiber Communication Lecture 4

Fiber Dispersion

From the movie

Warriors of the Net


Types of dispersion l.jpg

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

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


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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

Chromatic Dispersion Parameters of

a singlemode fiber

  • 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

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

b2<0 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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