Lecture3-228a s10-1

Lecture3-228a s10-1 - Lecture 3 Fiber Optics Modes Loss and...

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Lecture 3 - Fiber Optics – Modes, Loss, and Dispersion ECE 228A 3.1
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Lecture 2 Review •Step index: Simplest type of b fiber •(Most fiber these days is far more complex) •Cylindrical symmetry •Express Laplacian operator cylindrical coordinates in cylindrical coordinates 2 2 2 2 2 2 2 2 1 1 r •Separate variables (p. 31) z r r r ) ( ) ( ) ( ) , , ( z Z F z E z
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V parameter V<2.405 for single mode
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Fiber Modes
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Fiber-Optic Communication Systems 1 2 2 2 1 2 sin n n n n NA ext c The index difference between core and cladding determines the acceptance angle (numerical aperture) of fiber. 2 1 n n p.25
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Two primary limits to transmission Loss: Loss budget for loss limited transmission Dispersion: Dispersion budget for dispersion limited transmission.
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Comparison to electrical cable
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Loss in early optical fibers (now the O-H peaks around 1.4 m are small) 4 Rayleigh Scattering / C R
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Loss Budget p trans =transmitter power p rec =sensitivity of receiver Take 10 log of each side and express in dBm P L trans rec e p p P trans , P rec trans rec L P P rec trans P P L ax Example: max P trans =10 dBm P rec =-20 dBm L max =30 dB/0.2 dB/km=150 km
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Loss budget How does maximum distance change if a new laser is made with 10x the power? 100x the power? How does the maximum distance change if a new receiver is made with 10x better receiver (i.e. coherent receiver)? How does the maximum distance change with 0.05 db/km less loss (depressed cladding versus Ge-doped fibers)? How does the maximum distance change with ½ the loss (chalcogenide fibers)? How does the maximum distance change with periodic 30 dB amplifiers? How far apart should they be? What is the bit rate dependence of maximum distance between repeaters?
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Dispersion Inter modal – different modes have different β Intra modal (i.e. group-velocity dispersion) Material dispersion – silica refractive index is a function of wavelength Waveguide dispersion – V parameter is a function of wavelength Polarization-Mode Dispersion – bifrefringence induced by perturbations p.40
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Fiber-Optic Waveguides Step index fiber: Standard for single mode (small core size – 8 micron) Graded index fiber: Designed so all multimodes travel at the same velocity. p.24
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Multimode Dispersion For step index multimode fibers, the fiber bandwidth (in MHz km) is given by 2 2 c n B For graded index fibers, the fiber bandwidth in MHz km is given by 1 L n 2 8 c B 1 L n
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Multimode Dispersion For step index multimode fibers, the fiber bandwidth (in MHz km) is given by 2 2 c n B For graded index fibers, the fiber bandwidth in MHz km is given by 1 L n 2 8 c B 1 L n hat is the bit rate distance product in each case for delta=0 01? What is the bit rate distance product in each case for delta=0.01?
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Group-Velocity Dispersion The index of the mode is dependent on the wavelength (i.e. the fiber is
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This document was uploaded on 01/29/2011.

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Lecture3-228a s10-1 - Lecture 3 Fiber Optics Modes Loss and...

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