Lecture3-228a s10(3)

Lecture3-228a s10(3) - Lecture 3 - Fiber Optics Modes,...

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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 w . w . w . w ¡ •Separate variables (p. 31) z r r r w w w w I ) ( ) ( ) ( ) , , ( z Z F z E z U )
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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 ' | 0 T × The index difference between core and cladding determines the acceptance angle (numerical aperture) of fiber. 2 1 n n 0 ' 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 P m are small) 4 Rayleigh Scattering / O D 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 D L trans rec e p p 0 × P trans , P rec trans rec L P P 0 rec trans P P L 0 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 themaximum distance change if a new receiver ismadewith 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 ȕ × 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 ?
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This note was uploaded on 05/18/2010 for the course ECE 228a taught by Professor Bowers,j during the Spring '08 term at UCSB.

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Lecture3-228a s10(3) - Lecture 3 - Fiber Optics Modes,...

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