Lecture4-228a s10

# Lecture4-228a s10 - Fiber Optics Dispersion Manufacturing...

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Fiber Optics –Dispersion, Manufacturing Lecture #4 John Bowers ECE 228A Homework #1 due Thursday

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Numerical Aperture Definition: The light collecting capacity of the optical fiber is measured y the umerical Aperature (NA) by the Numerical Aperature (NA) NA n 0 sin T 0,max n 1 2 0 n 2 2 | n 2 ' (For small ' ) 1 Example: if we couple light from air into a fiber with = .01 and n 1 = 1.5 , then the NA Ő 0.2121 and 0,max Ő 12˚ The maximum acceptable angle when launching an optical beam into a fiber is consequently of the order of 0,max Ő ECE 228A 4.2 12˚
Multimode Fibers V 2 For large V, the number of modes N 2 g, propagating in a multimode fiber is approximately xample: multimode fiber with core diameter a=50vm 5x10 -3 d × Each mode will propagate in the fiber at as if it had its own index of Example: A multimode fiber with core diameter 2a=50vm, ' =5x10 and O =1.3vm supports about 160 modes refraction n . × The index of refraction for each mode n lies between n 1 and n 2 (from the solution of the Maxwell equations) × Intuitive explanation: each mode has different portions of the field overlap with different amounts of the core and cladding × Consequence: each mode will travel along the fiber at slightly different ECE 228A 4.3 speeds, giving rise to multimode fiber dispersion

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Bit Rate Limit for Multimode Fibers × When dealing with digital transmission, each pulse represent a bit × A pulse spreading leads to intersymbol interference (ISI) × Let’s assume a bit cannot spread by more than half the allocated bit period in order to have an acceptable ISI level ' T ? 1 2 B T B = 1/B T B = 1/B L c n 1 2 n 2 '? 1 2 B BL ? n 2 c 2 We can define the Bandwidth-Distance product (BL) for multimode fibers as: ECE 228A 4.4 2 n 1 '
Graded index fibers he multimode dispersion limit can be drastically changed by using a ×

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Lecture4-228a s10 - Fiber Optics Dispersion Manufacturing...

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