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 0,max n 1 2 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 0.2121 and 0,max 12 ˚ The maximum acceptable angle when launching an optical beam into a fiber is consequently of the order of 0,max ECE 228A 4.2 12 ˚
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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=50 μ m 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=50 m, =5x10 and =1.3 μ m 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
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Graded index fibers he multimode dispersion limit can be drastically changed by using a The multimode dispersion limit can be drastically changed by using a proper index of refraction profile di n 1 n 2 Step Index radius n 2 Graded Index radius n 1 ECE 228A 4.5
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Lecture4-228a s10 - Fiber Optics Dispersion, Manufacturing...

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