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FIBER OPTICS
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Agrawal-Fiber Optic Communications
Progress In Lightwave
Communication Technology
Agrawal-Fiber Optic Communications
Waveguide Basics
• Light is guided by a medium with a higher refractive index (Core)
surrounded by that with a lower one (Cladding)
• Step-index
• Numeric aperture (NA) = sine of the largest acceptance angle
NA  sin('m )  n12  n22  n1 2
n1  n2

n1
• Typically, NA=0.15 for single mode fiber (SMF) and 0.3 for
multimode fiber (MMF).
Optical Fiber
• Some Typical Dimension :
– Single Mode Fiber (SMF): 8 μm core
– MultiMode Fiber (MMF) : 50, 62.5, 85, 100, 200
μm core
• Index profile:
– Step vs. Graded vs. multi-step…
Graded index
8
8
Optical Loss
Agrawal-Fiber Optic Communications
Dispersion
• Different components of transmitted signal travel at
different velocities in the fiber and arrive at different
times at the receiver
– Modal dispersion: different modal components of a
pulse travel at different velocities
– Chromatic dispersion: different spectral components
of a pulse travel at different velocities
• Material dispersion: due to l-dependence in
doped silica (or any other core material)
• Waveguide dispersion: due to waveguide design
– Polarization dispersion: different polarization
components of a pulse travel at different velocities
Multimode Fiber Limitation:
Modal Dispersion
Multimode Fiber Limitation:
Modal Dispersion
Modal Dispersion is critical in MMF.