Guided Propagation Along the Optical Fiber The Nature of Light • Quantum Theory – Light consists of small particles (photons) • Wave Theory – Light travels as a transverse electromagnetic wave • Ray Theory – Light travels along a straight line and obeys laws of geometrical optics. Ray theory is valid when the objects are much larger than the wavelength (multimode fibers)
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Guided Propagation Along the Optical Fibersite.iugaza.edu.ps/mtastal/files/Pres3-fiberHO.pdf · Guided Propagation Along the Optical Fiber The Nature of Light • Quantum Theory –
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c We can visualize a graded indexfiber by imagining a stratifiedmedium with the layers of refractiveindices na > nb > nc ... Consider two
close rays 1 and 2 launched from Oat the same time but with slightlydifferent launching angles. Ray 1just suffers total internal reflection.Ray 2 becomes refracted at B andreflected at B'.
n decreases step by step from one layerto next upper layer; very thin layers.
Continuous decrease in n gives a raypath changing continuously.
TIR TIR
(a) A ray in thinly stratifed medium becomes refracted as it passes from onelayer to the next upper layer with lower n and eventually its angle satisfies TIR.(b) In a medium where n decreases continuously the path of the ray bendscontinuously.
– Scattering losses (Rayleigh, Raman and Brillouin…)
– Bending losses (macro and micro bending)
dB/km dB)(dB)0(
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All Wave Fiber for DWDM
Lowest attenuation occurs at
1550 nm for Silica A
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Escaping wave
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Microbending
R
Cladding
Core
Field distribution
Sharp bends change the local waveguide geometry that can lead to wavesescaping. The zigzagging ray suddenly finds itself with an incidenceangle that gives rise to either a transmitted wave, or to a greatercladding penetration; the field reaches the outside medium and some lightenergy is lost.
• Modal Dispersion: Different modes travel at different velocities, exist only in multimodal conditions
• Waveguide Dispersion: Signal in the cladding travel with a different velocity than the signal in the core, significant in single mode conditions
• Material Dispersion: Refractive index n is a function of wavelength, exists in all fibers, function of the source line width
Low order modeHigh order mode
Cladding
Core
Light pulse
t0 t
Spread,
Broadened
light pulse
IntensityIntensity
Axial
Schematic illustration of light propagation in a slab dielectric waveguide. Light pulseentering the waveguide breaks up into various modes which then propagate at differentgroup velocities down the guide. At the end of the guide, the modes combine toconstitute the output light pulse which is broader than the input light pulse.
All excitation sources are inherently non-monochromatic and emit within aspectrum, ² , of wavelengths. Waves in the guide with different free spacewavelengths travel at different group velocities due to the wavelength dependenceof n1. The waves arrive at the end of the fiber at different times and hence result in