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8/13/2019 10 Fiber Optics
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8/13/2019 10 Fiber Optics
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Optical communication systems
Optical communication systems
Optical-fiber cable
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AM, FM, and Digital
AM, FM, and Digital
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Light guidesLight guides
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Optical fibersOptical fibers
Step-index fiber Graded-index fiber
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10-4. Optics of propagation10-4. Optics of propagation
Cladding (n2)
Core (n1)
Air (n0)
Numerical aperture (N.A.)
Step index fiber
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Skip distance :
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10-5. Allowed modes : in slab (planar) waveguides10-5. Allowed modes : in slab (planar) waveguides
Modes (self-sustaining waves) must satisfy the condition.
Total number of propagating modes is the value of m when m c =
( )2 2
max 1 1 1
2 2 2
cos 1 . . 1 1cd d d
m n N A n n = + = + = +
(See p. 501)
m=0; straight-through mode at = 90o.
large m; high-order mode with near c
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Allowed modes : in fibersAllowed modes : in fibers
2
max
1. .
2
dm N A
=
max
d 2
2 ( . .)m N A <
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10-6. Attenuation (loss)10-6. Attenuation (loss)
Extrinsic losses : bending, defects,
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Intrinsic losses : absorption, Rayleigh scattering,
Silica glass fiber
4
1
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Attenuation (absorption) coefficientAttenuation (absorption) coefficient
[db/km]
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10-7. Distortion10-7. Distortion
Modal distort ion
L
L
Consider two extreme modes
when m = 0 and m = mmax
[sec/km]
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GRIN (graded index) fibersGRIN (graded index) fibers
Modal distortion
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Material dispersionMaterial dispersion
Pulse broadening
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Material dispersionMaterial dispersion
M : ps/nm.km
: temporal pulse spread (psec)
per unit spectral width (nm)
per unit length (km)
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Waveguide dispersionWaveguide dispersion
For a guided mode, the effective refractive-index of the mode is
1 1 1 2sin n sineffeff m cg
nc
n n n nv
= =