10 fiber optics

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

= =

10 fiber optics

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