Types Attenuation

8/8/2019 Types Attenuation

1/16

Folie 1.1


2/16

Folie 1.2

Refraction (and reflection)


3/16

Folie 1.3

Refraction (and reflection)

from optical thinnerto optical thicker medium

from optical thickerto optical thinner medium


4/16

Folie 1.4

Totalreflection

Special case of refractionwith ideal materials (no attenuation):

no loss in energy or power 100 % reflection

however: attenuation and loss due to material itself

optical thicker medium (n1)

optical thinner medium (n2)


5/16

Folie 1.5

Overview of fiber types

Type

Graded-index fiber

GI

Ray propagationProfile

Step-indexfiber

SI

Singlemodefiber

SM

Reflection

Refraction

Diffraction/Light-guiding


6/16

Folie 1.6

Construction of optical cable

Optical fiber

tension release(kevlar)

cable covering

secundary coating

primary coating

fiber axis

cladding

core

light ray


7/16

Folie 1.7

Totalreflection with light rays:basic mechanism in step-index fibers

core (nco)

cladding (ncl)

c

critical angle of totalreflection:

c = arc sin (ncl/nco)


8/16

Folie 1.8

Numerical apertureof SI-multimode fiber (using ray optics)

core (nco)

cladding (ncl)

a max: acceptance angle of meridional rays

defintion (used for microscope):

NA = sin a max = using Snells laws

c

a

max acceptance coneNA: describe the light-gatheringability


9/16

Folie 1.9

Light propagation in GI-fibersdue to refraction

Total reflection

n(r=0) = n0

Sine-wave, forlayer thickness 0

Snells law at allboundaries (refractedaway from lot)

n1

n2

n3

nN-2

nN-1

nN = ncl


10/16

Folie 1.10

Propagation of meridional rays

1

3

1

4

SI-fiber GI-fiber

12

3

44

4

3

2

2Meridional rays:


11/16

Folie 1.11

Acceptance angle and numerical apertureof multimode-fibres (meridional rays)

cladding (ncl)SI -fiber :

core (nco)

a

max

c: propagation anglecritical angle

NA = sin a max = nco - nclGI - fiber :

a

max

(r)

a max(0)

NA(r) = sin a max(r) = n(r) - ncl

cladding (ncl)

center (no)


12/16

Folie 1.12

Fiber attenuation

mainly local losses

Material-properties Fiber-properties(waveguide)

Absorptionintrinsic absorption (silica)ext. absorption

(impurities)

ScatteringRayleighRaman, Brillouinimperfections (local)

Additional lossUV-defects due to

ionizing radiationdiffusion effects (H2)

Disturbance of geometrymanufacturing

Bending losscurvatureexternal pressure

Influence of boundary

between core and claddingtensile stress


13/16

Folie 1.13

Back scattering method

Rayleigh scattering

Scattering of incoming light: particles or n-variationssmaller than wavelength of lightno wavelength shift

Silica as medium is optically inhomogenous:density variation leads to a spatial variation ofrefractive-index

Dominant mechanism of attenuation in modern silica-

based fibers Rayleigh scattering is strongly wavelength-dependent

a s~ 1/l4


14/16

Folie 1.14

Pulse spreading in fibers

3 effects for pulse-spreading (dispersion) material dispersion (M)

arises due to the variation of propagation velocitiesor delays with wavelength (bulk property)

modal dispersionarises due to different propagation velocities / delaysof different rays (or modes)

waveguide dispersion (W) in singlemode-fibresdetermined by the difference in propagation velocitybetween core and cladding

pulse-spreadingleads to

reduction of bandwidth/data rate of the system


15/16

Folie 1.15

1. High carrier-frequency (150 ... 400 THz), wavelength 1 m- higher modulation: > 10 GHz possible

2. Low loss/attenuation (< 0.3 dB/km )- longer transmission links without repeater (> 50km)

3. Small diameter (125 m)

- less material / lower weight- light and flexible cable

4. High resistance against electromagnetic waves

- no shielding necessary5. No interfering radiation to external

- no noticeable cross-talk6. Electrical insulator

- no problems with earthing / potential differences

Advantages of optical fibres


16/16

Folie 1.16

Fiber:1. Small diameter: difficulties with connections

2. Additional conducting line for electric power supplies

in remote terminals (if necessary)

3. Susceptibility of fibre to hydrogen, water and ionizing

radiation

Systems:

4. Poor source efficiency5. Nonlinearities of sources limit analog use

6. Difficulties with connections

7. High receiver noise

Disadvantages of optical fiber systems

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Types Attenuation