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Signal Degradation In Optical Fiber

Attenuation

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Degradation Mechanisms in OF

Attenuation Distortion

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Attenuation Power loss along a fiber:

P(z) is signal power at any distance z. P(0) is signal power launched in fiber. is fiber Attenuation coefficient and l is length of fiber.

Z=0P(0) mW

Z= llpePlP )0()( mw

zpePzP )0()( [1]

)()0(log10

lPPlp [2]

p

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Fiber loss in dB/km

The parameter is called fiber attenuation coefficient in a units of [dB/km] that is defined by:

[3]

p

)()0(log10]dB/km[

lPP

l

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Transmission of Light through Fiber

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Operating Wavelengths

First optical fiber proposed with the losses of 1000 dB/km in existing glass as compared to 5-10 db/km in coaxial cable.

It developed successfully in 1970, with attenuation of about 20dB/km.

This improvement was due to the removal of contamination in glass used.

Which employ compact GaAs semiconductor lasers.

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First Generation of Optical Communication

The first commercial system was developed, in 1977.

Operating wavelength around 0.8 µm. With GaAs semiconductor lasers. At a bit rate of 45 Mb/s. Repeater spacing of up to 10 km.

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Second Generation of Optical Communication

Launched in early 1980’s. Operating wavelength around 1.3 µm. With InGaAsP semiconductor lasers. In 1981 the single-mode fiber was developed. With further research continued upto 1987. At a bit rate of 1.7 Gb/s. Repeater spacing of up to 50 km.

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Third Generation of Optical Communication

Operating wavelength around 1.55 µm. The pulse spreading is controlled by using

dispersion-shifted fiber. It is the lowest attenuation range of about 0.2

dB/km. With InGaAsP semiconductor lasers. At a bit rate of 2.5 Gb/s. Repeater spacing of up to 100 km.

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Fourth Generation of Optical Communication

Operating wavelength around 1.55 µm. Employ optical amplification. With Wavelength-Division-Multiplexing. Bit rate reached at 10 Tb/s in 2001. Presently it is operating at 14Tb/s. Repeater spacing of up to 160 km.

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Fifth Generation of Optical Communication

It will be the next generation. Major concentration is on increasing the window

size of operation wavelength. Trying to use dispersion flattened fibers. Other developments include the concept of optical

solitons. Pulses that preserve their shape by counteracting

the effects of dispersion.

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Types of Attenuation

Absorption Loss:

Caused by the fiber itself or by impurities in the fiber.

Scattering Loss:

It is due to the fiber material & structural imperfection.

Radiative losses:

Loss induced by fiber geometry & structure.

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Material Losses

Absorption is related to the material composition & fabrication process of fiber. –Photons can make the valence electrons of an atom transition to higher energy levels.

–This energy can then• Be re-emitted (scattering)

• Frees the electron (photoelectric effects) (not in fibers)

• Dissipated to the rest of the material (transformed into heat)

– In an optical fiber optical power is effectively converted to heat dissipation within the fiber.

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Types of Absorption

Two types of Absorption mechanisms exist:

1- Intrinsic absorption by basic constituent atoms of fiber material.

2- Extrinsic absorption by impurity atoms in glass material

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Intrinsic Absorption

• It is caused by interaction of light with glass.

• Natural property of any substance.

Intrinsic absorption is very strong in the short-wavelength ultraviolet region of electromagnetic spectrum.

It’s effect decreases with increase in wavelength and becomes very less in operating wavelength region.

It contributes very little in the loss.

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Extrinsic Absorption

Extrinsic absorption is much more significant than intrinsic.

• Caused by impurities introduced into the fiber material during manufacturing.

• Two types of impurities are prominent.1. Transition metal ions as – Iron, nickel, cobalt, copper and chromium.

2. Dissolved water in the glass, as the hydroxyl or OH ion.

• Caused by transition of ions to a higher energy level associated in impurity atoms.

• Also due to the charge transition from one ion to another.

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Common Metallic Impurities in Silica Fiber

Peak attenuation wavelength and the attenuation caused by an impurity concentration of 1 in 109

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Extrinsic Absorption (OH ions)

The Absorption Spectrum of hydroxyl (OH) Group in Silica

•It gives rise to absorption overtones at 720, 950 and 1380 nm.

•Narrow windows at 800, 1310 nm and 1550 nm exist which are unaffected by this type of absorption.

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Scattering Losses in Fiber

• Scattering is absorption of light by the molecules followed by its re-radiation.

• All or some of the optical power in a mode is transferred into other mode. Frequently causes attenuation, since the transfer is often to a leaky or radiation mode.

•It occurs due to the variation in material composition like density.

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Types of Scattering Loss in Fiber

Scattering

Linear Scattering

Non-Linear Scattering

Rayleigh Scattering

Mie Scattering

Stimulated Brillouin

Scattering

Stimulated Raman

Scattering

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Linear Scattering

The process of power transfer in linear. The frequency/wavelength of light doesn’t change. Results as attenuation of transmitted power. This due to the non-ideal physical properties of fiber. Arise due to the density & compositional variation which are

frozen into the glass lattice on cooling. Compositional variation can be reduced by improved

fabrication processes. Density fluctuations are fundamental & cann’t be avoided.

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Rayleigh Scattering

Dominant scattering mechanism in low absorption window of silica fibers.

Wavelength dependent process. Scattering causes by inhomogeneities in the glass, of a

size smaller than 1/10th of wavelength. Inhomogeneities manifested as refractive index

variations, present in the glass after manufacture. It results light in almost all the directions.

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Rayleigh loss falls off as a function of the fourth power of wavelength:

4

1

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For single component glass it is given as:

n is refractive of medium. γR is Rayleigh scattering coefficient. p is average photoelastic coefficient. βc is the isothermal compressibility at fictive temperature (Tf). K is boltzmann’s constant in J/K.

The transmission loss factor due to Rayleigh scatteringis given by:

fcR KTpn 28

4

3

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lR

ReL

(1)

(2)

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•In exp (1) for any specific fiber cable

•Rayleigh coefficient falls off as a function of the fourth power of wavelength:

4r

kmAL

•The Rayleigh scattering coefficient Ar depends:

-The fibre refractive index profile

-The doping used to achieve a given core refractive index

•For a step index germanium doped fiber Ar is given by:

Ar = 0.63 + 2.06(NA)

•For a graded index near-parabolic profile fiber Ar is given by:

Ar = 0.63 + 1.75(NA)

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Mie Scattering Linear scattering due to the inhomogeinities of size

comparable to the guided wavelength.

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Occurs due to the non-perfect cylindrical structure of waveguide.

Fiber imperfection results because of: Irregularities in core-cladding interface. Non-uniform core-cladding RI difference. Diameter fluctuations. Strains and bubbles.

The scattering results light mainly in forward direction. Mie scattering can be reduced to insignificant level by

Removing imperfections in glass manufacturing process. Providing coating to the fiber. Increasing fiber guidance by increasing relative RI difference.

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Nonlinear Scattering

The optical power is transferred in forward or backward direction to the same or other modes at different frequency.

It depends on optical power density within the fiber.

Becomes significant above certain threshold power level.

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Stimulated Brillouin Scattering It can be considered as modulation of light through

molecular vibration within the fiber. The scattered light appears as lower and upper sidebands,

which are separated from incident light by modulation frequency.

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Stimulated Brillouin Scattering

It can be considered as modulation of light through molecular vibration within the fiber.

The incident photon produce a scattered photon at acoustic frequency.

This results a frequency shift. Threshold power in this scattering for source bandwidth

of v GHz is given by:

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Stimulated Raman Scattering It is similar to the Stimulated Brillouin Scattering except

that a high frequency optical photon is generated rather then an acoustic photon.

Here threshold power is higher then Stimulated Brillouin Scattering for particular fiber.

Threshold power in this scattering is given by:

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Radiative losses

It is the leakage of light from fiber. It takes place whenever fiber under goes a bend of

finite radius of curvature.

It is of two types: Macrobanding losses Microbanding losses

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Macrobending

The curvature of the bend is much larger than fiber diameter.

Lightwave suffers sever loss due to radiation of the evanescent field in the cladding region.

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As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius.

232

22

1

21

)(4

3

nn

nRc

For any radius a bit smaller than this point, the losses suddenly becomes extremely large.

Higher order modes radiate away faster than lower order modes.

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3

2

2232

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kRnRaiMM eff

Total number of modes supported by any fiber with bend of radius R can be given as:

Macroband losses can be reduced by: Designing fiber with large relative RI difference. Operating at shorter wavelengths.

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Microbending Loss

Microscopic bends are repetitive small-scale fluctuations in the radius of fiber axis.

Arises due to nonuniformities in manufacturing.

Nonuniform lateral pressure created during cabling of fiber.

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Also called as cabling or packaging loss. The power is dissipated because of the repetitive coupling

of energy between guided modes & the leaky or radiation modes in the fiber.

Can be minimized by providing compressive jacket over the fiber.

Reduction in loss is given as:

j

fm E

EabF

421)(

b is outer radius of jacket, Ef &Ej are Young’s modulus of fiber and jecket material.

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Exercise:

1. Silica has an estimated fictive temperature of 1400 K with an isothermal compressibility of 7x10-11. the RI and photoelastic coefficient for silica are 1.46 and 0.286 respectively. Calculate attenuation in db’s, at wavelengths 0.63, 1.00 and 1.30 μm. Boltzmenn’s constant is 1.381x10-23 J/K.

2. Show that for a graded index fibre with a numerical aperture of 0.275 operating at 1330 nm the Rayleigh scattering loss is approximately 0.36 dB/km.