Signal Degradation In Optical Fiber

Jun 25 • Notes • 8279 Views • 2 Comments on Signal Degradation In Optical Fiber

SIGNAL DEGRADATION IN OPTICAL FIBER

Losses in an optical fibre:-

The types of losses in a optical fibre are
Attenuation loss
Absorption
Scattering
Bending loss
Dispersion loss
Coupling loss

Attenuation:-
Attenuation is a loss of optical energy as it travels through fibre.It is measured in terms of dB/km.It is a transmission loss that can be measured as a difference between output signal power and input signal power.It is measured in dB as
Attenuation loss α=10log10(Pin/Pout)/L dB/km
Attenuation is caused both by scattering and absorption.

Absorption:-
Absoption is a phenomenon  in which the energy of an atom absorbed by the matter.Thus the light energy is transformed to other form of energy for example to heat.Absorption is of four kinds:-

1) Intrinsic absorption:-The reason of intrinsic absorption is basic fibre material properties.If an optical fibre is truely pure with no impurities or imperfections then absorption is intrinsic.The causes of intrinsic absorption are material and electron absorption.

2) Material absorption:-

It is a loss mechanism which results in dissipation of the transmitted power into heat.

3) Electron absorption:-

Intrinsic absorption in the ultraviolet region is caused by electronic absorption.It occurs when a photon absorbs energy from an electron and excites it to a higher level.
The effect of intrinsic absorption can be minimized using a suitable core cladding materials.

4) Extrinsic absorption
It is caused due to the impurities in the material.Metal impurities such as iron nickel and chromium are introduced in the fibre during fabrication.It also occurs due to the presence of hydroxyl ions due to presence of water vapour.
Extrinsic absorption can be reduced by glass refining techniques such as vapour oxidation and by using a glass fiber called dry fiber which reduces the OH ion concentration.

Scattering loss:-Scattering loss is a general physical process where some form of radiation is forced to deviate from a straight line by one or more localised medium through which they pass. Diffuse reflections are reflections that undergo scattering.Scattering losses are caused by interaction of light with density fluctuations in the fibres.The propagation of light through the fibre core is in the form of total internal reflection.Rough and irregular surface causes the light rays to scatter in different direction thus causing diffusion or scattering loss.

BENDING LOSSES:-

They are of two types:-

1) MACROBENDING LOSSES:-

The curvature of bend is larger than the fiber diameter.

2) MICROBENDING LOSSES:-

It arises when the fibers are incorporated into cables.

MODAL DELAY:-

– It is a distortion mechanism which take place in multimode fibres and other waveguides in which the signal is spread because the propagation velocity of the optical signal is not the same for all modes.

– It limits the bandwidth of the multimode fibres.

– It may be reduced but can never be eliminated.

– It can be reduced by using a core having a graded refractive index profile.

GROUP DELAY:-

– It is a measure of the time delay of the components of a signal and is a function of frequency.

-It is a useful measure of time distortion and is calculated by differentiating with respect to frequency.

– It is a measure of the slope of the phase response.

MATERIAL DISPERSION:-

– also called colour dispersion.

– It arises from the wavelength of the refractive index with wavelength.

 

WAVEGUIDE DISPERSION:-

– It is caused by the wavelength dependence of distortion of energy for the fundamental mode in fibers.

– It is mainly a problem for single mode.

– The mode of energy propagates in the cladding as the wavelength increases.

 

POLARIZATION MODE DISPERSION:-

– It is caused due to different velocity for each polarization mode because of lack of perfect symmetric fibers.

EXPECTED QUESTIONS WITH ANSWERS:-

1) WHAT ARE THE DIFFERENT TYPES OF LOSSES IN OPTICAL FIBERS?

Ans:- The types of losses in a optical fibre are
Attenuation loss
Absorption
Scattering
Bending loss
Dispersion loss
Coupling loss

2) WHAT IS ABSORPTION AND WHAT ARE ITS TYPES?

Ans:- Absorption
Absoption is a phenomenon in which the energy of an atom is taken by the matter.Thus the light energy is transformed to other form of energy for example to heat.Absorption is of two kinds:-

Intrinsic absorption
It is caused by basic fibre material properties.If an optical fibre is absolutely pure with no impurities or imperfections then absoption is intrinsic.The causes of intrinsic absorption are material and electron absorption.

Material absorption:It is a loss mechanism which results in dissipation of the transmitted power into heat.
Electron absorption:Intrinsic absorption in the ultraviolet region is caused by electronic absorption.It occurs when a photon absorbs energy from an electron and excites it to a higher level.
The effect of intrinsic absorption can be minimised using a suitable core cladding materials.

Extrinsic absorption
It is caused due to the impurities in the material.Metal impurities such as iron nickel and chromium are introduced in the fibre during fabrication.It also occurs due to the presence of hydroxyl ions due to presence of water vapour.
Extrinsic absorption can be reduced by glas refining techniques such as vapour oxidation and by using a glass fiber called dry fiber which reduces the OH ion concentration.

3) WHAT IS GROUP DELAY AND MODAl DELAY?

Ans:- MODAL DELAY:-

– It is a distortion mechanism occuring in multimode fibres and other waveguides in which the signal is spread because the propagation velocity of the optical signal is not the same for all modes.

– It limits the bandwidth of the multimode fibres.

– It may be reduced but can never be eliminated.

– It can be reduced by using a core having a graded refractive index profile.

GROUP DELAY:-

– It is a measure of the time delay of the components of a signal and is a function of frequency.

-It is a useful measure of time distortion and is calculated by differentiating with respect to frequency.

– It is a measure of the slope of the phase response.

5) WHAT ARE THE DIFFERENT TYPES OF DISPERSION IN OPTICAL FIBERS?

Ans:- MATERIAL DISPERSION:-

– also called colour dispersion.

– It arises from the wavelength of the refractive index with wavelength.

WAVEGUIDE DISPERSION:-

– It is caused by the wavelength dependence of distortion of energy for the fundamental mode in fibers.

– It is mainly a problem for single mode.

– The mode of energy propagates in the cladding as the wavelength increases.

POLARIZATION MODE DISPERSION:-

– It is caused due to different velocity for each polarization mode because of lack of perfect symmetric fibers.

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2 Responses to Signal Degradation In Optical Fiber

  1. Anonymous says:

    All the things well explained at one place
    very easy languagel

  2. Kanhaiya kumar says:

    How can I join m.tech & phd classes with reputed university

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