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Service d’Électromagnétisme et de TélécommunicationsService d’Électromagnétisme et de Télécommunications

Attenuation in optical fibres

5ème Electricité - Télécommunications II

Marc Wuilpart

Réseaux de transmission photoniques

22Service d’Électromagnétisme et de TélécommunicationsService d’Électromagnétisme et de Télécommunications

There are four main causes of attenuation

• Material absorption– Intrinsic

– Extrinsic

• Linear Scattering– Rayleigh

• Nonlinear scattering– Brillouin Scattering

– Raman Scattering

• Fibre bend loss


The attenuation parameter is expressed in dB/km

In uniform fibres, the optical power exponentially decays with distance

Where p is the attenuation coefficient expresses in km-1. When using the dB/km unit, it becomes :

Remark :


Material absorption is due to the fibre composition

Material absorption = loss mechanism related to the material composition and the fabrication process of the fibre :

Intrinsic absorption : interaction with one or more of the major components of the glass.

Extrinsic absorption : interaction with impurities within the glass.


Intrinsic absorption is caused by electronic and vibrational transitions

• Absorption due to atomic defaults negligible.

• Absorption due to electronic transitions in the glass ultraviolet absorption.

• Absorption due to molecular vibrations of :– Si-O (9.2µm)– Ge-O (11.0µm)– P-O (8.1µm)– B-O (7.2µm) infrared absorption.

from Senior, "Optical Fiber Communications", Prentice Hall, 1992


Extrinsic absorption is caused by metallic impurities

Arises from absorption by transition metal element impurities.

Element Peak Wavelength (nm) Attenuation in dB/kmfor 1 part in 10

9

Cr3+

625 1.6Cu

2+850 1.1

Fe2+

1100 0.68Fe

3+400 0.15

Ni2+

650 0.1Mn

3+460 0.2


Extrinsic absorption is also due to water into the glass

Absorption through molecular vibrations by water dissolved in the glass (hydroxil or OH-).

Absorption band centered on 1.38, 0.95 and 0.72µm

Resonance wavelength (µm) dB/km for 1 ppm OH- 1.38 65 0.95 1 0.72 0.05

Water contamination should be kept as small as possible during the fabrication process and during the fibre life cable design.


Extrinsic absorption due to water creates peaks in the spectrum


First overtone


Linear scattering is mainly due to Rayleigh scattering

• Rayleigh scattering is the dominant intrinsic loss mechanism in the low absorption window between the ultraviolet and infrared absorption


• Linear scattering = transfer of some or all the optical power contained within one propagating mode to another mode (guided or radiation modes).

• Linear means that there is no change of frequency or wavelength during the transfer.


Rayleigh scattering is the dominant loss mechanism in the short wavelength range

• It results from inhomogeneities of a random nature occurring on a small scale compared with the wavelength of the light.

• Inhomogeneities arise from density and compositional variations into the glass material refractive index fluctuations.

• Rayleigh attenuation has the form:A

CR ( )

4

• The light is scattered in every direction light is thus also backscattered which is used by OTDR (Optical Time Domain Reflectometry).


Total attenuation is the sum of all mechanisms



Attenuation is larger in multimode fibres

Multimode fibres Singlemode fibres

Loss are larger for multimode fibres because of higher dopant concentration.


from Keiser, "Optical Fiber Communications"

3 telecommunication windows

• 1310 nm window minimizes the chromatic dispersion

• 1550 nm window

minimizes the attenuation

• DSF fibres allows the use the third window with minimization of chromatic dispersion


Nonlinear scattering is important at high powers

• This effect causes the optical power from one mode to be transferred in either the forward or the backward direction to the same or other modes at a different frequency.

• It critically depends on the optical power density within the fibre and hence becomes only significant above threshold power levels.

• Brillouin Scattering.

• Raman Scattering.


Brillouin Scattering has the lowest threshold

• Interaction between acoustic waves and the optical signal.

• The scattered light appears as upper and lower sidebands which are separated from the incident light by the frequency of the acoustic wave.

• The incident photon produces by scattering a phonon of acoustic frequency as well as a scattered photon .

• The BS can only occur in backward direction.


Raman Scattering can occur in WDM systems

• Interaction between vibrational modes of molecules and the optical signal.

• The scattered light appears as upper and lower sidebands which are separated from the incident light by the frequency depending on the energy levels of vibration modes.

• The RS can occur in both forward and backward directions.


Nonlinear scattering create new optical frequencies in the spectrum



Fibre bend loss is an important practical parameter

• Optical fibres suffer from radiation losses at bends or curves on their paths.

• Due to the energy in the evanescent field at the bend exceeding the velocity of the light in the cladding radiation out of the fiber.



Microbending can couple guided modes into radiation modes

Microscopic random fluctuation coupling with

radiation modes


service d’Électromagnétisme et de télécommunications 1 1 attenuation in optical fibres 5 ème...

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