1. FIBER PROPERTIES
Transmission characteristics of a fiber depends on two important phenomena
Attenuation
Dispersion
Attenuation or transmission loss
Much less than those of metallic conductors
Expressed in logarithmic unit of decibel
Caused by absorption, scattering and bending losses

2. Attenuation - Loss of optical power as light travels along the fiber
Defined as ratio of input (transmitted) optical power Pi into fiber to the output (received) power Po
Signal attenuation = 10 log 10(Pi / Po) dB
Influenced by material composition, preparation and purification technique and the waveguide structure
Losses are categorized into
1. Material absorption losses
a. Intrinsic
b. Extrinsic
2. Linear scattering losses
3. Fiber bend loss

3. Material Absorption Losses
Related to material composition and fabrication process
Results in dissipation of transmitted power as heat
Absorption of light may be
Intrinsic – caused by interaction with one or more major components
of the glass
Extrinsic – caused by impurities within the glass

4. Intrinsic losses
Pure silicate glass has very little intrinsic absorption
In silica glass, the wavelength used is 700 nm to 1600 nm
This is between two intrinsic absorption regions
First in the UV region (< 400nm)
Second in the infrared region (>2000 nm)

5. Intrinsic absorption in the UV band
Due to electronic absorption (i.e.,) absorption occurs when a
light particle (photon) interacts with an electron and excites
it to a higher level.
Intrinsic absorption in infrared band
Absorption is caused by the vibration of Si-O bonds
Interaction between the vibration bond and the EM field of the optical
signal causes absorption.
Light energy transferred from the EM field to the bond

6. Extrinsic losses
Caused by impurities into the fiber material
Trace metal such as Iron, Nickel, Chromium
Electronic transition of these metal ions from one energy level to another causes absorption
Also occurs where hydroxyl ions (OH-) are introduced into the fiber and peak at 1383 nm, 1250 nm and 950nm
These absorptions define three windows of preferred operation, one centered at 1300 nm, second at 850 nm and third around 1550 nm

8. Scattering Loss
Caused by the interaction of light with density fluctuations within a
fiber.
Density changes are produced when optical fibers are manufactured.
During manufacturing, regions of higher and lower molecular density
areas, relative to the average density of the fiber, are created.
Light traveling through the fiber interacts with the density areas
Light is then partially scattered in all directions.

9. LIGHT SCATTERING

10. RAYLEIGH SCATTERING
In commercial fibers operating between 700-nm and 1600-nm wavelength,
the main source of loss is called Rayleigh scattering.
Rayleigh scattering is the main loss mechanism between the ultraviolet and
infrared regions.
Rayleigh scattering occurs when the size of the density fluctuation (fiber defect)
is less than one-tenth of the operating wavelength of light.
As the wavelength increases, the loss caused by Rayleigh scattering decreases.

11. FIBER LOSSES

12. MIE SCATTERING
If the size of the defect is greater than one-tenth of the wavelength of light - the scattering mechanism is called Mie scattering.
Mie scattering, caused by these large defects in the fiber core, scatters light out of the fiber core.
However, in commercial fibers, the effects of Mie scattering are insignificant.
Optical fibers are manufactured with very few large defects.

13. BENDING LOSS Bending the fiber also causes attenuation.
Bending loss is classified according to the bend radius of curvature:
microbend loss
macrobend loss
Macrobends are bends having a large radius of curvature relative to
the fiber diameter
During installation, if fibers are bent too sharply, macrobend losses will
occur
Microbend and macrobend losses are very important loss mechanisms.

14. MICROBEND LOSSES
Microbends are small microscopic bends of the fiber axis that
occur mainly when a fiber is cabled
Microbend losses are caused by small discontinuities or
imperfections in the fiber.
Uneven coating applications and improper cabling procedures
increase microbend loss.
External forces are also a source of microbends.
An external force deforms the cabled jacket surrounding the
fiber but causes only a small bend in the fiber.
Microbends change the path that propagating modes take up

15. MICROBEND LOSS

16. MACROBEND LOSS
Macrobend losses are observed when a fiber bend's radius of curvature is large compared to the fiber diameter.
Light propagating at the inner side of the bend travels a shorter distance than that on the outer side.
To maintain the phase of the light wave, the mode phase velocity must increase.
When the fiber bend is less than some critical radius, the mode phase velocity must increase to a speed greater than the speed of light.
However, it is impossible to exceed the speed of light. This condition causes some of the light within the fiber to be lost or radiated out of the fiber.

17. Two types of dispersion: Intramodal dispersion Intermodal dispersion
DISPERSION IN OPTICAL FIBERS
Two types of dispersion:
Intramodal dispersion
Intermodal dispersion
Dispersion leads to PULSE SPREADING
The varying delay in arrival time between different components of a signal "smears out" the signal in time.
This causes energy overlapping and limits information capacity
of the fiber

19. INTRAMODAL DISPESION
Pulse spreading that occurs within a single mode
Intramodal dispersion occurs because different colors of light travel through different materials and different waveguide structures at different speeds
Also called GROUP VELOCITY DISPERSION (GVD)
Occurs in all types of fibers
Two main causes : Material dispersion
Waveguide dispersion

20. Material Dispersion
Arises from variations of the refractive index of the core material as a function of wavelength
Spreading of a light pulse is dependent on the wavelengths interaction with the refractive index of the fiber core
Different wavelengths travel at different speeds in the fiber material and hence exit the fiber at different times
Material dispersion is a function of the source spectral width.
The spectral width specifies the range of wavelengths that can propagate in the fiber.
Material dispersion is less at longer wavelengths.

21. Waveguide Dispersion
Arises because a Single Mode Fiber confines only 80% of the optical power to the core
The other 20% tends to travel through the cladding and hence travels faster
This results in spreading of the light pulses
The amount of dispersion depends on the fiber design and the size of the fiber core relative to the wavelength of operation
In multimode fibers, waveguide dispersion and material dispersion are basically separate properties.
Multimode waveguide dispersion is generally small compared to material dispersion and is usually neglected.

22. INTERMODAL DISPERSION
Intermodal or modal dispersion causes the input light pulse to spread.
The input light pulse is made up of a group of modes (MULTIMODE)
As the modes propagate along the fiber, light energy distributed among the modes is delayed by different amounts.
The pulse spreads because each mode propagates along the fiber at different speeds.

23. Modes travel in different directions, some modes travel longer distances.
Modal dispersion occurs because each mode travels a different distance over the same time span
The modes of a light pulse that enter the fiber at one time exit the fiber a different times.
These conditions causes the light pulse to spread.
As the length of the fiber increases, modal dispersion increases

25. Optical Fiber Connection
Requires both jointing and termination of the transmission medium
Number of connections or joints is dependent upon the link length
Fiber to fiber connection with low loss and minimum distortion is important
Two major categories of fiber joint currently in use:
1.Fiber splices
2. Fiber Connectors

26. Splices and Connectors – Ideally couple all light propagating in one fiber into the adjoining fiber
Fiber Couplers
Branching devices that split all the light from the main fiber into two or more fibers
Couple a proportion of the light propagating in the main fiber into a branch fiber
Combine light from one or more branch fibers into a main fiber

27. Fiber alignment and Joint losses
Major consideration – Optical losses at the interface
Can be minimized if…..
Jointed fiber ends are smooth
Perpendicular to fiber axis
Two fiber axes are perfectly aligned
Still, a proportion of light – reflected back into the transmitting fiber
This phenomena is called FRESNEL REFLECTION

28. Magnitude of the partial reflection through the interface may be
Estimated using the classical Fresnel formula
r = [(n1 – n) / (n1 +n)]2