Download lecture 2 Fiber properties

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
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
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
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)
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
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
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.
LIGHT SCATTERING
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.
FIBER LOSSES
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.
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.
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
MICROBEND LOSS
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.
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
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
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.
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.
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.
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
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
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
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
Magnitude of the partial reflection through the interface may be
Estimated using the classical Fresnel formula
r = [(n1 – n) / (n1 +n)]2