3. Optical Fiber
• Optical fiber is the technology associated with data transmission using light
pulses travelling along with a long fiber which is usually made of plastic or glass.
• Optical fiber is a thin cylindrical fiber of glass or any transparent dielectric
medium.
• Optical Fiber uses the application of total internal reflection of light.
4. Attenuation
Attenuation means loss of light energy as the light pulse travels from one
end of the cable to the other.
Signal loss or Fiber loss
Attenuation is directly proportional to the length of the cable.
It is defined as the ratio of optical output power to the input power in the fiber of
length L .
∝= 10𝑙𝑜𝑔 pi/po [in db/km]
Where Pi = input power
Po = output Power
The various losses in the cable are due to
Scattering
Bending
Absorption
5. Scattering Losses
o Non – Linear Scattering
Stimulated Brillouin Scattering
(SBS)
Stimulated Raman
Scattering(SRS)
6. Bending Losses
• The loss which exists when an optical fiber undergoes bending is called bending
losses.
• There are two types of bending
(i) Macroscopic Bending
Type of bending in which complete fiber undergoes bends which causes certain
modes not to be reflected and therefore causes loss to the bending.
(ii) Microscopic Bending
Either the core or cladding undergoes slight bends at its surface and it causes
light to be reflected at angles when there is no further reflection.
7.
8. Absorption Losses
Absorption of Light energy due to heating of ion impurities results in dimming of
light at the end of the fiber.
There are two types of absorption losses
i) Intrinsic Absorption :- It caused by the interaction with one or more
components of the glass. It occurs when photon interacts with an electron in the
valence band and excites it to a higher energy level near the UV region.
ii) Extrinsic Absorption :- It is also called as impurity absorption.
9. Fiber Materials
Requirements for optical Fiber Materials :-
• It must be possible to make long thin , flexible fibers from the material.
• Material is transparent at a particular optical wave length in order for fiber
to guide light efficiently.
• Physically compatible materials that have slightly different refractive
indices for core and cladding must be available.
• This requirements are satisfied by two materials i.e. Glass and Plastic
materials.
• Majority of fibers are made of glass consisting of either silica or silicate.
• Plastic fibers are less widely used because of their higher attenuation.
10. Glass Fiber
• Glass is made by fusing mixture of metal oxides , sulfides or selenides . The
resulting material is a randomly connected molecular network rather a well
defined structure as found in crystalline materials.
• A consequence of this random order is glass does not have a well defined
melting point.
• When glass is heated it gradually begins to soften until it becomes a viscous
liquid
• Optical fiber are made from oxide glasses and most popular is silica which as
refractive index of 1.458 at 850 nm.
• Sand is the principle raw material for silica
• Glass composed of pure silica is referred to as either silica glass , Fused glass
11. Plastic Fibers
• Its demand is growing day by day for delivering high speed services to
workstations .
• Have greater optical signal attenuations than glass fiber.
• They tough and durable.
• Core diameter is 10-20 times larger .
