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Band width
Refractive Index
Wavelength
Information carrying capacity of optical fiber.
The ratio of velocity of light in vacuum to velocity of Light in other transmission medium.
The distance between crests ofelectromagnetic waveform measured in nm.
Wavelength
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1.ReflectionThe amount of light reflected away from the
surface.Two types of Reflections.
Diffuse Reflection: The reflection from rough surface.
Specular Reflection:The reflection from smooth surface. A good
mirror is specular reflection source. According to law of reflection The angle of incidence = Angle of reflection
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2. Absorption
It describes the amount of light that is absorbed by the surface when it strikes.
Absorption varies with different chemical substances.3. Transmission
The amount of electromagnetic radiations transmitted through a substance or media is called Transmission. The total of light transmitted towards a substance consists of some reflection, some absorption and some transmission.
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4. RefractionAs electromagnetic wave changes direction
at the interface of two mediums, if the angle of incidence is not 90 then the index of refraction of light is the sine of angle of incidence to the sine of angle of refraction. The refractive index is the function or wavelength. The Snells law is
n1/n2 = sinβ/ sinαWhen light moves from rare medium to denser medium it refracts towards normal and vice versa.
n1
n2
α
β
Away movement
Normal
Normal
Rare Medium
Rare Medium
Denser Medium
›
›
›
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5.Total Internal Reflection
If the light rays enter from optically denser
medium to optically rare medium, it will move
away from the normal. If the angle of incidence
is increased so that the angle of refraction
becomes 900 . The phenomena known as total
internal reflection will occur ,if angle of
incidence is further increased. The light instead
of refracting will reflect internally. The
phenomenon used in optical fibre
communication/ propagation.
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Jacket Cladding
(lower refractive index)
Light
Core
(higher refractive index)
α0
3
2
β0
Medium 1Medium 2
1
2 1
3
Rare Medium
Denser Medium
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6.Polarization
Light consists of two fields perpendicular to
each other electric field and magnetic field. In
normal light there are infinite number of
perpendicular osculating planes propagated in
the direction of travel of light.
The light can be polarized by reflection and
refraction. Example many people buy sunglasses
that reduce reflection from wet roads effect due
to sun.
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TE (Transmittance of electrically polarized
vector)
If the field intensity of electrical vector
remains unchanged and the field intensity of
magnetic vector decreases, it is called TE
polarized.
TM (Transmittance of magnetically polarized vector)
In which magnetic field intensity is
unchanged and electric field intensity is
minimized.
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Magnetic field M
Electric field
E
The fields propagate synchronously
E
M
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Laser Light enters the fibre strikes the core cladding boundary, the angle of incidence is such that the light is totally reflected in to the fibre core due to difference of refractive indices of core & cladding on the principle of total internal reflection. The light travels in a core of single/ multi mode step index fibres in this manner until it reaches at the other end of the fibre.
Core cladding boundary
Core cladding boundary
Cladding
Core
Core Normal
Normal
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Classification Of Optical Fibre
- According to light propagation
a. Single/ mono-mode
b. Multimode
According to refractive index
(a) Step index fibre
The refractive index of core is uniform and it undergoes abrupt change at the boundary of Core and Cladding. The light is propagated along fibre on the principle of total internal reflection.
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(b) Graded index fibre
The refractive index of the core is non uniform. It varies gradually along the radius of core. The light waves travel as sine waves along the core.
Types Of Propagation In Fibre
(a) Reflective type
- Single mode step index
- Multi mode step index
(b) Refractive type
- Multi mode graded index
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125 μm
50 μm
Multimode graded index
fibre
125 μm
10 μm
Singlemode step index
fibre
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S.No MM.SI MMGI SMSI(1) Core dia 50µm 50µm 10µm
(2) Used for short haul distance
used for short haul distance
used for long haul distance
(3) Core R.I constant
gradually varies
core R.I constant
(4) Signal is delayed
no delay no delay
(5) Power launching is
easy
easy difficult
(6) LED can be used
LED Laser
(7) Splicing is easy easy difficult
Comparison Of Various Fibres
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Cladding 125 µm
(Core)
9 µm
nSingle Mode (SM)
Step Index
Single Ray (Mode)n
M.M Graded Index
(Multimode)
Refractive Propagation
››
› ›
››
››
›› ›››› ››
n
M.M Step Index
(Multimode)
Reflection Propagation
Cladding
(125 µm)
Core(50 µm)
Section Index Profile
Beam Path In-Pulse Outs
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The Characteristics Of Single mode Step Index
Fiber
The single mode step index fibre has the
characteristics of low attenuation, less
dispersion, large bandwidth, low cost and
Recommended by ITU.T for long distance optical
transmission.
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Types Of Single Mode Step Index Fibres.There are four types of single mode step
index Optical Fibres recommended by ITU-T.G-652
It is single mode Non dispersion shifted fibre (NDSF) also called 1310nm property single mode fibre. The zero dispersion point is at 1310nm. It is used by PTCL.G-653
It is dispersion shifted fibre or 1550 nm property fibre. The zero dispersion point is shifted near to 1550 nm Optical window to minimize attenuation in this window and to achieve ultra high speed and ultra long distance for Optical transmission. It is also called dispersion shifted fibre (DSF).
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G-654It is cutoff wavelength shifted single
mode fibre. This type of fibre is designed to reduce attenuation at 1550 nm window. Its zero dispersion point is still near 1310 nm window. It is mostly used for submarine Optical Fibre communication to achieve long regenerator distance.G-655
It is non zero dispersion shifted fibre (NZDSF). It preserves some dispersion near 1550nm window to avoid four wave mixing phenomenon. It is most suitable for DWDM applications for bit rates >2.5 Gb /sec .
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Applications- Long Distance Network.- Local/ Junction Network.- Fibre Access Network .- Submarine Network.- Free Space Optics (FSO).- All Photonic Network.- Cable TV Network.- Medical Services.- Angiography/ Andoscopy .- Military Services.- Dense wavelength Division Multiplexing .
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Normalized Frequency (V)
The number of modes that can pass through fibre core are dependant on normalized frequency.
V= 2πa x √N12 - N2
2
λ
Modes Modes are possible route that light wave follow down in an optical fiber. One to hundred & even thousand of modes are transmitted N=V
2
2
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Numerical Aperture
This parameter describes the light gathering ability of fibre. The amount of optical power accepted by fibre.
The sine value of acceptance angle is called Numerical Aperture
sinβ = √N12
- N22
.
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n2
n1n0
αβ
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Mode Field Diameter
It describes the radial propagation of
fundamental mode.
A core diameter and a portion of cladding
is called MFD.
The mode field diameter of G652 fibre at
1310 nm is 8.6 to 9.6µm. The MFD for G655 fibre
at 1550 nm is 8 to 11µm with deviation less than
10%. It is a performance measure of fibre when
coupled to light source.
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Mode Field Concentricity Error
The distance between the core centre
and cladding centre divided by core diameter of
the interconnected fibre.
The connector loss is proportional to the
square of the mode field concentricity error. The
MFCE is used to reduce connector loss. The
MFCE value should not exceed .5 in both G652
and G655 fibres.
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Cut Of Wavelength
The cutoff wavelength can guarantee
single mode generation.
In shortest cable to suppress the occurrence of
higher order modes and to reduce power
penalty.
e.g. for G652 fibre the cut off wavelength
<1260 nm for 2m cable, for G655 the cut off
wavelength < 1480 nm for 2 m cable length.