Transcript Chapter2 Interaction Characteristics of Light

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Band width
Information carrying capacity of optical fiber.
Refractive Index
The ratio of velocity of light in vacuum to
velocity of Light in other transmission medium.
Wavelength
The distance between crests of
electromagnetic waveform measured in nm.
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Wavelength
1.Reflection
The 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
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The angle of incidence
= Angle of reflection
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
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transmission.
4. Refraction
As 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/ sinα
When light moves from rare medium to denser
medium it refracts towards normal and vice
versa.
Normal
Rare Medium
α
n1
n2
Denser Medium
β
Away movement
Rare Medium
Normal
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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
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communication/ propagation.
2
1
Denser Medium
3
α0
Medium 1
Medium 2
2
β0
Rare Medium
Cladding
(lower refractive index)
Light
Core
3
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(higher refractive index)
1
6.Polarization
Light consists of two fields perpendicular
each other electric field and magnetic field.
normal light there are infinite number
perpendicular osculating planes propagated
the direction of travel of light.
to
In
of
in
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
unchanged
minimized.
magnetic
and
electric
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field
intensity
is
field
intensity
is
Magnetic
field M
Electric
field
E
M
E
The fields propagate
synchronously
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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
Normal
Cladding
Core
Normal
Core
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Core cladding boundary
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. www.bzupages.com
(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
-
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Multi mode graded
index
Singlemode
step index
fibre
10 μm
125 μm
Multimode
graded index
fibre
50 μm
125 μm
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Comparison Of Various Fibres
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)
(4)
(5)
(6)
(7)
Core R.I
gradually
constant
varies
Signal is
no delay
delayed
Power
easy
launching is
easy
LED can be
LED
used
Splicing is www.bzupages.com
easy
easy
core R.I
constant
no delay
difficult
Laser
difficult
Index
Profile
Section
Beam Path
In-Pulse
Cladding
(125 µm)
n
Core
(50 µm)
M.M Step Index
Reflection Propagation
(Multimode)
n
M.M Graded Index
›
Refractive Propagation
(Multimode)
Cladding 125
µm
n
(Core)
9 µm
Single Mode (SM)
Step Index
Single Ray (Mode)
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›
Outs
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
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fibre (DSF).
G-654
It 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 www.bzupages.com
most suitable for DWDM
applications for bit rates >2.5 Gb /sec .
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.
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- Dense wavelength
Division Multiplexing .
Modes
Modes are possible route that light wave
follow down in an optical fiber. One to hundred 2
& even thousand of modes are transmitted N=V
Normalized Frequency (V)
2
The number of modes that can pass
through fibre core are dependant on normalized
frequency.
V= 2πa x √N12 - N22
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λ
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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n0
n1
n2
β
α
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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.
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