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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. AbsorptionIt 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.

n1n2

α

β

Away movement

Normal

NormalRare 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 13

Rare Medium

Denser Medium

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6.PolarizationLight 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

EM

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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 distanceused 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 IndexSingle 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 ApertureThis 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 = √N1

2 - N2

2

.

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n2n1

n0

αβ

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Mode Field DiameterIt 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 ErrorThe 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.