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    Introduct ion to Opt ical Fiber

    Transmiss ion

    and

    Character ist ics

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    Optical Fiber is new medium, in which information

    (voice, Data or Video) is transmitted through a glass in the

    form of light.

    Information is encoded into electrical signals.

    Electrical signals are converted into light signals.

    Light travels down the fibre.

    A detector changes the light signals into electrical signals.

    Electrical signals are decoded into information

    Transmission Sequence Though Fiber:

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    Optical Fibers are non conduc tive (Dielectr ic s)

    Electrom agnetic Immun i ty

    Large Bandw idth

    Low Loss

    Smal l, Ligh t weight and less cost cables

    Avai lable in Long length s

    Securi ty

    Safety

    No need of addi t ional equ ipment to p rotect against

    surge vol tage

    Advantages o f Fiber Opt ics

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    Common carr ier nat ionwide netwo rks.

    Telephone Inter-off ic e Trunk lines .

    Customer prem ise communication netwo rks. Undersea cables.

    High EMI areas (Power l ines , Railway, Roads).

    Factory communication.

    Contro l systems.

    High l igh tening areas.

    Mil itary app l icat ions .

    Classi f ied (secure) commun ications .

    Appl icat ion s of Fibers

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    Total Internal Reflect ion- The Reflection that Occurs

    when a Light Ray Travelling in One Material Hits a

    Different Material and Reflects Back into the Original

    Material without any Loss of Light.

    Principle of Operation

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    Light travelling from one material to another changes speed, which

    results in light changing its direction of travel. This def lect ion o f l ight is

    cal led Refract ion .

    As the angle of incidence increases, the angle of refraction approaches

    90o to the normal.

    The angle of incidence that yields an angle of refraction of90ois the

    cr i t ical angle.

    If the angle of incidence increases amore than the critical angle, the lightis totally reflected back into the first material so that it does not enter the

    second material.

    The angle of incidence and reflection are equal and it is called Total

    Internal Reflection.

    THEORY AND PRINCIPLE OF FIBRE OPTICS

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    1

    Angle of incidence

    n1

    n2

    2

    n1

    n2

    1

    2

    n1

    n2

    1 2

    Angle of

    reflection

    Light is bent away

    from normal

    Light does not enter

    second material

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    By Snell's law,n1 sin1= n2 sing

    2

    The critical angle of incidence c where 2 = 90o

    c= arc sing (n2/ n1)At angle greater than c the light is reflected, Because

    reflected light means that n1 and n2 are equal, 1 and 2

    are also equal.

    The angle of incidence and reflection are equal. These

    simple principles of refraction and reflection form the basis

    of light propagation through an optical fiber.

    Snell 's law

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    The optical fibre has two concentric layers called the

    core and the cladding.

    The inner core is the light carrying part.

    The surrounding cladding provides the difference

    refractive index that allows total internal reflection of light

    through the core.

    The index of the cladding is less than 1%, lower than thatof the core.

    Typical values for example are a core refractive index of

    1.47and a cladding index of1.46

    PROPAGATION OF L IGHT THROUGH FIBRE

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    Jacket

    Cladding

    Core

    Cladding

    Angle ofreflection

    Angle ofincidence

    Light at less thancritical angle is

    absorbed in jacket

    Jacket

    Light is propagated by

    total internal reflection

    Jacket

    Cladding

    Core

    (n2)

    (n2)

    Fig. Total Internal Reflection in an optical Fibre

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    The specific characteristics of light propagation through

    a fiber depends on The size of the fiber.

    The composition of the fiber.

    The light injected into the fiber.

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    125 8 125 50 125 62.5 125 100

    Core Cladding

    Typical Core and Cladding Diameters

    The size o f the fiber

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    The refractive Ind ex:The relation between the indices of the

    core and cladding.

    Step Index Graded Index

    - The step index fiber has a core with uniform index throughout

    the fiber.

    - The profile shows a sharp step at the junction of the core and

    cladding.- The graded index has a non-uniform core.

    - The Index is highest at the center and gradually decreases

    until it matches with that of the cladding.

    - There is no sharp break in indices between the core and thecladding.

    FIBRE TYPES

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    Three types of f ibers :

    Single- Mode Step Index fiber (Single Mode Fiber)

    Multimode Step Index fiber (Step Index fiber)

    Multimode graded Index fiber (Graded Index fiber)

    FIBRE TYPES

    Single Mode Step Index

    n1n2

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    Step index Mu lt imode Fiber

    High orderMode

    DispersionRefractiveIndex Profile

    Low Order ModeMulti mode Step Index

    InputPulse

    OutputPulse

    n1

    n2

    x d x N AN= 0.5 x (----------------------) 2

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    Graded index Mult imode Fiber

    Dispersion

    Multi mode Graded Index

    n1

    n2

    dxNAN= 0.25 x ( ---------------- )2

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    OPTICAL FIBRE PARAMETERS

    Wavelength

    Frequency

    Window

    Attenuation

    Dispersion

    Bandwidth

    Numerical Aperture

    Optical Fiber Characterist ic s

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    WAVELENGTH

    It is a characteristic of light that is emitted from the light

    source and is measures in nanometers (nm). Red Light has longer wavelength than Blue Light,

    Typical wavelength for fiber use are 850nm, 1300nm and

    1550nm all of which are invisible.

    FREQUENCYIt is number of pulse per second emitted from a light

    source. Frequency is measured in units of hertz (Hz). In

    terms of optical pulse 1Hz = 1 pulse/ sec.

    Optical Fiber Characterist ic s

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    WINDOW

    A window is defined as the range of wavelengths at

    which a fiber best operates.

    Window Operational Wavelength

    800nm - 900nm 850nm

    1250nm - 1350nm 1310nm

    1500nm - 1600nm 1550nm

    Optical Fiber Characterist ic s

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    ATTENUATION

    Attenuation is defined as the loss of optical power over a

    set distance, a fiber with lower attenuation will allow more

    power to reach a receiver than fiber with higher attenuation.

    Optical Fiber Characterist ic s

    Window Attenuation

    850nm 2.0 to 2.5 dB/km

    1310nm 0.5 to 1.0 dB/km

    1550nm0.2 to 0.25 dB/km

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    Optical Fiber Characterist ic s

    Loss in the Fiber Optics:

    Absorption loss :Natural Impurities in the glass

    absorb light energy.

    Scattering loss :Light rays travelling in the core

    reflect from small imperfections into a new pathway that

    may be lost through the cladding.

    Loss due to geometric effects or bending loss

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    DISPERSION Expressed in ps/km/nm

    Dispersion is the spreading of light pulse as its travels down

    the length of an optical fiber. Dispersion limits the bandwidth or information carrying

    capacity of a fiber.

    Dispersion increases along with the distance.

    There are three main types of dispersion

    Modal Dispersion

    Material dispersion

    Wave guide dispersion

    Optical Fiber Characterist ic s

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    MODAL DISPERSION

    The spreading of light is called modal dispersion.

    Modal dispersion occurs only in Multimode fibers Step

    index fibers.

    It occurs because rays follow different paths through the

    fiber and consequently arrive at the other end of the fiber at

    different times.

    Optical Fiber Characterist ic s

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    MATERIAL DISPERSION

    It occurs due to different wavelengths travel at different

    velocities through a fiber.

    Refractive Index changes according to the wavelength.

    This velocity variation is caused by some property of

    material available in glass.

    Optical Fiber Characterist ic s

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    WAVEGUIDE DISPERSION :

    It occurs because optical energy travels in both the core

    and cladding, which have slightly different refractive

    indices.

    The energy travels at slightly different velocities in the

    core and cladding because of the slightly differentrefractive indices of the materials.

    Optical Fiber Characterist ic s

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    BANDWIDTH

    In single mode fiber dispersion is most affected by the source's spectral

    width. The wider the source spectral width, the greater the dispersion.

    Conversion of dispersion to bandwidth can be defined by the followingequation.

    0.187

    BW = --------------------------

    (Disp) (SW) (L)

    Disp = Dispersion at the operating wavelength in psec/ nm/ km.

    SW = Spectral width of the source in nm.

    L = Fiber length in km.

    Optical Fiber Characterist ic s

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    NUMBERICAL APERTURE

    Numerical aperture (NA) is the "light - gathering ability"

    of a fiber.

    Light injected into the fiber at angles greater than the

    critical angle will be propagated. The material NA relates to

    the refractive indices of the core and cladding.NA = n1

    2 - n22

    where n1

    and n2

    are refractive indices of core and

    cladding respectively.

    Optical Fiber Characterist ic s

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    NUMBERICAL APERTURE

    NA is unit less dimension. It can also define as the angles at

    which rays will be propagated by the fiber. These anglesform a cone called the acceptance cone, which gives the

    maximum angle of light acceptance. The acceptance cone is

    related to the NA

    = arc sin (NA)

    NA = sin = sin /2

    where is the half angle of acceptance and is the full

    angle of acceptance.

    Optical Fiber Characterist ic s

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    Optical Fiber Characterist ic s

    NUMBERICAL APERTURE

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    1. G-652 (NDSF) : Non dispersion shifted fiber Operated

    in 1550nm region, Currently using for SDH and DWDM

    systems

    2. G-653 (NDSF) :Dispersion Shifted Fiber SDHsystems

    3 G-655 (NZDSF) :Non-zero dispersion-shifted fiber

    Good for both SDH and DWDM use in the 1550-nm

    region.

    Specially designed for DWDM applications to

    compensate the non-linearity (Four wave mixing and

    Polarization mode dispersion) presenting in DWDM.

    Types of Single mode Fibers

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    Cable Loss.

    Splice Loss.

    Connector Loss.

    Fiber Length.

    Continuity of Fiber.

    Fault Localizations/Break Fault.

    MAIN TESTS ON OPTICAL F IBRE CABLES

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    Optical Power Meter.

    Calibrated Light Source.

    Optical Attenuator.

    Optical Time Domain Reflectometer (OTDR).

    I NSTRUMENTS REQUIRED

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    TYPES:-

    Fixed Attenuators.

    Variable Attenuators.

    APPLICATIONS:-

    To Simulate the Regenerator Hop Loss at the FDF.

    To Provide Local Loop Back for Testing. To measure the Bit Error Rate by varying the

    Optical Signal at the Receiver Input.

    (RECEIVER SENSITIVITY)

    OPTICAL ATTENUATORS

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    Used for measuring

    Fiber Loss.

    Splice Loss.

    Connector Loss.

    Fiber Length.

    Continuity of Fiber.

    Fault Localization.

    OPTICAL TIME DOMAIN

    REFLECTOMETER (OTDR)

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    Thank You


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