Ofc Measurements

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    OPTICAL FIBRE : TESTS

    AND MEASUREMENTS.

    BY TX-I FACULTY

    A.L.T.T.C;GHAZIABAD

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    FEATURES BENEFITS * Low TX Loss. *Long repeater Spacing

    or Repeater lessN/W.

    * Wide Bandwidth. * Larger Chl. Capacity

    * Non-inductive. * No damage to Eqpt.due to surge

    voltage.* Immunity from * No shielding to Eqpt.

    Electro-magnetic no X-talk or Signalinterference. leakage.

    * Small size, * Easy to install,bending radius and reduction in spacelight weight. needed.

    * Difficult to tap. * High Security and

    Main Features and Benefits of Optical Fiber Cables

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    System Composition

    Transmitter E/OConverter

    O/EConverter Receiver

    Application area of Measuring InstrumentsIn Optical Fiber Communication system

    ElectricalSignal Optical

    Signal

    ElectricalSignal

    Data In Data Out

    DDF

    DDF

    FDF

    FDF

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    Cable Loss. Splice Loss. Connector Loss. Fibre Length.

    Continuity of Fiber. Fault Localizations/Break Fault.

    MAIN TESTS ON OPTICAL FIBRE CABLES

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    Calibrated Light Source. Optical Power Meter. Optical Attenuator. Optical Time Domain Reflectometer

    (OTDR ).

    INSTRUMENTS REQUIRED

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    Generates Light signals of knownpower and wavelength (LED or

    LASER).

    Wavelength variations to matchFiber's Wavelength.

    CALIBRATED LIGHT SOURCE

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    Measures Optical Power over wide range(Typically 1 nW to 2mW/-60dBm to + 3dBm)

    It is never measured directly, but measuredthrough Electrical conversion using Photo

    Electric conversion. It is known as OPTICALSENSOR of known Wavelength.

    The accuracy of the Optical Power meterdepends upon the stability of the Detectorspower to current conversion which changeswith Ageing.

    OPTICAL POWER METER

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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 OpticalSignal at the Receiver Input.(RECEIVER SENSITIVITY)

    OPTICAL ATTENUATORS

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    REQUIREMENTS OF ATTENUATORS

    Attenuation Range.

    Lowest Insertion Loss.

    Independent of Wavelength.

    Type of Connectors at the Input and Output.

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    Fiber

    Light Source

    Light Source

    100%

    Dark

    Light Receiver

    Fiber

    Motion

    0%Dark

    (VARIABLE ATTENUATOR)

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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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    One Port Operation . Works on the Principle of Back Scattering

    (Raleigh Scattering, see Figure ). Scattering is the main cause of Fiber Loss Scattering Coefficient=1/ 4

    An Optical Pulse is launched into one End of Fiber and Back Scattered Signals are detected.

    These Signals are approximately 50 dB belowthe Transmitted level.

    Measuring conditions and Results are displayed.

    OPERAING PRINCIPLES

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    Scattering in an Optical Fiber

    Light is scattered in all directions including back towards theSource in the Fiber.

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    FRESNEL REFLECTION

    It happens when there is a great change of RefractiveIndex:- Break Fault. Connecter Loss. Free Fiber-End.

    Received reflected signal depends on surfaceconditions.

    It is normally 14 db below Transmitted signals.Break

    FIBER CORE

    BREAK IN FIBER

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    n 2=1.5 n 1=1.0

    (n 2-n 1)2 (1.5-1.0)2

    (n 2+n 1)2 (1.5+1.0) 2= = 0.04 = 4% = - 14dB

    Fresnel Reflection

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    OTDR INSTRUMENT PRINCIPLE

    Fiber

    APD

    Signal

    Oscilloscope Amplifier

    Trigge r

    PulseGenerator Laser

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    BOX CAR AVERAGER AMPLIFIER It is provided to improve S/N of the RX. Signal in

    OTDR It is done by sampling the signal at each point in

    Time, starting at time, t=0.

    An Arithmetic Average is generated by a LowPass Filter (LPF). Then a variable delay is usedto move to the next point in Time t=1,2,3-------n.

    It scans the entire signal. Larger the No. of Samples (n),

    the smaller the Mean Square Noise Current:-i2noise = Constant /n

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    z t= t 1+ t

    frompulse front

    frompulse tail

    direction of pulse propagation

    BACKSCATTERING

    zbackscatteringfrom pulse front

    T= t 1

    Explanation of the Z/2 uncertainty of the OTDRSignal

    Z/2Z- Z/2

    C l l i f P l L h i Fib

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    For 100ns Pulse widthZ = Pulse Width (W) x Group Velocity

    = W x Speed of Light/Refractive Index.= 100x 10 -9 x 3 x10 8 /1.5= 20m.

    Z/2=10m i.e. 5mFor 1000ns Pulse Width:

    Z = Pulse Width (W) x Group velocity= W x Speed of Light / Refractive Index.= 1000 x 10 -9 x 3 x 10 8 /1.5= 200m.

    Z/2=100m i.e. 50m

    For 1000ns Pulse Width:Z = Pulse width (W) x Group velocity.= W x Speed of Light/Refractive Index.= 4000x10 -9x3x10 8 /1.5= 800m.

    Z/2 = 400m i.e. 200m

    Calculation of Pulse Length in Fiber

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    The amount of light scattered back to the OTDR is proportional tothe backscatter of the fiber, peak power of the OTDR test pulse andthe length of the pulse sent out.

    Length of OTDRPulse in the fiber

    Increasing the pulse width increases the backscatter level.

    OTDR pulse

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    Reflections show OTDRPulse Width and Resolution

    Connectors show bothLoss and Reflections

    Splices are usuallynot Reflective.Splices Loss

    Slope of trace shows Fiber

    Attenuation Coefficient

    OTDR Trace Information

    T i l Di l CRT f OTDR

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    Typical Display on CRT of OTDR2.0 km/DIV 4.0 db/DIV DR=36km

    Start point of Measurement

    Shifted distance0.000 km

    Starting point LOSS-----(LSA)Total loss =4.00 dbDistance = 4.000 kmLoss/km=1.00 db/km

    10.000 km --End point of MeasurementWavelength= 1.31, SM Type of fibre under testPW=100ns Pulse setting for transmissionREF= 1.5000 Refractive Index of Core under testGain= 5.0db Gain of Amplifier inside OTDR

    0

    0.000

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    Backscattered Light

    FresnelReflectionat connection

    Fresnel Reflection at

    near end connectorSplice

    Fresnel Reflection atFar-end or fault

    Loss(dB)

    Distance (km)

    General Waveform Analysis

    R f D d Z

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    X

    Y

    Reason for Dead Zone

    Dead Zone

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    Dead Zone depends on Pulse Width

    100ns 1 s

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    Splice Loss Measurement Principles

    The trace waveform at the Splice Point should be displayed as the dotted line in

    the figure below, but is actually displayed as the solid line. The waveform inputto the OTDR shows a sharp falling edge at the splice point, so the circuit cannotrespond correctly. The interval L gets longer as the pulse width becomes longer.

    Splice Point

    L

    Therefore, the Splice Loss can not be measured correctly in the Loss Mode.

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    In the Splice Loss mode, two markers are set on each side of the Splice

    Point and the lines L1 and L2 are drawn as shown below. The part of the straight

    line immediately after the splice point is the forward projection of the straight

    line, L2

    The Splice Loss is found by dropping a vertical line from the Splice Point to thisprojection of L2, and measuring the level difference between the Splice Point andthe intersection.

    x1

    x2

    x3

    x4

    L2

    Splice Loss

    Splice PointL1

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    Approximation Methods

    At Loss Measurement and Splice LossMeasurement, the loss is found by drawing animaginary line between two set markers. Thereare two methods for drawing the line.

    Least Square Approximation Method (LSA).

    Two Point Approximation Method (2PA).

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    In this method, the line is drawn by computingthe least square of the distance from all themeasured data between the two markers.

    LEAST SQUARE APPROXINATION METHOD (LSA)

    X1 X2

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    Measurement of Splice Loss by LeastSquares Method

    Splice Loss

    SpliceL1

    L2

    X2

    X3X4

    *X1

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    Splice Loss Measurement by Two Point Approximation

    Measured Value

    Splice

    X1

    True Value

    *

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    a. same fiber spliced

    actual losserror caused byfiber characteristics

    b. high loss fiber spliced to low loss fiber

    error caused byfiber characteristics

    actual loss

    c. low loss fiber spliced to high loss fibercan cause an apparent gain at a splice.

    Loss Errors in OTDR Measurements

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    Visible LightSource

    Visual Inspection:- Eye

    Light Source

    Optical PowerMeter

    Continuity Test:-

    Optical Fibre

    Optical Fibre

    Sensor

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    Receiver Sensitivity Test

    BER TestSet

    TransmitterDUT

    Receiver

    OF PatchCords

    Variable OpticalAttenuator

    Power Meter

    Optical PowerSplitter

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    Thank You

    Any Questions & Suggestions, please.