ofc measurements
TRANSCRIPT
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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.