Download OPTICAL FIBRE : TEST AND MEASUREMENTS

OPTICAL FIBRE : TESTS
AND MEASUREMENTS.
BY
TX-I FACULTY
A.L.T.T.C;
GHAZIABAD
Main Features and Benefits of Optical
Fiber Cables
FEATURES
* Low TX Loss.
* Wide Bandwidth.
* Non-inductive.
* Immunity from
Electro-magnetic
interference.
* Small size,
bending radius and
light weight.
* Difficult to tap.
BENEFITS
*Long repeater Spacing
or Repeater less N/W.
* Larger Chl. Capacity
* No damage to Eqpt.
due to surge voltage.
* No shielding to Eqpt.
no X-talk or Signal
leakage.
* Easy to install,
reduction in space
needed.
* High Security and
Copper resource savings.
System Composition
Electrical
Signal
D
D
F
Transmitter
Data In
E/O
Converter
Electrical
Signal
Optical
Signal
F
D
F
F
D
F
O/E
Converter
Application area of Measuring Instruments
In Optical Fiber Communication system
Receiver
D
D
F
Data Out
MAIN TESTS ON OPTICAL
FIBRE CABLES
•
•
•
•
•
•
Cable Loss.
Splice Loss.
Connector Loss.
Fibre Length.
Continuity of Fiber.
Fault Localizations/Break Fault.
INSTRUMENTS REQUIRED
•
•
•
•
Calibrated Light Source.
Optical Power Meter.
Optical Attenuator.
Optical Time Domain Reflectometer
(OTDR).
CALIBRATED LIGHT SOURCE
• Generates Light signals of known
power and wavelength (LED or
LASER).
• Wavelength variations to match
Fiber's Wavelength.
OPTICAL POWER METER
• Measures Optical Power over wide range
(Typically 1 nW to 2mW/-60dBm to + 3dBm)
• It is never measured directly, but measured
through Electrical conversion using Photo
Electric conversion. It is known as OPTICAL
SENSOR of known Wavelength.
• The accuracy of the Optical Power meter
depends upon the stability of the Detector’s
power to current conversion which changes
with Ageing.
OPTICAL ATTENUATORS
• 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)
REQUIREMENTS OF
ATTENUATORS
• Attenuation Range.
• Lowest Insertion Loss.
• Independent of Wavelength.
• Type of Connectors at the Input and Output.
0%
Dark
Light Receiver
Motion
Fiber
Light Source
Fiber
Light Source
100%
Dark
(VARIABLE ATTENUATOR)
OPTICAL TIME DOMAIN REFLECTOMETER
(OTDR)
• Used for measuring
– Fiber Loss.
– Splice Loss.
– Connector Loss.
– Fiber Length.
– Continuity of Fiber.
– Fault Localization.
OPERAING PRINCIPLES
•
•
•
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 below
the Transmitted level.
Measuring conditions and Results are displayed.
Scattering in an Optical Fiber
Light is scattered in all directions including back towards the
Source in the Fiber.
FRESNEL REFLECTION
• It happens when there is a great change of Refractive
Index:– Break Fault.
– Connecter Loss.
– Free Fiber-End.
• Received reflected signal depends on surface
conditions.
• It is normally 14 db below Transmitted signals.
Break
FIBER CORE
BREAK IN FIBER
Fresnel Reflection
n2=1.5
n1=1.0
(n2-n1)2 = (1.5-1.0)2 = 0.04 = 4% = - 14dB
(n2+n1)2 (1.5+1.0)2
OTDR INSTRUMENT PRINCIPLE
Pulse
Generator
Fiber
Laser
APD
Signal
Trigger
Oscilloscope
Amplifier
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 Low
Pass Filter (LPF). Then a variable delay is used
to 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
Explanation of the Z/2 uncertainty of the OTDR
Signal
BACKSCATTERING
z
backscattering
from pulse front
z

from
pulse front
T= t1
t= t1+ t
from
pulse tail
direction of
pulse propagation
Z-Z/2
Z/2
Calculation of Pulse Length in Fiber
For 100ns Pulse width
Z = Pulse Width (W) x Group Velocity
= W x Speed of Light/Refractive Index.
= 100x 10-9 x 3 x108/1.5
= 20m.
Z/2=10m i.e. ± 5m
For 1000ns Pulse Width:
 Z = Pulse Width (W) x Group velocity
= W x Speed of Light / Refractive Index.
= 1000 x 10-9 x 3 x 108/1.5
= 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.
= 4000x10-9x3x108/1.5
= 800m.
 Z/2 = 400m i.e. ± 200m
The amount of light scattered back to the OTDR is proportional to
the backscatter of the fiber, peak power of the OTDR test pulse and
the length of the pulse sent out.
Length of OTDR
Pulse in the fiber
OTDR pulse
Increasing the pulse width increases the backscatter level.
OTDR Trace Information
Reflections show OTDR
Pulse Width and Resolution
Connectors show both
Loss and Reflections
Slope of trace shows Fiber
Attenuation Coefficient
Splices Loss
Splices are usually
not Reflective.
Typical Display on CRT of OTDR
2.0 km/DIV 4.0 db/DIV
DR=36km
Start point of
Measurement
Shifted distance
0.000 km
0
Starting point 0.000
LOSS----(LSA)
Total loss =4.00 db
Distance = 4.000 km
Loss/km=1.00 db/km
10.000 km --End point of Measurement
Wavelength= 1.31, SM – Type of fibre under test
PW=100ns –Pulse setting for transmission
REF= 1.5000 – Refractive Index of Core under test
Gain= 5.0db– Gain of Amplifier inside OTDR
General Waveform Analysis
Fresnel Reflection at
Far-end or fault
Fresnel Reflection at Splice
near end connector
Fresnel
Reflection
at connection
Loss
(dB)
Backscattered Light
Distance (km)
Reason for Dead Zone
Y
Dead Zone
X
Dead Zone depends on Pulse Width
100ns
1s
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 input
to the OTDR shows a sharp falling edge at the splice point, so the circuit cannot
respond 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.
•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 this
projection of L2, and measuring the level difference between the Splice Point and
the intersection.
L1
Splice Point
x1
Splice Loss
x2
L2
x3
x4
Approximation Methods
At Loss Measurement and Splice Loss
Measurement, the loss is found by drawing an
imaginary line between two set markers. There
are two methods for drawing the line.
•Least Square Approximation Method (LSA).
•Two Point Approximation Method (2PA).
LEAST SQUARE APPROXINATION
METHOD (LSA)
In this method, the line is drawn by computing
the least square of the distance from all the
measured data between the two markers.
X1
X2
Two Point Approximation Method(2PA)
This method draws the line linking the two
measured data points at the two markers.
X1
X2
Measurement of Splice Loss by Least
Squares Method
Splice
L1
X1
X2
*
Splice Loss
X3
X4
L2
Splice Loss Measurement by Two Point
Approximation
Splice
X1
True Value
Measured Value
*
Loss Errors in OTDR
Measurements
a. same fiber spliced
actual loss
error caused by
fiber characteristics
b. high loss fiber spliced to low loss fiber
actual loss
error caused by
fiber characteristics
c. low loss fiber spliced to high loss fiber
can cause an apparent gain at a splice.
Visual Inspection:Visible Light
Source
Eye
Optical Fibre
Continuity Test:Sensor
Light Source
Optical Fibre
Optical Power
Meter
Receiver Sensitivity Test
BER Test
Set
Variable Optical Optical Power
Attenuator
Splitter
Transmitter
OF Patch
Cords
Power Meter
DUT
Receiver
Thank You
Any Questions & Suggestions,
please.