Transcript 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 E/O Converter F D F Optical Signal F D F Electrical Signal O/E Converter Receiver D D F Data In Application area of Measuring Instruments In Optical Fiber Communication system 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).
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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 Motion Light Receiver 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
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
OTDR INSTRUMENT PRINCIPLE
Pulse Generator Laser Fiber APD Signal Trigge
r
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: i 2 noise = Constant /n
Explanation of the
Z/2 uncertainty of the OTDR Signal
z BACKSCATTERING T= t 1 backscattering from pulse front z from pulse front from pulse tail Z-
Z/2
Z/2 t= t 1 + t
direction of pulse propagation
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 x10 8 /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 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 -9 x3x10 8 /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.
Start point of Measurement
Typical Display on CRT of OTDR
2.0 km/DIV 4.0 db/DIV DR=36km Shifted distance 0.000 km Starting point 0 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 near end connector Splice Fresnel Reflection at Far-end or fault Loss (dB) Backscattered Light Fresnel Reflection at connection Distance (km)
Y
Reason for Dead Zone
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.
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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 this projection of L2, and measuring the level difference between the Splice Point and the intersection.
Splice Point L1 x1 x2 Splice Loss L2 x3 x4
Approximation Methods
At Loss Measurement and Splice are two methods for drawing the line.
Loss Measurement, the loss is found by drawing an imaginary line between two set markers. There
•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
L1 X1 X2 Splice * Splice Loss X3 X4 L2
Splice Loss Measurement by Two Point Approximation
Splice True Value X1 * Measured Value
a. same fiber spliced error caused by fiber characteristics actual loss
Loss Errors in OTDR Measurements
b. high loss fiber spliced to low loss fiber error caused by fiber characteristics actual loss c. low loss fiber spliced to high loss fiber can cause an apparent gain at a splice.
Visual Inspection:-
Eye Visible Light Source Optical Fibre
Continuity Test:-
Light Source Optical Fibre Sensor Optical Power Meter
Receiver Sensitivity Test
BER Test Set Transmitter Variable Optical Attenuator Optical Power Splitter DUT Receiver OF Patch Cords Power Meter