OPTICAL FIBRE : TESTS AND MEASUREMENTS.

BY

TX-I FACULTY

A.L.T.T.C;

GHAZIABAD

FEATURES BENEFITS* Low TX Loss. *Long repeater Spacing

or Repeater less N/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 Signal

interference. leakage.

* Small size, * Easy to install,

bending radius and reduction in space

light weight. needed.

* Difficult to tap. * High Security and

Copper resource savings.

Main Features and Benefits of Optical Fiber Cables

System Composition

TransmitterE/O

ConverterO/E

Converter Receiver

Application area of Measuring InstrumentsIn Optical Fiber Communication system

Electrical Signal Optical

Signal

ElectricalSignal

Data In Data Out

DDF

DDF

FDF

FDF

• Cable Loss.

• Splice Loss.

• Connector Loss.

• Fibre Length.

• Continuity of Fiber.

• Fault Localizations/Break Fault.

MAIN TESTS ON OPTICAL FIBRE CABLES

• Calibrated Light Source.

• Optical Power Meter.

• Optical Attenuator.

• Optical Time Domain Reflectometer (OTDR).

INSTRUMENTS REQUIRED

• Generates Light signals of known power and wavelength (LED or LASER).

• Wavelength variations to match Fiber's Wavelength.

CALIBRATED LIGHT SOURCE

• 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 POWER METER

• 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

REQUIREMENTS OF ATTENUATORS

• Attenuation Range.

• Lowest Insertion Loss.

• Independent of Wavelength.

• Type of Connectors at the Input and Output.

Fiber

Light Source

Light Source

100%Dark

Light Receiver

Fiber

Motion

0% Dark

(VARIABLE ATTENUATOR)

• Used for measuring

– Fiber Loss.

– Splice Loss.

– Connector Loss.

– Fiber Length.

– Continuity of Fiber.– Fault Localization.

OPTICAL TIME DOMAIN REFLECTOMETER (OTDR)

• 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.

OPERAING PRINCIPLES

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

n2=1.5 n1=1.0

(n2-n1)2 (1.5-1.0)2

(n2+n1)2 (1.5+1.0)2= = 0.04 = 4% = - 14dB

Fresnel Reflection

OTDR INSTRUMENT PRINCIPLE

Fiber

APD

Signal

Oscilloscope Amplifier

Trigger

Pulse Generator Laser

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

z

t= t1+ t

from pulse front

from pulse tail

direction of pulse propagation

BACKSCATTERINGz

backscatteringfrom pulse front

T= t1

Explanation of the Z/2 uncertainty of the OTDR Signal

Z/2Z-Z/2

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. ± 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 108/1.5 = 200m. Z/2=100m i.e. ± 50mFor 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

Calculation of Pulse Length in Fiber

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

Increasing the pulse width increases the backscatter level.

OTDR pulse

Reflections show OTDRPulse Width and Resolution

Connectors show bothLoss and Reflections

Splices are usually not Reflective.Splices Loss

Slope of trace shows Fiber Attenuation Coefficient

OTDR Trace Information

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

Start point of Measurement

Shifted distance 0.000 km

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

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

00.000

Backscattered Light

Fresnel Reflection

at connection

Fresnel Reflection at near end connector

Splice Fresnel Reflection at Far-end or fault

Loss(dB)

Distance (km)

General Waveform Analysis

X

Y

Reason for Dead Zone

Dead Zone Dead Zone

Dead Zone depends on Pulse Width

100ns 1s

Splice Loss Measurement Principles

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

x1

x2

x3

x4

L2

Splice Loss

Splice PointL1

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).

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

LEAST SQUARE APPROXINATION METHOD (LSA)

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 Loss

SpliceL1

L2

X2

X3X4

*X1

Splice Loss Measurement by Two Point Approximation

Measured Value

Splice

X1

True Value

*

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 fiber can cause an apparent gain at a splice.

Loss Errors in OTDR Measurements

Visible Light Source

Visual Inspection:- Eye

Light Source

Optical PowerMeter

Continuity Test:-

Optical Fibre

Optical Fibre

Sensor

Receiver Sensitivity Test

BER Test Set

TransmitterDUT

Receiver

OF Patch Cords

Variable Optical Attenuator

Power Meter

Optical Power Splitter

Thank You

Any Questions & Suggestions, please.