optical time domain reflectometry (otdr) note

8/11/2019 Optical Time Domain Reflectometry (OTDR) Note

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Courtesy from Significant Technologies Sdn Bhd

OPTICAL TIME DOMAINREFELECTOMETER (OTDR):

PRINCIPLES


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Single ended test that...

...characterizes fiber from end-to-end

...locate and measure each event

...provides a detailed map of fiber

Why Test Fiber with an

T!"#


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Working Principles of OTDR

Typical OTDR applications

What can an OTDR tell you?

How Does an OTDR Work?

OTDRBlock Diagram

!nderstanding OTDR Specs

Typical OTDR Test

OTDR Report


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W(at can an OTDR tell you)

Total loss of a fiber

Total reflection of t(e system !OpticalReturn Loss " ORL$

Location an' type of eac( event

Loss an' Reflection of eac( event

*ttenuation of eac( fiber sections


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+o& Does an OTDR Work)

For eac( pulse sent, up to -. ...points are ac/uire'

+un're's of pulses are sent in t(efiber eac( secon'

Eac( point is t(en average't(ousan's of times

T(e longer t(e ac/uisition time, t(ebetter t(e results 0


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+o& Does an OTDR Work) OTDR performance comes from a blen' of tec(nologies

1 2uality, po&erful optics

1 Proper electronic 'esign

1 Efficient 'ata ac/uisition an' soft&are processing1 3ears of fine tuning

1 444 an' engineering magic touc(0

Laser 5 %oupler 5 P(oto'etector

Fresnel Reflections along t(e fiber are measure'

Rayleig( 6ackscatter along t(e fiber is measure'


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OTDR"6lock Diagram


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Function Diagrams

OTDR launches a series of optical pulses into the fiber, and

then measure the backward optical signal v.s. time taken for

the light travels through the fiber

The measurement trace, which is called fiber trace, is

shown on the !D panel.

Distance Range

d " #t$!% & #'$(OR% d, displa)ed distance

(OR, (nde* of Refraction

t, time for round trip

!, light speed in vacuum

The distance range

determines the time

interval of successive

pulse in OTDR

(+, bus

interface

R,- &Registers

,&Dconverters

Fibercoupler

,.D-icro/

controller

aser

Fiber

!onnector Fiber under test


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$nderstanding T!" Specs

!ynamic "ange!ynamic "ange

!ead %ones!ead %ones!istance &ccuracy!istance &ccuracy

Sampling "esolutionSampling "esolution'vent (ocation'vent (ocation

&ccuracy&ccuracy

(oss "esolution(oss "esolution(oss Threshold(oss Threshold

Splice (oss &ccuracySplice (oss &ccuracy"eflectance &ccuracy"eflectance &ccuracy


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!ynamic "ange

$sed for many years) to indicate attainable

distances

S*"+,

The most popular method of calculating

dynamic range method


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Dynamic Range = 28 dB

SNR=1

SNR: Signal to Noise Ratio.

Dynamic Range = 40 dB

!ynamic "ange S*"+,


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Dynamic range an' Distance

Distance covere' 'epen's of fiberteste'

Relate' to fiber attenuation, e78

9. '6 at .4: '65km ; :.. km ma74

9. '6 at .4:< '65km ; =>. km ma74

Re'uce' by event losses, e78< events of ='6 loss re'uce 'istance rangeby < '6 at .4: '65km ; :< km


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,/ us pulse


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0/ us pulse


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!ead %one

resulting fromreflectiveevent

a temporary 1blinding2 of the T!"

detector

!etector saturation due to high

reflectance

Two types of dead zones3

'ventand &ttenuationdead zones


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'vent !ead %one

The ability to !'T'4T an event

that closely follows a reflectiveevent

There is a !ead %one for each

reflective event

5igger reflectance saturatedmeans larger 'vent !ead %one

6easured on the trace at ,.7 d5

from the pea8 of the reflective

event

1.5 dB


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&ttenuation !ead %one

The ability to6'&S$"' anevent that closely follows a

reflective event

'9trapolate a line from the trace

points following the reflectiveevent towards the event

(ocate the point on the trace

that is /.7 d5 higher than the

e9trapolated line

The attenuation dead zone is

defined as the distance between

the beginning of the event and

the previously located point.

0.5 dB


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(oss "esolution

6inimal distance in d5 y-a9is between two points

:t;s a physical specification of the T!"

:t has nothing to do with loss or splice lossaccuracy.

0.001 dB


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(oss Threshold

The minimal loss difference between two splice

loss measurements3 /./, d5

The T!" can differentiate between a ,./7 and

,./< d5 splice) but not ,./7 and ,./7= d5 6inimal splice loss that can be detected by the

Tool5o9 analysis software is /./> d5

The T!" can measure splices /./> d5 and

greater with a resolution of /./, d5


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Splice (oss "esolution

?3 !oes this mean that the T!" will

detect every splice above /./> d5 # &nswer 3 *@

:t depends on the amount of noise@

:t depends on the pulse you are using@


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Splice (oss &ccuracy

The abilit) for the anal)sis software

to measure the loss of a splice

precisel).

Depends on man) things...

0oise

inearit)

Response to high reflectance

Distance from the front end connector.

ower #1 2.3 d4% at the limit of themeasurement range

4etter #1 2.23 d4% near the front end

connector.

Accurate

Noisy data(less accurate)

Linearity problems

(less accurate)


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Sampling "esolution

!istance "ange A *umber of acBuisition points

6ore points + better distance C loss precision

'9ample3 ,7/ 8m A >/ /// points + 7 meters

5 m


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Typical T!" sampling

resolutions and data points.

"&*D'Em Sampling "esolution !ata oints

.


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Sampling oints C

!istance &ccuracy


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!istance &ccuracy

!istance accuracy3

physical limitation of the T!"

not software related

:t gives the e9pected error on the location of

a point seen by the T!" in meters with

respect to its real physical location


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!istance &ccuracy

, m3 $ncertainty of the 1distance zero2location Fiber length in the unit) "ise Time 'lectronic (atency

Time

/.//07IJ!istance3 &cBuisition 4loc8 'rror

Sampling resolution Fiber :nde9 $ncertainty :" can also

induce distance reading variations

Spec.3 K ,m K /.//07I of dist. K Sampling res.

n ,7/ 8m3 K ,m K >.H7m K 7m + KL.H7 m


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Dont forget! Its also dependant on these two factors...

Helical factor Index of refraction

'vent (ocation &ccuracy

!epends on...

...the ability for the analysis software to locate anevent precisely

...noise level) distance from the front end connector)

and number of points.


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"eflectance &ccuracy

The ability for the analysis software to

measure reflectance precisely

&nritsu specifies K = d5.


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(inearity

The capability of the T!" to produce a straight

line for the entire trace.

& trace that is curved would give a higher spliceloss than what it is in reality.

&nritsu specifies /./7 d5 per d5.

1 dBBetween 0.95 and 1.05 dB


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$sing the T!"

Set the :nde9 f "efraction :"

Set the wavelength according to the system

under test

Set the pulse width

Set the distance range

Set the acBuisition time

Shoot the fiber.


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&uto-&nalysis using a 6ini-T!"

'vent table containing3for each event

Type of fault

!istance to fault

&ttenuation

ptical return loss

Splice loss.


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OTDR Test #etup


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Typical T!" Test


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C f Si ifi T h l i Sd Bhd

#ample8 OTDR *nritsu Report

optical time domain reflectometry (otdr) note

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