Fiber Optic Cable Measurement-OTDR.pdf

7/27/2019 Fiber Optic Cable Measurement-OTDR.pdf

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Cable Network

Measurement


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Optical Time Domain Reflectometers

(OTDR)

When Do You Use OTDRs?9 In LANs with splices between cables to

check if the fibers and splices are good

9To test the length of the fiber on a reelWarning

9OTDRs should not be used to measure cableplant loss. They can not show the actualcable plant loss that system will see

9The limited distance resolution of the OTDR


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OTDR

How does an OTDR work?

9The biggest factor in opticalfiber loss is scattering

9In the fiber, light is scatteredin all directions

9OTDR uses "backscattered

light" to make itsmeasurements

9 It sends out a very high

power pulse and measuresthe light coming back


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OTDR


Since it is possible tocalibrate the speed ofthe pulse as it passesdown the fiber, the

OTDR can correlatewhat it sees inbackscattered lightwith an actual location

in the fiber. Thus itcan create a display ofthe amount ofbackscattered light atany point in the fiber


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OTDR


The amount of lightscattered back to theOTDR is proportional to

the backscatter of thefiber, peak power of theOTDR test pulse and thelength of the pulse sent

out. If you need morebackscattered light toget good measurements,you can increase thepulse peak power orpulse width


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OTDR

Information in the OTDR trace

They say a picture is worth a thousand words !


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OTDR

Information in the OTDR trace

Reflective pulses can show you theresolution of the OTDR. You cannot seetwo events closer than is allowed by the

pulse width. Generally longer pulse widthsare used to be able to see farther alongthe cable plant and narrower pulses are

used when high resolution is needed,although it limits the distance the OTDRcan see.


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OTDR

Special Consideration For

Multimode Fiber

OTDRs only see the middle of the multimode fiber core

an OTDR will measure 6-7 dB of loss for a multimode cable

plant that tests at 10 dB with a source and power meter.


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OTDR

A Sample of High Performance

OTDR

FEATURES

9Simple one buttonoperation

9Pre-programmableprocedures

9Full on-line analysis and

remote operation9High-resolution and

long-range in one unit

9Small and lightweight9Exceptionally flexible


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OTDR

Two Rules in testing a fiber

Long Fiber

Short Fiber

More speed

More accuracy

Long Distance Rang

Long Pulse Width

Short Distance Rang

Short Pulse Width

lower Averaging Mode times

lower Distance Resolution settings

Higher Averaging Mode times

Higher Distance Resolution settings


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OTDR

A Sample of Mini-OTDR

9 Start-km: 0 km - 100 km

9 Span: 1 km - 400 km

9 Readout resolution: 0.08 m

9 Minimum sample spacing: 0.1m

9 Refractive index: 1.0000 - 2.0000

9 Backscatter coefficient: 20 - 60 dBat 1s

9 Length unit: km, kft or miles

9 Measurement points: up to 1600

9 Vertical scale: 0.1 - 10.0 dB/Div

9 Read-out resolution: 0.001 dB9 Reflectance range: -14 dB to -60 dB


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This section describes measurementprocedure for the typical measurementitems with the software program , one

the most important is the wintracesoftware . In this section ,these operationitems are briefly explained as

measurement procedure centering aroundthe sequence of execution . Note that thedistance Range , pulse width , and other

values set in this procedure are justexamples , and not actually requiredvalues . When we use this software firstof all , the follow specification chart haveto use .


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1 The title : operator choose for theoptical fiber link.

2 Date : Set the day of measuring .

3 Time : Set the time of measuring .

4 Device : The kind of measuringequipment .


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5 Lambda : Select wavelength .

6 Pulse : Set the pulse width ( 100ns ) it shows the pulse of OTDRlaser for measuring long hall .

7 Acquisition time : Time duration toaccess the graph .


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8 Range : Set the distance Range .

9 Index : Input the reflective index of the

fiber

( like as 1.465 , it is suitable for step indexSM )

10 Cable Number : Number of cable in theroute , show number of cable in this part .


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11 Fiber Number : The number of fiberin the cable .

12 Way : open ------ end OR open ------extrimity display the cursor indicates theabsolute distance from the output

connector to the fault location of fiber orend of fiber .


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TRACE ANALYZING

OTDR shows two events:

1-all reflection of the light whenpassing the length of the fiber.

2-Optical fiber atteuation.


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Trace Graph of the OTDR


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One Important Point:

All fiber losses is not frommanufacturing parameters. Some times

bad installation increase theattenuation of the fiber,

Because of:

Micro and macro bending

OH diffusion

Strength because of extra pushingand/or pulling.

Temperature increase.


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At the beginning of the graph,sometimes there are two peaks,the small one is not a real loss of

reflection and called ghost.


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Ghost caused by multiple reflection .If the reflected light at the

connector (connecting point) returnsback to the source and reflects again.In this case , optical pulse may seem tobe re- emitted from the source . Thisirregular reflection appears as a anextra peaks of loss.


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1 Slope

It shows the attenuation of the fiber


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2 Splice


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3 Reflection

Any connector or hot discontinuitymay occur a reflection which is apeak of loss in the trace.


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1 Distance : Shows between the

source and the events.


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2 Attenuation : Shows about theloss of events

3 Reflectance : Shows about theReflection of events


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4 - Real.dis : Shows distancebetween events

5 Slope : Shows slops betweenevents


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Field Test Instrument

1) OTDR2) TALK SET

3) LIGHT SOURCE & POWER METER4) FIBER SCOPE


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Fiber scope


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Fiber Connector Type


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DIN


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FC/PC


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SC


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FSMA


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SM


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E-2000


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Thank You

For Attention

Fiber Optic Cable Measurement-OTDR.pdf

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