testing fiber optic media lab last update 2014.07.25 1.1.0 copyright 2013 - 2014 kenneth m. chipps...

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Testing Fiber Optic MediaLab

Last Update 2014.07.251.1.0

Copyright 2013 - 2014 Kenneth M. Chipps Ph.D. www.chipps.com


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Testing Fiber Optic Media

• The basic test of a fiber optic cable based link is loss or attenuation

• This loss can be measured automatically by using a calculated loss limit or manually by just observing the power measured at the end of the cable


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Testing Fiber Optic Media

• The automatic method requires an OLTS with this capability built-in such as the Fluke DTX CableAnalyzer and SmartRemote set

• The manual method requires an OLTS with a power source and a power meter

• The Fluke DTX CableAnalyzer and SimpliFiber set does this


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Loss Limit

• Let’s first look at how the acceptable loss is calculated

• Many fiber optic cable testers such as the Fluke DTX will calculate the maximum loss that could occur for the link to still work

• For example in this test report the Loss Limit is 1.67

• Why is that


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Test Report


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Loss Limit

• In this case as the test report shows the length of the cable is 156 feet

• The standard loss for 62.5 multimode fiber optic cable is 3.5 dB per kilometer which is .0010167 dB of loss per foot

• 156 feet times .0010167 dBs per foot comes to .17 dB of loss due to attenuation in the cable


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Loss Limit

• It is assumed in this case by the DTX that there will be two connectors in the path since the test method selected was Method B

• Method B assumes two connectors• The standard maximum loss for a 62.5

multimode fiber optic cable connector is .75 dB, which as we will see is a very high value


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Loss Limit

• For two connectors this is 1.50 dBs• .17 plus .75 plus .75 is 1.67• So the computed limit of loss that can be

tolerated for this link to pass is 1.67• If the measured loss is found to be higher

than this, we need to find out why and fix it


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Loss Limit

• Now if we actually calculated this Loss Limit using these standard values for each part of the link under test in this example the Loss Limit would be 4.67 dBs broken down this way

• DTX Main – 1 Meter Patch Cable – 150 Roll of 62.5 Fiber Optic Cable – 1 Meter Patch Cable – DTX Remote

• The link looks like this


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Loss Limit

Loss Limit

Connector .75

Main Output Patch Fiber .0035

Connector .75

Connector .75

Roll of Fiber Fiber .17

Connector .75

Connector .75

Remote Input Patch Fiber .0035

Connector .75

Total Loss 4.67


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Loss Limit

• If we put a basic light source at the other end of the link instead of the SmartRemote, and then measure the received power for this link we find it is 19.18

• This means the real loss is .82• Why would there be such a large

difference between the expected loss of 4.67 and the actual loss of .82


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Loss Limit

• It seems the standard loss figures for connectors are way too high

• Corning for example states that the loss one should expect from a connector such as this is .1 as seen here


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Loss Limit


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Loss Limit


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Loss Limit

• If we adjust the table for these specification figures the Loss Limit now looks like this


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Loss Limit

Loss Limit

Connector .1


Connector .1

Connector .1

Roll of Fiber 150 Feet 62.5 Fiber .17

Connector .1

Connector .1


Connector .1

Total Loss .77


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Loss Limit

• Now this is much closer to the value we measured of .82

• The difference between the realistic Loss Limit calculated using the published specifications for these components and the loss actually found in the link is .05

• Regardless of what caused the difference it is so small a value it can be ignored as insignificant


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Loss Limit

• This is likely due to measurement error, more loss somewhere than the specifications state, dirt on end faces, or gaps between one or more of the connection ends


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So What

• The next question you should be asking yourself is - is this amount of received power good, bad, or what

• Further, just how sloppy can I really be in the installation

• Just how much loss can I measure without it causing a problem


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So What

• To answer this we must look at the specifications of the receiving port at the far end of the link

• How much power must it receive at a minimum to receive data


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Power Budget

• The power budget is the difference between the Launch or Transmit Power which is the energy level of the light as it leaves the transmitter and the Receive Sensitivity or Power of the receiving device which is the minimum energy required for the fiber receiver to detect an incoming signal


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Power Budget

• The Power Budget then is simply the result of subtracting the Receive Sensitivity from the Launch Power

• For example for a Cisco SFP being used on each end of a link such as this– GLC-SX-MM SFP– WS-G5484– For 1000BASE-SX– Using 850 nm


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Power Budget

• Minimum Transmitter Power -3• Minimum Receiver Sensitivity -17• Power Budget of -3 minus –17 or 14 dB


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So What

• Why do we care• The Power Budget is just a term used to

describe the total amount of light energy available for a certain link

• The Power Budget serves as a useful estimation to determine if sufficient optical power will remain on the receiver side of an optical link


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So What

• Why might sufficient optical power not be received at the far end of a link

• The Power Budget then also tells us how much loss we can tolerate in the link’s components such as the fiber attenuation, connector induced loss, dirt on end faces, and so on


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So What

• So in this case for a link of unknown length with an unknown number of splices and connectors and an unknown amount of dirt we have 14 dBs in our pocket to spend on this sort of stuff

• In our example above for a link made up of DTX Main – 1 Meter Patch Cable – 150 Roll of 62.5 Fiber Optic Cable – 1 Meter Patch Cable – DTX Remote


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So What

• We measured the real loss as .82• We then have some 13 dBs to spend on

adding more splices, connectors, dirt, a longer cable to extend the link, and so on

• For a typical installation we are unlikely to add anything to this link as why would we have calculated the expected loss at .77 in the first place if we expected to add devices later


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So What

• What we can spend these 13 dBs on is for junk on the end faces of the various connectors in the link

• In this case 6 connectors or end faces• For each one of these we could tolerate an

average of 1.11 dBs of crud covering the core

• Just how dirty does a end face need to be to produce 1.11 dBs of loss


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So What

• Dirt and other crud is commonly mentioned as causing loss but no one ever provides sample loss numbers for the various types of crud that could block the signal


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So What

• Cisco says in one document that a 1micrometer dust particle on a single-mode core can block up to 1% of the light based on 0.05 dB of loss

• Fluke in a BICSI presentation says the EPA says that typical dust particles in the office are from 2 to 10 microns in size


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So What

• 62.5 fiber is going to be able to still connect with a lot of dust particles if the loss for each one on singlemode fiber is .05 dB as 62.5 end faces are going to have a lot more surface area available to handle these particles


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So What

• If we just use the singlemode loss factor per dust particle of .05 dB we have at least room for 134 of these pesky little dust particles spread among six connectors before the link fails

• In short keep the connections clean and tight, but do not obsess over it


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The Test

• Let’s now perform a test to see how this is done

• We will do two types of tests– Automatic test using the DTX CableAnalyzer

and SmartRemote– Manual test using the DTX CableAnalyzer and

SimpliFiber light source


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Select the Link Type

• Regardless of whether the automatic or manual test is used the type of link being tested must be setup in the CableAnalyzer


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• In this case to do this select Setup• Select Fiber Loss

– In Tab 1 Select Fiber Loss• Press enter to select the Test Limit

– Select TIA568C Backbone MM

• Select the Fiber Type– Select Multimode 62.5 MBW=200

• Select the Remote End Source– Select SmartRemote


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– In Tab 2 Select Test Method• Method B

– Select Number of Adaptors• 2

– As we will be plugging into the link for the actual test– Thus there will be 6 adaptors in total– Two are accounted for at each end by setting the Test

Method to Method B– The other 2 - one at each end of the link under test - are

accounted for here


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Automatic Test

• The two steps for an automatic test are– Set the reference– Perform the test


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Set the Reference

• What color boot goes to which input or output depends on how the patch cables are made

• In this lab we are using a green patch cable at the CableAnalyzer end and a blue patch cable at the SmartRemote end

• The boots of these are both color coded so that red goes to red and black goes to black


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Set the Reference

• Regardless of the color of anything it does matter where the mandrels are

• They are always at the output port of each unit

• Therefore, in this lab connect the two units to each other in this manner


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Set the Reference

OUTPUT OUTPUT INPUT

REMOTE MAIN

GRAYMANDREL

GRAYMANDREL

INPUT


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Set the Reference

• With the cables connected as shown, on the CableAnalyzer select Special Functions

• Select Set Reference• Press Enter• Verify the connections• Press Test


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Set the Reference


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Set the Reference


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Set the Reference


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Set the Reference

• The result as displayed on the screen is


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Set the Reference


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Set the Reference

• Press Ok


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Automatic Test

• To perform the automatic test• Move the switch to AutoTest• In Room 227 connect the cables as shown

next• DO NOT disconnect the output port cables

from the OLTS


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General Connection Diagram


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Room 227Fiber Optic Cable Backbone

Loss Test Connection Diagram

INP

UT

OU

TP

UT

OU

TP

UT

INP

UT

WALL PORT PATCH PANEL PORT

REMOTE MAIN


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Room 227 Setup


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Room 227 Setup


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Room 227 Setup


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Room 227 Setup


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Room 227 Setup


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Automatic Test

• Press Test• See if the test passed or failed• A pass will look like this


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Room 226 Test Result


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Room 227 Test Result


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Manual Test

• Let’s see next how to do the manual test where a known amount of power is sent into a link, and then at the far end of the link the power received is measured

• If as discussed above this received level is high enough for the fiber modules to send and receive data then the link as installed can be considered to have passed


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Manual Test

• For this test we will use a 150 foot roll of 62.5 fiber optic cable

• We will need to calculate the actual expected loss before running the test so that we can compare it to the test results

• It will be up to us to then decide if the link passed or failed

• For this calculation we can just pull the figures from above down here


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Loss Limit

Loss Limit

Connector .1


Connector .1

Connector .1

Roll of Fiber 150 Feet 62.5 Fiber .17

Connector .1

Connector .1


Connector .1

Total Loss .77


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Loss Limit

• As the table shows the loss we expect to find for this link is .77 dB

• We are now ready to measure the actual loss of this link


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Manual Test

• The two steps for a manual test are– Set the reference– Perform the test


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Set the Reference

• What color boot goes to which input or output depends on how the patch cables are made

• In this lab we are using a blue patch cable only to set the reference and then both the green and blue cables for the test


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Set the Reference

• Regardless of the color of anything it does matter where the mandrel goes

• It is always at the output port of each unit• Therefore, in this lab connect the two units

to each other in this manner since the only power source is the remote end SimpliFiber unit


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SimpliFiber


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SimpliFiber


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SimpliFiber


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SimpliFiber

• Select CW – Continuous Wave using button 3 Mode

• The red light will stay lit• Select 850 nm using button 7 mode


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Set the Reference

INPUT OUTPUT OUTUT INPUT

SIMPLIFIBER MAIN

GRAYMANDREL


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Set the Reference Connections

• Change the Remote End Setup from Smart Remote to Far End Source


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Set the Reference


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Set the Reference


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Set the Reference

• With the cables connected as shown on the CableAnalyzer select Special Functions

• Select Set Reference• Press Enter• Verify the connections• Press Test• Press Test


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Set the Reference


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Set the Reference

• The result as displayed on the screen is


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Set the Reference


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Set the Reference

• Press Ok


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Manual Test

• When using the SimpliFiber each cable must be tested one at a time

• To carry out the test connect the cables in this manner


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Manual Test

INPUT OUTPUT INPUT

SIMPLIFIBER MAIN

GRAYMANDREL

OUTPUT


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Manual Test

• The AutoTest function of the CableAnalyzer can be used just as it was for the automatic test or the Power Meter function of the CableAnalyzer can be used

• This is what we will do in this case


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Manual Test

• Select Monitor• Select Power Meter• Select Test• Select 850 nm• Press Test• The result is a real time display of the

power level received by the CableAnalyzer• The received power is -18.78 dB


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Manual Test

• Now compare this received power level of -18.78 to the expected power level which is -20.00 minus .77 or 19.23

• So in this case we have .45 dB of unexpected and unexplained loss

• Do we need to worry about this .45 dB


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Manual Test

• To determine this we first need to see what level of power the device at the end of the link needs to receive in order to send and receive data over this link

• As we saw above in this example that value is -17

• So we have enough room as there is a difference of 1.82


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Manual Test


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Manual Test


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Manual Test


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Fiber Optic Cable Testing

• In this lab we have seen how to perform both an automatic and a manual test of multimode cable such as would be used for a backbone link

testing fiber optic media lab last update 2014.07.25 1.1.0 copyright 2013 - 2014 kenneth m. chipps...

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