FOA Basic Skills


Lesson 6: Fiber Optic Testing

Objectives: From this lesson you should learn:
What needs testing in fiber optics
How to trace fibers and locate faults with a visual fault locator
How to inspect and clean fiber optic connectors
How to measure optical power
How to measure insertion loss with a light source and power meter
Testing cable plants and patchcords
3 Methods of setting a "0dB" reference  
Factors affecting measurement uncertainty
How to use an OTDR properly
Setting up OTDR test parameters
Interpreting OTDR traces
Factors affecting OTDR measurement uncertainty

Tools And Components Needed
Visual Fault Locator
Fiber optic inspection microscope with adapters for connectors being inspected - typically 2.5mm ferrule (SC/ST/FC) and 1.25mm ferrule (LC)
Fiber optic power meter
with adapters for connectors being inspected
Fiber optic light source appropriate for fiber and cables being tested (850/1300nm LEDs for MM, 1310/1550nm laser for SM)
Jumper cables for use as reference test cables
appropriate for fiber and cables being tested (2+m long preferred)
OTDR
appropriate for fiber and cables being tested (850/1300nm for MM, 1310/1550nm laser for SM)
Launch and receive cables for OTDR testing (typically >100m, depending on the OTDR resolution and preferably from the same fiber)
Consult the Workbook section on Testing for equipment 
needed or refer to the CFOT Certification Lab Manual
 

Introduction


Fiber optic testing

Fiber optic testing is the least well understood aspect of fiber optic installation. That's what manufacturers say and exactly what we see on FOA certification exams. Granted fiber optic testing is not simple (mostly because of the complexity of the instruments) but it is straightforward. The principles are easily learned and if applied in everyday use, they become habits for the competent installer.

FOA Testing Book

Recognizing the complexity of fiber optic testing, the FOA devotes a chapter (Chapter 8) in our basic fiber and OSP textbooks and has created a complete textbook on fiber optic testing that expands on every subject in testing and covers some advanced topics that are not in our basic textbooks. If you are doing fiber optic testing, we highly recommend you get a copy of this book for reference. FOA Reference Guide To Fiber Optic Testing.


This lesson covers the basic fiber optic tests used to test components and installed cable plants. It includes six separate exercises that will each take some time to complete.

Make certain before you begin each exercise that you have everything you need - tools, test equipment and components. Refer to the check list on Lesson 2.
   

Safety

Lennie works safe Please Note: This is not the usual online course - it is intended to guide you as you learn new skills - the "hands-on" skills needed to install optical fiber cable plants. It involves using tools and components in a realistic manner. Some of the processes here can be hazardous, like working with sharp scraps of optical fiber and chemicals.

In Lesson 1 you should have familiarized yourself with the safety procedures - follow them all the time. Do not work with fiber without eye protection and a proper work area that is easy to clean up.  Always wear safety glasses when doing any of these exercises and dispose of all scraps properly.

Follow the safety procedures learned in lesson 1 with some special warnings for testing. Do not use a microscope to inspect connectors until you have tested the output of the connector with a power meter as the microscope can focus all the light into your eyes. Preferably use a microscope with a IR filter to remove potentially dangerous light. Beware of VFLs (visual fault locators) which have high output in the wavelength of light where eyes are very sensitive. Do not look into the VFL connector and look at the output of cables being tested with a VFL at an angle to see if the light is too bright. The output of laser sources may be high and they are invisible to the human eye. Follow the general polify of testing fibers with a power meter before looking at the end.
   
Download a FOA safety poster for your work area.



Background Review
This "skills" course assumes you have knowledge of fiber optic termination and splicing. If you are new to fiber optics, you should first  review the section of the FOA Guide on Testing, read the Workbook section on Testing and/or complete the
Fiber U Basic Fiber Optics: Testing  course before attempting the hands-on exercises here.


Hands-On Lab Instructions

Download the Workbook section on Testing and the VHO tutorials on Insertion Loss and OTDR. Watch the videos and/or read the references on the test you have available for practice. The
VHO  "Virtual Hands On" Tutorials take a "step-by-step" approach to the hands-on processes covered in this self-study program and the videos will show the processes in motion. The "Insertion Loss Simulator" uses a virtual source and power meter  to test a cable plant, step-by-step. They are both a good way to teach yourself the processes here - watch the video for the overview then follow the steps in the VHO web pages.



Lesson
Workbook Section (PDF)
VHO Tutorial (PDF)
Video (YouTube)
Lesson 6: Fiber Optic Testing Testing Insertion Loss Simulator  
Insertion Loss
OTDR
Reading An OTDR Trace (Interactive)
Insertion Loss Testing
Using an OTDR

Additional references are given for each of the exercises below.
 



Hands-On Assignments:
After reading the workbook section and VHO tutorial and watching the videos, complete these exercises using all the cable types available to you.

As you finish each section, fill in the worksheet in the back of the Workbook on Testing. Your completed worksheets are the records of your having successfully completed the exercises.





Visual Tracing And Fault Location

visual fault locator

Tools and equipment needed
Visual fault locator (VFL, laser based)
Visual tracer (LED flashlight based)
Patchcords and cables, both SM and MM

Warning: VFLs can be powerful enough to be an eye hazard. Do not look directly into the VFL connector or the end of a fiber illuminated by a VFL. Instead, aim the end of the fiber at a light colored object and look for the laser light.

YouTube Video:  FOA Lecture 13: Testing Fiber VisuallyFOA Lecture 21 Visual Fault Locator Demonstration 

Online Guide: Visual tracing & fault location


Connect a cable to the VFL or visual tracer, and check continuity by seeing if the fiber transmits light. Compare the amount of light transmitted through SM and MM fiber.

While the cable is connected to the VFL, put bends in the fiber as shown above (yellow jacket SM fiber shown) and note the loss in the cable. Can you tell a difference in the output of the fiber at the connector? Why?

If you have a cable
(preferably yellow jacket SM fiber) you are willing to damage, break the fiber and see what the cable looks like.






Visual Inspection With A Microscope

fiber optic microscope

Tools and equipment needed
Fiber optic inspection microscope, either optical or video (shown)
Fiber optic cleaning supplies
Patchcords and cables, both SM and MM

YouTube Video: Visual Inspection of Connectors With A Microscope,   FOA Lecture 13: Testing Fiber Visually

Online Guide: Microscope Inspection of Connectors 


Inspect the connectors on the patchcords or cables with the microscope immediately upon removal of the dust caps. What do you see?

Clean the connectors and inspect again. Is the ferrule surface clean? If not clean again. Use both wet and dry methods.

Replace the dust cap, remove it and check the connector again. Was it still clean?

Touch the end of the connector with your finger and inspect it. Clean and inspect again.

Drop a connector on a carpet (perhaps the one on the cable you broke for the VFL exercise) and
inspect it. Clean and inspect again.





 
Measuring Optical Power

Fiber optic power meter


Tools and equipment needed
Fiber optic light source and power meter, adapters for connectors used on patchcords and cables
Fiber optic cleaning supplies
Patchcords and cables, both SM and MM

YouTube Videos: FOA Lecture 14: Testing Optical Power 

Online Guide:
Measuring Optical Power,
Units of Measure (dB, dBm)
Calibration and Uncertainty in Fiber Optic Power Measurements 


Turn on the source and let the output stabilize.

Turn on the meter and familiarize yourself with the controls. Put the meter on dBm and set the wavelength to the wavelength of your source.

Measure the power output of the light source. Stress the cable by putting bends in it (as with the VFL exercise) and note the differences. Change the wavelength setting to another wavelength and note the changes. Why does the measured power change?

Measure the output of a 850nm or 1300nm LED source with both SM and MM fiber? What changes do you note?





Loss Budget

loss budget

YouTube Videos FOA Lecture 26, Loss Budgets

Online Guide:
What Loss Should You Expect?,
Link Loss Budgets And Power Budgets

Before measuring loss, it is important to calculate a loss budget so you have an idea of what the loss should be if the components are in good condition.

A: Calculate the loss budget for a long distance network: singlemode fiber, 75km long, 14 splices, LC connectors on each end. Fiber attenuation at 1550nm is 0.21 dB/km, splices average 0.05dB and connector losses are 0.3dB/km.

B: Calculate the loss budget for a metropolitan network: singlemode fiber, 6.5km long, 2 splices, one intermediate patch panel plus LC connectors on each end. Fiber attenuation at 1310nm is 0.40 dB/km, splices average 0.05dB and connector losses are 0.3dB/km.

C: Calculate the loss budget for a LAN backbone: OM3 fiber 210meters long, no splices, two intermediate patch panels plus SC connectors on each end. Fiber attenuation at 850nm is 3.10 dB/km and connector losses are 0.5dB/km.

Answers below.

How to calculate loss budgets easily: FOA has a free loss budget calculator for iPhones and iPads.

Calculate loss budgets for a cable plant you can make from the cables you have for the exercises. Remember you need two patchcords for reference test cables.





Single Ended Loss Test

single ended loss test

Tools and equipment needed
Fiber optic light source and power meter, adapters for connectors used on patchcords and cables
Fiber optic cleaning supplies
Patchcords and cables, both SM and MM, at least three of the same type for testing (2 reference test cables and one cable to test) and mating adapters

YouTube Videos FOA Lecture 16: Insertion Loss Testing ,

Online Guide: 
Installed Cable Plant Testing (FOA-1, OFSTP-7 & 14)
FOA Standard FOA-1
What Loss Should You Expect?,

VHO:
Insertion Loss

If you are not familiar with the operation of these instruments read the manuals and work with the instrument until you know how to change test parameters and interpret the display.

Attach one cable to the source which will be the reference "launch cable" and measure the output with the power meter. Set the power meter on the 'dB" range and push the button to zero the reading, setting the "0dB" reference.

Using a mating adapter, attach a second cable (the cable to test) to the launch cable and measure the loss. What are your measuring?

Reverse the cable to test and measure again. Is the loss different? Why?

Test other cables and record the results.

Modify the launch conditions with a mandrel wrap on the launch cable. Make a measurement of a cable under test and record the results. Wrap 5 turns of the cable around a 25mm (1 inch) mandrel (any round object will work) and repeat the test (set 0dB reference, check connection to receive cable, insert cable to test and record data.) Are the results different? Why? Repeat the test with a cable plant made from several cables and note the results.

Use a 62.5/125 micron cable for a launch cable and test a 50/125 cable. What is the result? Use a mandrel wrap on the launch cable and repeat the test. What is the result? Use the same
62.5/125 micron cable launch cable to test a WM cable. What is the result?

Use a 50/125 micron cable for a launch cable and test a 62.5/125 cable. What is the result?





Double
Ended Loss Test

Double ended loss test


Tools and equipment needed
Fiber optic light source and power meter, adapters for connectors used on patchcords and cables
Fiber optic cleaning supplies
Patchcords and cables, both SM and MM, at least three of the same type for testing (2 reference test cables and one cable to test) and mating adapters. The cable to test for the exercises should be long enough to also be used in the OTDR test exercise as you will compare the test data for each method.

YouTube Videos FOA Lecture 16: Insertion Loss Testing ,

Online Guide: 
Installed Cable Plant Testing (FOA-1, OFSTP-7 & 14)
FOA Standard FOA-1
What Loss Should You Expect?,

VHO: 
Insertion Loss.  

Insertion Loss Simulator allows you to use a virtual source and power meter  to test a cable plant, step-by-step. Use it with your source and meter to test insertion loss or do the test virtually using the simulator. Both singlemode and multimode versions are available and you see how Loss Budgets are used in testing.  

Insertion Loss Test Simulator
Testing cable with the insertion loss simulator


If you are not familiar with the operation of these instruments read the manuals and work with the instrument until you know how to change test parameters and interpret the display.

Attach one cable to the source which will be the reference "launch cable" and measure the output with the power meter. Set the power meter on the 'dB" range and push the button to zero the reading, setting the "0dB" reference.

Using a mating adapter, attach a second cable (the cable that will become the receive cable) to the launch cable and measure the loss. What are your measuring? DO NOT RESET the
"0dB" reference.

Detach the two cables and insert a cable to test using another mating adapter
and measure the loss. What are your measuring?

Reverse the cable to test and measure again. Is the loss different? Why?

Change the reference method to a "two cable reference," i.e. set the zero dB reference with both a launch and receive cable and repeat the test on the same cable you just tested. Record the results. What is different and why?

Change the reference method to a "three cable reference," i.e. set the zero dB reference with both a launch and receive cable and a third reference cable between them and repeat the test on the same cable you just tested. Record the results. What is different and why?

Compare your tests of the same cable with one, two and three cable reference methods.

Test other cables and record the results.

Modify the launch conditions with a mandrel wrap on the launch cable. Make a measurement of a cable under test and record the results. Wrap 5 turns of the cable around a 25mm (1 inch) mandrel (any round object will work) and repeat the test (set 0dB reference, check connection to receive cable, insert cable to test and record data.) Are the results different? Why? Repeat the test with a cable plant made from several cables and note the results.

Use a 62.5/125 micron cable for the launch and receive cable and test a 50/125 cable. What is the result? Why?






OTDR Testing

OTDR trace


Tools and equipment needed
OTDR with adequately long launch and receive reference test cables, preferably of the same fiber and equal lengths.
Fiber optic cleaning supplies
Cables of sufficient length to test with an OTDR, both SM and MM if the OTDR tests both, and mating adapters
(Optional - use the FOA
OTDR Simulator  or Fiberizer to simulate the test if you do not have an OTDR or adequate fiber to test.)

YouTube Videos FOA Lecture 17: OTDR Testing
FOA Lecture 18: OTDR Setup
FOA Lecture 19: OTDR Measurement Uncertainty     

Using An OTDR (Hands-On)  

Online Guide: 
OTDR testing ,

Frequently Asked Questions about OTDRs

VHO:
OTDR   

Software Download: OTDR Simulator


With an OTDR

If you are not familiar with the operation of this OTDR read the instrument manual and operate the instrument until you know how to set parameters manually, take traces and analyze them using both "two-point" and "LSA" methods.

Connect a launch cable to the OTDR and set the OTDR in the manual mode. Choose a range appropriate for the launch cable, typically the shortest range for the OTDR. Set the pulse width to the shortest option and averaging to a short time period or ~32-64 cycles. Set the wavelength to 850nm for MM or 1310nm for SM. Take a trace of the launch cable. Measure how long it takes for the trace to return to the baseline (the "dead zone"). Change the pulse width to longer options and measure the changes to the length of the dead zone.

Attach a cable to test to the launch cable and reset the OTDR to the shortest pulse. Take a trace. Can you see the end of the fiber in the cable under test or is it too noisy? If it is too noisy, go to a longer OTDR pulse or average for a longer time until you an clearly see the end of the cable under test. Using the two marker method, measure the loss of the connection between the launch cable and the cable under test and
the attenuation of the fiber in the cable under test. Record the results (including saving the traces for later comparison.) Repeat the tests using the LSA methods. Record the results and compare to the two marker method. Are there differences? Why? Measure the reflectance of the connection between the launch cable and cable under test. Can you measure the loss of the connector on the far end of the cable under test? If not, why not? Change the OTDR to the "autotest" mode and repeat the measurement. Compare the results. What is different?

Do you see any "ghosts" in the traces?

Attach a receive cable to the far end of the cable under test. Now using the two marker method, measure the loss of the connections on either end, the attenuation of the fiber in the cable under test and the total loss of the cable under test including the connections on each end. Record this data for comparison. 

Repeat the tests using the LSA methods. Record the results and compare to the two marker method. Are there differences? Why? Reverse the cables (remove the launch cable from the OTDR and connect the OTDR to the open end of the receive cable without disturbing the other connections) and repeat the tests. Record the results and compare to the two marker method. Are there differences? Why? Change the OTDR to the "autotest" mode and repeat the measurement. Compare the results. What is different?

Record the traces at one wavelength and test at the other (e.g if you have MM traces at 850nm, take a new trace at 1300nm, or SM at 1310nm, change to 1550nm.) Compare the traces. Is the fiber attenuation different? How about connections?


With The OTDR Simulator

Install the OTDR simulator on your PC. Open the program (ignore the warning that it is looking for the hardware of the PC OTDR it was written for.) Follow the manual to understand how the OTDR Simulator works.

Go to the folder "Parameter Traces" and open the traces w-850.sor and w-1300.sor and compare them. Each set of files with the same beginning to the name are meant to be compared. Do that with each set to see what happens when you change parameters on the setup menu of the OTDR. The folders Traces and Span Traces include more traces to analyze.


 


Test Your Comprehension - Online Quiz On Testing  

Have you completed the Workbook worksheets that are the records of your having successfully completed the exercises?


 



Loss Budget Answers:

A:
Calculate the loss budget for a long distance network: singlemode fiber, 75km long, 14 splices, LC connectors on each end. Fiber attenuation at 1550nm is 0.21 dB/km, splices average 0.05dB and connector losses are 0.3dB/km.

Loss = 75km x 0.21dB/km (=15.75dB) + 14 splices x 0.05dB (=0.70dB) + 2 connections X 0.30dB (=0.60dB) = 17.05dB

B:
Calculate the loss budget for a metropolitan network: singlemode fiber, 6.5km long, 2 splices, one intermediate patch panel plus LC connectors on each end. Fiber attenuation at 1310nm is 0.40 dB/km, splices average 0.05dB and connector losses are 0.3dB/km.

Loss = 6.5km x 0.40dB/km (=2.60dB) + 2 splices x 0.05dB (=0.10dB) + 3 connections X 0.30dB (=0.90dB) = 3.60dB

C:
Calculate the loss budget for a LAN backbone: OM3 fiber 210meters long, no splices, two intermediate patch panels plus SC connectors on each end. Fiber attenuation at 850nm is 3.10 dB/km and connector losses are 0.50dB/km.

Loss = 0.21km x 3.10dB/km (=0.65dB) + 4 connections X 0.50  dB (=2.00dB) = 2.65dB


     
Return to Fiber U Basic Skills Lab Lesson Plan



 

Table of Contents: The FOA Reference Guide To Fiber Optics

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