After the cables are installed and terminated, it's time
for testing. For every fiber optic cable plant, you will
need to test for continuity, end-to-end loss and then
troubleshoot the problems. If it's a long outside plant
cable with intermediate splices, you will probably want
to verify the individual splices with an OTDR also,
since that's the only way to make sure that each one is
good. If you are the network user, you will also be
interested in testing power, as power is the
measurement that tells you whether the system is
operating properly.
fiber optics.
Fibre Fault Finding and Testing
Even if you're an experienced installer, make sure you
remember these things.
1.
Have the right tools and test equipment for the job...
You will need:
Source and power meter, optical loss test set or test kit
with proper equipment adapters for the cable plant you
are testing.
Reference test cables that match the cables to be
tested and mating adapters, including hybrids if
needed
Fiber Tracer or Visual Fault Locator
Cleaning materials - lint free cleaning wipes and pure
alcohol
OTDR and launch cable for outside plant jobs
2.
Know how to use your test equipment
Before you start, get together all your tools and make
sure they are all working properly and you and your
installers know how to use them. It's hard to get the
job done when you have to call the manufacturer from
the job site on your cell phone to ask for help. Try all
your equipment in the office before you take it into the
field. Use it to test every one of your reference test
jumper cables in both directions using the single-
ended loss test to make sure they are all good. If your
power meter has internal memory to record data be
sure you know how to use this also. You can often
customize these reports to your specific needs - figure
all this out before you go it the field - it could save you
time and on installations, time is money.
3.
Know the network you're testing...
This is an important part of the documentation process
we discussed earlier. Make sure you have cable
layouts for every fiber you have to test. Prepare a
spreadsheet of all the cables and fibers before you go
in the field and print a copy for recording your test
data. You may record all your test data either by hand
or if your meter has a memory feature, it will keep test
results in on-board memory that can be printed or
transferred to a computer when you return to the
office.
Visual Tracing
Continuity checking makes certain the fibers are not
broken and to trace a path of a fiber from one end to
another through many connections. Use a visible light
"fiber optic tracer" or "pocket visual fault locator". It
looks like a flashlight or a pen-like instrument with a
lightbulb or LED soure that mates to a fiber optic
connector. Attach a cable to test to the visual tracer
and look at the other end to see the light transmitted
through the core of the fiber. If there is no light at the
end, go back to intermediate connections to find the
bad section of the cable.
A good example of how it can save time and money is
testing fiber on a reel before you pull it to make sure it
hasn't been damaged during shipment. Look for visible
signs of damage (like cracked or broken reels, kinks in
the cable, etc.) . For testing, visual tracers help also
identify the next fiber to be tested for loss with the test
kit. When connecting cables at patch panels, use the
visual tracer to make sure each connection is the right
two fibers. And to make certain the proper fibers are
connected to the transmitter and receiver, use the
visual tracer in place of the transmitter and your eye
instead of the receiver (remember that fiber optic links
work in the infrared so you can't see anything
anyway.)
Visual Fault Location
A higher power version of the tracer uses a laser that
can also find faults. The red laser light is powerful
enough to show breaks in fibers or high loss
connectors. You can actually see the loss of the bright
red light even through many yellow or orange simplex
cable jackets except black or gray jackets. You can
also use this gadget to optimize mechanical splices or
prepolished-splice type fiber optic connectors. In fact-
don't even think of doing one of those connectors
without one no other method will assure you of high
yield with them.
Visual Connector Inspection
Fiber optic microscopes are used to inspect
connectors to check the quality of the termination
procedure and diagnose problems. A well made
connector will have a smooth , polished, scratch free
finish and the fiber will not show any signs of cracks,
chips or areas where the fiber is either protruding from
the end of the ferrule or pulling back into it.
The magnification for viewing connectors can be 30 to
400 power but it is best to use a medium
magnification. The best microscopes allow you to
inspect the connector from several angles, either by
tilting the connector or having angle illumination to get
the best picture of what's going on. Check to make
sure the microscope has an easy-to-use adapter to
attach the connectors of interest to the microscope.
Measuring power
Power in a fiber optic system is like voltage in an
electrical circuit - it's what makes things happen. It's
important to have enough power, but not too much.
Too little power and the receiver may not be able to
distinguish the signal from noise; too much power
overloads the receiver and causes errors too.
Measuring power requires only a power meter (most
come with a screw-on adapter that matches the
connector being tested) and a little help from the
network electronics to turn on the transmitter.
Remember when you measure power, the meter must
be set to the proper range (usually dBm, sometimes
microwatts, but never "dB" that's a relative power
range used only for testing loss.) and the proper
wavelengths matching the source being used. Refer
to the instructions that come with the test equipment
for setup and measurement instructions (and don't
wait until you get to the job site to try the equipment)