Fiber Optic Cable Testing
After the cables are installed and terminated to your spec, it's time for testing. Cables need to be
tested for continuity, end-to-end
and any other potential problems. For long outside plant cables with intermediate splices, all individual splices need to be verified with an OTDR, since that's the only way to make sure that each one is good. Within the network testing for
is necessary as power is the measurement that tells whether the system is operating properly.
Fiber Cable Assemblies
Fiber Optic Test Instruments
Fiber Optic Hardware & Accessories
Fiber Testing Tutorial
Tools and test equipment for the job...
Source and power meter, optical loss test.
Reference test cables.
3. Fiber Tracer or Visual Fault Locator.
4. Cleaning materials - lint free cleaning wipes and pure alcohol.
OTDR and launch cable for outside plant jobs.
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.
And remember to check that no power is present in the cable before you look at it in a microscope protect your eyes!
Optical Power - Power or Loss? ("Absolute" vs. "Relative")
Practically every measurement in fiber optics refers to optical power. The power output of a transmitter
or the input to receiver are "absolute" optical power measurements, that is, you measure the actual
value of the power. Loss is a "relative" power measurement, the difference between the power coupled into a component like a cable or a connector and the power that is transmitted through it. This
difference is what we call optical loss and defines the performance of a cable, connector, splice, etc.
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)!
To measure power, attach the meter to the cable that has the output you want to measure. That can
be at the receiver to measure receiver power, or to a reference test cable (tested and known to be
good) that is attached to the transmitter, acting as the "source", to measure transmitter power. Turn
on the transmitter/source and note the power the meter measures. Compare it to the specified power for the system and make sure it's enough power but not too much.
There are two methods that are used to measure loss, which we call "single-ended loss" and
"double-ended loss". Single-ended loss uses only the launch cable, while double-ended loss uses a receive cable attached to the meter also.
Single-ended loss is measured by mating the cable you want to test to the reference launch cable and measuring the power out the far end with the meter. When you do this you measure 1. the loss of the
connector mated to the launch cable and 2. the loss of any fiber, splices or other connectors in the cable you are testing. This method is described in FOTP-171 and is shown in the drawing. Reverse
the cable to test the connector on the other end.
In a double-ended loss test, you attach the cable to test between two reference cables, one attached to the source and one to the meter.
This way, you measure two connectors' loses, one on each end, plus the loss of all the cable or cables in between. This is the method specified in OFSTP-14, the test for loss in an installed cable plant.
What Loss Should You Get When Testing Cables?
While it is difficult to generalize, here are some guidelines:
- For each connector, figure 0.5 dB loss (0.7 max) - For each splice, figure 0.2 dB - For multimode fiber, the loss is about 3 dB per km for 850 nm sources, 1 dB per km for 1300 nm. This roughly
translates into a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per 300 feet for 1300 nm. - For singlemode fiber, the loss is about 0.5 dB per km for 1300 nm sources, 0.4 dB per km for 1550 nm.
This roughly translates into a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet for 1300 nm.
So for the loss of a cable plant, calculate the approximate loss as:
(0.5 dB X # connectors) + (0.2 dB x # splices) + fiber loss on the total length of cable