- Fibre Optic Testing
Test
After the cables are installed and terminated, it's time for testing. For every Fibre 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.
You'll need a few special tools and instruments to test Fibre optics. See Jargon in the beginning of this Guide to see a description of each instrument.
Getting Started
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
Fibre 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 customise 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 Fibre you
have to test. Prepare a spreadsheet of all the cables and fibres
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.
A note on using a Fibre optic source eye safety...
Fibre optic sources, including test equipment, are generally too low in power to cause any eye damage, but it's still a good idea to check connectors with a power meter before looking into it. Some telco DWDM and CATV systems have very high power and they could be harmful, so better safe than sorry.
Fibre optic testing includes
three basic tests that we will cover separately:
Visual inspection for continuity or connector checking
Loss testing
Network Testing
Visual Inspection
Visual Tracing
Continuity checking makes certain the fibres are not broken and
to trace a path of a Fibre from one end to another through many
connections. Use a visible light "Fibre optic tracer"
or "pocket visual fault locator". It looks like a flashlight
or a pen-like instrument with a light bulb or LED source that
mates to a Fibre 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 Fibre. 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 Fibre 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 Fibre 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 fibres! And to make certain the proper fibres 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 Fibre 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 fibres 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 grey jackets. You can also use
this gadget to optimise mechanical splices or prepolished-splice
type Fibre 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
Fibre 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 Fibre will not
show any signs of cracks, chips or areas where the Fibre 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 Fibre 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.
Measuring power
Power in a Fibre 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.
Testing loss
Loss testing is the difference
between the power coupled into the cable at the transmitter end
and what comes out at the receiver end. Testing for loss requires
measuring the optical power lost in a cable (including connectors
,splices, etc.) with a Fibre optic source and power meter by
mating the cable being tested to known good reference cable.
In addition to our power meter, we will need a test source. The test source should match the type of source (LED or laser) and wavelength (850, 1300, 1550 nm). Again, read the instructions that come with the unit carefully.
We also need one or two reference cables, depending on the test we wish to perform. The accuracy of the measurement we make will depend on the quality of your reference cables. Always test your reference cables by the single ended method shown below to make sure they're good before you start testing other cables!
Next we need to set our reference power for loss ­ our "0 dB" value. Correct setting of the launch power is critical to making good loss measurements!
Clean your connectors and set up your equipment like this:
Turn
on the source and select the wavelength you want for the loss
test. Turn on the meter, select the "dBm" or "dB"
range and select the wavelength you want for the loss test. Measure
the power at the meter. This is your reference power level for
all loss measurements. If your meter has a "zero" function,
set this as your "0" reference.
Some reference books and manuals show setting the reference power for loss using both a launch and receive cable mated with a mating adapter. This method is acceptable for some tests, but will reduce the loss you measure by the amount of loss between your reference cables when you set your "0dB loss" reference. Also, if either the launch or receive cable is bad, setting the reference with both cables hides the fact. Then you could begin testing with bad launch cables making all your loss measurements wrong. EIA/TIA 568 calls for a single cable reference, while OFSTP-14 allows either method.
Testing Loss
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 Fibre, 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 generalise, here are some guidelines:
- -For each connector, figure
0.5 dB loss (0.7 max)
- -For each splice, figure 0.2 dB
- -For multimode Fibre, 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 Fibre, 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: - -For each splice, figure 0.2 dB
(0.5 dB x number of connectors) + (0.2 dB x number of splices) + Fibre loss on the total length of cable
Troubleshooting Hints
If you have high loss in a cable, make sure to reverse it and
test in the opposite direction using the single-ended method.
Since the single ended test only tests the connector on one end,
you can isolate a bad connector - it's the one at the launch
cable end (mated to the launch cable) on the test when you measure
high loss.
High loss in the double ended test should be isolated by retesting
single-ended and reversing the direction of test to see if the
end connector is bad. If the loss is the same, you need to either
test each segment separately to isolate the bad segment or, if
it is long enough, use an OTDR.
If you see no light through the cable (very high loss - only
darkness when tested with your visual tracer), it's probably
one of the connectors, and you have few options. The best one
is to isolate the problem cable, cut the connector off one end
(flip a coin to choose) and hope it was the bad one (well, you
have a 50-50 chance!)
OTDR Testing
As we mentioned earlier, OTDRs are always used on OSP cables
to verify the loss of each splice. But they are also used as
troubleshooting tools. Let's look at how an OTDR works and see
how it can help testing and troubleshooting.
How OTDRs Work
Unlike sources and power meters which measure the loss of the
Fibre optic cable plant directly, the OTDR works indirectly.
The source and meter duplicate the transmitter and receiver of
the Fibre optic transmission link, so the measurement correlates
well with actual system loss.
The OTDR, however, uses backscattered light of the Fibre to imply
loss. The OTDR works like RADAR, sending a high power laser light
pulse down the Fibre and looking for return signals from backscattered
light in the Fibre itself or reflected light from connector or
splice interfaces.
At any point in time, the light the OTDR sees is the light scattered
from the pulse passing through a region of the Fibre. Only a
small amount of light is scattered back toward the OTDR, but
with sensitive receivers and signal averaging, it is possible
to make measurements over relatively long distances. Since it
is possible to calibrate the speed of the pulse as it passes
down the Fibre, the OTDR can measure time, calculate the pulse
position in the Fibre and correlate what it sees in backscattered
light with an actual location in the Fibre. Thus it can create
a display of the amount of backscattered light at any point in
the Fibre.
Since the pulse is attenuated in the Fibre as it passes along
the Fibre and suffers loss in connectors and splices, the amount
of power in the test pulse decreases as it passes along the Fibre
in the cable plant under test. Thus the portion of the light
being backscattered will be reduced accordingly, producing a
picture of the actual loss occurring in the Fibre. Some calculations
are necessary to convert this information into a display, since
the process occurs twice, once going out from the OTDR and once
on the return path from the scattering at the test pulse.
There is a lot of information
in an OTDR display. The slope of the Fibre trace shows the attenuation
coefficient of the Fibre and is calibrated in dB/km by the OTDR.
In order to measure Fibre attenuation, you need a fairly long
length of Fibre with no distortions on either end from the OTDR
resolution or overloading due to large reflections. If the Fibre
looks nonlinear at either end, especially near a reflective event
like a connector, avoid that section when measuring loss.
Connectors and splices are called "events" in OTDR
jargon. Both should show a loss, but connectors and mechanical
splices will also show a reflective peak so you can distinguish
them from fusion splices. Also, the height of that peak will
indicate the amount of reflection at the event, unless it is
so large that it saturates the OTDR receiver. Then peak will
have a flat top and tail on the far end, indicating the receiver
was overloaded. The width of the peak shows the distance resolution
of the OTDR, or how close it can detect events.
OTDRs can also detect problems in the cable caused during installation. If a Fibre is broken, it will show up as the end of the Fibre much shorter than the cable or a high loss splice at the wrong place. If excessive stress is placed on the cable due to kinking or too tight a bend radius, it will look like a splice at the wrong location.
OTDR Limitations
The limited distance resolution of the OTDR makes it very hard
to use in a LAN or building environment where cables are usually
only a few hundred meters long. The OTDR has a great deal of
difficulty resolving features in the short cables of a LAN and
is likely to show "ghosts" from reflections at connectors,
more often than not simply confusing the user.
Using The OTDR
When using an OTDR, there are a few cautions that will make testing
easier and more understandable. First always use a long launch
cable, which allows the OTDR to settle down after the initial
pulse and provides a reference cable for testing the first connector
on the cable. Always start with the OTDR set for the shortest
pulse width for best resolution and a range at least 2 times
the length of the cable you are testing. Make an initial trace
and see how you need to change the parameters to get better results.
Restoration
The time may come when you have to troubleshoot and fix the cable
plant. If you have a critical application or lots of network
cable, you should be ready to do it yourself. Smaller networks
can rely on a contractor. If you plan to do it yourself, you
need to have equipment ready (extra cables, mechanical splices,
quick termination connectors, etc., plus test equipment.) and
someone who knows how to use it.
We cannot emphasise more strongly the need to have good documentation
on the cable plant. If you don't know where the cables go, how
long they are or what they tested for loss, you will be spinning
you wheels from the get-go. And you need tools to diagnose problems
and fix them, and spares including a fusion splicer or some mechanical
splices and spare cables. In fact, when you install cable, save
the leftovers for restoration!
And the first thing you must decide is if the problem is with
the cables or the equipment using it. A simple power meter can
test sources for output and receivers for input and a visual
tracer will check for Fibre continuity. If the problem is in
the cable plant, the OTDR is the next tool needed to locate the
fault.
