5 Standard Ways To Test Fiber Optic Cables
Abstract:
We often are asked questions about testing installed fiber optic cables
that indicate the industry standards are confusing, have little
information on measurement accuracy and no guidelines for
troubleshooting. This web page is an attempt to clear up some of this
confusion. But remember, as Bob Metcalfe, co-inventor of Ethernet,
says, "Standards are wonderful, because we have so many to choose from!"
Update:
In 2010, the old OFSTP-14 was replaced by a new ISO standard. The TIA
has adopted IEC 61280-4-1 as the replacement of OFSTP-14. Most of the
two documents is the same, with some important exceptions.
• For insertion loss, three reference methods are are
still approved, but the nomenclature is different - no more "Method A,
B or C" designations- it's now 1, 2 or 3 reference cables.
• OTDR testing is now an approved test method as long
as you use both launch and receive cables.
• Reference test cables with "reference grade connectors" are recommended.
• Methods are given for testing and verifying the loss of reference test cables.
• For multimode modal control, CPR with a mandrel
wrap is gone, replaced by "Encircled Flux," a complex - and not
completely debugged - method of measuring the source output.
Most of the changes are nomenclature. In the meantime, continue testing as usual.
There are five ways listed in various international standards from the
EIA/TIA and ISO/IEC to test installed cable plants. Three of them use
test sources and power meters to make the measurement, while the fourth
and fifth use an OTDR. The best way to understand them is to look
at the diagrams below.
The source/power meter method,
generally called "insertion loss," approximates the way the actual
network uses the cable plant, so one would expect the loss to be
similar to the actual loss seen by the network, which is preferable.
The OTDR is an indirect method, using backscattered light to imply the
loss in the cable plant, which can have large deviations from insertion
loss tests. OTDRs are more often used to verify splice loss or find damage to cables.
The differences in the three insertion loss
tests are in how we define "0 dB" or no loss. All three tests end up
with the same test setup (Figure 1), but the reference power can be set
with one, two or three cables as shown in the three setups below.
All four methods have measurement uncertainty. After we explain the
methodology, we will examine the measurement uncertainty.
Insertion Loss Per TIA OFSTP-14 (Multimode) and OFSTP-7 (Singlemode)
(and similar international standards)
Insertion loss testing with a test source and power meter
(right) simulates the way the cable plant will be used with an actual
link. The test source mimics the transmitter, the power meter the
receiver. But insertion loss testing requires reference cables attached
to the source and meter to connect to the cable under test. This
insertion loss test can use 1, 2 or 3 reference
cables to set the “zero dB loss” reference for testing. Each way
of setting the reference gives a different loss.
Generally network standards prefer the 1 reference
cable loss method, but it requires that the test equipment uses the
same fiber optic connector types as the cables under test. If the cable
has different connectors than the test equipment (e.g. LCs on the cable
and SCs on the tester), it may be necessary to use a 2 or 3 cable
reference, which will give a lower loss since connector loss is
included in the reference and will be subtracted from the total loss
measurement. Any of the three methods are acceptable, as long as the
method is documented. Be careful, however, as most network link losses
may assume a 1 cable reference, which can affect the acceptance of the
cable plant.
Reference per TIA OFSTP-14 (Multimode), 1 Cable Reference, Formerly Called Method B
This method uses only one reference cable attached between the
test source and the power meter. The meter, which has a large area
detector that measures all the light coming out of the fiber,
effectively has no loss, and therefore measures the total light coming
out of the launch reference cable. Once the reference is set, the
launch reference cable should not be removed from the source, as it may
have a different coupled power when reattached.
When
the cable is tested as shown above, the loss measured will include the
loss of the reference cable connection to the cable plant under test,
the loss of the fiber and all the connections and splices in the cable
plant and the loss of the connection to the reference cable attached to
the meter.
It is important to note with all these
tests the quality of the reference cables is important in the
uncertainty of the measurement. If one uses reference cables with bad
connectors, the losses when mated to the cable under test will be
higher than they should be, not a good result if you want the installed
cable to show the quality of your installation processes! Installers
should test all reference cables using FOTP-171 single-ended cable
tests to ensure their quality. Cables with losses higher than 0.5 dB
per end should be cleaned and retested, then discarded if they do not
meet the 0.5 dB max loss.
Dirt is always another
issue. If any of the connectors are dirty, measurements will show
higher loss and more variability.
Reference per TIA OFSTP-14 (Multimode), 2 Cable Reference, Formerly Called Method A
This method probably evolved from the telcos where lengths were
long and the instruments were brought together only once a day for
calibration, plus instruments often had connectors different from the
cable plant (e.g. biconics on the cable plant and SMAs on the
instruments.) Here the launch reference cable is attached to the
source, the receive reference cable to the meter, then the two cables
are mated to set the reference. Once the reference is set, the launch reference cable should
not be removed from the source, as it may have a different coupled
power when reattached, and one should probably do the same at the meter.
Setting the reference this way includes one connection loss (the
mating of the two reference cables) in the reference value. When one
separates the reference cables and attaches them to the cable under
test, the dB loss measured will be less by the connection loss included
in the reference setting step. That's approximate, by the way, since
the variations in mating alignment make the loss slightly different
each time two connectors are mated.
Anyway, this
method gives a loss that's less than the 1 cable reference, Method B (above) and because one
connection is included in settting the reference, it has a higher
variability. Dirt is always an issue. If any of
the connectors are dirty, measurements will show higher loss and more
variability. If the connectors are dirty when setting the reference but
cleaned afterwards before testing the cable, one gets a lower loss, or
even a gain!
Why would you
use this method? If your test equipment has different connectors than
the cable under test, but the connectors are all mateable with proper
mating adapters, this method can be used.
Reference per TIA OFSTP-14 (Multimode), 3 Cable Reference, Formerly Called Method C
Some of the newer connectors are male/female or plug/jack, not two
males that use a mating adapter to create a connection. One connector
is used on the jack in the wall or patch panel and one is used on a
patchcord. Examples are the MTP and MT-RJ. These connectors cannot be
mated to test equipment nor can two similar (
plug or jack ) be mated with a mating adapter. Reference cables
generally will be patchcords with plugs while the cable under
test will have jacks on either end. The only way to get a valid
reference is to use a short cable of known good condition as a
"stand-in" for the cable to be tested to set the reference. To test a
cable, replace the reference cable with the cable to test and make a
relative measurement.
Obviously this method
includes two connection losses in setting the reference, so the
measured loss will be less by the two connection losses and have
greater uncertainty. Dirt is again an issue. If any of the connectors are dirty,
measurements will show higher loss and more variability. If the
connectors are dirty when setting the reference but cleaned afterwards
before testing the cable, one gets a lower loss, or even a gain!
Why would you use this method? If your test equipment has
different connectors than the cable under test and the connectors on
the cable to test are not mateable, this method can be used. Since this
method works with any connector style, it has been chosen for several
international test standards.
Since
there is some confusion about what each method actually measures, the
FOA has a web page that explains it in simple diagrams and math. Read Loss Math.
OTDR Testing
OTDRs (right) test from one end of the cable using the backscatter
signature of the fiber to make an indirect measurement of the fiber.
OTDRs always require a launch cable for the instrument to settle down
after reflections from the high powered test pulse overloads the
instrument. OTDRs have traditionally been used with long distance
networks where only a launch cable is used, but this method does not
measure the loss of the connector on the far end. Adding a cable at the
far end allows measuring the loss of the entire cable, but negates the
big advantage of the OTDR, that it makes measurements from only one end
of the cable, since a tech is required to attach the receive cable to
each fiber as it is being tested. More on OTDR testing.
OTDR Test With Launch Cable Only
When testing with an OTDR using only the launch cable, the trace will
show the launch cable, the connection to the cable under test with a
peak from the reflectance from the connection, the fiber in the cable
under test and likely a reflection from the far end if it is terminated
or cleaved. Most terminations will show reflectance that helps identify
the ends of the cable.
Using markers A (set at the end of the launch cable) and B (set at the end of the cable under test) ,
the OTDR can measure the length of the cable under test and the loss of
the connection to the cable under test plus the loss of the fiber in
the cable under test and any other connections or splices in the cable
under test (not shown.)
This method does not,
however, test the connector on the far end of the cable under test
because it it not connected to another connector, and connection to a
reference connector is necessary to make a connection loss measurement.
OTDR Test With Launch And Receive Cable
If a receive cable is used on the far end of the cable under test, the
OTDR can measure the loss of both connectors on the cable under test as
well as the fiber in the cable and any other connections or splices in the cable under test (not shown.)
The placement of the B marker after the connection to the receive cable
means some of the fiber in the receive cable will be included in the
loss measured. Most OTDRs have a "least squares" test method that can
subtract out the cable included in the measurement of a single
connector, but this will not work on a double ended cable loss test.
Different Results From Different Reference Methods
So just how much does the loss of a cable plant change with the
different methods? The table below shows test results for a 520 meter
simulated cable plant with multimode 62.5/125 fiber tested four
ways at 850 nm using several different sets of reference cables to see
the results. Unfortunately, we did not use a receive cable to get data
for the fifth method mentioned above, but we did take data in both
directions. Using 10 different sets of reference cables and making
multiple measurements allowed calculating an average of each
measurement, comparable to what several different test crews might
find, plus calculating the standard deviation, a statistical indication
of the repeatability of the measurement.
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| Test Method | Results, Loss in dB, Standard Deviation | | 1 Cable Reference | 2.96 dB, +/-0.02 dB | | 2 Cable Reference | 2.66 dB, +/-0.20 dB | | 3 Cable Reference | 2.48 dB, +/-0.24 dB | | OTDR Launch Cable Only | 1.91 dB / 2.05 dB (reverse directions) |
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Note how the loss of our test
cable plant reflects the comments we made above. The one cable
reference method has higher loss than the other methods, but it also
has much less measurement uncertainty (standard deviation.) The 2 and 3
cable reference methods have less loss because we have subtracted the
connector loss(es) included when we set the reference for 0 dB loss,
and the uncertainty is higher because of the greater variance when
connected to the reference cables. And the OTDR measurement is
significantly lower than the other three methods plus is 0.14 dB
different according to which direction we are testing in. Note that if
we added to the OTDR loss the average connector loss difference in the
other tests, about 0.25 dB, the loss would still only be about 2.1 -
2.2 dB, significantly less than any of the insertion loss tests.
If you want to read more about what each insertion loss method actually
measures and why the uncertainty is different for each method, the FOA has a web page that explains it in simple diagrams
and math. Read Loss Math.
Interpreting Test Data
What is the loss of this cable plant? Well all four of those
measurements are strictly according to international standards, so you
can take your choice! However, it might cause a problem if you attach
some network electronics.
Most network
specifications have been written around a loss test that uses a one
cable reference method, as does the EIA/TIA 568B test method. If your
cable has been tested with any other method, the lower loss you measure
will give you a false measurement of system margin. If you are dealing
with new, fast networks like Gigabit Ethernet or Fibre Channel which
have much smaller operating power margins, you could be in trouble.
The different loss obtained in OTDR measurements comes from it’s
completely different methodology. OTDRs use backscattered light to
imply the measurement while insertion loss is measured directly by
transmission. All OTDRs use laser sources which inject light generally
only in the center of the core of the fiber where loss is lower. Some
claims for using mode conditioners to make OTDR and insertion loss data
correlate has not proven reproducible in independent tests conducted by
the FOA (Read the report here.)
A single-ended OTDR measurement, which most tech use, does not test the
connector on the far end. Recent standards mention and sometimes
require a receive cable for OTDR tests, which would have added 0.2-0.3
dB to the loss measured in the test detailed above. But having to use a
receive cable with an OTDR negates its advantage of being able to be
tested from one end by a single technician.
Which Test Method Should Be Used?
For insertion loss testing, it depends on the types of connectors on
your test equipment and the connectors on the cables you need to test.
If your cable plant has the same connector styles as your test set, or
all the connectors use the same 2.5 mm ferrule style common to ST, SC,
FC and some other connectors, you can use the one cable
reference method.
If your test equipment has ST or SC connectors and you must test LCs on
the cable plant, you may have no choice but to use a 2-cable
reference or 3 cable reference. To compare these test results with a 1 Cable Reference, you must add
an estimated loss for the connector included in the reference
measurement, say 0.3-0.5 dB for a typical connector on a high-quality
factory-made patchcord.
If your test equipment
has ST or SC connectors and you must test duplex male/female connectors
like the MT-RJ on the cable plant, you may have no choice but to use a
3-cable reference. To compare these test results with a 1 Cable Reference, you must add an estimated loss for the connectors included in the
reference measurement, say two times 0.75 dB for a typical MT-RJ
connector on a high-quality factory-made patchcord.
More on testing cables with different types of connectors.
Table 1. Reference methods for fiber optic loss testing.
| Reference Method (OFSTP-14) | Reference Cables | Connectors Included in Reference Measurement | Estimated reduction in measured loss | Estimated increase in errors | | 2 Cable Reference | 2, launch and receive | 1 | 0.2-0.75 dB | +/-0.2 dB | | 1 Cable Reference | 1, launch | 0 | 0 dB | 0 dB | | 3 Cable Reference | 3, launch, receive and “golden cable” | 2 | 0.4-1.5 dB | +/-0.25 dB |
Table 2. Recommended reference methods for connectors
| Connector Type | Mates to test equipment | Does not mate to test equipment | | Plug to Plug with mating adapter | 1, 2 or 3 Cable Reference | 1 or 3 Cable Reference* | | Plug to Jack | 1 or 3 Cable Reference | 3 Cable Reference |
*The 2 Cable Reference can be used if the connector under test can be adapted to the
connector interface on the test set – e.g. a tester with SC interface,
but ST and FC can be mated to SC with a hybrid mating adapter, so SC
reference cables can be used.
Additional Reading
Testing cables with different types of connectors.
Fiber Optic Measurement Accuracy
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