FIBEROPTICS BY JIM HAYES
The testing process begins in the
fusion-splicing machine. After fusing the
fibers, the machine pulls on the splice to
ensure it is mechanically strong. Most
splicing machines also provide the operator a display of splice loss, but it is only
an estimate based on the fiber alignment
method the splicer uses.
Most splicers today use “profile
alignment,” which is based on the video
images of the fibers being spliced. Splicers use video images from two axes
angled 90 degrees apart to analyze the
cores’ alignment (or sometimes the
cladding) of the fibers being spliced.
The analysis evaluates the geometry of
both fibers and the alignment to estimate the loss. If the fibers are within
specifications and of the same type, the
estimate should be reasonable. Splicing
mismatched fibers may make the estimate less reliable.
Some older splicers use a local injection and detection (LID) system to align
fibers and estimate loss. This system
bends the fibers to inject some light on
one side of the splice and detect it on the
other side. Fibers are aligned when the
maximum amount of light is transmitted through the joint between the fibers.
Since the coupling into and out of the
fibers is not always the same, absolute
loss measurements are impossible, so the
splicer can only estimate the splice loss.
Some types of fibers can fool both of
these methods, especially when splic-
ing different fiber types together. The
new bend-insensitive fibers may not be
a problem for some profile alignment
splicers but are a big problem with LID
The only legitimate method to
check splice loss is using an optical
time-domain reflectometer (OTDR).
If you know how to use one, you know
you need to check from both directions
and average to get trustworthy data.
The way OTDRs work—measuring
fiber backscatter over distance to imply
fiber attenuation—causes that quirk of
If you break a fiber and splice it back
together, the fibers will have the same
backscatter coefficient on both sides
of the splice, and the OTDR’s splice
loss measurement will be correct. If
you are splicing two fibers together
from different batches and have different backscatter coefficients, that
difference will cause directional variation in the measured loss, depending on
If you splice a high backscatter fiber
to a low backscatter fiber, there will
be directional differences. OTDR testing from high to low backscatter fibers
will show a higher loss than the real
loss. Testing from low to high backscatter fiber will show lower loss or even
a “gainer” where the OTDR sees gain
instead of loss, an impossible situation
in the real world, making it obvious that
the data is erroneous.
The solution is to test in both direc-
tions and average the results, which
removes the errors caused by backscat-
ter differences. In most cases, this is
impractical while the tech is making the
splices and will be performed only after
all of the cable plant has been spliced
together and final testing is done.
Knowing all of the problems, is there
a more practical solution? The best bet is
to use care in the splicing process and to
trust your splicing machine’s estimates,
then test and document consistently.
If you correctly maintain your splicer
and cleaver, they should operate prop-
erly. You can check the fiber cleaves
on the splicer display, and the machine
should automatically check and reject
bad cleaves. If your equipment starts
encountering problems, have it serviced
by an authorized dealer immediately.
Remember that the fusion splicer
has preprogrammed operational cycles
for each type of fiber that sets the
proper prefuse cycle, correct arc heat
and fusion time for the actual splice.
You always need to ensure the splicer
is properly set for the types of fibers
If you set up the machine properly
and follow the correct procedures, you
can be confident that the machine will
do its job and provide reliable results. If
you compare the machine’s splice estimates with some bidirectional OTDR
test results, you can confirm whether
the machine’s estimates are trustworthy.
That’s a good experiment for a dreary
day in the office when you are tired of
Testing 1, 2, 3
Fiber optic splices
ONCE A TECHNICIAN HAS SPLICED A FIBER OPTIC CABLE, he or she must
test the splice to verify it is strong and has low loss. The technician must add the
test data to the documentation for future reference and present it to the cable plant
owner to verify the installation has been done correctly.
HAYES is a VDV writer and educator and the president of the Fiber Optic Association. Find
him at www. JimHayes.com. W I