Understanding The Math Of
Insertion Loss Testing
In order to test “insertion loss” or
the direct loss of a fiber optic cable or cable plant
using a light source and power meter (LSPM in most
international standards or optical loss test set – OLTS –
in many articles), one must make an initial measurement to
determine the “0 dB” reference point with the LSPM and
reference cables to be used in making the measurement.
It has become apparent in technical and
standards meetings that what these measurements actually
measure are not well understood, even by experts, because
of the casual way the reference and test measurements
are often described. The purpose of this paper is to
use simple math is to clear up this issue.
Testing the cable plant
Standard test methods, use a light
source [LS] and reference “launch” cable on one end of the
“cable to test” and a “receive” reference cable connected
to a power meter [PM] on the far end. The test is intended
to measure the loss of the connections of the connectors
on either end to the reference test cables and the loss of
the rest of the cable (which may include splices or
additional connections in addition to the fiber.)
OFSTP-14/7: [LS]------ref cable------[A]------cable to
test------[B]------ref cable------[PM]
Three ways to set the
reference
All standards offer three methods of
setting a “0 dB” reference using one two or three
reference cables. It’s assumed that all reference cables
are short enough (generally less than 2m or 6 feet) that
the loss of the fiber in them can be ignored and that the
connectors are high quality – that is to say low loss.
It’s also assumed that the power meter has a large enough
detector that it gathers all the light from the end of a
fiber so it has a consistent connection to the fiber being
connected to its interface.
1 Cable Reference : [LS]-------ref cable-------[PM]
2 Cable Reference: [LS]-------ref
cable-------[1]-------ref cable-------[PM]
3 Cable Reference: [LS]-------ref
cable-------[1]-------ref cable-------[2]-------ref
cable-------[PM]
Why do we have three different reference methods?
The
reason for the existence of three methods is the
compatibility of test equipment to the cable plant;
whether the test equipment has connector interfaces that
allow direct connection to the cable under test.
The
options for use of these three methods are:
- If
the test equipment has connectors
compatible with the cable plant, a one-cable
method can be used.
- If
the test equipment does not have
connectors compatible with the cable
plant, a two- or three-cable method must
be used.
- If
the test equipment does not have
connectors compatible with the cable
plant and the connectors are the “plug and
jack” or “male, female” type, a
three-cable method must be used.
Mostly we believe the three options came about from three
issues: 1) Test equipment with fixed connectors that do
not match the connectors on the cable to test cannot be
used for the 1 cable reference, 2) Techs working at either
end of a length of cable and set the reference with a
local light source and power meter and 3) Belief that
since you need a launch and receive reference cable, you
should make the reference setting using both of them.
The 3 cable reference has become
the method of choice with plug/jack connectors like the
MT-RJ or MPO which are generally incompatible with test
instruments, but LC connectors may require a similar
approach if the power meter or OLTS does not have adapters
that allow connecting different types of connectors.
If you look at the schematic drawings
of the three methods, you can see that when you set the
reference, one or two connections are included. That means
that the loss of those connections are included in setting
the 0 dB reference. This has led to the misunderstanding
that the 3 cable reference , which includes two
connections when the reference is set, does not measure
the loss of the connectors on the ends of the cable
because two connections were included in setting the
0 dB reference and the 1 cable reference (the old
Method B) only measures one connector because one is
included in setting the reference. Unfortunately, it’s not
that simple - that's an improper generalization. Any
version of the test measures all the connector losses in
the cable under test, but subtracts the loss of
connections included when setting the “0 dB reference.”
Let’s do the math for each method and you will see what we
mean.
The 1 Cable Reference,
Formerly Method B
Set reference by Method B:
[LS]-------ref cable-------0-[PM]
where “0” equals the output of the reference cable
measured by [PM].
No connections are included in the
reference test (see next paragraph), so when we do the
test, we are connecting the connectors of the ref cables
directly to the connections on the cable to test and that
loss will be included in the measurement.
We consider the connection to the meter to
have "no loss" which may be true with a power meter with a
large area detector capturing ligth directly from the end
of a connected optical fiber cable. However, many OLTS,
especially those designed to make bidirectional tests, may
have that connection be fiber-to-fiber and may even have a
couple internal to the OLTS. That test set has a double
uncertainty, the connection to the fiber under test and
the variability of the coupler due to mode power
distribution in MM fiber.
OFSTP-14/7: [LS]------ref cable------[A]------cable to
test------[B]------ref cable------[PM]
Thus, with the power meter, we measure
loss “L”, we measure connection loss [A], the loss of the
fiber and any intermediate connections or splices in the
cable to test [CTT] and connection [B] or
L = 0 – ([A]+CTT+[B]) = loss of the cable
The 2 Cable Reference,
Formerly Method A
Set the reference with method A but do not change the
output of the LS.
2 cable reference : [LS]-------ref
cable------0-[1]-------ref cable-------(0-[1])[PM]
where “0” equals the output of the launch reference
cable measured by [PM] as in the 1 cable reference,
decreased by the loss of the connection [1] between the
reference cables, so the [PM] now measures (0-[1]).
OFSTP-14/7: [LS]------ref cable------[A]------cable to
test------[B]------ref cable------[PM]
Thus, with the power meter, we measure
loss “L”, we measure connection loss [A], the loss of the
fiber and any intermediate connections or splices in the
cable to test [CTT] and connection [B] but our “0 dB
reference” is now “0-[1]” and the loss measured is
L = (0- [1]) – ([A]+CTT+[B]) = measured loss of the cable
Note the measured loss is the same as
the 1 cable reference decreased by the loss of the
connection between the reference cables [1] when the “0 dB
reference” was set, which is not necessarily the same as
saying the loss measured does not include one of the
connectors, since the loss of [1] is not necessarily the
same as [A] or [B]. That unknown factor causes this method
to be more uncertain (many would say "less accurate") than
the 1 cable reference.
Is it possible to
compensate for the included connections?
Someone making this test who
understands this issue could compensate for the difference
by calibrating [1] this way:
Set reference by 1 cable reference :
[LS]-------ref cable-------0-[PM]
Add the receive reference cable and measure again:
2 cable reference : [LS]-------ref
cable-----0-[1]-------ref cable------[1][PM]
This test gives you a direct
measurement of [1] which can now be added back in when
testing the cable, giving you the same result as method B.
But in order to do so requires the ability of the
instruments to measure by the 1 cable reference , so it
just adds another step to the process – why not just
use 1 cable reference in the first place. And the
loss measured [1] will be slightly different each time the
two connectors are mated, adding to the uncertainty of the
measurement.
The 3 Cable Reference, Formerly Method C
Set the reference with method A but do
not change the output of the LS.
Method C: [LS]-----ref cable----0-[1]------ref
cable------[2]------ref cable------(0-[1]-[2])[PM]
where “0” equals the output of the launch reference
cable measured by [PM] as in the 1 cable reference,
decreased by the loss of the connections [1] and [2]
between the reference cables, so the [PM] now measures
(0-[1]-[2]).
OFSTP-14/7: [LS]------ref cable------[A]------cable to
test------[B]------ref cable------[PM]
Now with the power meter, when we
measure loss “L”, we measure connection loss [A], the loss
of the fiber and any intermediate connections or splices
in the cable to test [CTT] and connection [B] but our “0
dB reference” is now “0-[1]-[2]” and the loss
measured is
L = (0- [1]-[2]) – ([A]+CTT+[B]) = measured loss of the
cable
The measured loss now is the same as 1
cable reference decreased by the loss of the connection
between the reference cables [1] and [2] when the “0 dB
reference” was set, which is not necessarily the same as
saying the loss measured does not include one of the
connectors, since having two unknown connectors causes
this method to be more uncertain (many would say "less
accurate") than either of the other methods.
Putting some numbers on
this methodology
Consider that the test is being made by
someone who is careful with all the procedures and has
very good reference cables, which should have mating
losses of 0.2 dB or better. If we test a cable plant
with the 1 cable reference at 3.0 dB, with the 2
cable reference it will measure 2.8 dB and with the 3
cable reference, 2.6 dB.
We’ve done actual tests like this under
strictly controlled conditions and got 2.96 dB, 2.66
dB and 2.48 dB. See the table below.
The measurement uncertainty, called
“standard deviation” in statistics, can be determined by
making multiple measurements, averaging and calculating
standard deviation. What was found is this test here was 1
cable reference had a standard deviation of 0.02 dB, 2
cable reference had a standard deviation of 0.20 dB and 3
cable reference had a standard deviation of 0.24 dB. This
is expected because the 1 cable reference has no
variation caused by connections included in setting the
reference, while the 2 cable reference has one
and 3 cable reference has two and every fiber optic
connection has some variation each time it is connected.
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 |
Another unknown creates a problem with
these tests on multimode cable – mode power distribution
(MPD.) Even if we control the MPD in the launch cable
using a source of known MPD and a mandrel wrap as called
for in some standards, as soon as we add another
connector, we change the MPD in a fairly uncontrolled
fashion. Connectors act as mode modifiers, stripping off
higher order modes which are the most lossy in connections
but converting low order modes to higher order modes also.
So setting the reference with connections adds to the
uncertainty of mode power distribution in the testing.
Yet another issue is the cleanliness of
connectors. If one of the connectors is dirty when the
reference is set by the 2 or 3 cable reference methods,
but the connectors are subsequently cleaned or the dirt
falls off, the loss will change because of the extra loss
included in the reference setting process. This has even
been known to cause a LSPM test to show a “gain” which
sounds impossible.
“Fudging” the Methodology
Manufacturers of test equipment with
fixed connector interfaces have invented some creative
methods of "fudging" a 1 cable reference test. Some of
them do a 2 cable reference, add a hybrid adapter cable or
two to allow connection of the cable to test, then do some
behind the scenes software calculations and “kazam!” a
claim it is a 1 cable reference test because it includes 2
more connections in the test than in the reference!
Looking at the info above regarding the uncertainty of
measurements anyway, think about how uncertain a
measurement is when you try to guess at the loss of
several newly added connections!
"0" dB Reference: [LS]-------ref
cable------0-[1]-------ref cable-------(0-[1])[PM]
Test: [LS]------ref
cable------[X]--adapter---[A]------cable to
test------[B]------ref cable------[PM]
L = (0- [1]) – ([X]+[A]+CTT+[B]) = measured loss of the
cable
The uncertainty of the measurement is primarily determined
by the difference in the losses [1] and [X].
Which method to use?
Most cabling standards today recognize
the need for the 3 cable reference for some types of
connectors but prefer the 1 cable reference. Network
standards for link loss have referenced all methods, so
it's important to check the network specifications. Many
telcos still use the 2 cable reference. Whatever method
you use, the loss data you document must include the
method used to be valid and be comparable to other tests.
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