- Frequently Asked Questions On OTDRS And
Hints On Their Use
OTDRs, also known by their technical name optical time
domain reflectometers, are valuable fiber optic testers
when used properly, but improper use can be misleading
and, in our experience, lead to expensive mistakes for
the contractor. We have been personally involved in
several instances where misapplication of OTDR testing
has cost the contractor as much as $100,000 in wasted
time and materials. Needless to say, it's extremely
important to understand how to use these instruments
correctly.
What Is The Difference Between OTDR Testing And
Insertion Loss Testing?
An insertion loss test made with a light source and
power meter is a simple test that is similar in
principle to how a fiber optic link works. A light is
placed on one end of the cable and a power meter
measures loss at the other end, just like a link
transmitter and receiver use the fiber for
communications.
An OTDR, however, works like RADAR. It sends a pulse
down the fiber and looks for a return signal from fiber
backscatter and reflections from joints, creating a
display called a "trace" or "signature" from the
measurement of the fiber. From this trace, the OTDR
calculates fiber length, attenuation and joint loss. So
it does not “measure” loss directly, it implies it from
the trace.
When is OTDR Testing Appropriate?
The answer to this question has several aspects.
Let’s start with “What are we trying to test?”
OSP: In a long outside plant cable with many splices,
the OTDR is used to ensure that the cable has not been
damaged during installation and each splice is properly
made. Results are archived with other documentation to
be available if restoration is necessary in the future.
Later OTDR testing may be used for troubleshooting
problems like finding locations of cable breaks caused
by dig-ups. Generally, OSP networks are also tested with
a LSPM (light source and power meter) also called an
OLTS (optical loss test set.)
Premises: Premises cabling, however, has short cable
runs and almost never includes splices, so the
requirement of OTDR testing appears to be as an
alternative to insertion loss testing with a light
source and power meter, which in reality, is
inappropriate.
We recommend that insertion loss testing be done even
when OTDR testing is required by installation contracts.
In our experience, OTDR testing of premises cabling
systems is often required by users who do not really
understand when OTDR testing is appropriate or even what
an OTDR is. A knowledgeable contractor should be able to
educate the user on proper testing requirements.
New international testing standards, however, accept
both OTDR testing and insertion loss testing, even for
short premises multimode cable plants. The differences
in the measurement techniques used by OTDRs and a light
source and power meter means that OTDR testing,
especially on longer premises cable plants with higher
loss, may not be comparable to measured insertion loss
or the actual loss the communications system will
experience.
The international standard that allowed OTDR as well as
LSPM (light source and power meter) testing based that
decision on tests performed on cable plants appropriate
for 10G Ethernet which had losses of <2dB. In the
real world, multimode cable plants in premises
installations can have losses of 5-10 dB or more. OTDR
tests will generally not correlate with LSPM tests (or
often with other OTDRs!) which test the cable plant more
like how the cable plant is used by communications
equipment, and usually OTDR results are lower, setting
up the network owner for a problem when the
communications electronics are installed.
For this reason alone, we recommend that insertion loss
testing be done even when OTDR testing is required by
installation contracts.
Most of the technical calls we get at FOA regarding
problems using OTDRs on premises cabling systems are
caused by users who don't know what an OTDR is requiring
them for testing their installation just because
somebody told them to or they assume a bigger, more
expensive instrument gives better data.
Next: What Can The OTDR “Test”?
OTDRs use an indirect measurement process, have poor
length resolution and unique measurement errors that
limit its accuracy in testing cable plants. It is not
considered a replacement for insertion loss testing by
knowledgeable fiber optic personnel.
From a more technical standpoint, the first and most
important consideration for OTDR use is the length of
the fibers to be tested. Most OTDRs are designed for
long cable plants, especially singlemode OTDRs, and may
be inappropriate for short cables. Some multimode OTDRs
are now usable for short length multimode premises
cables but only if they are properly set up before use
and launch and receive cables have connections with low
reflectance. Furthermore, on short cables or even
relatively long ones with highly reflective events,
“ghosts” caused by the high reflectance
OTDR measurement of joint lost are directional,
dependent on the backscatter coefficient of the fiber,
causing measurement to vary by 0.25 dB or more depending
on direction.
If you are looking to test a cable plant to see if it
will support a communications system’s loss budget, you
do not want OTDR testing. If your network is short, the
OTDR will not give you valid data.
The second most common tech question we get at the FOA
is from people trying to use an OTDR when it’s
inappropriate.
Do I Need Training To Use An OTDR?
My OTDR manufacturer tells me its fully automatic and I
just push a button and it gives me a PASS/FAIL result
like my “Cat 5” tester. They say I don’t need any
training.
Let’s just say that the majority of calls we get at the
FOA involve OTDRs being used by people who are ignorant
of their use, either trying to use them for cable plants
that are too short or full of ghosts, their launch
cables are too short, the setup wrong, or they simply
don’t know how to interpret the OTDR trace. We have many
examples including one instance where over $100,000
worth of cable was rejected and pulled out when it was
perfectly OK but the OTDR user did not understand the
trace. We have had calls from people trying to test 70m
singlemode cables without a launch cable, MM cables with
SM OTDRs and vice versa, and many more.
If you are using an OTDR without training, you are going
to have big problems.
- Why Do I Need A Launch Cable
On The OTDR?
- OTDRs are always used with a launch cable and may
use a receive cable. The launch cable, sometimes also
called a "pulse suppressor," has two major reasons for
its use:
- 1. The launch cable allows the OTDR trace to settle
down after the test pulse is sent into the fiber so you
can analyze the beginning of the cable you are testing.
The large event you see right in front of the instrument
on the OTDR trace is caused by crosstalk within the
instrument and reflectance from the connector on the
face of the OTDR. The long recovery time from this
overload pulse means the OTDR cannot make any useful
measurements near the instrument itself. The launch
cable has also been called a "pulse suppressor" because
it allows time for the OTDR to settle down from this
initial overload. If possible, singlemode OTDRs should
have APC connectors on the front panel to reduce
reflectance. Also a short connection cable attached to
the OTDR before the launch cable that never gets removed
from the OTDR prevents excess wear on the panel
connector.
- 2, The launch cable provides a reference
connector for the first connector on the cable under
test to determine its loss. A receive cable may be used
on the far end to allow measurements of the connector on
the end of the cable under test also.
What Is The Resolution In Length Of The OTDR?
Most OTDRs have a display range digitized to about
10-20,000 parts, so on a 20km range, the display
resolution is 1m, or on a 2km range it would be 0.1m.
The actual resolution of the OTDR is usually thought of
as its ability to distinguish between two points on the
cable, like intermediate patchcords or closely spaced
splices. The actual resolution is determined by the
width of the test pulse and the bandwidth of the OTDR
receiver and is usually much longer than the display
resolution. A 100ns pulse, for example, equals about
20m, but the depending on the shape of the test
pulse, the OTDR may not be able to distinguish events
2-3 times that length.
I Have Heard The OTDR Measures Fiber Length, Not
Cable Length. How Do I Correct For That?
First, what are the sources of error? The OTDR uses the
speed of light in the fiber (from the index of
refraction) to calculate the length of the fiber. Also,
most cables have 1-2% excess fiber (less on ribbon
cables) to prevent fiber stress under cable tension.
Some manufacturers of cable can provide that information
for your testing. If you do not know the index of
refraction/velocity of propagation or the ratio of
excess fiber, you can estimate it or, if you have a long
spool of cable, calibrate it.
Just measure the fiber on the spool of cable with the
OTDR, then look at the cable jacket for length markings
to get the actual length of the cable from the printed
markings at each end of the cable. Use the OTDR’s
calibration feature to set the index of refraction to
the value that makes the OTDR read the same as the
marked length of the cable.
- Uncertainty of OTDR Test Results
- We received a call from a contractor who had tested
a cable plant for an end user using an OTDR. The user
had several others do the same test and got different
results, not widely different, but different enough to
make him wonder why. The same thing happens with OLTS
testing too, but for slightly different reasons. This
conversation inspired a short tutorial which follows:
- Two Types of Measurement Errors
- Measuring a physical parameter
always involves errors. Unfortunately you never know the
real value to begin with, so all you can do is to
estimate the error based on the possible sources of
error in making the measurement. There are two types of
errors, random and systematic.
- Random errors are what you see
when you make a measurement multiple times and get a
slightly different value each time. Hook up your
instrument and make the measurement, disconnect and try
again. Each time, the result will be slightly different.
Generally one should make several measurements, average
them and use the data to calculate the random error,
called standard deviation, to understand the uncertainty
of the measurement.
- Systematic errors are how the
average measurement differs from the real value, which
can be caused by some problem in setup or calibration.
Unfortunately, it’s hard to determine the systematic
error, but some possible ways exist sometimes.
- Let’s look at OTDR measurement uncertainty
from both a random and systematic viewpoint.
- Random Errors
- Testing loss with an OTDR
requires a launch cable that connects to the fiber in
the cable under test, taking a trace and analyzing the
trace, either manually or using some preprogrammed
auto-test function. This leads to several random errors
in loss measurement which may include:
- 1. Variation in loss of the
connection of the launch cable to the cable under test
caused by alignment variations each time it’s connected,
dirt, temperature, etc.
- 2. Changes in stress of the
launch cable or cable under test which can be caused by
temperature variations or physical movement of the
cable.
- 3. Changes in the mode power
distribution of launched pulses which can affect
both multimode and singlemode cables (short SM may not
be single-mode-it may take hundreds of meters!)
- 4. Noise in the OTDR trace, with
the effect greater effect with less averaging.
- 5. How the user sets the markers
on the trace for each measurement. This is affected by
pulse width (risetime) and the reflectance from an event
which can overload the OTDR and cause difficulties in
determining where the fiber baseline is located.
- Systematic Errors
- When you set up the OTDR, you
have to make certain set-up decisions, including range,
wavelength, fiber glass index of refraction, pulse
width, number of averages, etc. that affect the
measurement uncertainty for every measurement.
- Length Measurement
- 1. The range may affect the time
base of the OTDR which is used (along with index of
refraction) to calculate the length of the fiber.
- 2. The index of refraction (n) is
a direct expression of the speed of light in the fiber
(V=C/n). Distance is calculated as “time X speed.” Most
OTDR users use a generic value, but sometimes the actual
value for the fiber being tested is known.
- 3. Each cable has excess fiber,
typically ~1%, to prevent stressing the fiber when
pulled. The OTDR measures the length of the fiber, not
the cable. It can be calibrated for the cable under test
if one knows the length of the cable and uses that to
calculate a cable-specific index of refraction.
- 4. The pulse width may cause
systematic errors in the measurement of length, since
wider pulses have longer risetimes which make placing
the markers consistently more difficult.
- Loss
- 1. Setting markers for measuring
loss is affected by the width of the test pulse. Longer
pulses have longer risetimes which make setting markers
consistently more difficult. Wider pulses cause greater
reflectance from connectors and affect both the shape of
the reflected pulse and the shape of the return to the
fiber baseline, causing uncertainty on how the markers
are set. Noisy traces are wider, which can lead to
systematic errors.
- 2. Manually setting markers
generally will introduce random errors, as the operator
sets their location each time, but can introduce
systematic errors due to the way the operator typically
works.
- 3. Auto-test programming may
introduce systematic errors depending on the pulse
width, reflectance, range, averaging, etc. Generally
auto-test should not be used until it has been compared
to manual analysis.
- 4. Connectors on different launch
or receive cables will change the measurements
systematically.
- 5. The length of the launch cable
may affect SM or multimode measurements. A SM launch
cable should be 500-1000 m long to ensure the test
signals are singlemode. Multimode fiber will change mode
power distribution with length.
- 6. Any mode conditioning on a MM
cable (e.g. mandrel wraps) will affect the modal
conditioning on the downstream part of the test where
the test pulse from the OTDR goes out from the OTDR. On
the return backscattered light, the fiber modes will be
fully filled.
- 7. Instrument calibration will
cause systematic errors. Few users ever calibrate OTDRs,
but the time base and linearity of the amplifiers can
affect the measurements.
- Uncertainty of Results
- So what can you expect? Length may vary by several
percent on different OTDRs. Loss can vary by several
tenths of a dB on short lengths and proportionally more
on long cable plants for different OTDRs and at least as
much with the
I’m Troubleshooting A Break In A Long Cable Run But I
Don’t Know The Correction Factor For Fiber Vs Cable
Length. What Can I Do?
Here is a perfect example of why you need cable plant
documentation. If you have the data from the original
design and testing, you should have the actual length of
the cable plant. With that you can calculate the point
of the break very closely. Here is an example:
Let’s say we have a 10km cable with a break around 6km
from one end. From one end, the OTDR says the distance
to the break is 6500m and from the other end it says
it’s 4000m.
Total length of OTDR cable length: 6500+4000=10,500m
If the actual cable length is 10,000 m, the correction
factor is:
Actual length/measured length = 10000/10500 = 0.952 =
correction factor
Thus our 6500m measurement is actually 6500X0.952 =
6190m and from the other end it’s 3810m.
If you do not have documentation, try to calibrate the
OTDR using a section where you can get real length data
from the cable jacket.
Sometimes My Traces Show Big Reflections From The End
Of The Cable But Sometimes It Shows None At All. Why?
The reflection on the end of the cable depends on the
end of the fiber. If it’s broken and ragged, you will
see practically no reflection, but a perfectly cleaved
fiber will show a giant reflection peak.
Look at these three traces:
Note how the cleaved fiber has a high reflectance,
reaching saturation on the OTDR trace
The broken trace shows a small reflectance.
The shattered fiber shows virtually no reflectance.
Our thanks to FOA Master instructor Terry O’Malley
(http://www.fiberopticsolutions.biz/)
for his advice and work creating the sample traces.
- Want to learn more? Try the FOA
Self-Study Program on OTDRs at Fiber
U.
- Return To The FOA OTDR
Tutorial
- Download the free FOA OTDR
PC Simulator
To Learn How To Use An OTDR
- See
the FOA Virtual Hands-On OTDR Tutorial
- Return to the FOA
Online Reference Guide Table of Contents
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