Understanding The OTDR Measurement
Understanding the Physics (and Errors) of the Measurement
Don't let the title put you off,
it's pretty basic. The amount of light scattered back to the
OTDR for measurement is quite small, about one-millionth of what
is in the test pulse, and it is not necessarily constant. This
affects the operation and accuracy of OTDR measurements.
Since so little of the light comes back to the OTDR for analysis, the OTDR receiver circuit must be very sensitive. That means that big reflections, which may be one percent of the outgoing signal, will saturate the receiver, or overload it. Once saturated, the receiver requires some time to recover, and until it does, the trace is unreliable for measurement as shown in Figure 5.
The most common place you see
this as a problem is caused by the connector on the OTDR itself.
The reflection causes an overload which can take the equivalent
of 50 meters to one kilometer (170 to 3000 feet) to recover fully,
depending on the OTDR design, wavelength and magnitude of the
reflection. It is usually called the "Dead Zone". For
this reason, most OTDR manuals suggest using a "pulse suppresser"
cable, which doesn't suppress pulses, but simply gives the OTDR
time to recuperate before you start looking at the fiber in the
cable plant you want to test. They should be called "launch"
Do not ever use an OTDR without
this launch cable! You
always want to see the beginning of the cable plant and you cannot
do it without a launch cable. It allows the OTDR to settle down
properly and gives you a chance to see the condition of the initial
connector on the cable plant. It should be long, at least 500
to 1000 meters to be safe, and the connectors on it should be
the best possible to reduce reflections. They must also match
the connectors being tested, if they use any special polish techniques.
On very short cables, multiple
reflections can really confuse you! We once saw a cable that
was tested with an OTDR and deemed bad because it was broken
in the middle. In fact it was very short and the ghosted image
made it look like a cable with a break in the middle. The tester
had not looked at the distance scale or he would have noted the
"break" was at 40 meters and the cable was only 40
meters long. The ghost at 80 meters looked like the end of the
cable to him!
Backscatter Variability Errors
Another problem that occurs is a function of the backscatter coefficient, a big term which simply means the amount of light from the outgoing test pulse that is scattered back toward the OTDR. The OTDR looks at the returning signal and calculates loss based on the declining amount of light it sees coming back.
Only about one-millionth of the light is scattered back for measurement, and that amount is not a constant. The backscattered light is a function of the attenuation of the fiber and the diameter of the core of the fiber. Higher attenuation fiber has more attenuation because the glass in it scatters more light. If you look at two different fibers connected together in an OTDR and try to measure splice or connector loss, you have a major source of error, the difference in backscattering from each fiber.
However, if the fibers are different, the backscatter coefficients will cause a different percentage of light to be sent back to the OTDR. If the first fiber has more loss than the one after the connection, the percentage of light from the OTDR test pulse will go down, so the measured loss on the OTDR will include the actual loss plus a loss error caused by the lower backscatter level, making the displayed loss greater than it actually is.
Looking the opposite way, from a low loss fiber to a high loss fiber, we find the backscatter goes up, making the measured loss less than it actually is. In fact, this often shows a "gainer", a major confusion to new OTDR users.
The difference in backscatter can be a major source of error. A difference in attenuation of 0.1 dB per km in the two fibers can lead to a splice loss error of 0.25 dB! While this error source is always present, it can be practically eliminated by taking readings both ways and averaging the measurements, and many OTDRs have this programmed in their measurement routines. This is the only way to test inline splices for loss and get accurate results.
Another common error can come
from backscatter changes caused by variations in fiber diameter.
A variation in diameter of 1% can cause a 0.1 dB variation in
backscatter. This can cause tapered fibers to show higher attenuation
in one direction, or we have in the past seen fiber with "waves"
in the OTDR trace caused by manufacturing variations in the fiber
Overcoming Backscatter Errors
One can overcome these variations
in backscatter by measuring with the OTDR in both directions
and averaging the losses. The errors in each direction cancel
out, and the average value is close to the true value of the
splice or connector loss. Although this invalidates the main
selling point of the OTDR, that it can measure fiber from only
one end, you can't change the laws of physics.
The next thing you must understand is OTDR resolution. The OTDR test pulse, Figure 8, has a long length in the fiber, typically 5 to 500 meters long (17 to 1700 feet). It cannot see features in the cable plant closer together than that, since the pulse will be going through both simultaneously. This has always been a problem with LANs or any cable plant with patchcords, as they disappear into the OTDR resolution. Thus two events close together can be measured as a single event, for example a connector that has a high loss stress bend near it will show up on the OTDR as one event with a total loss of both events. While it may lead you to think the connector is bad and try to replace it, the actual problem will remain.
There is a tool that will help here. It is called a "visual fault locator". It injects a bright red laser light into the fiber to find faults. If there is a high loss, such as a bad splice, connector or tight bend stressing the fiber, the light lost may be visible to the naked eye. This will find events close to the OTDR or close to another event that are not resolvable to the OTDR. It's limitation is distance too, it only works over a range of about 2.5 miles or 4 km.
The visual fault locator is so
valuable a tool that many OTDRs now have one built into them.
If you are using an OTDR, you must have one to use it effectively.
Special Consideration for Multimode Fiber
Most OTDR measurements are made with singlemode fiber, since most outside plant cable is singlemode. But building and campus cabling are usually multimode fiber using light emitting diode sources for low and medium speed networks. The OTDR has problems with multimode fiber, since it uses a laser source to get the high power necessary to cause high enough backscatter levels to measure.
Figure 9. OTDRs only see the
middle of the multimode fiber core
Several projects have tried to
determine how to corellate OTDR measurements to source and power
meter measurements, without success. Our experience is an OTDR
will measure 6-7 dB of loss for a multimode cable plant that
tests at 10 dB with a source and power meter.
Measuring Fiber, not Cable Distance
And finally, OTDRs measure fiber not cable length. While this may sound obvious, it causes a lot of problems in buried cable. You see, to prevent stress on the fiber, cable manufacturers put about 1% more fiber in the cable than the length of the cable itself, to allow for some "stretch." If you measure with the OTDR at 1000 meters(3300 feet), the actual cable length is about 990 meters (3270 feet). If you are looking for a spot where the rats chewed through your cable, you could be digging 10 meters (33 feet) from the actual location!
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