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Dead Zone
Eliminator®
OTDR Launch Box / Delay Line / Pulse Suppressor
| Application Notes |
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In use, the
OTDR makes measurements on optical fibers by sending a very high intensity
pulse of light into the fiber and looking for the minute reflections that occur
along the length of the fiber as well as at all discontinuities at such places
as splices and connections. This launch pulse typically can be varied in length
to accommodate different lengths of fibers and measurement resolution.
Generally speaking, the longer the fiber to be tested, the greater the amount
of light that must be injected into the fiber. OTDR's cannot easily adjust the
intensity of the launched pulse, so they try to arrive at the same result by
keeping the pulse of light on for a longer period of time.
As an
example, a 100 nanosecond (100 nS=0.0000001 Seconds) pulse may be sufficient to
get a good signal back and make measurements on a 100 Meter length of fiber but
attempting to measure a 20,000 Meter length with the same pulse width may
result in an insufficient signal reflecting back to the OTDR from the far
lengths of the test fiber. Increasing the pulse width to 10 microseconds (10
uS=0.000010 Seconds) will allow more light to travel down the fiber and
consequently more light to be reflected back. In effect, increasing the pulse
width increases the signal to noise ratio and allows for an easier
measurement.
There is a significant trade-off between increased pulse
width and measurement resolution. Calculating how long a section of fiber a 100
nS pulse of light occupies, one arrives at approximately 20 Meters. Basically,
this tells us that by the time an OTDR shuts off a 100 nS light pulse, photons
from the beginning of the light pulse are already 20 Meters down the length of
the fiber. In effect, there is a 20 Meter bar of light traveling down the
fiber. Compare this to a 10 uS pulse of light, the bar of light is 2000 Meters
long! Some OTDR operators may not know that the length of this pulse or
subsequent bar of light is important to the operational use of the OTDR. As the
launch pulse leaves the OTDR, the reflection from the fiber optic connector on
the OTDR front panel being generally greater than the back-reflection from the
fiber itself, results in a saturation (overload) of the signal in the OTDR.
This reflection effectively "blinds" the OTDR for the duration of the launch
pulse. Since time equates to distance, we can say that the OTDR is effectively
“blind” to the first 20 Meters of fiber if we are using a 100 nS
launch pulse, and 2000 Meters if we are using a 10 uS launch pulse. In addition
to the above mentioned saturation, some older OTDR's may not handle the
intensity of the back-reflected signal from the beginning sections of the fiber
under test and result in an increase in the blind time. In these older OTDR's,
once the receiver saturates, it takes some finite amount of time for the
receiver to start reacting normally – increasing the overall blind time
some more. This blind time is generally referred to as the "dead zone".
During the dead zone time, the OTDR cannot measure signal amplitudes and
subsequently cannot properly measure fiber loss. In effect, we cannot measure
the loss of the beginning length of the fiber under test during this period of
saturation. Various OTDR manufacturers have developed novel ways of dynamically
adjusting the gain of the OTDR receiver, less in the beginning when the signal
is great, and more gain as the distance of the fiber increases. These
techniques, although very beneficial for specific measurements, still does not
allow us to measure the loss during the saturation or dead zone event. It is
important to note that distance measurements are not affected by the saturation
events as long as the user measures to the correct edge of the reflection. In
addition to dead zones from the front panel OTDR reflection, reflections from
subsequent connector to connector interfaces (patch panels) may result in their
own dead zone events.
Why not bury this unusable measurement time in a
piece of fiber that is not part of the fiber under test? That is exactly what
the DZE accomplishes. The DZE is available in various lengths and the perfect
length is one that is slightly longer than the optical length of the launch
pulse. In use however, it is not practical to have pulse suppressor lengths for
each launch pulse setting on the OTDR - so most users opt to have a couple of
different units on hand, each with a specific length.
By placing the
DZE in front of the fiber to be tested and shooting the OTDR through this
device, the receiver can be in saturation while the light is still in the DZE
and has not yet traveled into the fiber under test. In this manner, the
receiver returns to service while the launch pulse is still traveling in the
DZE. We are then able to make attenuation measurements starting at a point and
before the beginning of the fiber under test – but still not right at a
point inches from the start of the fiber under test. Why? Because, don't
forget, you somehow have to connect the pulse suppressor to the fiber under
test and this interface will cause a reflection that may result in another dead
zone. Remember - we cannot make loss measurements within these reflections. So
how does the Dead Zone Eliminator (DZE) allow us to measure through this dead
zone event? Easily, by allowing us to place a measurement cursor in the linear
(non-saturated) portion of the trace prior to the start of the fiber under
test, and within the length of the DZE. In this manner, we measure through the
dead zone events giving an indication of the loss of this section. When
measuring loss of fiber through these events it is important to realize that
the OTDR is measuring not only the fiber under test, but also a small length of
the DZE and the DZE to fiber connector pair. This method gives us a good
indication of the quality of the initial section of the fiber under test, a
loss indication that may not be available to us without the use of the DZE.
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