VDV Works Virtual Hands-On Training
Fiber Optics Testing: Using OTDRs
 
Part of the VDV Academy Fiber Optic Training Programs

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 Dos & Don'ts

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Take Data

Viewing the trace

Measure Loss

Modify Test Parameters

Compare Traces

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 Understanding OTDRs

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Modifying Parameters For Best Test Results

The dialogue box for Measurement, parameters displays the variety of settings that may be adjusted when collecting data from a fiber optic cable.

A complete description of the various setting may be found in Taking Data. The information that follows on this page will describe various scenarios of adjusting the parameters and provide suggestions of when to change the parameters to get the best measurements.


Averaging
OTDRs can take multiple samples of the trace and average the reThe two traces pictured here were captured from the same cable plant with all of the same settings except for the number of averages. Notice the difference in the distance that the signal travels before it the noise level becomes significant. In the image on the top nRep = 1 or only one sample trace was taken and the noise becomes significant at only 3 km.


One sample


1024 samples

In the lower trace, nRep= 1024 so the trace is the average of 1024 individual measurements. The signal travels for over 5 km before the noise becomes a problem.

Notice in the small locator window the amount of noise in each example. The absolute noise floor is significantly lower with more averaging. Closer looks at the trace shows almost 9 dB less noise which translates to 9 dB additional range!

If you are trying to get longer distances with high resolution, using more averages with a short test pulse will usually be the best choice. It will simply take longer to acquire each trace.


Pulse Width: Adjusting the pulse width is another way to get more measurement distance, but at the loss of resolution between events.

For example, in the traces, three different pulse widths were sent through the same cable plant. The blue line (bottom) represents the trace when Tp = 30 ns. The green line (middle) represents the trace when Tp = 90 ns. The red line (top) represents the trace when Tp = 330 ns.

Notice while the longer pulses yield traces with less noise and longer distance capability, the ability to resolve and identify events like the splice in the middle, becomes less, and the test pulse overloads the OTDR reducing its ability to see events nearby.

Here is a close up of the same splice at the three different pulse widths. Notice it almost disapears with the longest pulse.

pulse width

Set the pulse width at the shortest width that allows the OTDR to reach the end of the cable plant with a reasonable number of averages. Obviously there are tradeoffs. If you need short test times you may compromise on a longer pulse width to reduce the noise. If you need more resolution, average more with shorter test pulses.
Generally, premises cabling uses the shortest possible pulse and long haul uses a medium pulse for the first test. Then an analysis of the trace will tell the operator how to select the right compromise.

Other Options:

Wavelength: Since fiber has lower loss at longer wavelength, one can also use the longer wavelength source to make measurements at longer ranges with a better signal to noise ratio. You will see this effect in the next section on comparing traces.

Index of Refraction: The index of refraction is the calibration for the speed of light in the fiber which the OTDR uses to translate into distance in the fiber. Since fiber optic cable has about 1% excess fiber, the actual cable length is less than the fiber. The OTDR makes its measurements on the fiber, not the cable, so one must estimate the cable length. If you have a long length of cable with distances marked on it, you can measure it with the OTDR and use the index of refraction to calibrate to the actual cable length. If you do this, we suggest you make measurements on several fibers and average.











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