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

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Read About OTDRs

 Dos & Don'ts

 Tools

 Setup

Take Data

Viewing the trace

Measure Loss

Modify Test Parameters

Compare Traces

Troubleshooting

Quiz

More Useful References

 Understanding OTDRs

"Virtual Hands-On" Termination

"Virtual Hands-On" Testing


Taking Data - Acquiring A Trace Or Signature For A Fiber

In order to acquire a trace or signature for a fiber, you need to set the basic parameters on the OTDR for your fiber. First you need to set the wavelength to be tested and the length of the fiber. After you acquire the basic data, you can modify the parameters to get better traces.

You are looking to get a trace that looks like this:

Selecting SETUP from the menu allows the user to choose BIG BUTTONS or smaller ones, depending on one's preferences and monitor display resolution. If you choose BIG BUTTONS, you can toggle between all of the buttons with the far left button SWITCH PANELS.

Toolbar

Select Measurement, Parameters from the menu to prepare to take a trace. There are five important options to set in this dialogue box. The definitions and guidelines for each option are listed below.

Fiber Type - Wavelength - Choose the wavelength that you wish to use to test the fiber. Multimode fiber is tested at 850 and 1310 nm and singlemode fiber at 1310 and 1550 nm. The wavelengths available on your OTDR will be shown in the option window.

Refractive index (n) - N is the ratio of speed of light in a vacuum to speed of light in material. Specific values that should be set in an OTDR trace will range according to the manufacturer of a specific cable. This value is used by the OTDR to calcuate distance from measured time. If you are not sure what value to use, use the default value on the OTDR.

Distance Range (Lmax) - The maximum length of the OTDR trace. In an OTDR trace, the distance range should be at least twice the value of the cable's total length. For example, if the cable being tested is 5 km long, the range should be set to 10 km.

If you see a display like the one above, where the trace curves down to zero but there is no noise at the end of the trace, it means the range is set too short in the OTDR setup. Go back and change the range to longer distances until you get a proper signature.

Pulse Width (Tp) - The length of the light pulse that is initiated by the OTDR to travel through cables and create a trace. Longer pulse widths are effective for testing long distances while shorter pulse widths provide higher resolution. The following guide may help select a pulse width for cables of various lengths:

Number of Averages (Nrep) The number of repetitions that the OTDR will send the pulse width before calculating data and creating the trace. Averages can run in two different modes:
Normal mode: completes all repetitions and averages them before graphing the results
Fast (real time): redraws the trace and updates calculated numbers as each cycle is made. The signature will be redrawn before a final average is calculated.

A good place to start is 64 averages, which will give good range and fast trace acquisition. Use realtime displays for special uses like optimizing a mechanical splice, so you can see what is happening while you do it.

Capturing a Trace
After the parameters are set (Measurement, parameters) click on "OK" to run the trace. The screen should display a trace of the cable that's being tested that looks like this.

Now we're ready to analyze the trace.

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