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


Read About OTDRs

 Dos & Don'ts



Take Data

Viewing the trace

Measure Loss

Modify Test Parameters

Compare Traces



More Useful References

 Understanding OTDRs

"Virtual Hands-On" Termination

"Virtual Hands-On" Testing

Comparing Traces

Comparing two traces in the same window is useful for confirming data collection and contrasting different test methods on the same fiber.
All OTDRs offer this feature, where you can copy one trace and paste it on another to compare them. Here is an example of how you can use this feature.
To compare fibers, open one trace then choose COMPARISON from the menu to choose the second fiber or more,depending on the OTDR capability.
Measurements at Different Wavelengths

Note that the two traces  are taken from the same multimode fiber cable plant at different test wavelengths. The major difference in the slope of the traces displays the different attenuation coefficient of the fiber.
The blue line (top) represents the attenuation coefficient of the cable in at 1300 nm , the green line (bottom) represents the same cable measured at 850 nm.
There is also a noticable difference in the reflectance at the splice. Variations in reflectance due to the wavelength difference is not unusual.
Other reasons you might want to compare two traces includes:
Compare several fibers in the same cable to see if they are different.
Traces taken at different times to see if the cable has changed.
At different wavelengths, since fiber is more sensitive to stress at longer wavelengths, this allows finding stress points caused bu installation.
At different pulse widths (below) to decide which setting gives the best compromise between noise and resoluton or to find events lost with wide pulse widths.
comp PW



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