Do you really know how to make your OTDR sing?

I often refer to the OTDR (Optical Time Domain Reflectometer) as “the Swiss army knife of fiber testing tools”. It doesn’t do everything, but it can do more than any other test tool in your fiber arsenal.

While today many vendors today are focusing on simplifying the OTDR interface to make it easier to use by anyone, the real “magic” is accessible by those skilled deeper in the art of OTDR optimization and analysis.

First let’s look at what an OTDR can measure: 
For starters, it is the only fiber tool that provides a visual view of the fiber infrastructure from beginning to end. It can provide link measurement data (insertion loss, optical return loss, link length) as well as loss of individual passive elements in the link (connector and splice losses, point losses due to macro or micro-bending, and connector reflectance values). 
It will even show fiber mismatches (older fiber splice to newer fiber, or different fiber types within a span (G.652 / G.655)). 
You can even monitor splices live to confirm if good or bad, so that resplicing can be done immediately to avoid re-entering to fix later. 
Trace information is typically saved and can be used not only as a visual documentation record, but also as a troubleshooting tool later.

OTDRs can also be used to cross-corellate test results with other tests. For instance, OTDR link loss and ORL values can be compared with the official IL and ORL tests results to validate similar values. OTDR fiber mismatch data can be correlated with Chromatic Dispersion results to validate mixed fiber types.

Oh, and did you ever wonder why OTDRs almost always test at multiple wavelengths? That’s because with two or more wavelengths. we can confirm whether event losses include loss due to macro- or micro-bending. I often hear questions like “Ok, so is it a bend or a splice? — because there is a splice near that location.” In fact most bending related losses actually occur either in a splice tray or in the entry or exit areas. This could be either at the patch panel splice, a vault or mid-span splice closure. This is not to say that bending losses won’t occur in other locations (slack loops, transition points), but those are the most common.

Becoming OTDR savvy isn’t something that happens overnight. It takes a long time to develop a feel for how to analyze and interpret traces effectively. I’ve been doing it for over 25 years and I’m still challenged frequently with something different that can take some time to figure out.

A big part of our the CFCE (Certified Fiber Characterization Engineer) certification class we deliver for Optical Technology Training (OTT) is OTDR testing and analysis. Many of our students comment that the OTDR portion was the most valuable part of their learning experience. We offer these courses in North America throughout the year. The next one is scheduled in early April in Lowell, MA.

If you would like more information on this or other fiber testing topics, email us at: info@fiberinsight.com.

All the best,
Tim