My last two web exclusive articles (part 1 and part 2) have covered how optical time domain reflectometers (OTDR) work and make measurements. Now let’s get into several ways they try to confuse you. Have you heard of “gainers”? Gainers are the most obvious indication that OTDRs have errors measuring losses at splices and connectors.
Every OTDR tech is at some point confronted with a trace like this:
What looks like a splice on the trace (a connector would have a reflectance peak) shows a gain instead of the expected loss. This has been a source of confusion for OTDR operators since the instrument was invented. How can you get a gain? That implies that there is an amplifier there, but there is nothing but a fused joint between two fibers.
This phenomenon was a mystery until, by chance, I had a conversation with a fellow member of a standards committee in the mid-1980s. His company had hired a contractor to do an experiment to show that their single-mode fiber was compatible with fusion splicing to other companies’ fibers. As a result, he had data on many hundreds of splices between dissimilar fibers. What caught my attention was his mention of the number of splices showed gains.
I asked if he would share the data for me to analyze, and he did. My background as a physicist and astronomer meant I had a lot of experience analyzing data. Fortunately, he not only had splice data, he had bidirectional splice losses and data on every fiber including attenuation and mode field diameter, the measure of core diameter for single-mode fiber.
I put the numbers in a spreadsheet and began statistical analyses. I noticed immediately that about one-third of all splices showed a gain in one direction. In addition, the loss in the opposite direction was larger than one would expect.
The easiest way to analyze data like this is to graph it, so I graphed the difference between the loss in each direction against the attenuation coefficients of the fiber and mode field diameter. Here is what I saw. (This is a scan of the actual graphed data from my dot-matrix printer.)
There was an obvious correlation. When the OTDR was testing a splice between high-loss fiber to a low-loss fiber, the measured loss in that direction would be greater. In the opposite direction, from low-loss fiber to high-loss fiber, the loss would be less or even show a gain.
That made sense. The biggest factor in fiber loss was scattering, and the OTDR makes measurements using the fiber’s scattering. If you go from a high-loss fiber to a low-loss fiber, the OTDR sees the loss but also sees a lower amount of backscattering, making the trace of the fiber after the splice lower and adding that difference to the measured loss of the splice.
In the opposite direction, the backscatter is higher after the splice, reducing the splice loss or even causing the OTDR to show a gain. Here is a diagram of what happens. Remember that higher backscatter means the fiber has higher attenuation.
We also had data based on the mode field diameter (MFD) or effective core size of the fiber. That was a bit more confusing, since the gainers came from connecting larger MFD fibers to smaller ones. But fiber engineers explained that fibers with smaller MFDs concentrated the light in a smaller core, and the higher density of the light caused more backscatter.
Interestingly, 25 years later, Corning published graphed data just like we had, so I overlaid their data on mine and got this graph.
Their data matches my data quite well, except the fibers 25 years ago had a much greater variation in mode field diameter specifications. This is proof of how much better fiber is made today compared to the past.
So why OTDRs show gainers is not a mystery; it’s simply an artifact of the OTDR measurement technique, which is based on backscatter. Whenever you make a splice or connector loss measurement with an OTDR, it’s going to have some uncertainty due to the differences in fiber backscatter on either side of the joint.
One way to visualize the difference in backscatter is to look at this experiment done by one of the Fiber Optic Association schools to teach this principle to students. They fusion spliced G.652 single-mode fiber to short sections of G.655 fiber, which has a larger MFD. Here is what they saw with the OTDR trace.
We often get sent this kind of trace by puzzled techs splicing and testing fibers. They are working on a network based on G.655 fiber (generally used for longer single-mode networks often using wavelength-division multiplexing) where that fiber has been spliced to a length of G.652 fiber already installed or connected by mistake.
You can lessen the uncertainty of this effect in OTDR measurements, but it’s time consuming. You can make OTDR measurements from both directions and average the results. Averaging will remove the effect of the difference in backscatter giving you an average of the loss of the splice or connector. It is an average, however, because there will be actual directional differences in the splice loss caused by the difference in the MFD of the fibers, but that difference is generally small compared to the backscatter errors.
**In this article, I described this problem mostly in relation to single-mode fibers, which is where most OTDR tests are made. But the same problem can show up in multimode fiber measurements, as this trace shows.
This is a splice between regular and bend-insensitive multimode fiber, which has a larger effective core diameter. The effect is the same as with single mode.