Optical Time Domain Reflectometer (OTDR) Trainer

 

The FOA OTDR Trainer is a aid to learning how to use an OTDR without owning a real OTDR and spools of cable. The OTDR Trainer uses software but works just like a real OTDR. It uses software for OTDRs that are used for analyzing traces offline. FOA provides traces that illustrate how the OTDR works and traces of real cable plants to analyze. The traces are raw data that is analyzed just like a real OTDR.

Before you start using the OTDR Trainer, make sure you have read the FOA Guide page on OTDRs so you understand the basics of how an OTDR works and makes measurements. The Trainer will let you use software to view and analyze traces to make measurements from actual traces.

Part 1 Getting Started

Get The Software And Learn To Use It
First you need to set up the software on your device. We recommend using Fiberizer® from VeEX available free from their website. Fiberizer comes in several versions, Fiberizer Cloud - an online version that works with any device, Fiberizer Mobile for Android or iOS devices or Fiberizer Desktop Plus for Windows PCs. You may choose the version compatible with your device. We recommend Fiberizer Desktop Plus for instructors using Windows PCs since it runs without need for an Internet connection and is easy to project for a class. We will use Fiberizer Desktop Plus for our description on using the FOA materials for training but it works with any of the versions.

Be sure to download the appropriate manual - Fiberizer Desktop Plus User Manual  - and read it before downloading and installing the software - Fiberizer Desktop Plus Download.

Also download the zip file of FOA OTDR TRACES which we will use as part of the training exercises.

After you download the software, Follow Section 3. GETTING STARTED in the Fiberizer manual to install the software and set up your language and distance units.

Get started using the OTDR software. Open the Fiberizer program.

We've noted some items you need to know to use Fiberizer. We will note sections in the Fiberizer instruction manual that provide more details.

1. The first column shows the folders ("Collections") of files saved. (Sec 5, p 17)
2. This is folder of the samples provided. You will be adding other folders and traces (sor files).
3. This is where you select a trace or traces to display and analyze. (Sec 6, p 26)
4. This is the Map of the trace, At the top, make sure the Map is selected and the small map is shown below. You can grab the corners of the red square on the map and use it to zoom in on a section of the trace.
5. Is the name of the file or files (drop down menu) you are currently viewing.
6. There are vertical and horizontal sliders that also allow you to zoom in on the display.
7. Two point loss measurement (Sec 6.4, p 30)
8. Splice/connector loss measurement. (Sec 6.4, p 30)

The trace is the blue line on the left of the display and the dark blue segment in the bottom right of the display is the noise of the OTDR after the end of the fiber.

Viewing and Analyzing Traces (Sec 6.4, p 30)
The Fiberizer software comes with some sample traces to help you get started, Open one of the sample traces. We chose the first one.

Note the difference in splice loss - 0.332 dB with the 2 point method and 0.223 dB with the LSA method - the difference of 0.09 dB represents the loss of the fiber - 0.3 km x 0.3 dB/km. Always better to use LSA with splice and connector loss.

Automatic Analysis
The software can automatically analyze the trace. (sec 6.2 p 27)
Change the mode by hovering your pointer over the trace file and click on the pencil icon which appears on the right. Then click on the flask icon on the upper right corner of the screen. The Analysis thresholds window will appear. Click on Default and Analyze.

Parameter Demo Traces are 26 traces taken of the same multimode cable plant, with each of the OTDR parameters changed to allow you to see the effects of changing these parameters.  (We chose multimode fiber because its higher attenuation makes it easier to demonstrate some characteristics. There are plenty of singlemode fibers in the collections also and you can add any sor files you have yourself.) Traces highlight changes in parameters such as: wavelength, range, pulse width, fiber index of refraction, number of averages, and more. Traces and FTTx Traces are traces to analyze.


Comparing Traces
Our first lesson in learning about OTDRs involves understanding how important setting up the OTDR test parameters is to making good measurements. Along the way, we'll see some fiber characteristics demonstrated and lean some new things about fibers. To do this we need to know how to compare traces.

But first, we need to learn how to compare traces.

Go to the Parameter Demo Traces collection and click on the boxes in front of traces w-850 and w-1300 to select them for display. Click on the comparison icon above the list of traces. (It's faint but looks like several traces being compared.) That will give you a display of both traces. If the traces do not line up on the left like shown below, you can grab one of the traces and move it up or down until they align.

The large red arrow shows the drop down menu at the center of the display that shows the fiber being used for comparison and it's data table on the right. Choose the other fiber and it will show it's data table.

You can zoom using the red square, same as with a single trace.


 

Part 2 - Understanding OTDR Setup - Test Parameters

One of the first things to learn about OTDRs is how to set the options in test parameters. Range, wavelength, pulse width, averages, etc. all need to be understood so the operator knows what effect they have on the trace and how to choose the proper setup for a given test situation. Learning how to interpret traces also requires having access to large numbers of cables, including some very long ones. That's not easy in a lab and impossible at home. 

Test Wavelength
The attenuation of optical fiber varies over its wavelength range, so testing at the proper wavelength is important. The OTDR graphically demonstrates the difference in wavelength as shown in the display above we used to to show how to compare traces. While you have those two traces open, visually compare the slopes of the two traces, noting how the 850nm trace has a slope of over 3 dB/km shown in the data table. Use the drop down menu at the top center to change to the 1300 nm trace and see the attenuation of the fiber at 1310 nm. It's much lower. How about splice or connector loss? Is it sensitive to wavelength? Set up the loss test on the connection in the middle of the display and test it at both wavelengths too. Just set up on one wavelength, note the loss in the data table, then switch to the other trace. How close are the connection losses at the two wavelengths?

Measurement Range
Some OTDRs require setting the distance range manually. The range should be set at about twice the length of the cable plant being tested so the test pulse has time to go to the far end and return. (Remember time is distance in the OTDR.) If you set the range too short, you may get traces like the two at the bottom where the distance was set too short.

Averaging and Pulse Width
There are two setup parameters that affect the distance the OTDR can reach - how long a cable it can test, averaging and pulse width. Here are three traces showing the effect of averaging.

The pulse on the bottom was is the average of two test pulses sent down the fiber, the next one up was averaged over 64 tests and the top on was averaged 1024 times. Note how averaging reduces noise and  improves the "signal to noise" ratio so you can see further. The last quarter of the bottom trace is not measurable because of the noise, averaging 64 times improves the trace so the end can be measured, but with some uncertainty. The trace that is averaged 1024 times allows making measurements to the end, especially if a LSA measurement  is used. Here is a trade-off - more averages means longer distance capability and less noise in the trace to affect measurement uncertainty, but more averages takes more time, making testing take longer.

The other way to get more distance capability is to put more energy into the test pulse by widening it. Widening the pulse width means there is more energy - more signal - in each pulse. In the traces below, the lowest trace has the shortest pulse and the least energy, As you widen the pulse, the energy in the pulse increases, shown in the rising levels in the display. But there are trade-offs. A wider pulse has poorer resolution. You can easily see the connection in the lower pulses, but it becomes harder to see with the wider pulses in the two top traces. The flat beginning of the two top traces is because the widest pulses overload the OTDR receiver at short distances, creating a long dead zone.

You can see the effects of the pulse width in the traces here, as the lower pulse is the shortest with the best resolution but has the most noise. As we go to wider pulses, you get less noise and more range, but less resolution. In the top trace, we have lots of power, but the events are spread out so much to be hard to distinguish.

It is a matter of trade-offs. With longer distances, more averages and wider test pulses are often both necessary.

Index Of Refraction
Another parameter that can be set on the OTDR is the index of refraction of the fiber. The OTDR calculates the length of the fiber using the time to the even  and the speed of light in the fiber. The speed of light in the fiber is determined by the index of refraction of the glass which can vary by a few percent. This is the effect of a change in the index of refraction in the OTDR setup.

Here we have the same trace taken with an index of refraction of 1.4715 and 1.4915, within the range of standard fibers. The connection in the trace is showing a difference in distance of 42 meters (0.04216 km) at only about 3 km away from the OTDR. If you know the index of refraction of the fiber in the cable you are testing, you can set that in the OTDR parameters. If you don't know it, don't change it.

But remember that the OTDR measures fiber length, not cable length, and the typical cable has about 1% excess fiber length to prevent stressing the fiber when the cable is pulled. That too adds uncertainty. Some users suggest calibrating the OTDR for the cable you are testing - if you know the cable length, you can adjust the index of refraction so the OTDR reads that cable length, but it's only good for that cable. The real need to get accurate length measurements is when trying to locate an underground break, but then you can test from both ends and compare to the original length to get a better estimate of the location.

Real Traces

Examine traces of real fibers: The FOA Traces and PON Traces collections have over 40 traces of real cable plants and 5 traces of FTTH PON couplers for you to analyze. Few classrooms can offer this many different traces to analyze - and not many have a 180 km cable run with two  gainer sections.

And watch out for ghosts!

FOA wishes to thank VeEX for permission to use their Fiberizer software in our OTDR trainer. And our compliments to them for making the ap available on multiple platforms that ensure anybody can use it.