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OTDR Testing (Optical Time Domain Reflectometer)

OTDR Testing (Optical Time Domain Reflectometer)

Fiber optics provide the most powerful and reliable networking options in the modern world, but they come with certain costs.

Aside from the price point of the technology, fiber optic networking is much more complicated than copper-only systems, and installing and maintaining fiber optics requires specialized testing and troubleshooting.

In that pursuit, one powerful testing tool is known as an OTDR. This form of fiber optic testing can save time and improve results.

What Is OTDR?

An Optical Time Domain Reflectometer is a testing device that enables you to look at the integrity of fiber cables and junctions in a cable run. You can use it throughout the life of the cable. The device proves valuable when installing segments. You can apply it to network certification. It continues to provide value for routine maintenance and acute troubleshooting.

The specific advantage provided by an OTDR is its ability to virtually image the tested run. You can identify specific failure points and regions of signal loss. This test identifies issues with the cable and also with components along the run such as bends, connection points, and splices.

How Does It Work?

The benefits of OTDR testing clarify when you take a closer look at how the device works.

In principle, it utilizes light pulses to measure signals through a run of fiber optic cable. The OTDR itself creates the light pulses at one end of the cable. As light travels through the cable, some transmits while some scatters.

The OTDR can measure the signal of returning light (representing a fraction of the total scattering inside the fiber). By comparing the return signal to expectations and adjusting for specific characteristics of the cable (like length, width, transmission form, and more), it can calculate signal loss and attenuation through the tested run.

OTDR devices are sensitive enough to measure signal return times. This is what allows it to map signal viability throughout the run and create virtual imaging.

How Can You Use OTDR?

With such capabilities, OTDR tests are great for cable installation. When a splice is completed or a segment is added, you can quickly test signal integrity through the new installation to ensure viability. 

Beyond that, it’s a powerful maintenance tool as well. Regular OTDR tests verify cable performance or spot early signs of degradation in a network.

Of course, when unexpected problems arise, the OTDR test can assist with locating the specific point of failure in the cable.

With all of that said, OTDR tests require customization and a deep understanding of fiber optics. Because the tests rely on measuring scattering off of precisely timed signal pulses, you need to adjust each test for the specific network design under scrutiny.

Testing Parameters and Best Practices

Ultimately, OTDR tests boil down to setting the right parameters, and finding the right parameters depends entirely on what you are testing. All OTDR tests run along some length of deployed fiber optic cables (referred to as “test cables” in the sections below). 

Best practices revolve around adjusting the length of the test pulse, the width of the pulse, the wavelength of the signal, and the number of repetitions for your averages. Recommended parameters in these tips are all provided by the Fiber Optic Association (FOA).

Length

Your first essential parameter is pulse length (or range). You need to figure out the distances at play when the OTDR makes a measurement. As a rule, you want the range setting on the OTDR to double the length of the test cable.

That said, take into account the trade-offs you get when adjusting the test range. When you increase the range, you lower your trace resolution. Meanwhile, shorter ranges can introduce distortions in the trace. You can aim for the sweet spot between these problems, or you can run multiple tests, focusing on noise reduction in some and resolution in others.

Width

Your next parameter is the pulse width. Usually measured in nanoseconds, this determines the timing of each signal pulse.

Ideally, aim for the shortest pulse width you can for a given test. This provides the highest resolution. If a pulse width is too short, it can introduce errors in the test.

On average, pulse widths sit between 10 and 30 nanoseconds.

Wavelength

The signal wavelength depends on the cable being tested. As different types of fiber signals operate at different wavelengths, your test pulse needs to sit within the viable transmission wavelengths of your fiber cable. It’s pretty straightforward.

Typically, multimode fiber tests run at 850 nm wavelengths. Single-mode tests operate at 1310 wavelengths. 

To the extent possible, minimizing the test wavelength reduces backscatter and helps to eliminate signal noise. This typically provides a clearer indication of signal integrity.

In many cases, it helps to test across the viable wavelength spectrum. Simply average the results and get a more robust picture.

Averages

Speaking of averages, your OTDR can perform multiple pulses and average the results, all right there in the device. You can set how many measurements you want in a test. Naturally, more measurements provide more robust and reliable averages. The cost comes in time.

For perspective, industry standards put average test numbers between 16 and 64 for an OTDR test.

That covers the essentials of OTDR testing. With knowledge and best practices, you can put this technology to work and broaden your resources for installing, maintaining, and troubleshooting fiber optic lines.

Additional Learning Center Resources