How best to interpret OTDR traces?

Aug. 5, 2019
Improper use or interpretation of OTDR test results can result in wasted time, materials and money.

By NATE BROWN, FIS -- Optical Time Domain Reflectometers (OTDRs) are a valuable tool for measuring the performance of optical cables. They are often used to create a "picture" of a fiber optic cable when it is first installed so later comparisons can be made to help with troubleshooting network problems. How do they work? OTDRs send pulses of light into optical fibers at varying pulse widths. Then, they measure the small amounts of reflected light that are sent back from faults in the fibers. The device then determines the size and distance of the faults, and defines them as losses or changes in the cable's light-carrying capacity. 

However, there is a word of caution. Improper use or interpretation of OTDR test results can result in wasted time, materials and money. In fact, it is estimated that contractors lose as much as $100,000 annually due to improperly reading OTDR test results. It is critical to understand how to read an OTDR trace. To understand the results given by the OTDR, though, let's first dig deeper into how they work.

How Do OTDRs Work?

Think of an OTDR as radar that sends a pulse of light down the cable, looking for a return signal. When it finds one, the OTDR creates a display called a "trace" that can be used to identify breaks in the fiber optic cable, splices, connectors and excessive bends. In addition, it can measure the amount of light loss or attenuation in the fiber system. The OTDR trace is generated when a return signal is created by measuring the light scattered back toward the OTDR. To understand how this return signal is created, let's define two different ways that light is reflected. 

Reflectance: The peaks created on the OTDR trace are reflectance signals. These are produced when light is reflected back up the fiber from either the polished fiber end at a connector or a fault in the fiber. 

Backscatter: Backscatter refers to smaller signals that result from light interacting with the impurities in the optical fiber. When light hits the slight impurities that are inherently found in even the purest glass, it is scattered and a small amount goes back up the fiber link to the OTDR. The OTDR then amplifies and measures the backscatter. 

You will see both reflectance and backscatter measured in decibels (dB) on the vertical axis of your OTDR trace. The horizontal axis represents distance. How does the OTDR create a decibel measurement? It calculates the distance down the fiber using the speed of light in the glass fiber. This converts the trace from a simple length measurement into a graph of optical power in decibels.

How to Read Your Trace

OTDR displays will show a Y and X axis. The X axis measures distance, and the Y axis measures attenuation and reflection in dB. Before running your trace, select the appropriate fiber network length, pulse width and acquisition time. After starting the trace, the OTDR will display all connectors and splices as "events" along the fiber optic cable. These events should show a loss, as well as a reflective peak, if the event is showing significant reflection (such as a mated pair of connectors). The height of those peaks will indicate the reflection of that particular event. The slope of the fiber trace indicates the attenuation coefficient of the fiber cable, and is measured in dB/km.

What are the benchmark measurements you should look for?

Splice: <0.05dB for singlemode long distance and short links 

Mated Connector: <0.5dB for singlemode long distance and short links 

Reflectance:  >-40dB for single-mode 

Attenuation:  0.40dB/km at 1310nm, 0.25dB/km at 1550nm for singlemode long distance and short links

How can you measure splice or connector loss?

First place one of the markers or cursors (usually called 1 or A on your OTDR) just before the reflectance peak. Next, place the second marker (referred to as 2 or B on your OTDR) just after the reflectance peak. The OTDR will calculate the loss between the two markers. Reduce inaccurate measurements by making sure your markers are not placed on curved parts. What about measuring reflectance? Similarly, place the first marker before the reflectance peak and the second at the top of the peak.

Beware of Troublesome Events

One key to accurately reading your OTDR test results is to understand events that might present obstacles to your accuracy. For example, the loss of a good fusion splice is often too insignificant to be seen on the OTDR. To avoid confusion, know the lengths of all the fibers on your network so you'll know where unusual events might be located and won't get confused. Here are the top three troublesome events to look out for:

-- Ghosts are produced when there is a large reflection in a short fiber, which causes light to bounce back and forth. This event causes repetitions of a trace. You will know the difference because a ghost will not have any loss. Instead, it will be an equal distance from a highly reflective event. The key is to look for repetition. Also, note that ghosts tend to show up in the middle of noise after the end of the fiber optic cable. 

-- A nonreflective break occurs when a fiber cable has been shattered or has come in contact with liquid. This event prevents light from reflecting back to the OTDR, making it difficult to identify the break. 

-- A gainer refers to a splice in a fiber cable that displays as a gain in power. A splice does not cause a gain in light because it is a passive event that does not generate light. However, if the spliced fibers are mismatched, the splice may appear on the OTDR trace as a gain. Here's an example. If a splice goes from a larger core fiber to a smaller one, the difference in backscatter coefficients will show on the OTDR as a gain in light. 

OTDRs are valuable tools for testing fiber optic cables. Contact the experts at Fiber Instrument Sales (FIS) to learn more about the advantages of using this piece of test equipment. 

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Source: FIS Blog

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