Understanding the Main OTDR Event Types

Advanced device for testing and troubleshooting fiber optic communications, Optical Time Domain Reflectometer (OTDR) An OTDR is an essential tool that generates a range of insights into the performance and integrity of an optical fiber. An OTDR works by sending a pulse of light down the fiber, analyzing the scattered and reflected light, and generating a “trace” report that identifies and maps events along the fiber. This data is valuable, highlighting expected or common events such as known splices or connections to more serious issues such as fiber breaks. Therefore, an OTDR is useful for multiple testing operations from initial fiber deployment to routine maintenance and troubleshooting.

As we mentioned in a previous article titled “Important OTDR Parameters,” learning how to set up and use your OTDR to generate the most accurate results based on the network and fiber parameters specified during testing is essential. There are also many different types of OTDRs on the market today that offer different features and technical performance capabilities. However, the types of events they are all designed to identify are fairly universal, so understanding what they are is essential to using this equipment effectively.

Continuing to read, we will now briefly cover the main types of events identified by OTDRs, along with examples.

Reflective events

Reflective events typically occur when there is a significant change in refractive index, causing the signal to be attenuated as light is reflected back to the OTDR. These events are typically associated with:

Connectors: Often identified by a sharp peak on the OTDR path, connectors can cause reflections due to the air gap interface.
Mechanical connections: These connections are less precise than fusion connections and may not be perfectly aligned, causing a reflection event, although sometimes they do not have as much reflection as a connector.

fiber disconnectionFiber fracture can appear as a reflective or non-reflective event, depending on factors associated with the specific fracture.

Non-reflective events (Fresnel)

Non-reflective events refer to attenuation without a corresponding reflection, and are often associated with stress-causing factors and result in a small loss of light signal. Unlike reflective events that generate a sharp peak, these events appear as gradual dips or steps along the OTDR path. Common examples of non-reflective events include:

Fusion splices: Fusion splices are the most precise method of aligning the core to join two fibers, and often generate very little signal loss at the junction.
Large bends: Sharp bends in the fiber can also result in significant signal loss without reflection due to light leaking from the high pressure.

Profit Events (“Winner”)

Sometimes an OTDR trace may show a gain event, which shows a clear increase in signal strength on the trace. However, this is not an actual gain in signal strength, but rather a perception of gain due to inconsistencies in the fiber properties. One of the most common scenarios that leads to this type of event is:

Fiber Mismatch: Different types and grades of fiber are not manufactured in the same way and often have different core sizes, mode field diameter (MFD) sizes, or numerical apertures, so connecting mismatched fibers will result in inconsistencies as the optical signal passes through. For example, if a G.652D single-mode fiber with a mode field diameter of 10.2 µm at 1550 nm is connected to a dispersion compensation fiber with a smaller mode field diameter of only 5 µm at 1550 nm (i.e., the signal goes from larger to smaller), the resulting artifact will show what appears to be signal gain even though no gain actually occurs.

It is also important to note that the gain will appear on the trace as a loss when testing in the opposite direction (i.e. from a small MFD to a large MFD). Therefore, it is recommended to test in both directions, so that the values ​​can then be averaged to ascertain the actual change in value.

Ghost Events (“Echoes”)

Sometimes, an OTDR may encounter and record spurious events, which are events that do not actually exist, due to excessive reflections between other fiber events. These events can sometimes be difficult to distinguish from real events, and are referred to as:

Ghost or echo reflections: Multiple reflections between high reflection points can cause the OTDR to misinterpret signals, resulting in a false gain event. This is often caused by a high reflection event, such as a bad connector or air gap, reflecting light multiple times.

Fiber weakening events

When light is transmitted through a medium such as optical fiber, there is natural attenuation of the fiber along with other events that result in a gradual loss of signal over a distance. This type of attenuation can be caused by:

Overall Fiber Attenuation: All fibers experience some level of signal loss over some distance, which is represented by the downward slope seen in all OTDR traces.
Bends and stress points: In addition to the large bends already mentioned in this article, other physical stresses and strains on the fibers can lead to additional weakening, which may not always appear as distinct events but as an increase in overall loss.

End of Fiber / End of Fiber (EOF)

Fiber termination is a critical event that identifies the physical end of the fiber being tested. It is identified by a sudden drop in the OTDR trace, indicating no further reflections or scattering.

Example of an OTDR trace showing multiple events.

The events shown above are over 30 km of G.652D single mode fiber:

Event 1: Connection
Event 2: Connection
Event 3: Winner
Event 4: Mosul
Event 5: Connection
Event 6: End of Fiber

Learn how to identify or display OTDR events.