Fiber Optic Testing: Understanding Key OTDR Event Types

A sophisticated device for fiber optic communications testing and troubleshooting, the Optical Time Domain Reflectometer (OTDR) is an essential tool that generates a range of insights about the performance and integrity of optical fibers. An OTDR operates by sending a light pulse down a fiber, analyzing the scattered and reflected light, and generating a "trace" report that identifies and pinpoints events along the fiber length. This data is valuable, highlighting expected or common events like known connection points or splices to more serious issues like fiber breaks. Therefore, an OTDR is beneficial for multiple testing processes from the initial fiber deployment through routine maintenance and troubleshooting issues.

As we covered in a previous article, "Important OTDR Parameters," learning to set up and use an OTDR to generate the most accurate results based on the specific network and fiber parameters during testing is necessary. There are also many different types of OTDR devices in the market today that offer various 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 a key element to effectively using this device.

Reading on, we'll now briefly cover these key event types that are identified by OTDR devices, along with examples

Reflective Events

Reflective events typically occur when there is a significant change in the refractive index, resulting in signal attenuation with light reflection back to the OTDR. These events are usually associated with:

• Connectors: Often identified by a sharp peak on the OTDR trace, connectors can cause reflections due to the air-gap interface.
• Mechanical Splices: These splices are less precise than fusion splices and may not be perfectly aligned, causing a reflective event, although they sometimes do not have as large a reflection as a connector.
  
• Fiber Breaks: A fiber break can appear as a reflective or a non-reflective event, depending on factors associated with the specific break.

Non-Reflective (Fresnel) Events

Non-reflective events indicate attenuation without a corresponding reflection, often related to stress-inducing factors that lead to a small loss of the light signal. Unlike reflective events that generate a sharp peak, these events appear as gradual dips or steps on the OTDR trace. Common examples of non-reflective events include:

• Fusion Splices: The most precise core alignment method for joining two fibers, fusion splices often generate a very small signal loss at the splice point. 
• Macrobends: Sharp bends in fiber can also cause significant signal loss without a reflection due to light escaping from high stress.
  
  

Gain Events ("Gainer")

Occasionally, an OTDR trace may show a gain event, which shows an apparent increase in signal strength on the trace. However, it is not an actual gain in signal strength, but rather a perception of a gain that results from inconsistencies in the fiber characteristics. One of the most common scenarios that results in this type of event is a:

• Fiber Mismatch: Various types and classes of fibers are not constructed in the same manner and often have different core sizes, mode field diameter (MFD) sizes, or numerical aperatures, so joining mismatched fibers produces an inconsistency as the optical signal passes through. For example, if a G.652D single-mode fiber with a MFD of 10.2µm @ 1550nm is spliced to a dispersion compensating fiber with a smaller MFD of just 5µm @ 1550nm (ie, signal going from larger to smaller), the resulting trace will show what appears to be a signal gain despite a gain not actually occuring.
  
  It is also important to point out that a gain will appear on a trace as a loss when testing in the opposite direction (ie, small MFD to large MFD). Therefore, it is recommended that testing be performed in both directions, so the values can then be averaged to ascertain the actual change in value.

Ghost Events ("Echoes")

Occasionally, an OTDR may incur and register false events that do not actually exist, caused by excessive refelctions between other fiber events. These events can sometimes be difficult to differentiate from real events and are referred to as:

• Ghost or Echo Reflections: Multiple reflections between high reflectance points can cause the OTDR to misinterpret the signals, creating a false gain event. This is often due to a high reflectance event, such as a bad connector or air gap, reflecting light back multiple times.

Fiber Attenuation Events

When transmitting light across a medium like optical fiber, there is both a natural fiber attenuation event along with other events that produce a gradual loss of signal over a distance. This type of attenuation can be caused by:

• General Fiber Attenuation: All fibers experience some level of signal loss over distance, which is represented by the downward slope seen in all OTDR traces.
• Bends and Stress Points: In addition to macro bends already noted earlier in this article, other physical stresses and strains on the fiber can lead to additional attenuation, which may not always show as distinct events but rather as increased overall loss.

Fiber End / End of Fiber (EOF)

The end of the fiber is a crucial event that marks the physical end of the fiber under test. It is identified by a sudden drop in the OTDR trace, indicating no further reflections or scatterings.

An Example OTDR Trace Showing Multiple Events

Events shown above in a 30km length of G.652D single-mode fiber:

• Event 1: Splice
• Event 2: Splice
• Event 3: Gainer
• Event 4: Connector
• Event 5: Splice
• Event 6: Fiber End

Learn to Identify or Demonstrate OTDR Events