When using an Optical Time Domain Reflectometer (OTDR), one of the common challenges technicians face is distinguishing between a genuine fault and a reflection. While OTDRs are invaluable tools for troubleshooting fibre optic cables, they can sometimes make it difficult to tell whether an anomaly is due to a real issue in the network or merely a reflection caused by connectors or splices. Let’s break it down and simplify the process. 
 

Understanding the Basics of an OTDR 

An OTDR works by sending light pulses through a fibre optic cable and measuring the light that is reflected back. This reflection can come from various points along the cable, such as connectors, splices, or faults like breaks or sharp bends. A genuine fault is typically caused by a physical issue with the fibre, while a reflection is more often caused by a discontinuity in the fibre or connections. 

The Key Difference: Fault vs. Reflection 

The main difference between a genuine fault and a reflection is the nature of the reflection itself. Here’s what to look for: 

Size of the Reflection 

 Reflection: Often smaller and typically occurs at connection points (like connectors or splices). These reflections happen because of a difference in the refractive index between two materials (like the fibre and a connector). 

Fault: A genuine fault (like a break or a significant bend) will result in a sharp drop in signal, and sometimes a large reflection at the location of the fault. 

Location and Consistency 

Reflection: Reflections usually appear in specific, consistent locations that correspond to connectors or splices. They’ll remain at the same place across multiple tests. 

Fault: Faults are typically isolated and cause a sudden drop or loss of signal, with the OTDR showing a "dead zone" beyond the fault. 

Shape and Pattern 

Reflection: The trace will show a quick, sharp dip, which then stabilises. You might see this several times if there are multiple connections along the fibre. 

Fault: A genuine fault usually results in a longer "dark" area on the trace, showing a complete loss of signal beyond that point. 
 

Helpful Tips for Identifying Reflections 

Check the Event Markers: Modern OTDRs allow you to add event markers. If you spot an event at a connector or splice, you’re probably dealing with a reflection. 

Use Launch and Receive Cables: These cables can help reduce the effect of the initial reflection caused by the OTDR’s own launch pulse, helping you get a clearer trace. 

Analyse the Distance: Reflections tend to appear closer to the OTDR, while faults show a significant distance away from the tester. 

Example: Connector Reflection vs. Fibre Break 

Let’s say you’re testing a fibre optic cable, and the trace shows a reflection near 5 meters. You examine the OTDR and see a sharp drop, but the signal resumes shortly after. This likely indicates a connector problem rather than a fibre break. If you saw a long stretch of signal loss instead, it would suggest a fibre break or severe bend. 

Keep Your OTDR Skills Sharp 

Now that you know the key differences between faults and reflections, here’s a tip: always double-check your OTDR setup and use launch cables for more accurate results. Sometimes, a little reflection is just a part of the process, but knowing how to tell the difference will save you time and effort troubleshooting. 
 

 

FAQs 

 

What is the purpose of an OTDR? 

An OTDR is used to test fibre optic cables by sending light pulses through the fibre and measuring the time it takes for the light to return, allowing you to detect faults, splices, and other issues. 

How do I identify a connector reflection on my OTDR trace? 

A connector reflection usually appears as a small, sharp dip in the trace, close to the start of the test. If the dip is small and consistent, it is likely due to the connector. 

Can a reflection ever cause a real fault? 

No, a reflection itself does not cause a fault. However, a bad connection or poor splice that causes a reflection could be a sign of an underlying issue that needs to be fixed. 

What are launch cables, and why should I use them? 

Launch cables are specially designed cables used to extend the length of the fibre under test. They help reduce the effects of initial reflections from the OTDR's pulse, making it easier to get accurate readings. 

What should I do if I detect a fault in my OTDR trace? 

If you detect a fault, locate the exact spot using your OTDR, then perform a physical inspection of the fibre. You may need to replace the cable or repair the damaged section. 
 

 

By following these tips and examples, you can confidently differentiate between reflections and genuine faults in your OTDR trace, making your troubleshooting more efficient. Happy testing.