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What are Ghost Reflections in OTDR Traces?

Ghost OTDR events are often false reflections, non-physical events, that may appear on an OTDR trace under certain conditions, even though no actual connector, splice, or reflective interface exists at the indicated distance.

Ghost OTDR reflections are often misunderstood and misdiagnosed. These reflections are false, non-physical events that may appear on an OTDR trace under certain conditions, even though no actual connector, splice, or reflective interface exists at the indicated distance. These fake events are created by the test pulse bouncing back and forth between two highly reflective events.

Visual explanation of the multiple reflections that cause OTDR Ghost Events to appear on a fiber optics trace. (Results from a VeEX FX150+ mini OTDR)

In certain cases, the instruments' advanced (smart) trace analysis may not flag them as wrong, so it requires awareness, knowledge and experience to identify and remove these anomalies, when they occur. Understanding and correctly identifying Ghost Events is crucial because:

  • They can be mistaken for real connectors or breaks.

  • They can lead to incorrect alarming, diagnosis, unnecessary truck rolls and increased operational cost.

  • They may confuse automated event analysis in PON and long-haul links.

Ghost reflections that show up after the end of fiber are easier to identify and are often detected and ignored by the link map analysis and events table, however, they may still show on the trace if you zoom out to a longer span. Even though the test set is doing its job in these cases, inexperienced users may get confused when they look at the trace. The example below illustrates one of those cases.

OTDR trace with a Ghost Reflection event after the end of the fiber under test, showing the distance relationship between the events. (Trace taken with a VeEX FX150+ mini OTDR)

OTDR event analysis (link map view) of a trace with Ghost Reflections properly detected, identified and ignored by the automated analysis. (Result from a VeEX FX150+ mini OTDR)

Although the link map summary generated by the OTDR’s smart analysis correctly identifies all link elements and the fiber end, the presence of a ghost reflection in the trace still indicates a problem—either with the link itself (strong internal reflection) or with the measurement setup (test configuration or connections). In the example above, the result is a FAIL and indicates that you must inspect the connection at 68 ft (20.7 m).

How to Identify Ghost Reflections

These ghost reflections are generated by multiple reflections of a significant Fresnel event, such as a dirty or damaged connector, connector mismatch air gaps, or an open fiber end. These reflections re-enter the fiber, bounce back, and are detected again by the OTDR.

Ghost reflections on an OTDR trace may be identifiable by some of their specific characteristics:

  • They follow a strong reflective event. Ghost reflections occur when a connection has very high reflectance, generally worse than -40 dB and more typically when reflectance exceeds about -30 dB.

  • They occur at distances mathematically related to earlier reflections (often multiples of the fiber length between strong reflections).

  • They show reflection but no corresponding loss. 

  • They do not change when the fiber is physically inspected.

  • They tend to occur when testing short fibers.

Challenges

When present, accurate identification of ghost reflections is critical for reliable fiber certification and maintenance, yet it can be difficult and not always straightforward. Even the smart link map analysis in modern test sets can be misled by these events, and correctly interpreting the results still requires technician experience.

  • If there is only one highly reflective event within the fiber under test, the corresponding ghost reflection will appear at approximately twice the distance from the OTDR port. When multiple highly reflective events are present on the fiber link, the situation becomes more complex, and a ghost reflection may appear at approximately twice the distance from the preceding highly reflective event.
  • Ghost reflections that occur within the fiber span exhibit no associated loss. However, ghost reflections that appear beyond the physical end of the fiber may display measurable loss, particularly when they are located on the trailing edge of the end reflection.
  • Ghost reflections are less apparent when using a larger pulse width because the higher overall signal level increases backscatter, which can obscure or mask the ghost reflection.

Can Ghost Reflections Be Removed?

Once identified, ghost reflection could be eliminated by:

  • Proper receive cable termination.

  • Use launch and receive fibers (launch cables).

  • Thoroughly clean and inspect all connectors (eliminate highly reflective connections).

  • Use APC connectors where possible.

  • Reduce pulse width if dynamic range allows.

  • Avoid testing into open fiber ends.

  • Use proper fiber termination during testing.

  • Have a good idea of the link's characteristics, so you can detect anything that seems out of place.
  • If the separation between ghost events is a multiple of your launch cable length, then inspect all the connections, because one of them may be highly reflective.

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