Unexpected 'Live fiber detected' warning received

Applies to

FTBx-700 and MAX-700 series OTDRs incorporating SML optical ports


When operating an Optical Time Domain Reflectometer (OTDR), a 'Live fiber detected' warning may sometimes be unexpectedly displayed by the instrument, even when performing testing using the SingleMode Live (SML) port of the device.

This article combines information from various sources into a list of troubleshooting steps that can be followed to help localize issues.

Although primarily intended for users experiencing issues with 'Live fiber detected' warnings being received whilst performing testing using the SML port of a device, the information in this article may nevertheless still be of use when testing using ('dark') SingleMode (SM) optical ports.

Troubleshooting steps

Possible Cause 1: Fiber Under Test (FUT) is connected to incorrect optical port.

Proposed Solution

  • Check markings on instrument to identify optical port with a 'Live' designation.
  • Consult introductory sections of the OTDR user guide.


Possible Cause 2: High internal reflectance caused by poor connection, damaged or dirty optical port.

Check whether an acquisition is possible with the FUT disconnected.

If a 'Live fiber detected' warning continues to be displayed, stop the acquisition and continue with the actions below.

Proposed Solution


Possible Cause 3: Power of received optical signal is too high.


Received signals with an optical power in the range -65dBm to -40dBm will affect OTDR acquisitions and can cause added electronic noise, affecting the Dynamic Range of the instrument and therefore its ability to successfully resolve events.

Signals received at the SM Live port of a device with an optical power greater than 10dBm could damage instruments.

Signals received at the SM ('dark') port of a device with an optical power greater than -20dBm could damage instruments.

Proposed Solution

  • Using an appropriate Optical Power Meter (OPM), check the absolute power of expected wavelength/s to ensure they are within allowable limits.
  • Attempt an acquisition using reduced pulse width/s.
  • Consider connecting a suitable attenuator to the launch fiber or applying a mandrel wrap.


Possible Cause 4: Optical characteristics of Network Under Test (NUT) are affecting acquisitions.

Energy in the sidelobes of the spectral band of sources, active optical devices, Raman effects and abnormal Fresnel reflections can all effect an OTDR's ability to perform an acquisition, as can the manufacturing and tolerance differences between optical components.

Proposed Solution

  • Confirm that the issue is not specific to the current NUT / cable plant being tested.
  • Confirm that the issue is not specific to the instrument being used.
  • Consider connecting a suitable external filter to the launch fiber.
  • Check that transmission wavelengths greater than 1600nm are not present in the NUT.
  • If applicable, consider testing using an instrument with an optical configuration incorporating a bandpass filter with different rejection characteristics.
  • Contact the appropriate Technical Support Group for further advice.


Possible Cause 5: Device is defective.

Proposed Solution

  • Ensure that the relevant system / application software installed on the instrument is the most recent available.
  • Attempt a reset / recovery of the instrument.
  • Consider requesting an RMA for repair of the instrument.